WO2001018552A1

WO2001018552A1 - Volume accelerometer

Info

Publication number: WO2001018552A1
Application number: PCT/NL1999/000548
Authority: WO
Inventors: Michiel Wilbert Rombout Maria Van Overbeek; Jan Willem Verhey
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno
Priority date: 1999-09-03
Filing date: 1999-09-03
Publication date: 2001-03-15
Also published as: AU5536099A

Abstract

Measurement device and method for measuring the volume acceleration of a predetermined sub-are a of an acoustically radiating surface (10), comprising a termination end and side walls (4), the side walls (4) defining a volume of air between the termination end and the acoustically radiating surface (10) in operation, and measurement means (5) for measuring the sound pressure in the volume of air. The termination end is formed by a flat piston (2) of a first material; and the measurement device (1) further comprises actuator means (6) for bringing the flat piston (2) into vibration such that the sound pressure in the volume of air is kept at a minimum level and acceleration sensing means (8) for measuring the normal acceleratin of the flat piston (2).

Description

Volume accelerometer

The present application relates to a measurement device for contactless measurement of acceleration of a vibrating surface. More specifically, the present application relates to a measurement device for measuring the volume acceleration of a predetermined surface area of an acoustically radiating surface, comprising a termination end and side walls, the side walls defining a volume of air between the termination end and the acoustically radiating surface in operation, and measurement means for measuring the sound pressure in the volume of air. Such a measurement device is known from the article "Application of an area- integrating vibration velocity transducer" by K.R. Holland and F.J. Fahy, Proceedings of Internoise 96, Liverpool, UK, 1996, pp. 2581-2584.

Conventional methods for measuring the structural vibrations which are causing the acoustic radiation often have practical objections. Conventional accelerometers only provide information on the acceleration (and thus velocity) of a point on a surface and do not integrate over a predetermined area. To be able to measure the volume acceleration of the vibrating surface, integration over a grid of accelerometers is necessary, the number and spacing depending on dimensions of the surface and Nyquist's criterion for the highest frequency to be determined. Another known method for determining the surface vibration velocities operates contactless by using laser equipment. However, this laser equipment is generally large and can not be used easily to measure vibrating surfaces in places which are difficult to reach (e.g. under car seats). Moreover, this kind of equipment is expensive.

The article of Holland and Fahy, referred to above, describes a volume velocity transducer to determine the instantaneous, area integral of the normal velocity of a vibrating surface. These kinds of transducers are used to determine the acoustic noise generated by sound radiating structures. The transducer consists of a square-section tube, which is open at one end and connected to an anechoic termination at the other. Close to the termination end, a miniature microphone is present to sense the plane wave resulting from the vibrating surface when the open tube end is brought close to it. The plane wave generated by the vibrating surface at the open end propagates along the tube towards the termination end and has an amplitude, which is proportional to the instantaneous integral of the normal velocity of the surface beneath the tube end. The wavelength in the vibrating surface is in general smaller than the wavelength in air, as the sound speed in the material of the surface will generally be smaller than the sound speed in air. Only certain wavelengths of sound will cause a vibrating surface to radiate noise, as only these wavelengths allow an efficient transfer of the sound to the air mass. The transfer of sound therefore concerns two major parameters, i.e. the amount of vibration (amplitude) and radiation efficiency. The method used in the measurement device described above uses an integrating measurement technique to determine those vibration components on the surface of a structure, which cause this structure to radiate sound into its surroundings. Vibrating surfaces may be the cause of noise pollution, e.g. in the interior of transport means (cars, trains, aircraft,...). In general, more than one vibrating surface will simultaneously contribute to the noise causing the nuisance. To prevent or minimise the nuisance, it is required to know which vibrating surfaces contribute to the noise, and at what level. Therefore, it is required to measure the acoustic radiation of the vibrating surfaces in realistic circumstances, i.e. without having to take away parts to enable installation of measurement equipment. In general, the vibrating surfaces are flat or only slightly curved. An important method to characterise the acoustic radiation of a surface is to subdivide the area of a vibrating surface into a limited number of piston-like sub-areas. For each of these sub-areas the volume acceleration is determined by integrating the accelerations over all points within the sub-area. This integration over the sub-area acts as a spatial low-pass filter. Measurement equipment, which performs this integration on the site, thus enables the direct measurement of those vibration components, which determine the radiated noise. So far, these kinds of measurement methods cannot be implemented in practice, as good and practical equipment is not yet available.

The integrating transducer according to the article of K.R. Holland e.a. as described above, has certain limitations, which may turn out to be disadvantageous in certain applications. Firstly, the dimensions of the transducer need to be relatively large compared to the dimensions of the cross-sectional area of the open tube from which the volume velocity is measured. The length of the tube has to be at least about twice as large as its width to obtain a plane sound wave at the place of the miniature microphone. Also, the width dimensions of the anechoic termination needs to be at least approximately three times as large as the width of the tube. Secondly, the resistive layer which is used to realise a space-efficient anechoic termination is relatively transparent to incident sound waves from other directions than from the open tube end. This makes the volume velocity sensor relatively sensitive for unwanted sound inputs.

The object of the present invention is to provide a measurement method and a measurement sensor to determine the integrated volume acceleration (and thus volume velocity) of an acoustically radiating surface, in order to determine the acoustic radiation characteristic of the vibrating surface. Another object is to provide a measurement sensor which is compact, robust, reliable, manageable (i.e. having a small size and weight) and which can be manufactured economically. This object has been achieved by a measurement device according to the preamble of claim 1, characterised in that the termination end is formed by a flat piston of a first material and the measurement device further comprises actuator means for bringing the flat piston into vibration such that the sound pressure in the volume of air is kept at a minimum level. The measurement device according to the present invention has the advantage that it can be used to determine the acoustic radiation characteristic of the vibrating surface without disturbing the natural behaviour of the vibrating surface. Furthermore, it is very practical in use because of its relatively shallow dimensions. The measurement device according to the present invention can e.g. be used under a car seat without the need to remove any obstacles in the car. Furthermore, because the measurement device is not in direct contact with the surface of which the vibration is to be measured, the surface does not need to be perfectly flat.

In a preferred embodiment of the measurement device according to the present invention, the flat piston is suspended in the measurement device such that it can move freely with respect to the measurement device, at least in a direction perpendicular to the surface of the flat piston. This will allow a movement of the flat piston in a direction essentially perpendicular to its surface, in other words parallel to the movement of the vibrating surface. The movement of the flat piston will enable an accurate determination of the normal acceleration of the vibrating surface, integrated over the measurement area.

In a further embodiment according to the present invention, the measurement device further comprises acceleration sensing means for measuring the normal acceleration of the flat piston. The acceleration sensing means may comprise an accelerometer fixedly mounted to the flat piston, preferably in a central position for reasons of symmetry. The acceleration means directly provide a signal proportional to the integrated normal acceleration of the vibrating surface.

In further embodiment of the measurement device according to the present invention, the first material is a sufficiently stiff material such that the flat piston can be brought into vibration, the flat piston in its entirety moving substantially in phase in a direction perpendicular to the surface of the flat piston. The frequency characteristics of the flat piston are determined by the material of the flat piston and the surface area of the flat piston. For a frequency range from 30 to 300 Hz, the flat piston preferably has a surface area between 17500 mm² and 70000 mm².

In a preferred embodiment of the present invention, the actuator means drive the flat piston electromechanically. As actuator means, the driving coil of a loudspeaker can be used. By modifying a generally available loudspeaker, a very low cost measurement device can be provided. The measurement means comprise a sound pressure sensor in a further embodiment, which is preferably positioned in an opening in the flat piston and directed towards the air cavity between the flat piston and the surface of which the vibration is to be measured. In a further preferred embodiment, the measurement means are positioned centrally on the flat piston for reasons of symmetry. Generally available, low cost sound pressure sensors, e.g. microphones, can be used to enable production of a low cost measurement device.

A second aspect of the present invention relates to a measurement arrangement comprising a measurement device according to the present invention. The measurement arrangement further comprises controlling means connected to the measurement means and actuator means, and the controlling means are arranged for controlling the actuator means such that the sound pressure in the volume is kept at a minimum level. Preferably, the controlling means are also arranged to further process the measurement data, for storage or presentation of the measurement results.

The measurement device according to the present invention can also be used as a sound detector for an anti-sound system.

A third aspect of the present invention relates to a method for determining the acoustical radiation of a vibrating surface by measuring the spatially integrated normal acceleration of the vibrating surface characterised by the steps of bringing a flat piston over the vibrating surface into in phase vibration, controlling the in phase vibration of the flat piston such that the sound pressure in a substantially sealed space between the flat piston and the vibrating surface is minimal, and determining the volume acceleration of the vibrating surface from the normal acceleration of the flat piston. In this way, the volume acceleration (and thus volume velocity) of an acoustically radiating surface can be determined directly, without the need for further processing of the measurement data.

The present invention will now be described more detailed in the following description of preferred embodiments with reference to the accompanying drawings, in which:

Fig. 1 shows a sectional view of measurement device according to the present invention;

Fig. 2 shows a sectional view of a further embodiment of the measurement device according to the present invention; Fig. 3 shows a block diagram of a measurement arrangement according to the present invention.

Noise pollution can be caused by vibrating surfaces 10. This may occur in the interior of transport means such as cars, trains, aircraft, but also in industrial buildings where noise may be generated by different kinds of machinery. Usually, the noise is caused by a number of vibrating surfaces, all contributing to the noise pollution. In order to be able to take preventive measures against the noise pollution, it is required to accurately know which surface is contributing to the noise and at what level. In order to determine this, the acoustic radiation of each of the vibrating surfaces has to be measured in realistic surroundings. In a car, e.g. the internal noise has to be measured without necessitating removal of the seats or coverings. One principal method to characterise the acoustic radiation of a vibrating surface 10 is to integrate its normal acceleration (and hence velocity) over a sub-area of a vibrating panel.

Fig. 1 shows a sectional view of measurement device 1 according to the present invention. The measurement device 1 can be used to determine the integrated normal acceleration of a sub-area of a vibrating surface 10 which can be part of a larger structure, e.g. the interior of a vehicle.

The measurement device 1 according to the present invention comprises a flat piston 2 and a structure 4 supporting the flat piston 2. The structure 4, preferably made of a flexible material, can be placed on the surface 10 to be measured, such that in operation a sealed volume of air between the surface 10 and the flat piston 2 is formed. In operation, the surface of the flat piston 2 is parallel to the surface of which the vibration is to be measured. The measurement device 1 further comprises actuator means 6 for bringing the flat piston 2 into vibration. The flat piston 2 is made from a sufficiently stiff material, such that the flat piston 2 in its entirety moves substantially with the same amplitude in a direction perpendicular to the surface of the flat piston 2 when brought into vibration. In this manner, the volume of air will act as a spatial low- pass filter for the transfer of the vibrations of the vibrating surface 10 to the flat piston 2, thereby integrating the normal acceleration of the vibrating surface 10. The flat piston 2 is made of a stiff material with such a thickness and dimension, that the frequency response of the measurement device 1 is substantially linear between preferably 30 and 300 Hz, as these frequencies are of major importance in noise pollution. The measurement device 1 is provided with measurement means 5 for measuring the sound pressure in the volume of air, positioned on the flat piston 2. Preferably, the measurement means 5 are positioned centrally on the flat piston 2 for reasons of symmetry. The actuator means 6 are controlled in such a way, that the sound pressure in the volume of air is kept at a minimal value, preferably zero. In these conditions, the flat piston 2 will move in correspondence with the in phase part of the vibrations on surface 10 and the normal acceleration of the flat piston 2 is a direct measure for the spatially integrated normal acceleration of the surface 10 to be measured. The measurement means 5 are preferably formed by a generally available sound pressure sensor, such as a microphone. The normal acceleration of the flat piston 2 can be derived from acceleration sensing means 8 (see Fig. 1). The acceleration sensing means 8 can be formed by a generally available accelerometer, fixedly attached to the flat piston 2 and centrally positioned on the flat piston 2 for reasons of symmetry.

The pressure in the air cavity between the flat piston 2 and the surface 10 during operation can be expressed as:

in which α is a sensitivity constant, Q_s is the volume acceleration of the sub-area 10 (and equals Qa_sdS), a is the acceleration and S the area (suffixes p and s denote piston and sub-area, respectively).

The sensitivity constant can be expressed by the following equation: _{a =} βKS_p _ β_K (2πf)² V ~ (2πf)² in which β (<1) is dependent on the percentage of boundary leakage, K is the compression modulus (= 1 bar), S_p is the piston area, f is the frequency, V is the cavity volume, and S V ~\/h, h being the height of the cavity in operation. From this it follows that, in case of no leakage (β=l) and a small height h, the sensitivity α is the highest. The measurement device 1 according to the invention has only modest dimensions and is easy to use in all circumstances, such as measuring in small, confined spaces. Also, the measurement device can be used as sound detector in an anti-sound system.

Fig. 2 shows a sectional view of a preferred embodiment of the measurement device 1 according to the present invention. In this embodiment, the measurement device 1 is a modified speaker, making the measurement device 1 easy and economically to produce.

The measurement device 1 comprises a flat piston 2, which can be brought into vibration by the actuator means 6, in this embodiment formed by the driving coil 6 of the speaker cone 12 in co-operation with a permanent magnet 1 1 of the speaker. The flat piston 2 is attached to the speaker cone 12 such that the flat piston 2 is driven by the driving coil 6, and preferably has a dimension substantially equal to the aperture 3 of the speaker. When the modified speaker is of the type having double driving coils 6, one of the driving coils can be used to drive the speaker cone 12, and the other driving coil can be used to provide a measurement signal representing the normal velocity of the flat piston 2. In this embodiment, the acceleration sensing means 8 can be omitted.

The cone 12, and thus the flat piston 2, is suspended in structure 4 of the measurement device 1, such that the flat piston 2 can move freely up and down, substantially in a direction perpendicular to the surface of the flat piston 2. In operation, the flat piston 2 then moves in a direction perpendicular to the vibrating surface 10. The circumference of the structure 4 of the measurement device 1 is closed, at least between the suspension of the flat piston 2 and the aperture 3 of the measurement device, such that a substantially sealed volume of air is enclosed between the flat piston 2, the structure 4 and the vibrating surface 10 when the measurement device 1 is put in its operative position with the aperture 3 on the vibrating surface 10. Preferably, the outer edge of the structure 4 which in operation contacts the vibrating surface 10, is provided with sealing and damping means 9. The sealing and damping means 9 serve both to seal the volume of air between the flat piston 2, the vibrating surface 10 and the structure 4 and to isolate the measurement device 1 (more precisely, the structure 4 of the measurement device 1) from vibrations of the vibrating surface 10.

The measurement device 1 further comprises, as in the embodiment shown in Fig. 1, a pressure sensor 5 and an accelerometer 8. As shown in Fig. 2, the pressure sensor 5, e.g. in the form of a generally available microphone, is positioned in a central opening in the flat piston 2. The signal leads 13, 14 of the measurement means 5 and acceleration sensing means 8, respectively, can be guided out of the measurement device 1 through a hole 16 in the driving coil 6. This hole 16 also serves to keep the air pressure inside the cone 12 equal to the outside air pressure, thus preventing possible non-linear response of the measurement device 1. The measurement device 1 according to the embodiment shown in Fig. 2 can be produced very economically and can be made to have dimensions sufficiently small to be used in small confined spaces, such as under a car seat. As shown in Fig. 3, the measurement device 1 is preferably used in conjunction with a measurement arrangement comprising controlling means 7 connected to the measurement means 5 and actuator means 6 by means of signal leads 13 and 15, respectively. The controlling means 7 are arranged for controlling the actuator means 6 such that the sound pressure in the volume of air is kept at a minimum level. In a preferred embodiment, the controlling means 7 are also connected to the accelerometer 8 by means of the signal lead 14 and the controlling means 7 are arranged to process the measurement signals, e.g. for data storage or data presentation.

Claims

1. Measurement device for measuring the volume acceleration of a predetermined sub-area of an acoustically radiating surface, comprising a termination

5 end and side walls, the side walls defining a volume of air between the termination end and the acoustically radiating surface in operation, and measurement means for measuring the sound pressure in the volume of air, characterised in that the termination end is formed by a flat piston (2) of a first material; and the measurement device (1) further comprises actuator means (6) for bringing the flat 0 piston (2) into vibration such that the sound pressure in the volume of air is kept at a minimum level.

2. Measurement device according to claim 1, characterised in that the flat piston (2) is suspended in the measurement device (1) such that it can move freely with respect 5 to the measurement device (1), at least in a direction perpendicular to the surface of the flat piston (2).

3. Measurement device according to claim 1 or 2, characterised in that the measurement device (1) further comprises acceleration sensing means (8) for measuring o the normal acceleration of the flat piston (2).

4. Measurement device according to claim 1, 2 or 3, characterised in that the first material is a sufficiently stiff material such that the flat piston (2) can be brought into vibration, the flat piston (2) in its entirety moving substantially in phase in a direction 5 perpendicular to the surface of the flat piston (2).

5. Measurement device according to one of the preceding claims, characterised in that the flat piston (2) has a surface area between 17500 mm² and 70000 mm².

0 6. Measurement device according to one of the preceding claims, characterised in that the actuator means (6) drive the flat piston (2) electromechanically.

7. Measurement device according to one of the preceding claims, characterised in that the measurement means (5) comprise a sound pressure sensor.

8. Measurement device according to one of the preceding claims, characterised in that the measurement means (5) are positioned in an opening in the flat piston (2).

9. Measurement device according to one of the claims 1 through 7, characterised in that the measurement means (5) are positioned centrally on the flat piston (2).

10. Measurement device according to one of the claims 3 through 9, characterised in that the acceleration sensing means (8) comprise an accelerometer fixedly mounted to the flat piston (2).

11. Measurement arrangement comprising a measurement device according to one of the claims 1 through 10, characterised in that the measurement arrangement 5 further comprises controlling means (7) connected to the measurement means (5) and actuator means (6), the controlling means (7) being arranged for controlling the actuator means (6) such that the sound pressure in the volume is kept at a minimum level.

12. Anti-sound system for suppressing sound, including a measurement device 0 according to one of the claims 1 through 10.

13. Method for determining the acoustical radiation of a vibrating surface by measuring the spatially integrated normal acceleration of the vibrating surface characterised by the steps of: 5 bringing a flat piston (2) over the vibrating surface (10) into in phase vibration, controlling the in phase vibration of the flat piston (2) such that the sound pressure in a substantially sealed space between the flat piston (2) and the vibrating surface (10) is minimal, and determining the volume acceleration of the vibrating surface (10) from the normal o acceleration of the flat piston (2).

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