GB2204402A - Audio location of a sound source - Google Patents

Abstract

A pair of matched microphones (5) mounted either side of an acoustic obstacle (3) are used to give approximate location of a sound source by stereophonic effect. The microphones (5) are rotated as a pair towards the source and microphone output signals are compared during this rotation until they are of equal amplitude or, are co-incident in time, or both, according to sound frequency, to give more accurate location of the sound source. The microphones (5) may be mounted on the outside of a helmet. Alternatively, the microphones (5) may be mounted orthogonal to a sight (7), the line of sight intersecting the axis of rotation. Artificial pinnae of different size may be mounted around each microphone transducer, to provide front to rear spatial discrimination. <IMAGE>

Description

METHOD AND APPARATUS FOR AUDIO LOCATION AND PARTS THEREOF This invention concerns a method and apparatus for audio location, and, parts used in apparatus construction, in particular, microphone sub-systems.

iEen in an enclosed environment - for example, in a tank turret, or a submarine hold - an observer is isolated from all otrds directed from external sources. With only visible cues, the observer's perception of the external surroundings is severely limited, being confined to what is usually a narrow and directed field of view, such as may be provided by a window or periscopic sight.

Similarly, an observer wearing a protective helmet is isolated from external sounds, and when using a headset incorporated in the helmet, he is wholly reliant upon sound cues produced by the earpieces of the headset.

In the recording or relaying of sound, to produce stereophonic effect, it is conventional to use a pair of microphones and to reproduce or relay transduced sound signals via separate channels to a pair of earpieces or spaced loudspeakers. The paired microphones usuaily have a separation closely matching average ear separation ~ 20 cm. They may be free standing, but in some recording studios they have been mounted on either side of a fixed duimny head to give improved separation of the sterophonic sounds incident upon the fixed microphones.

Audio location Apparatus was used 1939-19L as an Tti-aircraft aid. A typical apparatus then comprised four microphones mounted in T-formation on a rotatable gantry. kdhilst upon rotation of the gantry, changes in signal amplitude could be monitored and used to detect the direction of a source of sound, the apparatus did not provide immediate albeit approximate sound location. There was therefore a significant delay before the source of sound could be located with accuracy, often following an inordinate and unnecessary amount of rotation.

The invention is intended to provide a remedy for these problems.

According to the invention there is provided a method for locating the direction of a source of sound comprising:firstly, using a pair of matched microphones mounted each side of an acoustic obstacle, (which obstacle serves to shadow either one of the microphones from high frequency sound incident upon the other microphone, and to increment the time delay between low frequency sounds reaching both microphones), to locate the approximate direction of the source; and, secondly, rotating the pair of microphones towards the source and comparing microphone output signals to locate more accurately the direction of the source.

According to the invention there is provided apparatus for audio location comprising:a pair of matched microphones mounted either side of a support body; each microphone being adapted for front to rear spatial discrimination of sound, the support body being capable of rotation in azimuth and serving as an acoustic obstruction to shadow either one of the microphones from high frequency sound incident upon the other microphone, and to increment the time delay between low frequency sounds reaching both microphones; the support body being provided with means for locating the microphones in position relative to a sight, such that the line of the sight shall be orthogonal to the microphone pair and shall intersect the axis of rotation.

The apparatus may be in the form of a helmet support body having microphones mounted externally, one on each side of the helmet. In this case, the helmet is provided with internal fittings so that it can be located in position on the head of the wearer, the pair of microphones then being located orthogonal to the line of sight of the wearer - ie orthogonal to the direction of relaxed gaze(ie when the eyes of the wearer are directed straight forazards, the eye muscles being relaxed).Using this helmet together with matched amplifiers and earpieces, the wearer is then able to determine the approximate location of a source of sound without need to turn the head, However, once the wearer has determined the approximate location of the source, he can then turn his head towards the source to obtain a more accurate f.x on the source using his aided ears, balancing the volume of sound received in each ear, and/or reducing the time delay between these sounds. Filters may be used to eliminate extraneous noise.

Alternatively the apparatus mey incorporate a sight. The sight is mounted between the microphones with line of sight orthogonal to the microphone pair, and intersecting the axis of rotation.

Such apparatus, when used in conjunction with a balanced stereophonic amplifier and headset (or signal detector), provides an observer with audible cues (or indication) sufficient to enable him to locate a source of sound. He may then aim the sight in the perceived direction of the sound, by a rotation of the support. Thr continued monitoring of sound, during this rot at ion, he may then utilise phase gradient and intensity information to assist him in more accurate location of the source direction.

Each microphone may be directional, and may be combined with other directional microphones, and adapted to give front to rear spatial discrimination by weighted summation of the microphone output signals.

Alternatively each microphone may be omnidirectional and adapted with an acoustic attenuator for providing the required discrimination.

The attenuator may be in the form of artificial pinnae.

Alternatively, each microphone mar be mounted in a cavity-attenuator eg an aerodynamically shaped heavy metal block with a small orifice facing outwards or, it may be mounted behind an aperture, with its diaphragm normal to the aperture.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drwinEs of which: FICJES 1: is a schematic plan view of an audio location apparatus embodying the features of this invention; FIGUEE 2: is a perspective view of a microphone sub-system of the apparatus shown in figure 1; FIGURE 3: is a cross-section view in the plane A-A of figure 2 of the microphone sub-system with pinnae and other accessories included; FIGURE 4 is a diagrammatic circuit of audio location apparatus including a stereophonic amplifier and headset.

As shown in the drawings, the audio location apparatus 1 comprises: a rotatable frame, the cupola of a tank turret 3; two microphone sub-systems 5; and, a sight 7. The two sub-systems 5, shown in detail in figures 2 and 3, are mounted on the cupola 3 at diametrically opposite positions, spaced about a metre apart. This spacing is about five times greater than the separation of the human ears - the interaural separation being typically about 20 cm. The sight 7, in the example, is the uppermost part of a periscope 9 by which means the observer in the turret of the tank can view his external surroundings. The sight 7 is fixed on the cupola 3 between the two microphone sub-systems 5 with its optic axis, its line of sight L, passing through the centre of rotation of the cupola 3 and parallel to the ground plane.

Each sub-system 5 includes a miniature omnidirectional piezoceramic or electret microphone 11 supported in a cylindrical metal housing 13. The microphone 11, including integrated preamplifier solid-state circuitry, has an essentially flat response between 100 Hz and 6 kHz ie it has a flat response over a substantial region of the audible acoustic waveband. Typically, it has a sensitivity of 60 dB below 1 volt for a sound pressure of 0.1 No.:2 at a frequency of 1 kHz.

The microphone 11 is located in a central position of the housing 13 and faces outwards. It lies inside a protective ball of plastics foam 15 which serves both as a microphone windshield, and to damp the effects of mounting vibration and housing cavity resonance.

The ball 15 has a thin loose fitting outer skin 17 of impervious membrane to prevent ingress of water, and is protected from flying dust particles by a hemispherical screen 19 of wire gauze. The foam ball 15 is held in position inside the housing 13 by spikes 21 carried on a bracket 23. A hemispherical steel lid 25 is pivoted on the bracket 23 and serves to cover and protect the foam ball 15 when the microphone 11 is not in use. The microphone is shielded by rubber pinnae 27, 29. These serve to reduce wind noise, butt what is more important, they modify the acoustic response of the microphone 11 to give front to back spatial discrimination of sounds.

The pinnae 27, 29 are part of a rubber moulding which can fit neatly over the housing 13 at one end. One of the pinnae, the front pinna 27 is shorter than the other, the rear pinna 29. The pinnae are designed such that the difference in response to sound from front and rear directions is approximately 6 debts over a wide range of frequencies centred on 1 kHz, so to approximate the natural discrimination of the hurnan ear. The three-wire lead 31 from the microphone 11 and its integrated preamplifier is fed through the centre of one of the hollow swivel pins which support the lid 25, and taken through a terminal block from the interior of the cupola 3 to its interior. The lead 31 is connected to the input of one channel of a balanced stereophonic high fidelity amplifier 33.

The microphone 11 of the other microphone sub-system 5 is similarly connected to the other channel of this amplifier 33. The parallel outputs of this amplifier 33 feed a high quality headset 35 a pair of matched earpieces - through matched parallel filters 37. To minimise distortion, the amplifier has a flat frequency response between say 20 Hz and 6 kHz. The headset provided has an essentially flat response between 100 Hz and 5 kHz. In order that the observer, carrying out a binaural listening task, shall not suffer hearing impairment as a result of listening to long duration amplified continuous high level acoustic signals or very high level impulsive noises, the output electrical signals from the amplifier 37 are peat clipped before they reach the observer's headset.The filters 37 are optional and may be used to filter out unwanted sounds, such as local engine noise, wind noise or bird noise. These may be selected by switch operation as required. For example, a fourth order Butterworth high pass filter cutting at 1 kHz could be used to filter out wind noise; and, a fourth order Butterworth low pass filter cutting at 2 kHz to reduce interference from bird song.

The electrical signal outputs from these filters 37 may also be interfaced with communications channels from other equipment.

It is known that there are two distinct physical mechanisms whereby an unaided observer is able to binaurally identify the direction of a sound source. These are by intensity discrimination at high frequency, say above 1500 Hz when the mean diameter of the head is of the order of the wavelength, and by time delay discrimination at frequencies below 1500 Hz. Also, although localisation is mainly attributed to intensity differences between the ears above 1500 Hz, time delay discrimination is also used in conjunction with differential intensity information up to 4000 Hz and beyond.The human is accustomed to listening to complex sounds which arrive at his right and left ears with different time delays and with different frequency amplitude spectra as the low frequency sounds are diffracted round his head while the high frequency sounds suffer selective attenuation due to shadowing-by the head. The higher the frequency, the greater the attenuation by the head.

It is found, surprisingly, that with the aid of microphones spaced at distances significantly greater (eg x 5) than the interaural separation, though the time delays and amplitude differences are scaled, the human observer is able to adapt to, and utilise, sounds entering the ears via headphones or earpieces, and to identify the direction of the sounds. Thus once the observer has registered his own frame of reference with the frame of reference defined by the combined optical system - the periscope 9 and acoustic system - the microphone sub-systems 5 - he does this when peering through the periscope eyepiece - he is able to determine the location of sound sources relative to the line of sight L. He may then rotate the cupola 3 so that the line of sight L is directed towards the identified source. In bringing the source into line he can rely not only on sighting, but also on audible cues. In this way he relies on cues similar to those the unaided observer would use when turning his head towards the source - he aims to balance signal loudness in both ears, or to zero the time delay between the signals received at his two ears, or both, depending on signal frequency.

The apparatus described above could also be used to provide electrical signals for automatic processing and indication. For example, the electrical signals at the outputs of the amplifier 33 could be fed to a signal detector which at selected frequencies could detect phase or amplitude differences to give an indication of source direction. Such indication could then be used to reinforce the observer's subjective judgement.

Microphone arrays instead of single microphones could also be used to improve the signal to noise performance of the apparatus.

whilst in the above example only a periscopic sight has been described as the apparatus sight 7, it is possible to use, instead, a directed infra-red tube or camera, or other directional sight eg sonar or radar beacon used for visual display of the outside environment.

Claims (6)

1. Claims

   What I claim is:1. A rnethod fol locating the direction of a source of sound comprising:firstly, using a pair of matched microphones mounted one each side of an acoustic obstacle (which obstacle serves to shadow either one of the microphones from high frequency sound incident upon the other microphone, and to increment the time delay between low frequency sounds reaching both microphones), to locate the approximate direction of the source; and, secondly, rotating the pair of microphones towards the source and comparing micrdphone output signals, to locate more accurately the direction of the source.
2. 2. Apparatus for audio location comprising:a pair of matched microphones mounted one either side of a support body; each microphone being adapted for front to rear spatial discrimination of sound, the support body being capable of rot at ion in azimuth and serving as an acoustic obstruction to shadow either one of the microphones from high frequency sound incident upon the other microphone, and, to increment the time delay between low frequency sounds reaching both microphones; the support body being provided with means for locating the microphones in position relative to a sight, such that the line of sight shall be orthogonal to the microphone pair and shall intersect the axis of rotation.
3. 3. Apparatus as claimed in claim 1 wherein the support body is in the form of a helmet1 the microphones being mounted on the outside of the helmet, the helmet including internal fittings for locating it in position on the human head.
4. 4. Apparatus for audio location comprising:a pair of matched microphones mounted one either side of a support body; each microphone being adapted for front to rear spatial discrimination of sound, the support body being capable of rotation in azimuth and serving as an acoustic obstruction to shadow either one of the microphones from high frequency sound incident upon the other microphone, and, to increment the time delay between low frequency sounds reaching both microphones; and a sight mounted between the pair of microphones, the line of sight being orthogonal to the microphone pair and intersecting the axis of rotation.
5. 5. Apparatus as claimed in any one of the preceding claims 2 to 4 wherein each microphone is adapted for front to rear spatial discrimination by the provision of artificial pinnae.
6. 6. Apparatus for audio location constructed, adapted and arranged to operate substantially as described hereinbefore with reference to and as shown in figures 1 to 4 of the accompanying drawings.

   6. Apparatus as claimed in any one of the preceding claims 2 to 5 including matched amplifiers and earpieces connected to the microphone outputs.

   7. Apparatus as claimed in claim 6 including filters connected to remove extraneous noise.

   8. Apparatus for audio location constructed, adapted and arranged to operate substantially as described hereinbefore with reference to and as shown in figures 1 to 4 of the accompanying drawings.

   9. A microphone assembly for use in any one of the apparatus as claimed in claims 2 to 7, comprising an omnidirectional microphone and different pinnae mounted one to the front and one to the rear of the microphone.

   10. A microphone assembly substantially as described hereinbefore and as shown in figures 2 and 3 of the accompanying drawings.

   Amendments to the claims have been filed as follows CLAIMS What I claim is:1. A method for locating the direction of a source of sound comprising: firstly, reproducing in real time a stereophonic sound field using a pair of matched microphones mounted each side of an acoustic obstacle (which obstacle serves to shadow either one of the microphones from high frequency sound incident upon the other microphone, and to increment the time delay between low frequency sounds reaching both microphones), a stereophonic amplifier, and a headset including a pair of earpieces; estimating from the sound amplitude imbalance in each ear, and/or from the sound time delay for each ear, the approximate direction of the source of the sound, and, secondly, rotating the pair of microphones towards the direction so estimated until the sound amplitude in each ear is balanced, and/or the perceived time delay is nulled, to locate more accurately the precise direction of the source.

   2. Apparatus for audio location by the method claimed above, and comprising:a stereophonic amplifier; and, head apparel including both a pair of matched microphones, each adapted for front to rear spatial discrimination of sound, and mounted for fitting symmetrically each side of the head of a listener; and, a pair of matched earpieces, mounted for fitting adjacent to the ears of the listener; the microphones and the earpieces being connected to corresponding input and output channels, respectively, of the stereophonic amplifier.

   3. Apparatus for audio location by the method claimed above, and comprising:a pair of matched microphones mounted one either side of a support body; each microphone being adapted for front to rear spatial discrimination of sound, the support body being capable of rotation in azimuth and serving as an acoustic obstruction to shadow either one of the microphones from high frequency sound incident upon the other microphone, and, to increment the time delay between low frequency sounds reaching both microphones; a sight mounted between the pair of microphones, the line of sight being orthogonal to the microphone pair and intersecting the axis of rotation; a stereophonic amplifier, each channel thereof being connected to a corresponding one of the microphones; and, a pair of matched earpieces, each connected to a corresponding output of the stereophonic amplifier.

   4. Apparatus as claimed in either of claims 2 or 3, wherein each microphone has artificial pinnae, and is thereby adapted for front to rear spatial discrimination.

   5. Apparatus as claimed in any one of the preceding claims 2 to 4 wherein the amplifier includes matched filters in each channel for suppressing its response to extraneous noise.