WO2006103441A1

WO2006103441A1 - Wind noise rejection apparatus

Info

Publication number: WO2006103441A1
Application number: PCT/GB2006/001166
Authority: WO
Inventors: David Herman
Original assignee: Audiogravity Holdings Limited
Priority date: 2005-03-30
Filing date: 2006-03-30
Publication date: 2006-10-05
Also published as: EP1872619A1; EP1872619B1; DE602006012030D1; ATE456906T1

Abstract

An apparatus for reduction of wind noise comprised of an electro-acoustic transducer arrangement with at least three bi-directional transducer elements disposed in three dimensions at an angle to each other and pointing towards a common sound source. The exposed structure is covered with a thin layer of acoustic-resistive material. The electrical outputs of the elements are added together to provide an output signal with increased signal to noise ratio.

Description

WIND NOISE REJECTION APPARATUS

The present invention relates to use of electro-acoustic transducers and more particularly to an arrangement in conjunction with an electronic circuit which reduces the effects of wind noise in the case of a microphone.

The problem with wind noise in relation to microphones is well known and many solutions have been proposed. Such proposals have often required the use of complex signal processing equipment which increases the cost of a microphone quite considerably.

Simpler solutions such as providing the microphone with a wind screen of some sort have also been proposed which can be effective, however, they are extremely bulky. The present invention provides an electro-acoustic transducer arrangement comprising three or more bi-directional transducer elements disposed at an angle to each other in three dimensions and pointing generally towards a common sound source. The outputs of the elements are added together to provide an output signal with increased signal to noise ratio.

An advantage of the present invention is that there is no requirement for there to be a desired sound source present for the invention to work.

In order that the present invention be more readily understood, an embodiment thereof will now be described by way of example only with reference to the accompanying drawings, in which:-

Fig 1 shows diagrammatically a first embodiment of microphone in accordance with the present invention;

Fig 2 shows a block diagram of a circuit for use with the microphone of Fig 1 ;

Fig 3 shows diagrammatically two dimensional relative angles between microphones for explanatory purposes;

Fig 4 shows a block diagram of a further arrangement including modified circuitry according to the present invention.

Embodiments of the present invention comprise at least three bi-directional transducer elements. A bi-directional transducer element is one where there are front and rear ports in a housing with the diaphragm of the transducer disposed within the housing so as to be influenced by sound waves entering the housing through the front and rear ports. With one embodiment of the present invention, the elements are ideally positioned relative to each other such that their front ports are equidistant from a desired source and a projection of the central axis of each element forms a unique angle at the back port. In one such arrangement, the elements can be located on the surface of an imaginary sphere so that they are all equidistant from the desired sound source. The sensitivities of the front ports of the bidirectional microphones overlap as much as possible and the sensitivities of the rear ports overlap as little as possible as indicated in fig. 3. The desired sound source is located generally in the overlap of the front ports to where the most gain will be achieved. The wind noise rejection effect is not significantly affected by the direction that the wind is blowing. And the desired sound is usually located generally in front of the front ports, though this is not critical. The microphones shall not be mounted in a physical hemispherical or parabolic device normally used to focus the desired sound to a common focus because this will create unacceptable differences in wind pressure on the front and back ports which will adversely reduce the wind noise rejection. In another possible configuration of three bidirectional microphones, the axis of each may correspond to the perpendicular axis of each dimension of three dimensional space (X₅Y₅Z) and therefore rio two microphones share a common axis angle. The microphones can be mounted in an enclosed container and exposed through a common hole. Furthermore, the microphones should be shielded from the wind with a thin acoustic resistive material that may surround them or at least placed over the exposed hole(s) common to all microphones. This material can be thin felt or acoustic foam similar to that used to cover the ear pieces of headphones. This will now be explained in more detail in relation to the transducer element being a microphone element.

A bi-directional microphone element has the same sensitivity front and back and will produce an electric signal as a result of the difference of the sound pressure entering the front and back ports of the element.

If one considers Fig 1, three bi-directional microphone elements are present but rather than being disposed relative to each other so that they are physically in three dimensions and pointing at a common sound source, the B and D elements in Fig 1 are physically disposed in the same plane but the axes of the elements B and D pointing generally at a zone containing the sound source. In other words, the axes of the two elements are in the same plane but at different angles. The middle element C is physically above the plane as the elements B and D but it is tilted. Thus, it is pointing at the zone containing the sound source but with its front polar response diagram substantially overlapping those of the elements B and D. Consequently, the advantages of the present invention can be obtained with this arrangement.

Turning now to Fig 2, it will be seen that with this arrangement of microphone elements, it is possible to arithmetically add the outputs of all the microphone elements.

The electrical outputs can be added together in any convenient manner using any suitable analogue or digital procedure such as digital signal processing.

Although the preferred embodiment utilises three bi-directional microphones, three, or more microphone elements may be used. There is a limitation to the number of usable microphone elements however. This limit will now be explained with reference to polar sensitivity diagrams representing the microphone elements performance as measured in a full circle however it is understood that in practice, the actual arrangement is always three dimensional and not two dimensional. This is because the wind may arrive from any direction in three dimensional space. As stated above, the elements are bi- directional. Consequently, each element's performance can be represented by front and back polar response diagrams centered around an axis. In the ideal situation where the elements are equidistant from a desired sound source, the front polar response diagrams will overlap to a considerable degree. However, the rear polar response diagrams ideally should not overlap and the microphone elements are disposed relative to each other to achieve this. Thus, the overlap of the sensitivity diagrams is a factor to this limit of the number of microphone elements. While the above explanation assumes that the front ports are equidistant from the desired sound source and the rear ports have polar diagrams which do not overlap, this can be approximated while still achieving desirable results as will now be described in relation to Fig 3. This figure shows any two microphone elements for clarity in view of the fact that a third microphone element will be in a plane normal to the plane of the paper.

Consider first the front ports ie those closest to the desired sound source. Although in principle one disposes the microphone elements such that their central axes will ideally coincide with the focus of the geometric surface on which they are theoretically disposed, all that is required, in practice, is that there is substantial overlap of the front facing polar sensitivity diagrams of the elements. Bearing in mind that the polar sensitivity diagram represents a three-dimensional space, as long as the desired sound emanates from the volume contained by the overlapping portions of the respective polar diagrams, the advantages of the present invention will be achieved. It is thus also apparent that strict equidistance is not required. A large degree of difference can be accommodated, defining a zone or volume. With further reference to Fig 3, it will be noted that the bi-directional elements are located within a housing provided with or formed by a layer of acoustic resistant material. Alternatively, the bi-directional elements may be located in a case with one or more holes, in which case only the holes need be covered with a layer of acoustic resistive material. Further, this material may be of a very thin variety such as that normally associated with headphones and therefore not burden the practical manufacturability of the invention. It is to be noted that both the front and back ports are open with the rear ports acoustically exposed so that the axes extend through the front and back ports away from the element in free space. And, mounting this within a physical hemisphere or parabolic structure to focus the desired sound to a common point is not permitted as this will impede the operation of the rear ports, adversely affecting the wind noise rejection effect. While the above embodiment discloses the use of microphone elements disposed on the surface of an imaginary sphere, it is also possible to locate the microphone elements on the imaginary surface of any other geometric figure which has a focus. The elements can be disposed as a matrix or cruciform shape (a three dimensional array) or as depicted in Fig 1.

One intended use is that the microphone elements will be mounted in some manner so that the three dimensional array is in a relatively fixed position with respect to the desired sound source. In the case of a microphone for use with a person, the microphone could be attached to the end of a boom which itself is part of an ear piece or headset. Alternatively, the microphone could be mounted in a helmet which may have an oxygen feed acting as an internal source of unwanted wind noise, or it could be used to replace the existing microphone in existing outside broadcast arrangements where the microphone is located within a cage which is arranged to be held against the face of a user with the microphone itself spaced from the user's mouth by a distance. Applications include wired or Bluetooth PHF (Personal Hands Free) for use with a mobile phone. The microphone may be used with a digital or video camera such that the desired sound is coming from approximately in front of the camera. The people speaking may be non- stationary or moving without affecting the desired affect wind noise rejection.

It is to be emphasised that the microphone elements described in relation to Fig. 3 will enhance any sound signal emanating from the zone represented by the overlapping front polar response diagrams whether or not the desired sound source is physically located in the zone. Thus, precise location of the microphone with respect to, say, the mouth, is not required and it has been found that an array of microphone elements as described in relation to Fig 1 or 3 will function satisfactorily even if the array is dangling near a suitable sound source and consequently receiving only off-axis signals. Fig. 1 is useful to explain an optional modification which can be made to the circuitry which is fed by the microphone arrangement. The wind direction is represented by the arrows in Fig 1 and it will be seen that the microphone element B has its axis lying normal to the wind direction. Thus, wind noise will act equally on both the front and rear ports of element B and consequently radically affect the ability of the element B to react appropriately to sound from the zone containing the sound source. It is possible, therefore, to detect electronically such a condition and to inhibit the signal from this element being processed leaving the signals from the elements C and D to be added together. In other words, the signal from the most affected element is excluded and only signals from the remainder of the elements are further processed. Fig 4 shows a block diagram of a microphone array with electronic circuitry for carrying out the inhibition and signal processing mentioned above. There are many other methods for achieving this using either analog or digital solutions. In this figure, the microphone elements B,C and D are shown covered by a common thin layer of acoustic resistive foam material 50. The outputs of the elements are fed to a selector circuit 51 where the signals are compared and the signal from the worst affected element is inhibited. Thereafter, the signals are fed through level and frequency control circuits 52, signal conditioning circuits 53 and then added together and fed to an output 55 after processing in a filter circuit 54 which applies band pass filtering below 200Hz. Other notch and band pass filtering can be provided to compensate for slight low frequency drop off in the voice frequency band. The array of microphone elements replaces a conventional microphone and thus can be used as a direct replacement for such a microphone by being incorporated into equipment during manufacture. This may be achieved by incorporating the microphone elements and the associated signal addition circuitry as components of the larger equipment during manufacture. Alternatively, the microphone elements could be packaged in such convenient manner with or without their associated signal addition circuitry and provided to manufacturers as a module. The microphone elements may be mounted at the appropriate angle to each other and in the correct orientation by use of an over-molded high temperature polymer. Care has to be taken to ensure that the rear ports are unobstructed and that the front to back port wind pressure is minimally affected by the mounting structure. Furthermore, the mounting structure should minimize the effect of altering the direction of the wind as picked up by the one microphone with respect to the others. In other words, all microphones should detect the wind coming from the same general direction.

The array of bi-directional transducer elements, whether in modular form or not may be mounted in a housing which may be waterproof but is provided with an array of perforations covered by a thin layer of acoustic resistive material. The housing may be provided with means for attaching the array of elements to another piece of equipment on a user, e.g. by means of a spring clip. The present invention has wide application either as component parts of a larger piece of equipment or as a module for the larger equipment. To give some indication of the various applications, a number of different implementation will now be described. This is not an exhaustive list. One implementation is to replace an outside broadcast microphone as indicated previously. Another is to replace the microphone in a mobile phone or part of a personal hands-free kit for a mobile phone. Another is to replace the microphone in portable recording devices.

A further implementation is to replace the microphone in a digital camera or video camera, video camera-phone, or another portable communication device. This can be either the microphone which is pointed at the user so that the user can comment on the scene being photographed. While the above arrangements are all disclosed with reference to wind and microphones, the same principles can be applied to other fluids such as water, in which case the transducer is normally termed a hydrophone. Further, the bidirectional transducer elements can be fabricated using semiconductor techniques which allows the array of elements to occupy very little space. A MEMs microphone sometimes referred to as a SiMIC (Silicon Microphone) will require the addition of a rear aperture or apertures to enable bi-directionality.

Using miniature bidirectional microphone elements in an appropriate array permits a version of the invention to be utilised in a hearing aid that is suitable for use outdoors and in breezy or windy conditions.

While the above description refers to the use of bi-directional elements, it is to be noted that uni-directional microphone elements can also be used as they also exhibit the same properties and effects but to a much lesser degree.

Claims

CLAIMS:

1. An electro-acoustic transducer arrangement comprising at least three transducer elements having open front ports and open back ports and being disposed relative to each other such that lines joining the front and back ports of respective elements lie in three dimensions at an angle to each other and pointing generally towards a common sound zone with a means for receiving the outputs of the elements and for adding the outputs together, and a thin acoustically resistive material covering at least a portion of the common volume that all the transducers are exposed to the wind.

2. A transducer arrangement according to claim 1, wherein the transducer elements are located in an array forming part of an imaginary sphere or parabola.

3. A transducer arrangement according to claim 1 or 2, wherein the elements are microphone elements and are located on a boom attached to a user's head so as to be located pointing at the user's mouth.

4. A transducer arrangement according to claim 1 or 2, wherein the elements are microphone elements and are located on a helmet so as to be pointing at a user's mouth.

5. A transducer arrangement according to any one of claims 1 to 4 wherein the elements are mounted at the appropriate angle and orientation by use of an over-moulded high temperature polymer which will be placed by the standard SMT process.

6. A transducer arrangement according to any one of claims 1 to 5 wherein the plurality of elements are manufactured using semiconductor micro fabrication techniques.

7. A transducer arrangement according to any one of claims 1 to 6, wherein each of the transducer elements is a bi-directional element.

8. A transducer module comprising a housing within which is provided a transducer arrangement as claimed in any of the preceding claims, wherein an outer surface of the housing is semi-permeable in one direction and is waterproofed.

9. A module as claimed in claim 8, wherein an array of perforations is provided in said waterproofing housing adjacent to each microphone.

10. A module as claimed in claim 9 is adjustable in direction all planar direction normal to the microphone face, the said orientation is fully adjustable, and self positions with adequate friction.

11. A module as claimed in any one of claims 8, 9 or 10, wherein mounting means are provided in the form of an over-moulded package.

12. A module according to claim 8, wherein said waterproofing material is bonded to the rear of the microphone package and forms an elliptical core, which is in clearance of all other points.

13. A module as claimed in 8, wherein said housing is provided with a resilient snap back clip.

14. A module as claimed in any one of claims 8 to 13 wherein electrical connections and said transducer arrangement are secured electrically in the other equipment.

15. A camera incorporating a transducer arrangement according to any one of the preceding claims.

16. A portable communication device incorporating a transducer arrangement or module according to any one of the claims 1 to 14.

17. A portable communication device according to claim 16, wherein the device communicates data, as well as sound.

18. A hearing aid incorporating a transducer arrangement according to claim 6.

19. A recording device incorporating a transducer arrangement or module according to any one of claims 1 to 13.