EP1430298A1 - Endfire beampattern processing for acoustic interferometer - Google Patents

Endfire beampattern processing for acoustic interferometer

Info

Publication number
EP1430298A1
EP1430298A1 EP02763089A EP02763089A EP1430298A1 EP 1430298 A1 EP1430298 A1 EP 1430298A1 EP 02763089 A EP02763089 A EP 02763089A EP 02763089 A EP02763089 A EP 02763089A EP 1430298 A1 EP1430298 A1 EP 1430298A1
Authority
EP
European Patent Office
Prior art keywords
acoustic
signal
sample
reflection coefficient
interferometer
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP02763089A
Other languages
German (de)
French (fr)
Inventor
Franciscus Petrus Gijsbertus Driessen
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Original Assignee
Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO filed Critical Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Publication of EP1430298A1 publication Critical patent/EP1430298A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/34—Generating the ultrasonic, sonic or infrasonic waves, e.g. electronic circuits specially adapted therefor
    • G01N29/348—Generating the ultrasonic, sonic or infrasonic waves, e.g. electronic circuits specially adapted therefor with frequency characteristics, e.g. single frequency signals, chirp signals
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04—Analysing solids
    • G01N29/041—Analysing solids on the surface of the material, e.g. using Lamb, Rayleigh or shear waves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/04—Analysing solids
    • G01N29/11—Analysing solids by measuring attenuation of acoustic waves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/22—Details, e.g. general constructional or apparatus details
    • G01N29/223—Supports, positioning or alignment in fixed situation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/22—Details, e.g. general constructional or apparatus details
    • G01N29/225—Supports, positioning or alignment in moving situation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/34—Generating the ultrasonic, sonic or infrasonic waves, e.g. electronic circuits specially adapted therefor
    • G01N29/346—Generating the ultrasonic, sonic or infrasonic waves, e.g. electronic circuits specially adapted therefor with amplitude characteristics, e.g. modulated signal
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/44—Processing the detected response signal, e.g. electronic circuits specially adapted therefor
    • G01N29/449—Statistical methods not provided for in G01N29/4409, e.g. averaging, smoothing and interpolation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/44—Processing the detected response signal, e.g. electronic circuits specially adapted therefor
    • G01N29/46—Processing the detected response signal, e.g. electronic circuits specially adapted therefor by spectral analysis, e.g. Fourier analysis or wavelet analysis
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
    • G01N29/44—Processing the detected response signal, e.g. electronic circuits specially adapted therefor
    • G01N29/48—Processing the detected response signal, e.g. electronic circuits specially adapted therefor by amplitude comparison
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00—Indexing codes associated with group G01N29/00
    • G01N2291/01—Indexing codes associated with the measuring variable
    • G01N2291/015—Attenuation, scattering
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00—Indexing codes associated with group G01N29/00
    • G01N2291/01—Indexing codes associated with the measuring variable
    • G01N2291/018—Impedance
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00—Indexing codes associated with group G01N29/00
    • G01N2291/04—Wave modes and trajectories
    • G01N2291/042—Wave modes
    • G01N2291/0423—Surface waves, e.g. Rayleigh waves, Love waves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00—Indexing codes associated with group G01N29/00
    • G01N2291/04—Wave modes and trajectories
    • G01N2291/044—Internal reflections (echoes), e.g. on walls or defects
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00—Indexing codes associated with group G01N29/00
    • G01N2291/10—Number of transducers
    • G01N2291/103—Number of transducers one emitter, two or more receivers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2291/00—Indexing codes associated with group G01N29/00
    • G01N2291/26—Scanned objects
    • G01N2291/263—Surfaces
    • G01N2291/2632—Surfaces flat

Definitions

  • the present invention relates to a method for determination of an acoustic reflection coefficient by an acoustic interferometer as defined in the preamble of claim 1. Also, the present invention relates to an arrangement for determination of an acoustic reflection coefficient by an acoustic interferometer as defined in the preamble of claim 12.
  • the reflection coefficient of such a sample relates to the reflection of acoustic waves on its surface, and can be defined as the ratio of the amplitudes (or energy) of the reflected and incoming acoustic waves.
  • the reflection coefficient is closely related to the acoustic impedance of the sample material, and correlated to acoustical properties of the measured material such as the sound propagation velocity and physical properties as mass density and compression modulus. Further, the reflection coefficient is influenced by a number of wave related properties such as frequency and angle of incidence, and environmental conditions such as temperature, pressure, and (properties of) surrounding medium.
  • An acoustic interferometer for measuring the reflection coefficient of a material consists of four basic elements viz., an acoustic transmitter element for generating an acoustic wave, a tube in which the acoustic wave propagates, the specimen, and an acoustic receiver element for receiving reflected acoustic waves.
  • the most frequently encountered acoustic interferometer comprises a tube either filled with air or water.
  • water filled acoustic interferometers may be capable of measuring both acoustic signals being reflected or transmitted by the sample by an acoustic receiver element placed before and after the sample, respectively.
  • acoustical mode is able to propagate inside the tube, being a plane wave propagating along the inner axis of the tube.
  • This cut-off frequency is related to the shape and dimensions of the inner cross section of the tube.
  • acoustic interferometers are operated below the cut-off frequency, and only the reflection coefficient related to normal incident waves can be measured.
  • the most straightforward method for determining the reflection coefficient is to transmit a short continuous wave (CW) pulse, and to measure the amplitude of both the transmitted and reflected wave.
  • the pulse has to be short enough to prevent interference with its own reflection from the specimen.
  • low frequency (and therefore long wavelength) CW pulses cannot be used, since this is usually conflicting with the cut-off frequency related to the inner dimensions of the cross section of the tube. Due to this and the fact that the envelope of short CW pulses can be highly distorted by the acoustic properties of the specimen and of the measurement equipment itself, this method is generally not very useful.
  • SWR Standing Wave Ratio
  • ASTM C384-90 standard test method as published by the American Society for Testing Materials in: "ASTM C 384-90 Standard Test Method for Impedance and Absorption of Acoustical Materials by the Impedance Tube Method".
  • ASTM C 384-90 Standard Test Method for Impedance and Absorption of Acoustical Materials by the Impedance Tube Method uses the standing wave pattern that will arise due to interference of the reflected front end of a relative long CW pulse and the not yet reflected back end of the same pulse.
  • the shape of the pulse will be changed (distorted) due to the reflection. Especially near absorption peaks (often areas of interest) this distortion can be severe. For CW pulses this distortion will be mainly at the beginning and the end of the pulse. For reliable measurements, the pulse should be long enough to have a long enough 'middle part'. Due to multiple reflections at the end and the beginning of the tube, this will put a restriction on the lower bound of the frequency range that can be measured. Such a restriction also applies to SWR and related methods.
  • Source ringing relates to the fact that, generally, an acoustic transmitter (a source) cannot stop emitting acoustic power stepwise.
  • the acoustic transmitter will need some extra time in order to decrease the signal gradually. This extra transmission is called ringing, and the amount of ringing is determined by the 'Q-factor' of the acoustic transmitter and its suspension.
  • the present invention provides a method for determination of an acoustic reflection coefficient by an acoustic interferometer arrangement, as defined above, characterised in that the method comprises the following steps: measuring by the at least one acoustic signal receiver at each of a plurality of intermediate positions between the acoustic signal transmitter and the sample holder, an acoustic signal comprising a direct signal portion from the acoustic signal transmitter and a reflected signal portion from the sample; recording the acoustic signal for each intermediate position as a function of the intermediate position, separating the direct signal portion and the reflected signal portion from the acoustic signal measured at each intermediate position of the at least one acoustic signal receiver by endfire beampattern processing to render a separated direct signal portion and a separated reflected signal portion, and determining the a
  • the method of the present invention is capable of strongly suppressing the measured effects relating to source ringing and spurious reflections by applying an endfire beampattern processing technique. Moreover, the method of the present invention provides the ability to measure the reflection coefficient of a material over a broad frequency range, without the need for any further data manipulation such as smoothing or averaging.
  • the endfire beampattern processing method has the capability to separate direct signals, coming from the acoustic source, and reflected signals that travel in opposite directions inside the tube, even if they are strongly interfering. Also, due to the separation, the complete acoustic signal of both direct and reflected signals (including the source ringing and spurious reflections) as received by an acoustic receiver, is available for processing. Hence it is possible to use the middle part of the pulses, which is not possible with the SWR-like methods known from the prior art.
  • the present invention relates to an arrangement for determination of an acoustic reflection coefficient of a sample by an acoustic interferometer as described above, characterised in that the at least one acoustic signal receiver is arranged to measure at each of a plurality of intermediate positions between the acoustic signal transmitter and the sample holder, an acoustic signal comprising a direct signal portion from the acoustic signal transmitter and a reflected signal portion from the sample; and that the signal processing means are arranged:
  • the present invention relates to a computer program product to be loaded by a signal processing means in an arrangement for determination of an acoustic reflection coefficient of a sample by an acoustic interferometer, the acoustic interferometer comprising a tube, a sample holder for holding the sample, an acoustic signal transmitter, and at least one acoustic signal receiver, the sample holder being located inside the tube, the acoustic signal transmitter being located inside the tube, characterised by the at least one acoustic signal receiver being arranged to measure at each of a plurality of intermediate positions between the acoustic signal transmitter and the sample holder, an acoustic signal comprising a direct signal portion from the acoustic signal transmitter and a reflected signal portion from the sample; the computer product, after being loaded, providing the signal processing means with the capacity: - to record the acoustic signal for each intermediate position as a function of the intermediate position,
  • the present invention relates to a data carrier provided with a computer program product as described above.
  • Figure 1 shows schematically an acoustic interferometer arrangement which uses the arrangement and method according to the present invention
  • Figure 2 shows schematically a computer system for use in the arrangement and method for acoustic interferometry according to the present invention
  • Figure 3 shows a signal plot of CW pulse measurements by acoustic interferometry from the prior art
  • Figure 4 shows a plot of direct and reflected CW pulses according to the arrangement and method for acoustic interferometry of the present invention, aligned in the time domain
  • Figure 5 shows a plot of direct and reflected CW pulses according to the arrangement and method for acoustic interferometry of the present invention, aligned in the time domain;
  • Figure 6a shows a plot of direct and reflected CW pulses processed in accordance with the arrangement and method for acoustic interferometry of the present invention
  • Figures 6b shows a plot of reflected CW pulses processed in accordance with the arrangement and method for acoustic interferometry of the present invention
  • Figure 6c shows the plot of Figure 6b on an enlarged scale
  • Figure 7 shows a plot of the reflection coefficient as a function of CW pulse frequency, determined by the arrangement and method for acoustic interferometry according to the present invention.
  • FIG. 1 shows schematically an acoustic interferometer arrangement which uses the arrangement and method according to the present invention.
  • the acoustic interferometer arrangement comprises a tube 1, a sample holder 2, an acoustic transmitter 3, and an acoustic receiver 4.
  • the sample holder 2 is mounted on one end. It is noted that, if required, the sample holder may also be at some location inside the tube 1.
  • the sample holder 2 holds a sample 2' during measurements.
  • the acoustic transmitter 3 is provided for generating acoustic excitations. If required, the transmitter may also be located at some location inside the tube 1.
  • the acoustic receiver 4 for receiving direct acoustic signals from the acoustic transmitter 3 and reflected acoustic signals from the sample 2', (and possibly spurious reflections from e.g., the receiver 4) is mounted inside the tube 1.
  • the acoustic receiver 4 can be shifted along a guiding device 5, parallel to the length of the tube 1.
  • the guiding device 5 comprises means (e.g. a thin guiding cable driven by a stepping motor) to move the acoustic receiver 4 inside the tube 1 to a plurality of predetermined positions.
  • the guiding device 5 itself is preferably located outside the tube 1.
  • the acoustic transmitter 3 is connected to a signal generator 7 which produces the desired acoustic excitation for either the CW pulse method or the SWR method. Other signal patterns may be generated, if required.
  • the acoustic receiver 4 is connected to signal measuring equipment 8 which in some way measures (and stores) the output of acoustic receiver 4 that is based on an acoustic signal received by the acoustic receiver 4.
  • a control device 9 is connected to the guiding device 5 to control the movement of the acoustic receiver 4 along the length of the tube 1.
  • the signal generator 7, the signal measuring equipment 8 and the control device 9 are connected to (or, are incorporated in) a computer system 6, that is capable during experiments to control and monitor the functions of each apparatus 7, 8, 9, and to process measured signals.
  • the signal generator 7 may be implemented as an arbitrary waveform generator, which can generate any desired acoustic excitation.
  • the signal measuring equipment 8 may be a suitable voltage measurement device such as an analog/digital voltage converter, capable of measuring at a required sampling frequency.
  • the acoustic interferometer tube 1 used in the present invention has a length of 3 m with a constant circular inner diameter of 51 mm.
  • the tube 1 is preferably filled with water, although it is also possible to do measurements in air or other gases or liquids.
  • the acoustic interferometer tube 1 is mounted on a platform (not shown) and has a tilting angle ⁇ of approximately 30° with the horizontal plane for degassing purposes.
  • the tube 1 may comprise provisions (not shown) for adjustment of the temperature and pressure of the medium within the tube 1, as is known to persons skilled in the art.
  • FIG. 2 shows a general overview of a computer system 6 for use in the arrangement and method for acoustic interferometry according to the present invention, comprising host processor means 21 with peripherals.
  • the host processor means 21 are connected to memory units 18, 19, 22, 23, 24 which store instructions and data, one or more reading units 30 (to read, e.g., floppy disks 17, CD ROM's 20, DVD's, etc.), a keyboard 26 and a mouse 27 as input devices, and as output devices, a monitor 28 and a printer 29.
  • Other input devices like a trackball, a touch screen or a scanner, as well as other output devices may be provided.
  • an input/output (I/O) device 30 is provided for a connection to a network 31.
  • the memory units shown comprise RAM 22, (E)EPROM 23, ROM 24, tape unit 19, and hard disk 18. There may be provided more and/or other memory units known to persons skilled in the art. Moreover, one or more of them may be physically located remote from the processor means 21, if required.
  • the processor means 21 are shown as one box, however, they may comprise several processing units functioning in parallel or controlled by one main processor, that may be located remotely from one another, in any way known to persons skilled in the art.
  • the computer system 6 further comprises (the connections to) the signal generator 7, the signal measuring equipment 8 and the control device 9 to enable automated measurements and automated signal processing.
  • Figure 3 shows a signal plot of CW pulse measurements by acoustic interferometry from the prior art.
  • the logarithm of the envelope of the measured signal for each measurement is represented by a grey value indexed as shown in the vertical grey scale.
  • the vertical axis represents the hydrophone position index, i.e., the position of the acoustic receiver 4 along the guidance 5 relative to the starting position (which is a function of the actual frequency of the pulse signal and the acoustic characteristics of the tube 1). Shifting distance between consecutive hydrophone positions (i.e., index differs by 1) is 7 mm in the particular set up of this measurement.
  • the pulse length may be longer than would be possible in the SWR method.
  • the longer pulse length provides enlargement of the allowable frequency range (towards lower frequencies).
  • the method presented here uses an acquisition geometry comparable to that of the SWR method, but a different processing technique.
  • the processing technique is based on endfire beampattern processing (i.e., endfire beamforming), which is a special kind ofbeamforming.
  • endfire beampattern processing i.e., endfire beamforming
  • the beam pattern B of a receiver array can be calculated using the following general equation [1] as published in: R.O. Nielsen, "Sonar signal processing", Artech House, Norwood MA, USA 1991 :
  • the acoustic measurement is repeated for different positions of the acoustic receiver 4 in the tube.
  • the positions of the acoustic receiver 4 in the tube 1 may be chosen equidistant.
  • a synthetic array of receivers is formed, in which the measured signal for each receiver (position) represents a measured signal for the respective receiver in the synthetic receiver array.
  • acoustic waves travelling in the two opposite directions in the tube can be separated. This is caused by the fact that for each endfire beamforming operation the wave travelling in the opposite direction will be suppressed.
  • the suppression factor S can be calculated using the following equation [2]:
  • weighting for the received signals at different positions (i.e., the a k parameters in eq. [1]) can give significant improvement to the suppression factor S. Often the weighting is called shading or windowing.
  • An example of a suitable windowing function is a Hamming window (see R.O. Nielsen as referenced above). In some cases, especially when the number of wavelengths over the synthetic array is small (usually this is the case for low frequencies), it is better not to apply a windowing function, which is equivalent to have all au parameters equal to unity
  • the method according to the present invention is as follows:
  • the movable acoustic receiver 4 mounted inside the tube 1 is used.
  • the measurement is carried out under control of the computer system 6, and is repeated for different, preferably, equidistant receiver positions, so by combining the signals obtained at all these receiver positions, the overall measured signal for a synthetic receiver array is formed.
  • a wide range of frequencies may be measured.
  • the acquisition time window should be large enough to record the front of the direct CW pulse (i.e. directly originating from the acoustic transmitter 3, and not from reflections by the sample 2' or obstacles in the tube 1) and the end of the reflected pulse.
  • the CW pulses are generated by the acoustic transmitter 3 under control of the signal generator 7, preferably under control of the computer system 6.
  • the full wave i.e.
  • acoustic excitation generated by the acoustic transmitter 3 may also be a chirp signal or an arbitrarily shaped acoustic signal comprising a range of frequencies.
  • the computer system 6 preferably transfers all signal recordings from the signal measuring equipment 8 to the memory means 18, 19, 22, 23, 24 for storage and further signal processing.
  • the number of receiver positions and the distance between successive receiver positions are dependent on the acoustic characteristics of the tube and the pulse frequency used. Therefore, when performing a repeated measurement at a plurality of (possibly equidistant) positions, it will be appreciated that it may not be necessary at each position to measure signals at all desired frequencies. Moreover, the starting position for each frequency may be different. Using this information, it is possible to reduce the time to carry out all measurements in the synthetic receiver array, and to reduce the amount of collected data.
  • the sampling rate of the signal measuring equipment 8 may be adjusted to the actual frequency to be measured. For low frequency signals the sampling rate can be lower than for higher frequency signals. In this way, the amount of sampled data for each receiver position can also be kept relatively low.
  • the pulse height and pulse length of the input signal may also be adjusted to the actual sampling frequency.
  • the processing of the recorded signals in order to determine the reflection coefficient can be subdivided into three stages as described below:
  • the direct signal is the acoustic signal directly originating from the acoustic source (the acoustic transmitter 3). All recorded signals are given a time shift (in either the time domain or the frequency domain) to align the start of the direct pulse of all signals at the same point in time. In the frequency domain this is accomplished by using the exponential term of equation [1].
  • the processing means 21 of the computer system 6 fetch the recorded measurements, and aligns all recordings internally on the start of the direct pulse by adjusting the recorded time for each measured signal by a time-shift.
  • the start of the direct pulse can be deduced from the recorded signals itself, or can be calculated from the synchronisation information, receiver positions, and wave propagation velocity. Due to this operation the direct pulses will be aligned, but the reflected pulses are not aligned, since they travel in the opposite direction.
  • the processing means 21 calculate the sum (or average) of the aligned signals, which leads to a signal containing an unaffected direct pulse and a low amplitude residue of the reflected pulse, substantially equal to zero. Possibly, a weighting function is applied in the summing or averaging operation. It is noted that since the endfire beampattern processing technique uses an averaging operation, in the method of the present invention the signal to noise ratio is enhanced by a factor of about 10 10 log(M), with M being the number of receiver positions.
  • Figure 4 shows a plot of direct and reflected signals from CW pulses according to the arrangement and method for acoustic interferometry of the present invention, aligned in the time domain. It should be noted that only the first 5 ms of the measured signals are shown and that the full waveform of each signal is displayed instead of its envelope.
  • the reflected signal is defined as the signal reflected by the sample 2' or any obstacles in the tube 1.
  • the reflected signal originates from reflections of the direct signal on sample 2' or obstacles in the tube 1.
  • the same procedure as described for endfire beampattern processing of the direct signal is followed, with the exception that now the start of the reflected pulses are aligned at the same point in time. After averaging over all receiver positions, the reflected signal will be unaffected, and the direct signal will almost disappear.
  • Figure 5 shows a plot of direct and reflected CW pulses according to the arrangement and method for acoustic interferometry of the present invention, aligned in the time domain.
  • the ratio of the reflected signal processed by endfire beampattern processing (stage b) and the direct signal processed by endfire beampattern processing (stage a) determines the reflection coefficient.
  • the pulses should be long enough and the amplitude should be determined in the middle part of the pulses.
  • the mid-pulse detection only uses the middle part of a pulse in order to measure a signal having a constant amplitude in the measurement time frame.
  • the processing means 21 determine the middle part of the reflected signal processed in stage b and of the direct signal processed in stage a. Then, the ratio of the two signal levels (reflected and direct) in the middle part of the respective signals is calculated by the processing means 21.
  • Figure 6a shows the plot of the direct CW pulses processed in accordance with the arrangement and method for acoustic interferometry of the present invention.
  • Figure 6b shows the plot of the reflected CW pulses processed in accordance with the arrangement and method for acoustic interferometry of the present invention.
  • Figure 6c shows the plot of Figure 6b on an enlarged scale.
  • the processing means 21 apply a Hamming window to the signal in the vertical direction.
  • the suppression of the undesired signals is clearly shown since the remainder of the incoming (direct) signal in figure 6b and figure 6c is negligible and hardly noticeable.
  • the vertical scale has been enlarged to display the signal in more detail.
  • the reflected signal shown in Figure 6b is not noticeable in Figure 6a.
  • the suppression factor S is calculated by the processing means 21 using equation
  • the suppression factor S (eq. [2]) amounts to approximately 60.
  • Figure 7 shows a plot of the reflection coefficient of some arbitrary sample 2' as a function of CW pulse frequency, determined the arrangement and method for acoustic interferometry according to the present invention.
  • the endfire beampattern processing method has the capability to separate direct and reflected signals that travel in opposite directions inside the tube, even if they are strongly interfering. Also, due to the separation, the complete acoustic signal of both direct and reflected signals (including the source ringing and spurious reflections) as received by the acoustic receiver 4, is available for processing. Hence it is possible to use the middle part of the pulses, which is not possible with the SWR-like methods known from the prior art.
  • the method is performed using a single acoustic receiver 4, for recording both the direct pulse and the reflected pulse, there is no need for complicated calibration procedures of many separate receivers. Moreover, the positioning of the single acoustic receiver 4 by the guiding device 5 provides a flexible arrangement which can geared to very specific experimental conditions. However, it will be appreciated by persons skilled in the art, that instead of a synthetic receiver array a real receiver array comprising a plurality of acoustic receivers may be used as well.

Landscapes

  • Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Acoustics & Sound (AREA)
  • Probability & Statistics with Applications (AREA)
  • Mathematical Physics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Length Measuring Devices Characterised By Use Of Acoustic Means (AREA)
  • Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)

Abstract

Arrangement for determination of an acoustic reflection coefficient of a sample by an acoustic interferometer, the interferometer including a tube, a sample holder, a transmitter, at least one receiver and signal processing means, the sample holder and the transmitter located inside the tube. In this arrangement the receiver is arranged to measure at a plurality of intermediate positions between transmitter and sample holder, an acoustic signal including a direct signal from the transmitter and a reflected signal from the sample; and the signal processing means are arranged: -to record the acoustic signal as a function of the intermediate position, -to separate the direct signal and the reflected signal from the measured signals at each intermediate position by endfire beampattem processing, and -to determine the acoustic reflection coefficient from an amplitude ratio of the separated reflected signal over the separated direct signal.

Description

Endflre beampattern processing for acoustic interferometer
Field of the invention
The present invention relates to a method for determination of an acoustic reflection coefficient by an acoustic interferometer as defined in the preamble of claim 1. Also, the present invention relates to an arrangement for determination of an acoustic reflection coefficient by an acoustic interferometer as defined in the preamble of claim 12.
Prior art
Methods and arrangements for acoustic interferometry to measure a reflection coefficient of a sample are well known.
The reflection coefficient of such a sample relates to the reflection of acoustic waves on its surface, and can be defined as the ratio of the amplitudes (or energy) of the reflected and incoming acoustic waves. The reflection coefficient is closely related to the acoustic impedance of the sample material, and correlated to acoustical properties of the measured material such as the sound propagation velocity and physical properties as mass density and compression modulus. Further, the reflection coefficient is influenced by a number of wave related properties such as frequency and angle of incidence, and environmental conditions such as temperature, pressure, and (properties of) surrounding medium. An acoustic interferometer for measuring the reflection coefficient of a material consists of four basic elements viz., an acoustic transmitter element for generating an acoustic wave, a tube in which the acoustic wave propagates, the specimen, and an acoustic receiver element for receiving reflected acoustic waves. The most frequently encountered acoustic interferometer comprises a tube either filled with air or water. Generally, water filled acoustic interferometers may be capable of measuring both acoustic signals being reflected or transmitted by the sample by an acoustic receiver element placed before and after the sample, respectively.
Below a certain frequency, only one acoustical mode is able to propagate inside the tube, being a plane wave propagating along the inner axis of the tube. This cut-off frequency is related to the shape and dimensions of the inner cross section of the tube. Usually, acoustic interferometers are operated below the cut-off frequency, and only the reflection coefficient related to normal incident waves can be measured.
From the prior art, two main methods for measuring the reflection coefficient are known. The most straightforward method for determining the reflection coefficient is to transmit a short continuous wave (CW) pulse, and to measure the amplitude of both the transmitted and reflected wave. The pulse has to be short enough to prevent interference with its own reflection from the specimen. In order to fulfil this requirement low frequency (and therefore long wavelength) CW pulses cannot be used, since this is usually conflicting with the cut-off frequency related to the inner dimensions of the cross section of the tube. Due to this and the fact that the envelope of short CW pulses can be highly distorted by the acoustic properties of the specimen and of the measurement equipment itself, this method is generally not very useful. To overcome the above mentioned problems the Standing Wave Ratio (SWR) method has been developed. This method is standardised in the ASTM C384-90 standard test method as published by the American Society for Testing Materials in: "ASTM C 384-90 Standard Test Method for Impedance and Absorption of Acoustical Materials by the Impedance Tube Method". This method uses the standing wave pattern that will arise due to interference of the reflected front end of a relative long CW pulse and the not yet reflected back end of the same pulse.
Although this method gives an improvement in measurement stability and accuracy, there is still a need for improvement. Due to the fact that the SWR method uses many positions for the acoustic receiver, SWR is very time consuming. A lot of research has been dedicated to find improvements of the SWR method and to find alternative methods, such as 1-, 2-, or 3-point transfer function methods. See for example: C. Peng, "The Measurement of Low Frequency Impedance Using an Impedance Tube", Journal of Low Frequency Noise, Vibration and Active Control, Vol. 17 No. 1, 1998; J. Kriiger, "Determination of Acoustic Absorber Parameters in Impedance Tubes", Applied Acoustics, Vol. 150 No. 1, 1997; and W. Chu, "Impedance Tube Measurements - A Comparative Study of Current Practices", Noise Control Engineering Journal, July-August 1991).
For one of these methods a second ASTM standard exists, published by the American Society for Testing Materials in: "ASTM E 1050-86 (1986): Standard Test Method for Impedance and Absorption of Acoustical Materials using a Tube, Two Microphones, and a Digital Frequency Analysis System".
When performing measurements in an acoustic interferometer of the prior art, several problems can be encountered that will have a negative influence on the accuracy of the results. Effects that require special attention are e.g., pulse distortion, source ringing and spurious reflections.
Due to the fact that the reflection coefficient of a specimen is frequency dependent, the shape of the pulse will be changed (distorted) due to the reflection. Especially near absorption peaks (often areas of interest) this distortion can be severe. For CW pulses this distortion will be mainly at the beginning and the end of the pulse. For reliable measurements, the pulse should be long enough to have a long enough 'middle part'. Due to multiple reflections at the end and the beginning of the tube, this will put a restriction on the lower bound of the frequency range that can be measured. Such a restriction also applies to SWR and related methods. Further, in the method for an acoustic interferometer arrangement of the prior art, it is often found that two other phenomena degrade the performance of such an acoustic interferometer arrangement: viz., source ringing and reflection by the acoustic receiver.
Source ringing relates to the fact that, generally, an acoustic transmitter (a source) cannot stop emitting acoustic power stepwise. The acoustic transmitter will need some extra time in order to decrease the signal gradually. This extra transmission is called ringing, and the amount of ringing is determined by the 'Q-factor' of the acoustic transmitter and its suspension.
Acoustic reflection occurs due to the fact, that every imperfection in the tube, such as the presence of an acoustic receiver (hydrophone), will give rise to extra reflections of the generated acoustic wave. These reflections have two negative implications for measurements: l)The amplitude of the original pulse will decrease, and thus an artificial enhancement of the reflection coefficient of the sample is created.
2) After reflection at the source side of the tube, the spurious reflections will interfere with the original pulse reflected at the specimen.
Although both source ringing and spurious reflections are often hardly noticeable, they can have a significant effect on measurements since they give rise to extra interference, especially when a low reflection coefficient is to be measured, i.e, relating to a material with a high absorption of acoustic wave energy.
The discussed phenomena of source ringing and spurious reflections will manifest itself by highly unstable and not reproducible results in the CW pulse and SWR methods.
Summary of the invention
It is an object of the arrangement and the method of the present invention to overcome the disadvantages of source ringing and spurious reflections as observed for acoustic interferometry of the prior art. As a solution the present invention provides a method for determination of an acoustic reflection coefficient by an acoustic interferometer arrangement, as defined above, characterised in that the method comprises the following steps: measuring by the at least one acoustic signal receiver at each of a plurality of intermediate positions between the acoustic signal transmitter and the sample holder, an acoustic signal comprising a direct signal portion from the acoustic signal transmitter and a reflected signal portion from the sample; recording the acoustic signal for each intermediate position as a function of the intermediate position, separating the direct signal portion and the reflected signal portion from the acoustic signal measured at each intermediate position of the at least one acoustic signal receiver by endfire beampattern processing to render a separated direct signal portion and a separated reflected signal portion, and determining the acoustic reflection coefficient by calculating an amplitude ratio of the separated reflected signal portion over the separated direct signal portion. Advantageously, the method of the present invention is capable of strongly suppressing the measured effects relating to source ringing and spurious reflections by applying an endfire beampattern processing technique. Moreover, the method of the present invention provides the ability to measure the reflection coefficient of a material over a broad frequency range, without the need for any further data manipulation such as smoothing or averaging.
Further, the endfire beampattern processing method has the capability to separate direct signals, coming from the acoustic source, and reflected signals that travel in opposite directions inside the tube, even if they are strongly interfering. Also, due to the separation, the complete acoustic signal of both direct and reflected signals (including the source ringing and spurious reflections) as received by an acoustic receiver, is available for processing. Hence it is possible to use the middle part of the pulses, which is not possible with the SWR-like methods known from the prior art. Also , the present invention relates to an arrangement for determination of an acoustic reflection coefficient of a sample by an acoustic interferometer as described above, characterised in that the at least one acoustic signal receiver is arranged to measure at each of a plurality of intermediate positions between the acoustic signal transmitter and the sample holder, an acoustic signal comprising a direct signal portion from the acoustic signal transmitter and a reflected signal portion from the sample; and that the signal processing means are arranged:
- to record the acoustic signal for each intermediate position as a function of the intermediate position, - to separate the direct signal portion and the reflected signal portion from the acoustic signal measured at each intermediate position of the at least one acoustic signal receiver by endfire beampattern processing to render a separated direct signal portion and a separated reflected signal portion, and
- to determine the acoustic reflection coefficient by calculating an amplitude ratio of the separated reflected signal portion over the separated direct signal portion.
Moreover, the present invention relates to a computer program product to be loaded by a signal processing means in an arrangement for determination of an acoustic reflection coefficient of a sample by an acoustic interferometer, the acoustic interferometer comprising a tube, a sample holder for holding the sample, an acoustic signal transmitter, and at least one acoustic signal receiver, the sample holder being located inside the tube, the acoustic signal transmitter being located inside the tube, characterised by the at least one acoustic signal receiver being arranged to measure at each of a plurality of intermediate positions between the acoustic signal transmitter and the sample holder, an acoustic signal comprising a direct signal portion from the acoustic signal transmitter and a reflected signal portion from the sample; the computer product, after being loaded, providing the signal processing means with the capacity: - to record the acoustic signal for each intermediate position as a function of the intermediate position,
- to separate the direct signal portion and the reflected signal portion from the acoustic signal measured at each intermediate position of the at least one acoustic signal receiver by endfire beampattern processing to render a separated direct signal portion and a separated reflected signal portion, and
- to determine the acoustic reflection coefficient by calculating an amplitude ratio of the separated reflected signal portion over the separated direct signal portion.
Furthermore, the present invention relates to a data carrier provided with a computer program product as described above.
Brief description of drawings
Below an arrangement and a method according to the present invention will be explained with reference to some drawings, which are intended for illustration purposes only. The scope of protection of the present invention as defined in the accompanying claims is not to be limited by these drawings.
Figure 1 shows schematically an acoustic interferometer arrangement which uses the arrangement and method according to the present invention;
Figure 2 shows schematically a computer system for use in the arrangement and method for acoustic interferometry according to the present invention; Figure 3 shows a signal plot of CW pulse measurements by acoustic interferometry from the prior art;
Figure 4 shows a plot of direct and reflected CW pulses according to the arrangement and method for acoustic interferometry of the present invention, aligned in the time domain; Figure 5 shows a plot of direct and reflected CW pulses according to the arrangement and method for acoustic interferometry of the present invention, aligned in the time domain;
Figure 6a shows a plot of direct and reflected CW pulses processed in accordance with the arrangement and method for acoustic interferometry of the present invention; Figures 6b shows a plot of reflected CW pulses processed in accordance with the arrangement and method for acoustic interferometry of the present invention;
Figure 6c shows the plot of Figure 6b on an enlarged scale; Figure 7 shows a plot of the reflection coefficient as a function of CW pulse frequency, determined by the arrangement and method for acoustic interferometry according to the present invention.
Description of preferred embodiment Figure 1 shows schematically an acoustic interferometer arrangement which uses the arrangement and method according to the present invention. The acoustic interferometer arrangement comprises a tube 1, a sample holder 2, an acoustic transmitter 3, and an acoustic receiver 4. In the tube 1 the sample holder 2 is mounted on one end. It is noted that, if required, the sample holder may also be at some location inside the tube 1. The sample holder 2 holds a sample 2' during measurements. On the other end of tube 1 the acoustic transmitter 3 is provided for generating acoustic excitations. If required, the transmitter may also be located at some location inside the tube 1. The acoustic receiver 4 for receiving direct acoustic signals from the acoustic transmitter 3 and reflected acoustic signals from the sample 2', (and possibly spurious reflections from e.g., the receiver 4) is mounted inside the tube 1. The acoustic receiver 4 can be shifted along a guiding device 5, parallel to the length of the tube 1.
The guiding device 5 comprises means (e.g. a thin guiding cable driven by a stepping motor) to move the acoustic receiver 4 inside the tube 1 to a plurality of predetermined positions. The guiding device 5 itself is preferably located outside the tube 1. The acoustic transmitter 3 is connected to a signal generator 7 which produces the desired acoustic excitation for either the CW pulse method or the SWR method. Other signal patterns may be generated, if required. The acoustic receiver 4 is connected to signal measuring equipment 8 which in some way measures (and stores) the output of acoustic receiver 4 that is based on an acoustic signal received by the acoustic receiver 4. Further, a control device 9 is connected to the guiding device 5 to control the movement of the acoustic receiver 4 along the length of the tube 1. Preferably, the signal generator 7, the signal measuring equipment 8 and the control device 9 are connected to (or, are incorporated in) a computer system 6, that is capable during experiments to control and monitor the functions of each apparatus 7, 8, 9, and to process measured signals.
The signal generator 7 may be implemented as an arbitrary waveform generator, which can generate any desired acoustic excitation. The signal measuring equipment 8 may be a suitable voltage measurement device such as an analog/digital voltage converter, capable of measuring at a required sampling frequency.
In one embodiment, the acoustic interferometer tube 1 used in the present invention has a length of 3 m with a constant circular inner diameter of 51 mm. In the present invention, the tube 1 is preferably filled with water, although it is also possible to do measurements in air or other gases or liquids. The acoustic interferometer tube 1 is mounted on a platform (not shown) and has a tilting angle α of approximately 30° with the horizontal plane for degassing purposes. The tube 1 may comprise provisions (not shown) for adjustment of the temperature and pressure of the medium within the tube 1, as is known to persons skilled in the art.
Figure 2 shows a general overview of a computer system 6 for use in the arrangement and method for acoustic interferometry according to the present invention, comprising host processor means 21 with peripherals. The host processor means 21 are connected to memory units 18, 19, 22, 23, 24 which store instructions and data, one or more reading units 30 (to read, e.g., floppy disks 17, CD ROM's 20, DVD's, etc.), a keyboard 26 and a mouse 27 as input devices, and as output devices, a monitor 28 and a printer 29. Other input devices, like a trackball, a touch screen or a scanner, as well as other output devices may be provided.
Optionally, an input/output (I/O) device 30 is provided for a connection to a network 31.
The memory units shown comprise RAM 22, (E)EPROM 23, ROM 24, tape unit 19, and hard disk 18. There may be provided more and/or other memory units known to persons skilled in the art. Moreover, one or more of them may be physically located remote from the processor means 21, if required. The processor means 21 are shown as one box, however, they may comprise several processing units functioning in parallel or controlled by one main processor, that may be located remotely from one another, in any way known to persons skilled in the art. The computer system 6 further comprises (the connections to) the signal generator 7, the signal measuring equipment 8 and the control device 9 to enable automated measurements and automated signal processing.
Figure 3 shows a signal plot of CW pulse measurements by acoustic interferometry from the prior art. In Figure 3 the logarithm of the envelope of the measured signal for each measurement is represented by a grey value indexed as shown in the vertical grey scale. Along the horizontal axis, the time elapsed after CW pulse generation is plotted. The vertical axis represents the hydrophone position index, i.e., the position of the acoustic receiver 4 along the guidance 5 relative to the starting position (which is a function of the actual frequency of the pulse signal and the acoustic characteristics of the tube 1). Shifting distance between consecutive hydrophone positions (i.e., index differs by 1) is 7 mm in the particular set up of this measurement.
In Figure 3 the results of 50 measurements are shown using a 1 ms CW pulse of 8811 Hz. Just behind each CW pulse (time between 2 and 3 ms) source ringing is visible. The source ringing induces an undesirable vertical interference pattern in the reflected pulse (time between 3 and 4 ms).
According to the present invention a method is applied which suppresses the negative effects of source ringing and spurious reflections. In the method according to the present invention the pulse length may be longer than would be possible in the SWR method. The longer pulse length provides enlargement of the allowable frequency range (towards lower frequencies).
The method presented here uses an acquisition geometry comparable to that of the SWR method, but a different processing technique. The processing technique is based on endfire beampattern processing (i.e., endfire beamforming), which is a special kind ofbeamforming. The beam pattern B of a receiver array can be calculated using the following general equation [1] as published in: R.O. Nielsen, "Sonar signal processing", Artech House, Norwood MA, USA 1991 :
M
B = ak exp( j '2τtf \k -X)d {cos φ ~ cos φQ)l c) [1]
4=1 where / is the frequency of a signal, Ma number of receiving elements in a receiver array, d the distance between receiver elements, φo steering angle (90° is normal to the array), an array amplitude weighting factor.
With endfire beamforming the steering angle is defined as a direction parallel to the array of receivers, so for a bearing φ=0° (parallel to the tube length), this implies the value for φ0 is 0° or 180°.
In the present invention, the acoustic measurement is repeated for different positions of the acoustic receiver 4 in the tube. The positions of the acoustic receiver 4 in the tube 1 may be chosen equidistant. By grouping all successive receiver positions, a synthetic array of receivers is formed, in which the measured signal for each receiver (position) represents a measured signal for the respective receiver in the synthetic receiver array. By performing the beamforming on the synthetic receiver array twice, for steering angles of 0° and 180°, acoustic waves travelling in the two opposite directions in the tube (towards and from the sample) can be separated. This is caused by the fact that for each endfire beamforming operation the wave travelling in the opposite direction will be suppressed. The suppression factor S can be calculated using the following equation [2]:
Using appropriate weighting for the received signals at different positions (i.e., the ak parameters in eq. [1]) can give significant improvement to the suppression factor S. Often the weighting is called shading or windowing. An example of a suitable windowing function is a Hamming window (see R.O. Nielsen as referenced above). In some cases, especially when the number of wavelengths over the synthetic array is small (usually this is the case for low frequencies), it is better not to apply a windowing function, which is equivalent to have all au parameters equal to unity
After the endfire beamforming, a straightforward method of determination of (mid)- pulse amplitude ratio is used for calculating the reflection coefficient. By using mid- pulse, the middle part of a signal is used without the preceding and trailing parts of the acoustic signal, in which the amplitude is not constant.
The method according to the present invention is as follows:
For the acquisition of the signals the movable acoustic receiver 4, mounted inside the tube 1, is used.
The measurement is carried out under control of the computer system 6, and is repeated for different, preferably, equidistant receiver positions, so by combining the signals obtained at all these receiver positions, the overall measured signal for a synthetic receiver array is formed. At each receiver position, a wide range of frequencies may be measured. During measurement, the acquisition time window should be large enough to record the front of the direct CW pulse (i.e. directly originating from the acoustic transmitter 3, and not from reflections by the sample 2' or obstacles in the tube 1) and the end of the reflected pulse. The CW pulses are generated by the acoustic transmitter 3 under control of the signal generator 7, preferably under control of the computer system 6. The full wave (i.e. including amplitude, phase information, and time) is received by acoustic receiver 4 and converted into an electric output signal provided to the signal measuring equipment 8 that records this electric output signal together with time-related information. It is noted that the acoustic excitation generated by the acoustic transmitter 3 may also be a chirp signal or an arbitrarily shaped acoustic signal comprising a range of frequencies.
The computer system 6 preferably transfers all signal recordings from the signal measuring equipment 8 to the memory means 18, 19, 22, 23, 24 for storage and further signal processing.
It is noted that the number of receiver positions and the distance between successive receiver positions are dependent on the acoustic characteristics of the tube and the pulse frequency used. Therefore, when performing a repeated measurement at a plurality of (possibly equidistant) positions, it will be appreciated that it may not be necessary at each position to measure signals at all desired frequencies. Moreover, the starting position for each frequency may be different. Using this information, it is possible to reduce the time to carry out all measurements in the synthetic receiver array, and to reduce the amount of collected data.
Also, the sampling rate of the signal measuring equipment 8 may be adjusted to the actual frequency to be measured. For low frequency signals the sampling rate can be lower than for higher frequency signals. In this way, the amount of sampled data for each receiver position can also be kept relatively low.
Furthermore, the pulse height and pulse length of the input signal may also be adjusted to the actual sampling frequency.
The processing of the recorded signals in order to determine the reflection coefficient, can be subdivided into three stages as described below:
a) Endfire beampattern processing of the direct signal:
The direct signal is the acoustic signal directly originating from the acoustic source (the acoustic transmitter 3). All recorded signals are given a time shift (in either the time domain or the frequency domain) to align the start of the direct pulse of all signals at the same point in time. In the frequency domain this is accomplished by using the exponential term of equation [1]. The processing means 21 of the computer system 6 fetch the recorded measurements, and aligns all recordings internally on the start of the direct pulse by adjusting the recorded time for each measured signal by a time-shift.
It is noted that the start of the direct pulse can be deduced from the recorded signals itself, or can be calculated from the synchronisation information, receiver positions, and wave propagation velocity. Due to this operation the direct pulses will be aligned, but the reflected pulses are not aligned, since they travel in the opposite direction.
After aligning the signals, the processing means 21 calculate the sum (or average) of the aligned signals, which leads to a signal containing an unaffected direct pulse and a low amplitude residue of the reflected pulse, substantially equal to zero. Possibly, a weighting function is applied in the summing or averaging operation. It is noted that since the endfire beampattern processing technique uses an averaging operation, in the method of the present invention the signal to noise ratio is enhanced by a factor of about 10 10log(M), with M being the number of receiver positions.
Figure 4 shows a plot of direct and reflected signals from CW pulses according to the arrangement and method for acoustic interferometry of the present invention, aligned in the time domain. It should be noted that only the first 5 ms of the measured signals are shown and that the full waveform of each signal is displayed instead of its envelope.
b) Endfire beampattern processing of the reflected signal:
The reflected signal is defined as the signal reflected by the sample 2' or any obstacles in the tube 1. The reflected signal originates from reflections of the direct signal on sample 2' or obstacles in the tube 1. The same procedure as described for endfire beampattern processing of the direct signal is followed, with the exception that now the start of the reflected pulses are aligned at the same point in time. After averaging over all receiver positions, the reflected signal will be unaffected, and the direct signal will almost disappear. Figure 5 shows a plot of direct and reflected CW pulses according to the arrangement and method for acoustic interferometry of the present invention, aligned in the time domain.
c) Determination of the amplitude ratio of the direct and reflected wave signal:
The ratio of the reflected signal processed by endfire beampattern processing (stage b) and the direct signal processed by endfire beampattern processing (stage a) determines the reflection coefficient. In order to prevent problems with pulse distortion, the pulses should be long enough and the amplitude should be determined in the middle part of the pulses. As mentioned above, the mid-pulse detection only uses the middle part of a pulse in order to measure a signal having a constant amplitude in the measurement time frame.
First, the processing means 21 determine the middle part of the reflected signal processed in stage b and of the direct signal processed in stage a. Then, the ratio of the two signal levels (reflected and direct) in the middle part of the respective signals is calculated by the processing means 21.
Here the results of the averaging process for the direct and reflected signals are shown after endfire beampattern processing.
Figure 6a shows the plot of the direct CW pulses processed in accordance with the arrangement and method for acoustic interferometry of the present invention. Figure 6b shows the plot of the reflected CW pulses processed in accordance with the arrangement and method for acoustic interferometry of the present invention. Figure 6c shows the plot of Figure 6b on an enlarged scale.
Before the averaging process the processing means 21 apply a Hamming window to the signal in the vertical direction. The suppression of the undesired signals is clearly shown since the remainder of the incoming (direct) signal in figure 6b and figure 6c is negligible and hardly noticeable. In figure 6c the vertical scale has been enlarged to display the signal in more detail. Similarly, the reflected signal (shown in Figure 6b) is not noticeable in Figure 6a. The suppression factor S is calculated by the processing means 21 using equation
[2].
In the example of Figure 6a, 6b, 6c the suppression factor S (eq. [2]) amounts to approximately 60. Using equation [2], with c=1460 m/s, d=l mm, /=8811 Hz, and the weighting factors α& are according to a Hamming window, the theoretical suppression factor is S=\1A (without application of a Hamming window suppression factor S would be S=2 >).
Figure 7 shows a plot of the reflection coefficient of some arbitrary sample 2' as a function of CW pulse frequency, determined the arrangement and method for acoustic interferometry according to the present invention.
In Figure 7, a final result of the calculation of the reflection coefficient is shown of several measurements at different frequencies (with a logarithmic spacing). At each position of the acoustic receiver 4, measurements have been done for a series of pulses with different frequencies. The measured signals for each position and each frequency have been recorded in the computer system 6. After completion of the measurements, the computer system has processed the measured signals for each measured frequency according to the method of the present invention to determine the reflection coefficient at each frequency. In Figure 7, the reflection coefficient determined for each frequency is thus plotted as a function of frequency.
It is noted that the resulting reflection coefficient values as calculated with the endfire beampattern processing method, are plotted here without any further modifications. No smoothing operation has been applied.
By processing signals in accordance with the three stages a, b and c mentioned above, the endfire beampattern processing method has the capability to separate direct and reflected signals that travel in opposite directions inside the tube, even if they are strongly interfering. Also, due to the separation, the complete acoustic signal of both direct and reflected signals (including the source ringing and spurious reflections) as received by the acoustic receiver 4, is available for processing. Hence it is possible to use the middle part of the pulses, which is not possible with the SWR-like methods known from the prior art.
It is also noted that the most harmful spurious (receiver) reflections travel in the opposite direction of the actual reflected pulse of interest. So the influence of these spurious reflections is strongly suppressed by the endfire beampattern processing which separates the signals going in opposite directions.
Since the method is performed using a single acoustic receiver 4, for recording both the direct pulse and the reflected pulse, there is no need for complicated calibration procedures of many separate receivers. Moreover, the positioning of the single acoustic receiver 4 by the guiding device 5 provides a flexible arrangement which can geared to very specific experimental conditions. However, it will be appreciated by persons skilled in the art, that instead of a synthetic receiver array a real receiver array comprising a plurality of acoustic receivers may be used as well.

Claims

Claims
1. Method for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, said acoustic interferometer comprising a tube (1), a sample holder (2) for holding said sample (2'), an acoustic signal transmitter (3), at least one acoustic signal receiver (4) and signal processing means (6, 8), said sample holder
(2) being located inside said tube (1), said acoustic signal transmitter (3) being located inside said tube (1), characterised in that said method comprises the following steps: measuring by said at least one acoustic signal receiver (4) at each of a plurality of intermediate positions between said acoustic signal transmitter (3) and said sample holder (2), an acoustic signal comprising a direct signal portion from said acoustic signal transmitter (3) and a reflected signal portion from said sample (2'); recording said acoustic signal for each intermediate position as a function of said intermediate position, separating said direct signal portion and said reflected signal portion from said acoustic signal measured at each intermediate position of said at least one acoustic signal receiver (4) by endfire beampattern processing to render a separated direct signal portion and a separated reflected signal portion, and determining said acoustic reflection coefficient by calculating an amplitude ratio of said separated reflected signal portion over said separated direct signal portion.
2. Method for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, according to claim 1, characterised in that said acoustic interferometer includes one acoustic signal receiver (4), and that said method comprises the following step: positioning said one acoustic signal receiver (4) at said plurality of intermediate positions between said acoustic signal transmitter (3) and said sample holder (2) to form a synthetic acoustic signal receiver array.
3. Method for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, according to claim 1, characterised in that said acoustic interferometer comprises a plurality of acoustic signal receivers (4) located at said plurality of intermediate positions between said acoustic signal transmitter (3) and said sample holder (2), said plurality of acoustic signal receivers (4) making up an acoustic signal receiver array.
4. Method for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, according to claim 1, 2, or 3 characterised in that said method comprises the following step: determining said acoustic reflection coefficient by calculating an amplitude ratio of a middle part of said separated reflected signal portion over a middle part of said separated direct signal portion.
5. Method for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, according to claim 4, characterised in that said endfire beampattern processing comprises the following steps:
- aligning said acoustic signal measured at each intermediate position internally to a starting point of the direct signal portion to render an aligned direct signal portion;
- aligning said acoustic signal measured at each intermediate position internally to a starting point of the reflected signal portion to render an aligned reflected signal portion;
- either summing or averaging said aligned direct signal portion at each intermediate position to render said separated direct signal portion;
- either summing or averaging said aligned reflected signal portion at each intermediate position to render said separated reflected signal portion.
6. Method for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, according to claim 5, characterised in that said summing or averaging of said aligned direct signal portion at each intermediate position comprises application of a weighting function.
7. Method for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, according to claim 5 or 6, characterised in that said summing or averaging of said aligned reflected signal portion at each intermediate position comprises application of a weighting function.
8. Method for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, according to claim 1, characterised in that said acoustic reflection coefficient of said sample (2') is determined at a plurality of frequencies by performing said method at each of said plurality of frequencies.
9. Method for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, according to claim 1, characterised in that said direct signal portion generated by said acoustic signal transmitter (3) comprises a chirp signal or an arbitrarily shaped waveform.
10. Method for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, according to claim 1, characterised in that said tube (1) comprises a sample surrounding medium.
11. Method for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, according to claim 5, characterised in that said sample surrounding medium is air, gas or water.
12. Arrangement for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer, said acoustic interferometer comprising a tube (1), a sample holder (2) for holding said sample (2'), an acoustic signal transmitter (3), at least one acoustic signal receiver (4) and signal processing means (6, 8), said sample holder (2) being located inside said tube (1), said acoustic signal transmitter (3) being located inside said tube (1), characterised in that said at least one acoustic signal receiver (4) is arranged to measure at each of a plurality of intermediate positions between said acoustic signal transmitter (3) and said sample holder (2), an acoustic signal comprising a direct signal portion from said acoustic signal transmitter (3) and a reflected signal portion from said sample
(2'); and that said signal processing means (6, 8) are arranged:
- to record said acoustic signal for each intermediate position as a function of said intermediate position, - to separate said direct signal portion and said reflected signal portion from said acoustic signal measured at each intermediate position of said at least one acoustic signal receiver (4) by endfire beampattern processing to render a separated direct signal portion and a separated reflected signal portion, and - to determine said acoustic reflection coefficient by calculating an amplitude ratio of said separated reflected signal portion over said separated direct signal portion.
13. Arrangement for determination of an acoustic reflection coefficient of a sample (2') by an acoustic interferometer according to claim 12, characterised in that said arrangement comprises a guiding device (5) for shifting one acoustic signal receiver (4) inside said tube (1), a signal generator (7) for generation of a signal for said acoustic transmitter (3), a signal measuring equipment (8) for recording said acoustic signal, and a computer system (6), said one acoustic signal receiver (4) being linked to said guiding device (5), said acoustic transmitter (3) being connected to said signal generator (7), said one acoustic signal receiver (4) being connected to said signal measuring equipment (8), said computer system (6) being connected to said signal generator (7) for controlling said generation of an input signal for said acoustic transmitter (3) and to said signal measuring equipment (8), said computer system (6) further being connected to said guiding device (5) for controlling said shifting of said intermediate position of said one acoustic signal receiver (4).
14. Computer program product to be loaded by a signal processing means (6, 8) in an arrangement for determination of an acoustic reflection coefficient of a sample (21) by an acoustic interferometer, said acoustic interferometer comprising a tube (1), a sample holder (2) for holding said sample (2'), an acoustic signal transmitter (3), and at least one acoustic signal receiver (4), said sample holder (2) being located inside said tube (1), said acoustic signal transmitter (3) being located inside said tube (1), characterised by said at least one acoustic signal receiver (4) being arranged to measure at each of a plurality of intermediate positions between said acoustic signal transmitter (3) and said sample holder (2), an acoustic signal comprising a direct signal portion from said acoustic signal transmitter (3) and a reflected signal portion from said sample (2'); said computer product, after being loaded, providing said signal processing means (6, 8) with the capacity:
- to record said acoustic signal for each intermediate position as a function of said intermediate position,
- to separate said direct signal portion and said reflected signal portion from said acoustic signal measured at each intermediate position of said at least one acoustic signal receiver (4) by endfire beampattern processing to render a separated direct signal portion and a separated reflected signal portion, and - to determine said acoustic reflection coefficient by calculating an amplitude ratio of said separated reflected signal portion over said separated direct signal portion.
15. Data carrier provided with a computer program product as claimed in claim 14.
EP02763089A 2001-09-24 2002-09-23 Endfire beampattern processing for acoustic interferometer Withdrawn EP1430298A1 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
NL1019020 2001-09-24
NL1019020A NL1019020C2 (en) 2001-09-24 2001-09-24 Endfire beam pattern processing for acoustic interferometer.
PCT/NL2002/000612 WO2003027663A1 (en) 2001-09-24 2002-09-23 Endfire beampattern processing for acoustic interferometer

Publications (1)

Publication Number Publication Date
EP1430298A1 true EP1430298A1 (en) 2004-06-23

Family

ID=19774056

Family Applications (1)

Application Number Title Priority Date Filing Date
EP02763089A Withdrawn EP1430298A1 (en) 2001-09-24 2002-09-23 Endfire beampattern processing for acoustic interferometer

Country Status (3)

Country Link
EP (1) EP1430298A1 (en)
NL (1) NL1019020C2 (en)
WO (1) WO2003027663A1 (en)

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10851641B2 (en) * 2018-09-05 2020-12-01 Saudi Arabian Oil Company Acoustic testing of core samples
US11920460B2 (en) 2021-12-08 2024-03-05 Saudi Arabian Oil Company Identifying formation layer tops while drilling a wellbore
US11920467B2 (en) 2022-01-13 2024-03-05 Saudi Arabian Oil Company Minimization of drill string rotation rate effect on acoustic signal of drill sound
US12578492B2 (en) 2022-08-24 2026-03-17 Saudi Arabian Oil Company Evaluation of density and seismic impedance values of geologic layers using drill bit sound during drilling

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4274048A (en) * 1978-12-18 1981-06-16 General Dynamics Corporation, Electronics Division Three-dimensional measurement of radiant energy scattering characteristics with a turntable-type scanning interferometer
US5373742A (en) * 1993-02-01 1994-12-20 General Electric Company Ultrasonic interferometer

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO03027663A1 *

Also Published As

Publication number Publication date
WO2003027663A1 (en) 2003-04-03
NL1019020C2 (en) 2003-03-25

Similar Documents

Publication Publication Date Title
US11391863B2 (en) Method of free-field broadband calibration of hydrophone sensitivity based on pink noise
US5383366A (en) Ultrasonic two probe system for locating and sizing
US5621699A (en) Apparatus and method of calibrating vertical particle velocity detector and pressure detector in a sea-floor cable with in-situ passive monitoring
CN102590349B (en) Method for measuring insertion loss/ transmission coefficient of underwater sound passive material based on time reversal focusing
KR101917374B1 (en) Apparatus and method for processing 3d ground penetrating radar signal
Willardson et al. Time reversal focusing of high amplitude sound in a reverberation chamber
WO1997005502A1 (en) Broadband sonar method and apparatus for use with conventional sonar sensor arrays
CN110426460B (en) Traveling wave tube measuring device and method for decoupling characteristic parameters of underwater acoustic material
EP1277065A1 (en) Acoustic sounding
Davies et al. An impulse method of measuring normal impedance at oblique incidence
US4415996A (en) Nonwavelength-limited holographic sound field reconstruction
WO2003027663A1 (en) Endfire beampattern processing for acoustic interferometer
Johnston et al. Special Report of the SEG Technical Standards Committee1 SEG standards for specifying marine seismic energy sources 2
Henderson et al. Seafloor profiling by a wideband sonar: Simulation, frequency-response optimization, and results of a brief sea test
Robb et al. Measurement of the in situ compressional wave properties of marine sediments
KR101703104B1 (en) Method and system for measuring acoustic wave velocity and acoustic attenuation for sediment sample
Fricke et al. A standard quantitative calibration procedure for marine seismic sources
Schock et al. Spatial and temporal pulse design considerations for a marine sediment classification sonar
CN110208778B (en) Device and method for measuring broadband complex response of transducer based on logarithmic variable window function
Linné et al. Development of calibration technique for underwater transducers in free field conditions below 1000 Hz with results on an acoustical recorder
US5493540A (en) System for estimating far-field acoustic tonals
Li et al. Underwater acoustics beamforming based on acousto-optic deflection
Muzi et al. Frequency based noise coherence-function extension and application to passive bottom-loss estimation
Li et al. Marine compressed air source array acoustic field characterization from at-sea measurements: Long-range propagation
US3597962A (en) Traveling-transducer method of measuring cross correlation

Legal Events

Date Code Title Description
PUAI Public reference made under article 153(3) epc to a published international application that has entered the european phase

Free format text: ORIGINAL CODE: 0009012

17P Request for examination filed

Effective date: 20040326

AK Designated contracting states

Kind code of ref document: A1

Designated state(s): AT BE BG CH CY CZ DE DK EE ES FI FR GB GR IE IT LI LU MC NL PT SE SK TR

AX Request for extension of the european patent

Extension state: AL LT LV MK RO SI

17Q First examination report despatched

Effective date: 20071218

REG Reference to a national code

Ref country code: DE

Ref legal event code: 8566

GRAP Despatch of communication of intention to grant a patent

Free format text: ORIGINAL CODE: EPIDOSNIGR1

RIC1 Information provided on ipc code assigned before grant

Ipc: G01N 29/32 20060101ALI20100630BHEP

Ipc: G01N 29/22 20060101ALI20100630BHEP

Ipc: G01N 29/36 20060101ALI20100630BHEP

Ipc: G01N 29/04 20060101AFI20100630BHEP

STAA Information on the status of an ep patent application or granted ep patent

Free format text: STATUS: THE APPLICATION IS DEEMED TO BE WITHDRAWN

18D Application deemed to be withdrawn

Effective date: 20101119