GB2455300A

GB2455300A - Accurate ambient noise sensing and reduction of wind noise

Info

Publication number: GB2455300A
Application number: GB0723638A
Authority: GB
Inventors: David Herman
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-12-03
Filing date: 2007-12-03
Publication date: 2009-06-10
Also published as: GB0723638D0; WO2009071896A1; TW200931399A

Abstract

An ambient noise reduction apparatus employing a transducer arrangement comprising a plurality of microphone elements B, C, D, with means for adding 17 the respective outputs and filtering or otherwise processing the combined signal 18,19. The ambient noise signal provided by this transducer arrangement is used as an input to an active noise reduction system, with particular application to windy conditions. The microphone elements may be omni-directional and disposed in a common housing which may comprise an acoustic resistive covering material such as mesh, foam or porous plastic. The apparatus may be used in audio or communication devices possibly comprising earphones, headphones or loudspeakers.

Description

APPARATUS FOR ACCURATE AMBIENT NOISE SENSING AND REDUCTION IN

THE PRESENCE OF WIND

The present invention relates to arrangements for sensing ambient noise, in particular for use in ambient noise reduction systems and particularly used outdoors, in a breeze or wind.

Noise cancelling arrangements are well known and are designed to remove unwanted noise from the sound perceived by a listener, who may be either listening to a wanted audio signal in the presence of unwanted noise or simply exposed only to such unwanted noise. One particular source of noise is ambient noise either at the location of the listener or at the location of the source of the audio signal, or both.

There have been many proposals for dealing with this problem but one common method is simply to have a microphone specifically for sensing ambient noise, generating an electrical signal from the microphone indicative of the ambient noise, inverting and adding this signal to the wanted audio signal in such a way that the resultant signal, when provided to the listener, to some extent cancels the ambient noise to which the listener is exposed, by a process of interference.

Currently, most attention is being directed to the signal processing circuitry utilised in noise cancelling arrangements. These attempts do not address noise in a wind or breeze. It is known that noise cancelling arrangements do not perform well in windy conditions or even in a light breeze.

The present invention proposes to improve the transducer arrangement used for sensing ambient noise in order to address the problem of ambient noise in a windy environment. It is proposed to utilise the improved transducer arrangement with all types of noise cancelling systems which utilise a separate ambient noise transducer.

In order that the present invention can be more readily understood, an embodiment thereof will now be described with reference to the accompanying drawings in which, Fig 1 shows a block diagram of a noise cancelling apparatus; and Fig 2 shows the construction of a transducer arrangement according to the present invention.

The present invention is based on the use of an improved construction for a transducer which is used as the ambient noise pick up transducer in a noise cancelling system. Such a system is shown generally in Figure 1 where a wanted audio signal I is being fed to an output audio transducer, in this case an earpiece or earphone used by a listener. An ambient noise transducer 3 generates an electrical signal indicative of the ambient noise and this signal is sent to a processing circuit 4 where it is combined with the wanted signal. For example, the ambient noise signal is typically inverted and then added to the wanted signal. One possible application of this system is to feed a sound to the listener so that the acoustic wave produced by the earpiece interferes with the actual acoustic ambient noise in such a way that the total sound heard by the listener consists of the wanted signal together with a reduced magnitude or cancelled ambient noise. The location of the noise sensing transducer 3 is dependent on the location of the noise which it is wished to remove from the signal presented to the user's earpiece. For example, it is known to produce noise cancelling headphones which can be used to reduce the effect of environmental noise in an aircraft when a passenger is listening to an aircraft entertainment channel. Alternatively, when utilising a hands-free kit in a car fitted with a mobile telephone, the user of the mobile telephone in the car does not want to transmit the ambient noise from within the car to the receiver of the called party and it is common to provide ambient noise suppression systems at the origin of the telephone call in this case. These two examples exhibit the fact that the transducer 3 can be located at a number of different locations depending on the type of system in operation.

Turning now to Figure 2, this shows a transducer arrangement designed to improve the quality of the signal produced by the ambient noise transducer 3. This transducer arrangement may be used to produce an improved ambient noise signal in any noise cancelling system which detects ambient noise and may be used to reduce the ambient noise heard by a user listening to, for example, an audio player (such as a CD or MP3 player) or other media player, or a communication device (such as a mobile telephone). Alternatively, a user may not be listening to a particular sound source, but may be using a noise cancelling system to remove ambient noise with the intention of hearing silence.

The improved transducer arrangement in particular addresses the problem of wind noise in the ambient noise. When subjected to wind, the signal from a microphone easily becomes saturated by the air flow into the microphone, which impairs the ability of the microphone to produce a signal which accurately conveys the sound to which the microphone is exposed. In the case of a microphone being used to detect the ambient noise for use in a noise cancelling apparatus, the effect of such exposure to wind is that the microphone does not accurately pick up the ambient noise, and the effect of the noise cancellation using the detected ambient noise signal is therefore greatly reduced.

A further problem arises when a microphone is used to detect the ambient noise in a noise cancelling system, when the user is listening to earphones or headphones in the presence of wind, e.g. outdoors. In this environment, the user may be exposed to various forms of ambient noise (e.g. traffic noise), but is in fact shielded to some extent from the actual wind noise by the presence of the earphones or headphones. In the absence of the noise cancelling system, the user therefore hears any signal being supplied via the earphones (or none where no sound is being played), together with the ambient noise, but no significant wind noise. When the noise cancelling system is employed, the microphone detects the ambient noise together with the wind noise, and this signal is then inverted and mixed with the wanted signal. What the user hears is therefore the wanted signal and ambient noise, together with the inverted ambient noise signal, which includes the inverted wind noise. Since the user is shielded from the actual wind noise, the effect of this is to actually introduce the inverted wind noise into the signal provided to the user's earphones, greatly reducing the effectiveness of the noise cancelling system, and possibly resulting in the total signal heard by the user being significantly worse than if ambient noise cancellation were not used at all.

The purpose of the improved transducer arrangement used in the present invention is therefore to improve the detection of the ambient noise, in particular in the presence of wind, such that the noise reduction apparatus functions more effectively when using the detected ambient noise signal to produce a signal intended to cancel out the ambient noise heard by the user. The system works best where the user is listening to the signal produced by the noise reduction system via earphones or headphones with both ears, since the user is then less exposed to the actual wind noise, and the improved detection of the ambient noise, with reduced wind noise, more accurately cancels the ambient noise actually experienced by the user.

However, the improved transducer arrangement is not limited to use where the listener is wearing earphones or headphones, since it will provide a more faithful representation of the ambient noise as an input to any noise cancellation/reduction system which detects ambient noise. For example, noise cancelling systems may also be used where a listener is listening to a wanted sound using loudspeakers, particularly if the source of the noise and the associated ambient noise sensor are outside the acoustic feedback path of the loudspeakers.

The transducer arrangement comprises two or more transducer elements the outputs of which are summed to produce a single output. The transducer elements in this example are located within the volume defined by a shaped acoustic resistive material so as to be maximally exposed to ambient conditions. This is achieved by the elements being spaced from the support surface to which the acoustic resistive material is attached. The important point is that the transducer elements are exposed to the ambient conditions, and the wind noise reduction is most effective when the exposure of the elements to the wind is maximised. It is preferable to utilise omni-directional transducer elements but unidirectional or bidirectional elements may also be used but with reduced performance levels. An omni-directional transducer element is one where there is a single port in a housing with the diaphragm of the transducer disposed within the housing such that it responds equally to sounds from different directions. The disposition of the elements of the transducer arrangement with respect to one another is not significant as the wind noise reduction effect achieved by summing the outputs can be obtained irrespective of the direction that the elements face with respect to the sound source, although optimal performance is achieved when the elements face in different directions.

Furthermore, the microphones should preferably be shielded from the surrounding environment with a thin resistive material that may surround them. This material advantageously consists of a mesh with perforation sizes about 125 microns or smaller, which may be combined with a thin felt or foam. The foam can be similar to that used to cover the ear pieces of headphones, although other arrangements are also effective. Preferably, the material should not significantly adversely affect the frequency response of the elements.

Referring now to Fig. 2, this shows an arrangement which comprises a plurality of omni-directional transducer elements covered with a layer of resistive material 10 in the form of a mesh where the holes are of the order of less than 125 microns, preferably less than around 75 microns, and more preferably 40 -50 microns. The mesh may, for example, be made of wire. Also a special porous plastic may be used. If desired, the mesh may be combined with a layer of thin felt or acoustic foam similar to that usea to cover the ear pieces of headphones. The shaped mesh and layer of felt or foam may be combined in a number of different ways, not simply with the mesh covering the felt or foam. For example, the felt or foam may cover the mesh or there may be alternating layers of mesh and felt or foam in any combination to achieve better wind noise rejection at the expense of adding bulk. As such, the material 10 does not affect the frequency response of the element or elements.

In the present embodiment, a plurality of transducer elements is provided, and the outputs of the elements are fed through buffer circuits 16 and added together by a summation circuit 17. After summation, the signals may be filtered by a high pass or band pass filter circuit 18 where lower cut off frequency is almost 200 Hz before being fed to an output buffer 19, although the use of any such filtering and the selected cut off frequency may depend on the application and the nature of any expected ambient noise, since the wind noise rejection in the detected ambient noise signal should not take place at the expense of accurately detecting the ambient noise itself.

In the embodiment of Fig 2, three omni-directional microphone elements are present and are disposed relative to each other so that they are physically orientated in three dimensions and point in different directions. The elements are covered with material 10 as described above. In the particular arrangement shown in Fig 2, the B and D elements are physically disposed in the same plane but the ports of the elements B and D point generally at a common point. In other words, the ports of the two elements are in the same plane but point at different angles. The middle element C is physically above the plane containing the elements B and D but it is tilted. Thus, it is also pointing at the common point.

The shape of the acoustic screen comprising a combination of mesh and felt or foam has an effect on the wind noise rejection performance with optimum performance being achieved with a piuraiity of convex shapea portions. Preferably, the convex shaped portions constitute a three dimensional generally hyperbolic shape. In particular, forming the screen with pinched portions between the shaped portions has been found to disrupt wind effectively.

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 with or without their associated signal addition circuitry and provided to manufacturers as a module.

The omni-directional transducer element(s) can be fabricated using semi-conductor techniques which allows the array of elements to occupy very little space. A MEMs microphone is sometimes referred to as a SiMIC (Silicon Microphone).

Using a plurality of miniature omni-directional microphone elements in an appropriate array as described above, permits a version of the invention to be utilised in apparatus which may be used in breezy or windy conditions, for example outdoors, in view of the fact that we have found the wind noise reduction characteristics of the arrangement as shown in Fig 2 to be excellent. In other words, the arrangement will result in excellent ambient noise and wind noise reduction characteristics. APPARATUS FOR ACCURATE AMBIENT NOISE SENSING AND REDUCTION IN

THE PRESENCE OF WIND

Using a plurality of miniature omni-directional microphone elements in an appropriate array as described above, permits a version of the invention to be utilised in apparatus which may be used in breezy or windy conditions, for example outdoors, in view of the fact that we have found the wind noise reduction characteristics of the arrangement as shown in Fig 2 to be excellent. In other words, the arrangement will result in excellent ambient noise and wind noise reduction characteristics.

Claims

1. Ambient noise reduction apparatus for providing an output audio signal to an output audio transducer for reproducing a wanted sound to a listener, comprising an ambient noise sensing transducer arrangement for sensing ambient environmental noise, and a processor circuit arranged to receive an ambient noise signal from the ambient noise sensing transducer arrangement and process the ambient noise signal to produce the output audio signal whereby to reduce the amount of ambient noise perceived ty me listener, wrerein me ambient noise sensing transducer arrangement comprises a plurality of microphone elements and means for receiving the outputs of the elements and adding the outputs together.

2. Apparatus according to claim 1, wherein the ambient noise sensing transducer arrangement further comprises acoustic resistive material covering at least a portion of the common volume exposed to the environment, whereby all the elements are maximally exposed to the environment.

3. Apparatus according to claim I or 2, further comprising means for receiving a source of wanted audio signals, wherein the processor circuit is arranged to receive wanted audio signals from the source and to combine the wanted audio signals with the processed ambient noise signal to produce the output audio signal.

4. Apparatus according to claim 1, 2 or 3, wherein the output audio transducer comprises earphones or headphones or loudspeakers.

5. Apparatus according to any of claims 1 to 4, wherein the ambient noise sensing transducer arrangement is arranged to sense the ambient environmental noise at the location of the source of the wanted audio signals.

6. Apparatus according to any of claims 1 to 4, wherein the ambient noise sensing transducer arrangement is arranged to sense the ambient environmental noise at the location of the listener.

7. Apparatus according to any preceding claim, wherein eacn microphone element is facing a unique direction.

8. Apparatus according to any preceding claim, wherein the microphone elements are omni-directional elements.

9. Apparatus according to any preceding claims, wherein the microphone elements are manufactured using semiconductor micro fabrication techniques.

10. Apparatus according to any preceding claim, wherein the acoustic resistive material is in the form of a mesh or porous plastic.

11. Apparatus according to claim 10, wherein the mesh has holes less than approximately 125 microns.

12. Apparatus according to claim 10 or 11, and comprising a layer of foam material.

13. Apparatus according to any preceding claims, wherein the outputs of the elements are subjected to filtering in order to reduce noise.

14. Apparatus according to claim 13, wherein the filtering utilises a high pass filter.

15. Apparatus according to claim 13 or 14, wherein the filter passes frequencies above about 200 Hz.

16. Apparatus according to any one of tne previous claims wnerein the acoustic resistive material is shaped to have at least a part formed in the shape of a convex curve.