GB2234137A

GB2234137A - A device for the detection and location of sound waves

Info

Publication number: GB2234137A
Application number: GB9015256A
Authority: GB
Inventors: Oskar Bschorr
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1989-07-18
Filing date: 1990-07-11
Publication date: 1991-01-23
Anticipated expiration: 2010-07-11
Also published as: GB9015256D0; GB2234137B; FR2650466B1; DE3923740C1; FR2650466A1

Abstract

In order to increase the noise gap between incident- and interference signals, in particular wind noise, microphone (5) is arranged inside a resonator (2) which is tuned to a certain resonance frequency. The resonator can also be a silator (2a). The impedance break at the resonance frequency allows alignment to the impedance of the surrounding medium so that optimal large antenna cross-sections can be used. The sensitivity of the microphone is also increased by the sound- pressure increase at resonance. Since the shape of the resonators can be freely chosen, they can be integrated into existing vacant spaces, for example wings of search drones. <IMAGE>

Description

AJW080690 1 A DEVICE FOR THE DETECTION AND LOCATION OF SOUND WAVES This

invention relates to a device for detection and location of sound waves comprising a microphone, and a resonator for suppression of interference, in particular wind noise.

The resolution and the range of microphones are restricted by interfering signals, and in the case of external microphones in particular by wind noise. Conventional wind screens and nose cones improve the signaltonoise ratio. However, further suppression of wind noise by 6-8 dB would double the hearing distance.

The so called antenna cross-section of known types of microphones, such as condenser microphones, dynamic microphones, piezo microphones etc., is extremely small due to high acoustic flow impedance. This is also a requirement for standard microphones so that the sound field to be measured is not distorted by the measuring means. However, in many observation tasks, this factor is not critical for example in the detection of sound waves.

An object of the invention is to provide a device of the above discussed type (i.e. comprising microphone and resonator) which is especially adapted for measurement in waves and whereby a considerable increase of the signal-to- -1 AJW080690 2 - noise ratio from intelligence signal to interference signal, in particular wind noise, will be achieved.

This object is achieved, according to the invention, in that the microphone is arranged inside the resonator.

The microphone may be a conventional pressure microphone, which is located inside a resonator, such as a Helmholtz- or X /4 resonator or a silator. Henceforth, that entire device will be described as a resonator microphone.

It is known that by means of resonators of the aforementioned type, the absorption plane at resonance frequency can be reached at resonance frequency up to X1 -17o' /TFin the free field and 9/2 n an echo wall, where \represents the sound wavelength at resonance. The absorption plane and thus the antenna cross-section of such a microphone, for example tuned to 20 Hz, is 22.8 M2, or 45.6 M2 respectively. Maximum achievable antenna crosssections are determined by, amongst other things, the spatial correlation length of the sound signal. This length differs between sound- and wind-pressure interference, i.e. it is considerably shorter for wind noise than for intelligence signals. Thus, an increase in the acoustic antenna cross-section improves the ratio of intelligence-tointerference signal.

a AJW080690 According to a further aspect of the invention, several such resonator microphones, tuned to different frequencies, can be used for the detection of broadband sound waves. Advantageously, these resonators are given internal attenuation in a manner which is already known so as to cover a broader frequency range and achieve faster transient oscillation.

In certain embodiments in accordance with the invention, tunable resonators are used wherein frequency and flow resistance are adjustable by way of resonator mass, resonator compliance and damping. It is thus possible to adjust to doubleshift of the target object. Subsequently, frequency oscillation around the direction-finding frequency has to be performed.

A partially evacuated silator can be used as a hermetically sealed resonator. This is a lens-shaped chamber in which the chamber walls act as a compliance/masssystem. The lens-shaped curvature together with the negative-pressure load result in reduced inherent rigidity.

In other apertured embodiments, to further reduce wind noise the resonator aperture may advantageously be protected by wire grids, or by foam material and/or by thin-walled foils in single or multiple layers.

4 AJW080690 Since wind-pressure interference is only of short correlation length. Helmholtz resonators with several apertures may advantageously be used with the spacing between respective apertures being approximately equal the correlation length of the wind-pressure interference. The acoustic paths to the individual apertures should, however, be smaller than half wavelength at resonance so as to avoid interference loss.

Certain embodiments in accordance with the invention make use of Helmholtz resonators with several apertures which are sealed by diaphragms of different rigidity and mass. Such an arrangement results in several resonance frequencies at unchanged resonator volume.

For direction-finding purposes two resonator microphones tuned to the same frequency can be used. The incident direction of the sound can be determined by thei-r mutual spacing from one another - the basis - and by the phase relationship of aggregate signal and differential signal. The two or more resonance microphones which are used can rotate along with their direction-finding basis.

Several resonance microphones can be arranged in arrays for the purpose of direction finding.

The invention will be described further, by way of j AJW080690 - example, with reference to the embodiment accompanying drawings, in which:

illustrated in the Fig.1 is a schematic illustration of the basic arrangement of a resonance microphone according to the invention; Fig.2 is an arrangement of several, differently tuned resonance microphones; Fig.3 is a resonance microphone, according to the invention, with a tunable resonator; Fig.4 is a schematic illustration of a resonance microphone, according to the invention, in which the resonator is a silator; Fig.5 is a schematic illustration of a resonance microphone, according to the invention, with an additional wind shield; Fig.6 is a resonance microphone in which the resonator has several resonator apertures; Fig.7 is a schematic illustration of a resonance microphone in which the resonator has several resonator apertures which are sealed AJW080690 by diaphragms; Fig.8 is an arrangement of two identically tuned resonance microphones according to the invention; and Fig.9 is an arrangement of two resonance microphones according to the invention, arranged to be rotatable around the centre point of a common direction-finding basis.

In the Figures, equivalent parts or parts of equivalent function are identified by the same reference numbers.

Fig.1 shows in cross-section the basic arrangement of a resonance microphone 1, consisting of resonator 2 (in the example given, a Helmholtz resonator) having a resonator aperture 3 and a resonator volume 4, i.e. the space inside the resonator 2, and a microphone 5 arranged in the interior space 4 of the resonator. The dimensions of the resonator aperture 3 and the resonator volume 4 are tuned to the desired resonance frequency, damping and flow impedance, in a manner already well known.

The resonator aperture 3 as well as the mass of air contained therein and the inherent compliance/dampingg property of the resonator volume 4 form an acoustic filter 11 AJW080690 - 7 allowing passage in the region of the set resonance frequency. At the same time, amplification of the sound signal, entering via the resonator aperture 3 into the microphone 5, occurs by way of the resonator 2.

The alignment of the flow impedance of the resonance microphone to the field impedance of the environmental medium, for example air, is significant. It is already known that absorption cross-sections up tog/47Fcan be achieved in free-field conditions, where/\ represents the sound wavelength at resonance. Since turbulent windpressure interference is only correlated over a reduced stretch, different antenna crosssections have the effect of improving the incident-to-noise ratio of the signal during sound and turbulence.

Fig.2 shows schematically an arrangement of four resonance microphones la, lb, lc and ld, in which respect each individual resonance microphone is arranged as in Fig.1 to include a respective resonator 2 with a resonator aperture 3, and a microphone 5. The individual resonators are tuned to different resonance frequencies by appropriate dimensioning of their resonator volumes and apertures, so that the microphone arrangement shown in Fig.2 can pick up a broadband frequency spectrum.

The resonance microphone 1 shown in Fig.3 again AJW080690 includes a resonator 2 with a resonator aperture 3, as well as a microphone 5, arranged at the bottom of resonator 2. Below the resonator aperture 3 a diaphragm 6 is arranged parallel with the bottom surface of the resonator. This diaphragm 6 is connected to an adjustment mechanism outside resonator 2 by means of which it can be moved in the direction of the double arrow towards or away from the resonator aperture 3.

By adjusting the diaphragm 6 in this way the aperture cross-section of the resonator 2 and/or its aperture length can be adjusted, thereby varying the resonator volume. In this way, this resonator can be adjusted to any desired resonance frequency within a certain frequency spectrum.

Fig.4 shows a resonance microphone 1 in which the resonator is a silator 2a. This silator is crosssectionally of approximately lens-shaped configuration, and its walls 9 are relatively rigid but light in construction, consisting for example of a honeycomb structure. At the periphery of the silator 2a a support ring 10 is provided which seals the walls around the outside. In this respect, the silator consists of a system which is completely hermetically sealed towards the outside, and the inside space of which is at least partially evacuated. In this inside space is disposed a microphone 5. A resonator aperture is not required with this construction. Sound ii, S AJW080690 7 9 - waves hitting the resonance microphone 1 are directed into the inside by the resonance frequency of the silator, amplified and picked up by the microphone 5. The curvature of the silator wall 9 together with its tension load result in a low resonance frequency.

Fig.5 shows another resonance microphone 1, the basic construction of which, with resonator 2 and microphone 5 corresponds to that in Fig.l. However, in and above the resonator aperture 3 an additional wind shield 11, for example of open-pored foam material, is provided. Grid-like covers and thin-walled foils, which may be arranged in several layers one on top of another to increase protection from wind, are effective in the same manner.

The resonance microphone 1 shown in Fig.6 again includes a resonator 2 with a resonator volume 4, and a microphone 5 arranged at the bottom of the resonator 2. In place of a single resonator aperture, several - in this case three - resonator apertures 3a, 3b and 3c, are provided. The mutual spacing of the individual resonator apertures can and should be equal to or larger than the correlation length of expected wind-pressure interference.

The resonance microphone 1 shown in Fig.7 is modified from the arrangement in Fig.6 in that the resonator apertures 3a, 3b and 3c are additionally each sealed by a AJW080690 diaphragm 12, each of different mass- and inherent compliance properties. Constructing a resonance microphone in such a way allows it to be tuned to different resonance frequencies.

In the resonance microphones of Figs. 1, 3, 5, 6 and 7, the resonator 2 is in each case shown as a rectangular box. This shape is not in any way essential and it plays no significant part as far as the resonance frequency is concerned. The shape can, of course, be adapted to suit the individual application. For example, the supporting surface, or at least part of the supporting surface of an acoustic search drone, can be constructed so as to be a resonator.

Fig.8 shows a dipole resonance microphone 1 consisting of two identically tuned resonance microphones le and lf, each once again consisting of a resonator 2 with a resonator aperture 3, and a microphone 5. The bottom surfaces of the two are connected to one another so that the resonator apertures 3 are positioned diametrically opposite one another. The space between the resonator apertures 3 determines the so-called directionfinding basis of the resonance microphone and should be chosen to be reasonably large. For example, basis length of half the wavelength of the sound wave to be picked up is well suited to direction finding of a sound signal emitted by a sound source. For z AJW080690 direction finding, the sum- and differential signals of the two resonance microphones le and lf are evaluated in a manner which is already known. This microphone arrangement can also be arranged to be rotatable around a central axial A which lies vertically on the connecting line, between the resonator apertures 3. Such a direction-finding resonance microphone can be used for probing the surrounding area for sound sources.

A similar arrangement of resonance microphone is shown in Fig.9, where two resonance microphones lg and 1h are mounted on a common direction- finding basis 13. Here the resonator apertures 3 of the two resonators 2 each point in the same direction, in this case perpendicularly relative to the basis 13, and the basis 13 is rotatable around a centre axis A in the direction of the arrow. This microphone arrangement can also be used for probing the surrounding area for sound sources.

Numerous other combinations of any of the previously J types of individual resonance microphones are describe. possible to provide a device or assembly for detection of sound waves/sound sources in accordance with the invention. However, when such an arrangement is required for directionfinding it is essential that each of two combined resonance microphones are tuned to the same resonance frequency.

9 1 AJW080690

Claims

1. A device for the detection and location of sound waves comprising a microphone, and a resonator for the suppression of interference, the microphone being arranged inside the resonator.

2. A device as claimed in Claim 1 wherein the resonator is a Helmholtz or/\/resonator having at least one resonator aperture.

3. A device as claimed in Claim 1 or 2, wherein the resonator is a tunable resonator.

4. A device as claimed in Claim 1, 2 or 3 wherein the resonator has several resonator apertures.

5. A device as claimed in Claim 4 wherein each resonator aperture is sealed by a diaphragm which is tuned to a predetermined inherent frequency.

6. A device as claimed in Claim 2 or 4 further including for the or each resonator aperture, a wind guard consisting of open-pored foam material, or of wire grids or of foils in one or several layers.

7. A device as claimed in Claim 1 wherein the resonator W AJW080690 1 is a silator which is at least partially evacuated.

8. A device for detection and location of sound waves comprising several resonance microphones as claimed in any one or more of the preceding claims, and having different resonance frequencies, combined into one microphone arrangement.

9. A device for detection and location of sound waves comprising at least two identically tuned resonance microphones, constructed in accordance with any one or more of the preceding claims and combined into a common microphone arrangement, which also includes one directionfinding basis.

10. A device as claimed in claim 9 wherein the microphone arrangement is rotatable around a central rotary axis of the direction-finding basis.

11. A device for the detection and location of sound waves substantially as hereinbefore described with reference to and as illustrated by any one of the Figures of the accompanying drawings.

Published 1991 at 7 tie Patent Office. tate House. 56171 hi.:1 Londrr WC I R 4171P Further copies maybe Obtained frerr, Sales Branch. Unit 6. Nine Mil. Point. C%47nfelinfacl-,. Cross Keys. Newpon. NT I 7H2' Printed by Niulup;cx techniques lid. St Cr.-, Ker.: