JP2003207328A - Surface area measuring method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method to measure the surface area of any object simply. <P>SOLUTION: A diffuse sound field is formed in a cavity surrounding the surface of the object whose surface area is to be measured, and the dissipation rate of the acoustic energy in the cavity is measured, and based on the obtained rate, the surface area of the object is determined. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION Surface area is an important measure in many fields. For example, in the manufacturing sector, the cost of plating and painting is proportional to the surface area of interest. Surface area is also an important parameter in thermal design because the thermal time constant of the device is determined by the surface area of the device. In the field of acoustic engineering, the surface area of sound absorbing material has become an important parameter for determining acoustic characteristics. In physiology, surface area is necessary to estimate the basal metabolism of animals and plants. In food safety inspections in the field of insurance administration, trace chemical substances eluted from tableware, packaging materials, toys, etc. are measured, but the amount of trace chemical substances eluted is proportional to the contact area between the target and the medium. Is required to be measured. The invention relates in these fields to methods for measuring the surface area of an object.

[0002]

2. Description of the Related Art As mentioned above, despite the fact that surface area is a measurement quantity required in many fields, there is no suitable measurement means. If the shape of the target object is simple, it is possible to measure the surface area by measuring it with a three-dimensional shape measuring instrument, but for a target object with a complicated shape such as a hidden depression, It is difficult to measure accurately. A method of applying a liquid such as glycerin to the surface of the target and measuring the amount of the liquid may be performed, but the applicable target is very limited. In addition to these methods, a method of applying a sinusoidal pressure fluctuation with a constant frequency to a container containing an object and measuring the acoustic impedance of the container to estimate the surface area of the object has been developed (Torikoshi, Ishii:
Surface area measurement using sound, Transactions of the Society of Instrument and Control Engineers, 34
Vol. 3, No. 3, p. 182, 1998). However, in this method, it is necessary in principle to use a very low frequency so that the pressure can be considered to be uniform throughout the container. However, since the thickness of the boundary layer becomes large when the frequency is low, this method has a problem that an error occurs when the curvature of the surface to be measured is large, that is, the measurement error becomes large when the object has a complicated shape. Have a point.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for measuring the surface area of an object having a complicated shape simply, quickly and in a non-contact manner.

[0004]

According to the present invention, a diffuse sound field is formed in a cavity surrounding the surface of an object whose surface area is to be measured, and the rate at which acoustic energy in the cavity is dissipated is measured. The surface area of the object is calculated based on.

When the gas in the space is vibrated by the sound source, the vibration of the medium particles propagates as a wave in the space. With the vibration of the medium particles, pressure fluctuation, that is, sound pressure is generated,
This is perceived as a sound in our ears. Further, the vibration of the medium particles is accompanied by the temperature change of the medium. In free space, where there are no obstructions to the sound in space, the attenuation of acoustic energy is caused by the internal loss of the medium. On the other hand, when an object exists in the space where sound propagates, acoustic energy is lost on the wall surface of the object. This is because the vibrating resistance causes the particle vibration velocity to become zero on the wall surface, and the temperature fluctuation also becomes zero on the wall surface, causing heat conduction between the medium and the wall surface (particle velocity and temperature fluctuations The layered region near the wall that is affected by the presence of is called the "boundary layer"). In a large free space, the main cause of acoustic energy loss is the internal loss of the medium, but inside the cavity, which is not so large, the dominant factor of acoustic energy loss is the wall loss. At this time,
The rate of dissipation of acoustic energy in the cavity is greater for larger wall areas. Furthermore, in the case where the inside of the cavity is a sound field with a uniform acoustic energy density and the flow of acoustic energy occurs in all directions with equal probability, that is, a diffuse sound field, the rate of dissipation of acoustic energy is Proportional to the wall surface area of. By utilizing this phenomenon, it becomes possible to know the area of the wall surface of the cavity by measuring the rate of dissipation of acoustic energy.

[0006]

As described above, since the dissipation rate of acoustic energy is proportional to the wall surface area in the diffuse sound field, the diffuse sound field is formed in the cavity surrounding the surface of the object whose surface area is to be measured. The rate at which the acoustic energy in the cavity is dissipated is measured, and the surface area of the object is determined based on the rate. The diffuse sound field in the cavity is formed by radiating noise from a sound source such as a speaker into the cavity. The rate of dissipation of acoustic energy in the cavity is measured by detecting the sound pressure in the cavity using a microphone and processing the sound pressure signal. Other than this, there is also a method of measuring the electric energy supplied to the sound source while driving the sound source so that the acoustic energy density in the cavity is in a steady state. In order to calculate the wall surface area from the measured ratio, normally, the relationship between the dissipation ratio and the area is calibrated in advance using a target object having a known surface area. When the surface area to be measured is the wall surface area of the cavity on the object, a diffuse sound field may be formed in this cavity and the above method may be used. On the other hand, when the surface area to be measured is the outer surface area of the target object, a container having a known inner surface area is prepared and the target object is placed therein, and the total wall surface area of the cavity formed by the container and the object is calculated. The surface area to be measured is calculated by subtracting the known surface area inside the container from the value.

[0007]

EXAMPLES (First Example) Hereinafter, examples will be described. FIG. 1 is a first embodiment of the present invention. In FIG.
4 is a container. Reference numeral 5 denotes a lid of the container 4, which can be attached and detached. Reference numeral 1 is a cavity formed by a container and a lid. Remove the lid 5 and place the measured object 6 in the container 4,
At the time of measurement, the lid 5 is closed again. A support projection is provided on the bottom of the container 4, and the object to be measured has a structure in which it is supported by the point of the tip of this projection. This structure also makes it possible to measure the surface area of the lower surface of the object to be measured. 2 is a speaker attached to the lid 5,
Noise is emitted from this speaker into the cavity. Reference numeral 3 is a microphone attached to the container 4 for detecting the sound pressure in the cavity. Reference numeral 7 is a white noise generator, 8 is a bandpass filter, and 9 is an amplifier. These devices generate bandpass noise radiated from the speaker. Reference numeral 11 is a timing control circuit, and the control signal generated by this circuit determines the timing of noise emission and the like. 10 is a root mean square for calculating the acoustic energy density level from the signal of the microphone 3, 12 is a reverberation time measuring device,
Reference numeral 13 is a display device for estimating and displaying the surface area from the obtained reverberation time.

As shown in the upper graph of FIG. 2, the acoustic emission in this embodiment is controlled in the order of emitting noise for a certain period of time and then suddenly stopping the emission. Since the radiated sound is band noise, a standing sound wave is not generated and a diffuse sound field is created in the cavity. The root mean square 10 squares the sound pressure signal detected by the microphone 3 and then calculates a short-time average. The sound pressure fluctuation of noise is very fast as compared with the time change of the acoustic energy density level. By the action of the root mean squarer, the instantaneous sound pressure can be averaged over a short time interval to obtain the acoustic energy density level in the cavity at that time. The change over time of the acoustic energy density level is approximately equal to the lower graph in FIG.
The waveform is as shown in the reverberation curve. Even if the acoustic emission from the loudspeaker is stopped, the acoustic energy density level in the cavity approaches zero exponentially instead of zero. The time required for the acoustic energy density level to decay to one-millionth of the steady-state level is called the "reverberation time" and is one of the indicators of the rate of dissipation of acoustic energy in the cavity. In the reverberation time measuring device 12, the reverberation time is obtained from the time-varying waveform of the acoustic energy density level. When the cavity has a diffuse sound field, the reverberation time of the cavity is inversely proportional to the size of the internal surface area.
In the present embodiment, the relationship between the wall surface area in the cavity and the reverberation time is calibrated in advance and stored as a table in the display device 13, and the wall surface area of the cavity is obtained by referring to this. Since the inner surface area when the container is empty is also stored in the display device 13 in advance, the surface area of the measured object 6 is displayed by subtracting the empty inner surface area from the total inner surface area when the object is put in. ing.

In the present embodiment, the speaker 2 is attached to the lid 5 and the microphone 3 is attached to the container 4. However, it goes without saying that the number of these devices and the attaching positions are not limited to this. When the measurement target is the inner surface area of the cavity itself, the wall surface area can be measured by mounting the speaker and the microphone on the lid and directly covering the measurement target without preparing the container 4. Also, the reverberation time can be estimated from the initial slope of the reverberation curve in addition to the method of actually measuring the time until the acoustic energy density level reaches one millionth, and this method can also be used. it can.

(Second Embodiment) In the first embodiment, the reverberation time in the cavity is measured as defined to estimate the wall surface area of the cavity, but the method of measuring the rate of dissipation of acoustic energy in the cavity is used. Is not limited to the method of the first embodiment. FIG. 3 shows a second embodiment of the present invention. In FIG. 3, 14 is a multiplier, 15 is a gate signal generator, 16 is an integrator, and 17 is a display device.

The output noise of the white noise generator 7 is multiplied by the gate signal of the gate signal generator 15 to form burst noise of short duration, which is radiated from the speaker 2 into the cavity 1. The sound pressure in the cavity is detected by the microphone 3, converted into an acoustic energy density level by the root mean square device 10, and integrated by the integrator 16. Unlike the normal integrator, the integrator 16 of the present embodiment outputs not the result of integrating the signals from the past to the present but the result of integrating the signals from the future to the present by reversing the time axis. Is configured. It is known that the reverberation curve of the cavity can be obtained by integrating the acoustic energy density level signal when the sound is radiated into the cavity in burst (MRSchroader, New Method of Measuring R
everberation Time, Journal of the Acoustical Socie
ty of America, Vol.37, p.409, 1965). After the reverberation curve is obtained by the integrator 16, the display device 17 measures the rate of energy dissipation from the initial slope of the reverberation curve, estimates the wall surface area, and outputs it. In this embodiment,
The slope of the reverberation curve is used as an index showing the rate of acoustic energy dissipation, without obtaining the reverberation time. In addition,
When the signal-to-noise ratio is low and a reliable reverberation curve cannot be obtained by one measurement, burst noise is repeatedly emitted and the reverberation curve is averaged.

(Third Embodiment) FIG. 4 shows a third embodiment of the present invention in which the rate of energy dissipation is measured using an irregular signal and a correlation method. In FIG. 4, 18 is a carrier signal generator, which generates a binary pseudo-random signal.
A switching signal generator 19 generates a binary pseudo-random signal that is statistically independent of the carrier signal.
These two pseudo-random signals are multiplied by a multiplier 14 and then passed through a bandpass filter 8. The output signal of the bandpass filter is band-limited pseudo white noise. This noise is radiated from the speaker 2 into the cavity 1. The output signal of the microphone 3 is converted into a sound energy density level by a mean squarer 10,
It is input to the correlator 20 together with the switching signal.

The output signal of the root mean squarer 10 is the acoustic energy density level signal of the noise in the cavity, as described with respect to the first embodiment. It is known that the cross-correlation between the acoustic energy density level signal and the switching signal is equal to the response to impulse power injection into the cavity = impulse power response (Sagayama et al., Reverberation by M-sequence modulation correlation method). Time measurement, Proceedings of the Acoustical Society of Japan, p.83, 1974). A reverberation curve is obtained by integrating this response in the opposite direction of the time axis as in the second embodiment. After the reverberation curve is obtained by the integrator 16, the display device 17 calculates the rate of energy dissipation from the initial slope of the reverberation curve, estimates the surface area, and outputs it.

(Other Embodiments) The sound field formed in the cavity may be a strict diffuse sound field, and a diffuser plate may be installed in the cavity for the purpose of improving the measurement accuracy (not shown). . Further, during the period of measuring the ratio of acoustic energy dissipation, the speaker or the diffuser plate or both may be moved to change the direction in which sound is emitted or reflected to form a diffuse sound field. Further, the surface area measuring method of the present invention, other measurement amount utilizing the result of measuring the surface area of the target, for example, the surface roughness of the target object, the coating thickness of the target,
It can be applied to a device for measuring the basal metabolic rate of a living body, the concentration of a substance in a medium, or the like, or for correcting a surface area error when precisely measuring a volume using sound (not shown).

[0015]

The thickness of the boundary layer of sound waves generated near the wall surface becomes thinner as the frequency becomes higher, and becomes thicker as the frequency becomes lower. As described above, in the method of measuring the acoustic impedance by applying sinusoidal pressure fluctuation to the container, it is necessary to select a low frequency so that the pressure inside the cavity is uniform, so the boundary layer thickness becomes thick. However, an error occurs when measuring the surface to be measured having a large curvature. On the other hand, in the present invention in which a diffuse sound field is formed in the cavity, noise including a high frequency component is radiated for measurement. Therefore, the thickness of the boundary layer becomes thin, and the error can be suppressed to be small even if the curvature of the target object becomes large.

The operation required for the measurement according to the present invention is only that the object to be measured is placed in a container to emit sound, or the sound is emitted into the cavity surrounding the object to be measured, and the subsequent processing is performed by a microcomputer. It can be automatically executed by. In the process of measurement, there is no need to touch the object, and there is no destructive influence. In summary, the present invention makes it possible to measure the surface area of an object having a complicated shape whose geometrical dimensions are difficult to measure easily, quickly and accurately.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a noise emission timing and a sound energy level in the first embodiment.

FIG. 3 is a diagram showing a second embodiment of the present invention.

FIG. 4 is a diagram showing a third embodiment of the present invention.

[Explanation of symbols]

1 cavity 2 speakers 3 microphones 4 containers 5 lid 6 Object to be measured 7 White noise generator 8 band filter 9 amplifier 10 mean square 11 Timing control circuit 12 Reverberation time measuring device 13 Display 14 Multiplier 15 Gate signal generator 16 integrator 17 Display 18 Carrier signal generator 19 Switching signal generator 20 Correlator

Claims (2)

[Claims] 1. A diffuse sound field is formed in a cavity surrounding the surface of an object whose surface area is to be measured, the rate at which acoustic energy in the cavity is dissipated is measured, and the surface area of the object is determined based on the rate. A surface area measuring method characterized by: 2. A sound field forming means for forming a diffuse sound field in a cavity surrounding a surface of an object whose surface area is to be measured, a measuring means for measuring a rate at which acoustic energy in the cavity is dissipated, and the measuring means. And a means for determining the surface area of the object based on the ratio measured by the surface area measuring device.