NL1008006C1 - Particle speed sensor. - Google Patents

Description

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985061 / Me / MMA

Short designation: Particle speed sensor.

The present invention relates to an apparatus and method for determining acoustic sound intensity using a plurality of acoustic flow sensors. Current commercially available measurement systems are based on pressure measurement according to IEC standard 1043 with two pressure microphones, or a combination of a pressure microphone and an indirect particle velocity sensor.

To measure the sound intensity, the amount of sound, two components of an acoustic phenomenon must be measured: the acoustic pressure variation and the so-called particle velocity, also known as the acoustic flow. Acoustic pressure variations are measured with a conventional pressure microphone such as, for example, a standard magnetodynamic pressure microphone or with a condenser pressure microphone. The particle velocity must be measured with a particle velocity microphone.

It is difficult to measure particle velocity directly. Instead of a direct particle velocity measurement, a pressure gradient is often measured with a so-called pressure gradient microphone. Use is also made of opposed ultrasonic radiators, which can measure the particle velocity between the radiators in one direction due to a Doppler shift. An example of this is a Norwegian Electronics Type 216 P-U intensity probe, also named 25 in the book: F.J. Faby "Sound Intensity", E&F Spon, London 1995, 105, 115.

A hot wire has also been used in the past to measure particle velocity. However, this appears not to go well because the hotwire sensor can only determine the absolute quantity and not the direction. Both a flow in the positive and negative directions cool the wire and also the hot wire microphone is hardly sensitive in the acoustic range (which is defined 1008006 - 2 - is between 20 Hz and 20 kHz from 0 dB SPL). Reference is made here to I.V. Lebedeva and S.P. Dragan, "Calibration of Hot wire Anemometers in Soundfields" in a report by Dantec, H.F. Olson, Acoustical Engineering, Van Nostrand 5 Company Inc, Princeton, New York, 1967.

The disadvantage of the previous solutions to measure the particle speed is the fact that the sensor is large compared to the wavelength of the acoustic phenomenon, which among other things gives scattering of the sound field at 10 higher frequencies and it is also difficult to calibrate the sensor.

Another method to measure particle velocity and also measure sound intensity is to use two pressure sensors (microphones) placed close to each other. The particle velocity appears to be estimated from the difference signal of the two pressure sensors. This system requires two identical pressure sensors (microphones) and is very sensitive to so-called phase errors between the two microphones.

20 Two methods are used in a conventional manner to measure acoustic sound intensity: 1. measure and multiply the particle velocity and the acoustic pressure variations (in standard IEC 1043 this method is referred to as P-U method); or 25 2. estimate the particle speed using two spatially separated pressure sensors and also measure the average pressure with both pressure sensors (in standard IEC 1043 this method is referred to as P-P method).

The main errors occurring in the determination of sound intensity with the various aforementioned methods are now discussed. Using the P-P method, the main errors are: 1. The unwanted phase difference between the two pressure microphones leads to an error in particle velocity determination; 2. Since the pressure is determined by the sum of the two pressure microphones, an extra error occurs: the average pressure is determined and not the pressure in one place.

1008006 - 3 -

The conclusion is that both the pressure and the particle velocity are not properly measured, which leads to an error in the determined sound intensity.

The main error inherent in the use of a P-U measuring instrument is the phase error between the acoustic pressure sensor and the particle velocity sensor. This is usually much greater than the phase error made in a P-P measuring instrument, because the two sensors are based on completely different transduction principles. Although the pressure and the particle velocity are therefore well measured, the sound intensity cannot be properly determined.

It is an object of the invention to provide a very small acoustic sound intensity measuring device to minimize problems with calibration and influence of the phenomenon (noise) to be measured. In order to achieve this objective, use is made of sensors and means which are known per se for measuring fluid flows. A characteristic example of such a sensor is given in the article: 20 H.E. de Bree et al .: "The Microflown, A novel device measuring acoustic flows", Sensors & Actuators: A Physical,

Volume SNA054 / 1-3, biz. 552-557 and international patent application PCT / NL95 / 00220. The particle velocity sensors described in these publications are hereinafter referred to briefly as "Microflown".

As the aforementioned article shows, there are three types of Microflowns. The first type is a so-called SHS type Microflown where three elements in a line are used to measure the particle velocity on that line. This type consists of two temperature sensors S, which may or may not generate heat and a heating element H, which is explicitly intended to generate heat. The second type is a so-called SS type Microflown where two elements in a line are used to measure the particle speed. This type does not use a heating element and therefore heat is generated with the temperature sensors S itself. With both of the aforementioned types (SHS and SS), the particle speed is determined from the difference signal 100800 © '- 4 - of the two temperature sensors S. The third type is a so-called SH type Microflown where two elements in a line are used to measure the particle velocity d. The sensor S is directly sensitive to the particle speed 5, provided that a heater is placed nearby.

The present invention provides a sound intensity sensor comprising at least one temperature sensor for generating an electrical signal corresponding to the temperature and at least one other temperature sensor or a heating element. It should be noted here that it is possible to have a temperature sensor function as a heating element.

The invention is elucidated with reference to the annexed drawing, in which: fig. 1 schematically shows a partial cross-section in side view through a device according to the invention, partly in the form of a block diagram; Fig. 2 schematically shows an alternative sensor-20 configuration; Fig. 3 schematically shows an alternative sensor configuration; Fig. 4 schematically shows a partial cross-section in side view through a device according to the invention, which device is partly shown in the form of a block diagram; and Fig. 5 schematically shows a partial cross-sectional side view through a sensor configuration.

Fig. 1 shows the sound intensity sensor in a 30-fold form. Both the assembly of temperature sensors Sl and S2, for example consisting of one- or two-sided clamped micro-bars, executed in IC technology, as | the assembly of temperature sensors S3 and S4 concerns a possible embodiment of the Microflown, indicated by 2 and 35, respectively. 4. For the functioning of the invention it is necessary that both sensors are more than a certain distance apart: thus it is a spatially distributed sensor. This distance is called 1008006 - 5 - separation distance. It can be demonstrated by means of mathematical proof and measurements that the difference signal from both particle velocity sensors 2 and 4 is proportional to acoustic pressure variations. The minimum separation distance between the sensors is determined by the minimum wavelength of the acoustic wave and thus the highest frequency to be measured. The distance must be many times smaller than the minimum wavelength. For 20 kHz, the minimum wavelength in air is 16 mm. A minimum separation distance of 300 micrometers is assumed. A smaller separation distance is also possible.

The particle velocity is measured using the Microflowns 2 (S1 / S2) and 4 (S3 / S4). The sum of the two signals from the Microflowns, which is obtained with a per se known electronic summing circuit 6, which is not further elaborated here, is proportional to the average particle velocity which is obtained with an electronically known per se, not further elaborated hereinafter. operation circuit 8. After all, the two Microf-20 lowns emit a different signal because of the geometric distribution. For this reason, the acoustic pressure can be determined from the difference between the two signals by means of a per se known electronic differential circuit 10, which is not further elaborated here. 12. The sound intensity is now determined by multiplying the average particle velocity and the determined acoustic pressure in a known electronic multiplication circuit 14, which is not further elaborated here. This is called the UU or differential flow method (U is for particle velocity).

The system S2 / S3 (fig. 1) can also be seen as a Microflown, but with the special feature that the distance between the sensors S2 and S3 is greater than the minimum separation distance. By means of the mathematically processed difference signal of the two Microflowns S1 / S2 and S3 / S4 which gives an electrical signal proportional to the 1008006 - 6 - acoustic pressure and the particle velocity signal originating from the Microflown S2 / S3, an electrical signal can be obtained by multiplication that is proportional to the sound intensity. It is preferable to measure the particle velocity with the Microflown consisting of the assembly S2 / S3 due to the fact that now the average particle velocity is not measured. This is called the improved U-U or improved differential flow method, which will be discussed in more detail below with reference to Fig. 4.

It is striking that it is possible to measure particle speed with the assembly S2 / S3 because the separation distance is very large. An expert in the field would expect that this is not possible because of the large distance between the two sensors. The particle velocity sensor, as already described in the literature (PCT / NL95 / 00220), preferably has a distance between the elements (temperature sensors or heating elements) of about 40 micrometers.

20 Literature (see: Lammertink et al., "Micro-liquid flow sensor", Sensors and Actuators A, 37-38 (1993), biz. 45-50) shows that the sensitivity of the sensor to the particle velocity is dependent of the distance between the two sensors. In Fig. 5 of the article by Lammertink et 25 al., A relationship is derived for this sensitivity when the flow sensor is enclosed in a channel. However, this channel does not make a significant contribution to the flow sensor, as has already been argued in PCT / NL95 / 00220. It can now be assumed that this analysis is also valid for the Microflown; After all, the sensor design is the same, except that the channel is missing. From the said fig. 5 of the article it appears that there is a maximum sensitivity for a certain distance between the two temperature sensors shown. This particular optimal distance is calculated and | 35 determined on 30 micro meters according to scientific insights.

Furthermore, it can be seen in the aforementioned fig. 5 of the article that, if the distance increases by a factor of ten, "* 008006 - 7" is compared to the optimum distance, which in the case of a Microflown is 300 micrometers, the sensitivity will decrease very much, which would lead to an unusable design.

Within the scope of the invention it has now been found that the models as mentioned in the article are not correct, or do not apply to the Microflown. It has been found that the Microflown actually works better the longer the distance is chosen that the aforementioned 30 or 10 40 micrometers. This completely unexpected insight leads to the outdated models published so far. A new theory about the operation of the Microflown has been developed.

The applications that a Microflown with a large distance (i.e. at least 300 micrometers) entails are different from the applications for a Microflown with a small (i.e. less than 300 micrometers) distance. This is because the great distance can be in the order of magnitude of the wavelength of the sound to be processed. One can think of the already mentioned application in the improved U-U probe, because the two sensors herein have to be a small separation distance of 300 micrometers or more from each other in order to serve as a U-U probe. Furthermore, a greater distance will lead to a greatly reduced transfer if the wavelength of the particle velocity wave becomes smaller than the distance; this creates a low-pass character in the frequency domain. This character can be used very well in, for example, telecommunications where signals must be filtered after 30 3400 Hz, a property that

Microflown can now be inherent, so that a separate filter can be made redundant.

Returning to Fig. 1: measurements have thus shown that it is very possible to measure particle velocity with the sensor S2 / S3 with the sensor distance 35 greater than or equal to the minimum separation distance and thus see this system as a Microflown.

The distances between the different parts of * 006006 1 - 8 - the sound intensity sensor as shown in fig. 1 are defined as follows: the distance AF is the so-called separation distance and is given as a minimum of 300 micrometers. The distances GH and BC are called the spacing of a Mi-5 croflown and are preferably the same and on the order of 40 micrometers. The sensor width (AB, CD, IH, GF) is preferably 40 micrometers.

To improve the signal, a heating element can be placed between the sensors, see fig. 2 (heat sources are indicated in fig. 2 by H1, H2, H3). The principle effect will not change as a result; the structures thus created are still Microflowns.

By placing several systems one behind the other, it is in principle possible to more accurately determine the sound intensity. When using two Microflowns, the acoustic pressure can only be determined from one difference signal between the Microflowns. If, on the other hand, three Microflowns are used, a difference can be measured in three ways. If the width of the assembly 20 remains smaller than the smallest wavelength to be measured, a better estimate of the acoustic pressure can be made from these three difference signals (see also fig. 3). The three difference signals which are proportional to the acoustic pressure arise from the difference signal of Microflowns S1 / S2 and S3 / S4, the difference signal of Microflowns S3 / S4 and S5 / S6 and also the difference signal of Microflowns S1 / S2 and S5 / S6. The particle velocity is now measured, for example, by means of a microflown which is created by the assembly of the sensors S2 and S5 and the heat source consisting of the sensors S3 and S4. The minimum separation distances in Figure 3 (DE and H1) need not be the same. It should be clear that in such a way systems with a larger number of Microflowns can also be made.

Two methods are usually used to measure sound intensity. The so-called P-P method (where P stands for acoustic pressure variations). The P-P method assumes a probe with two pressure microphones (as already described above). The P-U also exists

1008006 - 9 method. The P-U method assumes a probe with a pressure microphone and a particle speed microphone. The reason that the U-U method has not been described in the literature to date is the fact that no suitable part-5 velocity microphones existed. The (ultrasonic) particle speed microphones (as in the P-U measuring instrument from Norwegian electronics) are already spatially distributed sensors and this makes it difficult to make a spatially distributed U-U 10 measuring instrument with this type of particle velocity sensors. The assembly would simply become too large.

Because the P-P and U-U method (can) be called each other, since pressure and particle velocity are coupled in an acoustic wave (by means of the acoustic impedance), both methods are also coupled. The method which is the subject of this invention, the U-U method, will in principle introduce the same errors as the P-P method.

1. The phase error between the two particle velocity sensors will lead to an error in the estimation of the acoustic pressure.

2. The spatial separation of the two particle velocity sensors will lead to a measurement of the average particle velocity.

However, the method according to the invention introduces a smaller error (compared to the PP method) due to the fact that: 1. the phase error between the two particle velocity sensors is very small because they are made in one (IC-technological) process ( we will come back to this later);

2. the particle velocity can be measured by the Microflown created by the pair of sensors S2 / S3 as shown in fig. 1. This shows once again that this improved U-U method has advantages over the U-U

35 method which is the exact dualogon of the PP method, because the particle velocity is not determined by the sum of both Microflowns but from an assembly of S2 / S3, so from parts of the sensors that determine the acoustic pressure 1006006 - 10 - .

Here is a further explanation of the operation of the improved UU measuring instrument with reference to Fig. 4. The sensor pair S1 / S2 forms a Microflown if the signal 5 is subtracted from both sensors with a circuit shown with the functional block "A "and the same goes for the pair S3 / S4 and a circuit shown with the functional block" C ". However, the pair S2 / S3 and the circuit shown with the functional block "B" can also be used to measure the particle velocity while the spacing is greater than the minimum separation distance.

The signals from the functional block "A" and the functional block "C" are subtracted from each other with a circuit shown with the functional block "D". The result of this represents the gradient of the particle velocity in the direction X. After integration and multiplication of the result of block "D" by a constant value by means of a circuit represented by the functional block "E", the signal is proportional with the acoustic pressure. After multiplication by a circuit shown with the functional block "F" of the output signal of the functional block "E" (a signal proportional to the acoustic pressure) and the functional block 25 "B" (a signal proportional to the particle velocity) ont is a signal proportional to the acoustic sound intensity in direction X.

For the description of the IC technological production processes by which the present sound intensity measuring instrument can be made, reference is made to various articles and books which have been published in this field. It is known that with these production processes it is quite possible to realize exactly the same structures.

35 An embodiment with which! it is possible to measure acoustic sound intensity.

The system shown in fig. 5 is an acoustic sound intensity probe consisting of three sensors S1, S2 1008006 -lien S3, of which in certain cases sensor S2 is also used as heating element H2. The combination SI and S2 forms Microflown 1 and the combination S2 and S3 forms Microf-lown 2. The acoustic pressure can be determined from the mathematically processed difference signal of 5 Microflown 1 and Microflown 2. The particle speed is measured by the combination of sensor SI, heater H2 and sensor S3. The minimum separation distances, the distance BC and DE in Fig. 5, are again 300 micrometers and are preferably the same. 10 The product of acoustic pressure and particle velocity determines the sound intensity.

All of the aforementioned intensity measuring instruments measure the particle velocity and acoustic pressure related to an acoustic phenomenon in the direction of the line on which the sensors are located. In order to fully determine both the magnitude and the direction of the sound intensity, three such measuring instruments must be positioned perpendicular to each other in three directions. A Cartesian coordinate system indicates three perpendicular directions with, for example, x, y and z. It can also be proposed that three of the aforementioned measuring instruments are placed on each of the x, y and z axes. Generally speaking, it is of course also possible to position the three measuring instruments at any angle with one another and not in the same plane. In this way, the particle velocity vector is completely determined and the actual total acoustic pressure is determined, with which the sound intensity is therefore only completely determined. Thus, by using three linearly independent measuring instruments, the total sound intensity vector is determined, while the sound intensity measuring instruments determined by the aforementioned types of sound intensity are only determined in one direction.

It is possible to construct a 3-dimensional Microflown consisting of three ordinary Microflowns. The total particle velocity vector can now be measured. A combination with an omnidirectional pressure microphone then provides the possibility to determine the total sound intensity * 008006 - 12 - vector according to the three-dimensional P-U method.

It is also possible to determine the complete sound intensity with three U-U probes, ie with six Microflowns. So now the U-U method or the improved U-U method has been extended to three dimensions.

All of the aforementioned intensity probes measure the total or total sound intensity vector in one place. The sound intensity field is measured by placing several of the aforementioned systems in a 2-dimensional or 3-dimensional matrix. This is an important factor in determining how sound propagates in or around, for example, household appliances or an exhaust from, for example, a car. It is also advantageous to measure the sound intensity in terms of size and direction, in particular in the area of very low frequencies, in the vicinity of, for instance, airports and noise barriers, or to detect and measure leaks from acoustic spaces (for example, discos).

1008006

Claims (11)

An apparatus for measuring acoustic sound intensity, comprising: at least two aligned particle velocity sensors, each comprising at least one temperature sensor 5 and arranged to receive a first resp. to issue a second velocity signal corresponding to the particle velocity; first means for determining a difference signal of the speed signals from the respective particle velocity sensors 10; second means for performing a first mathematical operation on the difference signal to form a pressure signal corresponding to the acoustic pressure; Third means for determining a sum signal of the speed signals from the respective particle velocity sensors; fourth means for performing a second mathematical operation on the sum signal to form a third velocity signal corresponding to the particle velocity; fifth means for forming a product signal of the pressure signal and the third velocity signal, which product signal corresponds to the sound intensity. 25 2. Apparatus for measuring acoustic sound intensity, comprising: at least two aligned particle velocity sensors, each comprising at least one temperature sensor 30 and arranged to receive a first resp. to issue a second velocity signal corresponding to the particle velocity; first means for determining a difference signal from the speed signals from the respective particle velocity sensors 35; second means for performing a first 1 08006-14 mathematical operation on the difference signal to form a pressure signal corresponding to the acoustic pressure; sixth means for determining a fourth speed signal using at least one temperature sensor of one of the particle speed sensors and a temperature sensor or a heating element of the other particle speed sensor; seventh means for forming a product signal of the pressure signal and the fourth velocity signal, which product signal corresponds to the sound intensity. 3. Device according to claim 1 or 2, characterized in that the particle velocity sensor comprises two temperature sensors. Device according to claim 3, characterized in that the particle velocity sensor comprises a heating element. 20 Device according to claim 1 or 2, characterized in that the particle velocity sensor comprises a temperature sensor and a heating element. Device according to any one of claims 3-5, characterized in that the minimum distance between a temperature sensor and another temperature sensor or a heating element of a particle speed sensor is 300 micrometers. 30 Device according to any one of claims 1 to 6, characterized in that the first mathematical operation comprises an integration and a multiplication by a factor. Device according to claim 1, characterized in that the second mathematical operation comprises determining an average value. * 008006 <- 15 - Apparatus for measuring the magnitude and direction of the acoustic sound intensity, comprising three measuring devices according to any one of claims 1-8, wherein the lines on which the particle velocity sensors in each measuring device are arranged are all angled to one another and do not lie in the same plane. A method of measuring acoustic sound intensity, comprising the steps of: determining a first particle velocity with a first particle velocity sensor and a second particle velocity with a second particle velocity sensor, both of which particle velocity sensors are arranged in a line; determining the difference of the first and the second particle velocity to determine the acoustic pressure; determining the average of the first and second particle velocities; determining the product of the acoustic pressure and the average particle velocity to determine the sound intensity. A method of measuring acoustic sound intensity, comprising the steps of: determining a first particle velocity with a first particle velocity sensor and a second particle velocity with a second particle velocity sensor, the two particle velocity sensors being aligned; determining the difference of the first and the second particle velocity to determine the acoustic pressure; determining a third particle velocity using a particle velocity sensor consisting of at least a temperature sensor of the first particle velocity sensor and a temperature sensor or a heating element of the second particle velocity sensor; determining the product of the acoustic pressure and the third particle velocity to determine the sound intensity. 1008006