NOISE REDUCTION This invention relates to noise reduction and in particular, but not exclusively, to apparatus for reducing the level of wind noise perceived by the wearer of a helmet, and to associated methods of noise reduction. In many high speed sports and activities such as motorcycling, cycling, skiing, skydiving, etc., the person taking part in the activity is well advised to wear a helmet for reasons of personal safety. However, at high speed, the wind flow over, around and through the helmet and the neck and shoulder region of the wearer often gives rise to a wind noise, or whistle or hiss. As the speed increases the wind noise tends to become louder. Even for short periods and at low levels the wind noise is a distraction and irritant, but at higher levels and/or for prolonged periods, the noise level poses a major safety hazard both in terms of potential long term damage to the wearer's hearing, and also contributing to fatigue of the wearer, with a consequent reduction in the level of concentration. It is already known to use earplugs in an attempt to protect the wearer from the effects of wind noise but the use of earplugs is problematic. One difficulty is that they attenuate across a broad spectrum and therefore tend to block out not only wind noise but also other information which is crucial to the wearer of the helmet such as ambient traffic noise, sirens on emergency vehicles, speech and so on, therefore increasing the hazard risk. They are also easily lost and need continual replacement. Also they are not hygienic and can give rise to ear infections. Therefore a need exists for an effective wind noise reduction device and associated methods for reducing the level of wind noise perceived by a wearer of a helmet, in studies conducted by the applicant, it has been shown that wind noise is essentially random in nature but with a reasonably predictable frequency spectrum that is predominantly comprised of low frequency noise up to about 1 KHz. Surprisingly the frequency domain characteristics change little with changes in speed. It is sinusoidal in form. Also, when looking particularly at wind noise induced by interaction of a helmet and the adjacent air flow, the frequency characteristics of the 'noise' are not only reasonably well defined (as mentioned above) but the noise source is also located close to the user's ear. Accordingly, in one aspect this invention provides a wind noise reduction apparatus for being worn in conjunction with a helmet to reduce the level of wind noise perceived by the helmet wearer, said apparatus comprising: a microphone for detecting in use the ambient wind noise in the vicinity of the helmet and for producing an electrical output signal, active noise cancellation circuitry responsive to said microphone output signal to produce a noise cancellation signal, and a speaker responsive to said noise cancellation signal, to emit a sound tending to reduce in use the level of wind noise perceived by the wearer. In this invention the ambient wind noise in the vicinity of the helmet is detected and used to produce a noise cancellation sound to reduce the level of wind noise perceived by the wearer. In use, for optimum noise cancellation the microphone should be located close to the ear canal and ideally well within 10% of one wavelength of the highest frequency. In some particular instances, the noise cancellation circuitry may include frequency isolator means for searching for and selecting a bandwidth within which said wind noise is detected, the circuitry being further operable to output a noise cancellation signal in which said bandwidth predominates. Preferably, the noise cancellation circuitry is analogue with little or no substantial delay in treatment of the signal from the microphone to drive the speaker. Preferably the microphone is located adjacent the speaker and in substantially the same horizontal or vertical plane as the speaker. Thus the microphone may be located close to the side of the speaker or, where appropriate, centrally behind the speaker. Preferably the apparatus is operable to cause the speaker to emit a setting or confidence tone so that, in use, the wearer may determine whether the speaker is correctly aligned with their ear. Preferably the apparatus comprises one cancellation set for each ear, each cancellation set including a microphone, active noise cancellation circuitry and a speaker as set out above and powered by respective power packs or a shared power pack. In one arrangement, the noise cancellation circuitry may include means responsive to the magnitude of the ambient sound to switch said circuitry between a standby or quiescent condition in which no noise cancellation signal is output and an active condition in which said noise cancellation signal is output. Preferably, said active noise cancellation circuitry includes phase inverting means. Preferably, said noise cancellation circuitry is operable to output an inverted signal to cancel or attenuate sound signals in the range from about 50Hz to 1.5KHz. In one arrangement, the apparatus may include a wall or cup of foam material designed to contact the user's head around the ear to provide passive noise reduction. The foam is preferably closed cell. In some instances, the foam wall or cup of foam material may provide passive cancellation at frequencies beyond that at which the effect of the electronic noise cancellation reduces. In this fashion a natural effect may be achieved where, whilst providing effective noise cancellation of the unwanted noise, a 'muffled1 effect is avoided. The foam attenuation in conjunction with a high fidelity amplifier can reduce or avoid an artificial sound or artefacts. The foam also can help assure a predetermined spacing of the speaker and microphone from the ear. Advantageously, the amount of phase inversion or phase shifting with higher frequency sounds decreases above 2 or 3 KHz or so, thereby to reduce attenuation of sound signals such as sirens used by emergency services. In one arrangement, to locate the microphone/speaker assemblies securely and repeatedly one against each ear, the apparatus includes an item of headgear for being worn by the user. The headgear may be an item made of suitable stretchy material such as e.g. a cap, a balaclava etc. This arrangement provides a secure comfortable fitting of the assemblies in the desired positions over the ears, and allows a helmet to be slid over with minimal risk of discomfort. Preferably, said active noise cancellation circuitry includes a preamplifier. The preamplifier, the phase inverting means and the frequency isolator may be arranged in a different order to process the signal. In one arrangement, the apparatus may be powered by an electric battery; in other arrangements, the device may be powered by other means such as voltage sources driven by body heat, solar energy or kinetic energy or any other suitable power source. The device may comprise more than one microphone; if more than one, each microphone may provide a signal to a common shared active noise cancellation circuit, or each may provide a signal to be processed by respective different active noise cancellation circuits. Likewise there may be more than one speaker, each of which may be driven by a common signal or separate individual signals. Depending on the particular design of the helmet, various arrangements are possible as alternatives to the above arrangenment. In full face helmets or other helmets having respective ear shells or cavities which enclose the a pair of devices may be installed within the helmet, one in each ear shell respectively. In these embodiments the applicants make use of the fact that full face helmets and other helmets which cover the ear regions effectively define an enclosed area adjacent the user's ear within which a noise cancellation device may be located. In still further arrangements the apparatus may be fitted to the ear by a hook or clip which clips around the pinna of the ear. Alternatively the device may be designed to fit within the ear canal of the wearer. The device preferably is contained within a housing which contains the microphone, the speaker, and said active noise cancellation circuitry. The housing is preferably designed to maintain the speaker and the microphone at a predetermined distance from the ear. The housing preferably fits at least partially within the ear shell of a helmet. The outer housing may comprise suitable attachment means such as Velcro® or other hook and loop type attachments. The apparatus may comprise a headstrap which connects two housings so that they are held in the correct position next to the wearer's ears. The connection of the headstrap to the housings may be pivoted to allow the housings to fit closely against the ears in use. In another arrangement, the housings may be connected to the headstrap such that they are held in a non- parallel, convergent configuration such as to match or conform to the inclination of the pinnae relative to a notiona! parallel sided head. Thus the inclination may be say between 0° and 35° on each side. Where provided the headstrap and/or housings are suitably shaped so that the wearer may fit the helmet over the apparatus, with the helmet gliding over the housings. In order to maintain the correct orientation and spacing of the housings they may have hook means associated therewith to hook around the pinna to locate the housing. The invention also extends to a helmet temporarily or permanently incorporating the noise reduction apparatus as described above. In another aspect this invention describes a method of reducing the wind noise perceived by the wearer of a helmet which comprises using a local microphone to detect in use the ambient wind noise in the vicinity of the helmet to produce an electrical output signal, supplying said electrical output signal to active noise cancellation circuitry for producing a noise cancellation signal and using the noise cancellation signal to cause a speaker to emit a sound tending to reduce the level of wind noise perceived by the wearer. In an arrangement in which the helmet encloses the ears of the wearer, the sound signal is preferably emitted in respective chambers defined by each ear of the wearer and the adjacent inner surface of the helmet. Whilst the invention has been described above it extends to any inventive combination of the features set out above or in the following description. The invention may be performed in various ways, and an embodiment thereof will now be described by way of example only, reference being made to the accompanying drawings in which: Figures 1 (a) and 1 (b) are respective front and side views of a first embodiment of wind noise reduction device in the form of a balaclava; Figures 2(a) and (b) are front and side views respectively of the microphone and speaker assembly and its location in the balaclava for one side of the device; Figure 3 is a general view of a futher embodiment of wind noise reduction devices; Figure 4 is a detailed view of the arrangement of the microphone and speaker for one side of the device; Figure 5 is a circuit diagram of an anti-noise circuit for use in the above embodiments; Figure 6 is a schematic view showing possible orientation of the speaker/microphone relative to the wearer's ears; T/GB2005/002798

8 Figure 7 is a schematic view showing a fixing arrangement in which left and right noise cancelling pouches are fitted into a motorcycle helmet; Figure 8 is a schematic view of another embodiment of wind noise reduction device in accordance with this invention; Figure 9 is a schematic side section view through the ear shell region of a helmet, including a noise cancellation device in accordance with this invention; Figure 10 is a block diagram showing an example of a first embodiment of circuit for use in the invention; Figure 11 is a block diagram showing an example of a second embodiment of circuit for use in the invention, and Figure 12 is a block diagram showing an example of a third embodiment of circuit for use in the invention. The embodiments described herein are intended to provide attenuation of wind noise in a motorcycle helmet or the like, in these embodiments an analogue noise cancellation circuit emits a suitable sound wave in close proximity to the user's left and right ears to attenuate the wind noise component whilst allowing transmission of other noise sounds and in particular sirens etc. used by the emergency services. In particular, in these embodiments, the microphone is located within 10% of the wavelength of highest frequency of interest and the microphone is in the same horizontal or vertical plane as the speaker. Referring to Figure 1 there is shown an embodiment of headgear in the form of a balaclava 1 with microphone/speaker assemblies 2 held by the balaclava in repeatable close proximity to each ear of the user. The balaclava 1 is made of a stretchy moisture control material such as a 'moisture wicking' material which avoids a build-up of heat and perspiration. The balaclava 1 is cut and fashioned so that the assemblies 2 fit closely against each ear. Instead of the full face balaclava, it will be appreciated that other designs could be used such as a simple headband which passes around the temple and the back of the head of the user, a cap such as a skull cap or beanie cap which fits over each ear of the user maintaining the assembly 2 in the required position and so on. Where required, a chin strap may be provided which could be attached by Velcro® or suitable attachments such as a buckle, popper, clip or just a simple tie off. Referring now to Figures 2(a) and (b), each microphone/speaker assembly 2 comprises a foam ring 3 of closed cell material dimensioned to fit against the head of a user around their ear so their ear fits fully inside with the surface of the ring 3 bearing against the side of the head. The assembly 2 is made up of the speaker 4 and an adjacent microphone 5 in a suitable housing 6 which may be rigid or flexible. The assembly is attached to the inside of the balaclava by means of an inner cloth ring 7 stitched around its outside edge. The design is such that the assembly 2 may be prised out of the pocket formed by the cloth ring 7 and the adjacent material of the balaclava 1. The arrangement is such as to place the speaker in close proximity to the ear canal of the wearer, at less than 10% of the wavelength of the highest frequency to be cancelled. The operation of the device is controlled by means of a circuit board 8 and the operation of the circuit may conveniently follow that described with reference to Figure 5 below. T/GB2005/002798

10 Referring now to Figure 3 there is shown a headset 10 designed to fit under the helmet and to position two wind noise cancellation speaker assemblies 12 against the wearer's ears. The headset 10 is powered by a battery pack 14 with a suitable on/off switch 16 and the calibration button 18. Each wind noise cancellation speaker assembly 12 comprises a foam rim 20 designed to seal against the user's head around the ear to define a space within which are located a speaker 22 and a microphone 24. Holes 26 are provided in the shell 28 to allow passage of ambient noise and the shells 28 and headband 30 are designed so that once fitted to the head of a wearer the helmet can be easily passed over the top without disturbing the position of the shells 28 relative to the ear. Referring now to Figure 4, it will be seen that the speaker 22 and the microphone 24 are located side by side so that both are as close as possible to the ear canai 25 when worn. Referring now to Figure 5, there is shown one possible noise cancellation circuit. The output for the microphone 24 passes via a coupling capacitor 26 to a preamplifier 30. The preamplifier 30 is supplied with positive and negative voltage by a power supply unit 32. The gain of the preamplifier may be adjusted during assembly. The output from the preamplifier 30 passes via a coupling capacitor 28 to a power amplifier 34 which is of fixed gain and which again is supplied with positive and negative voltages from the power supply unit 32. The amplifier drives the speaker 22 directly (i.e. without a coupling capacitor) to provide a DC coupled arrangement. This has the advantage that it has good operating characteristics at low frequency although we do not exclude the possibility of capacitor coupling. The speaker 22 is relatively high impedance of at least 16 ohms and typically 32 ohms. Power for the circuit is provided by a battery 34 with an on/off switch 16. The circuit is designed to have good phase shift/phase inversion characteristics for signals in the range of 100Hz to 1.5KHz. Beyond 1.5KHz and particularly from frequencies of 3KHz and higher, there is only a slight phase shift with the result that sounds in the spectrum above 3KHz are only slightly if at all attenuated. Referring now to Figure 6, because of the physiognomy of a typical user's head, the disposition of the wind noise cancellation speaker assembly 12 within the shell 28 may be shifted so that it is aligned with the user's ear. The shell may therefore be narrower at the front end and wider at the rear. An advantage of this arrangement is that it reduces contact with the ear, reducing possible pressure and that the speaker is also more closely directed to the ear. In a variation (not shown), instead of, or in addition to, the headband 30, the wind noise speaker assemblies 12 may be held in proximity to the ear by hook around ear clips of the form of a base clef currently used for personal stereos, hands-free kits etc. Referring now to Figure 7, instead of using a headband 30 or a hook around clip, the wind noise cancellation speaker assemblies 12 could be located within the ear shell of the motorcycle helmet in a resiliently suspended configuration such that they are in a predetermined position with respect to the ear when the helmet is applied. Thus in this arrangement the wind noise cancellation speaker assembly 12 is held in a pouch 40 made up of two layers of 98

12 cloth or other fabric 42 joined around their periphery over the ear cavity 44 of a helmet. When the helmet is applied the speaker assembly 12 is held firmly but resiliently against the ear of the user. The objective of the arrangements shown in Figures 3 and 7 are to ensure that the wind noise cancellation speaker assembly 12 is held adjacent the ear at substantially the same position each time the helmet is applied. Other arrangements may be used to achieve the same effect. Referring now to Figure 8, a further embodiment 110 of noise cancellation device is designed to fit in the ear shell or cavity region 112 of a helmet, as shown in Figure 7. The device 110 comprises a casing housing a microphone 114, a loudspeaker 116, noise cancellation circuitry (not shown), a battery or other power source (not shown), and an on/off switch 118. The microphone 114 may be any suitable microphone for example an electret condenser microphone. The speaker 116 may typically be a small, eiectro-magnetically driven speaker of the form commonly available in electronic equipment, but other suitable speakers may be used for example those using piezo-electro transducers. As seen in Figure 0, the device is designed to fit within the ear shell of a helmet 120, next to the user's ear. The device 110 may be conveniently attached to the outer foam by means of Velcro® or similar. Referring now to this embodiment of noise cancellation circuitry shown in Figure 10, the microphone 110 receives power from a power supply unit 122 (e.g. a battery) which also supplies power to the other elements of the circuit requiring power. The output signal from the microphone 110 is passed to a sound pressure level trigger 124. The trigger operates to maintain the circuitry downstream of it in a standby or quiescent mode until a particular noise pressure level is exceeded. A typical threshold figure is 55 dB(A). If and when this sound pressure level is exceeded, the signal picked up by the microphone is passed to a frequency isolator 126. The frequency isolator 126 therefore searches within the frequency band from about 10 Hz to 20 kHz and then selects it for further processing. The signal containing the isolated frequency is then passed to a non-inverting preamplifier 128. The amplified signal then passes to a unity gain phase inverter 130 which can be bypassed by a bypass switch 132, and then to an inverting headphone amplifier 134. In normal noise cancelling operation the bypass switch 132 is closed so that signal from the non-inverting amplifier 128 is not inverted before it passes to the inverting headphone amplifier 134 and so the sound output by the speaker 116 is substantially in anti-phase with the original noise source picked up by the microphone, and of similar magnitude. !n this embodiment, in a set up mode, the bypass switch 130 may be opened such that the signal from the non- inverting amplifier 128 passes to the phase inverter 132 where it is inverted, and then to the inverting headphone amplifier 134 where it is inverted again. In this condition the signal at the speaker is substantially in phase with the signal detected by the microphone. This allows the circuitry to be tested with the bypass switch open to check for proper operation of the remainder of the circuit components and on passing the check one bypass switch closed to configure the circuitry into noise cancellation mode. The combination of the natural wind noise and the inverted and amplified sound output by the speaker 116 has a nulling effect so that the user experiences a significant reduction of wind noise. The circuit continues to operate in this manner until such time as the sound pressure level falls below the relevant threshold for the trigger 124, whereupon the circuitry ramps down the sound output by the speaker 116 and returns to a standby mode until the level is exceeded again. The circuitry is preferably designed to provide a 'soft' start and stop function so that the sound output from the speaker gradually increases from zero on starting up to avoid an audible thump, and likewise gradually decreases back to zero on stopping. In this way the device operates in a fully automated fashion, only switching itself on when the wind noise level increases beyond a safe and predetermined threshold and so the need for an ON/OFF switch may be obviated. In conjunction with the frequency isolator 128, this provides an effective power saving and fully automated device which reduces power consumption when the device is not needed. Referring to Figures 11 and 12, it will be appreciated that the order in which the frequency isolator 126, pre-amplifier 128, and phase inverter 132 treat the signal may be rearranged to provide a similar effect. The design may be modified to cater for different requirements. For example, for general purpose use, the demands on electrical power, duration of use and noise intensity may be relatively low and so the power consumption means that battery life may be expected to be of many months' continuous use. In another embodiment, designed for high speed use where the intensity and duration of the wind noise is greater, the demands on electrical power, duration of use and noise intensity will tend to be higher, and for this a larger battery may be supplied with a consequent modification of the housing. In yet another example, intended for professional sporting use, the device may be modified to include multiple microphone and multiple speaker arrays to provide, with associated active noise cancellation circuitry, an enhanced level of noise reduction. For a configuration with one microphone and one speaker, there will only be one noise cancellation circuit. However for configurations having multiple microphones and multiple speakers there may be a common noise cancellation circuit or there may be multiple noise cancellation circuits (e.g. in a phased array). It is also possible that the circuits for each configuration may be slightly different, and may well have differing component values (for example to speed up or slow down individual arrays or because the embodiment application may require more or less power, or because of other factors affecting the performance of the device. The design of the housing may also be different. In this arrangement, the demands of battery life are not a major consideration and so a higher power battery may be used. In other arrangements, instead of the device being semi-permanently attached within the helmet, it may be an integral part thereof. In yet other embodiments, where the helmet does not cover the ear of a user (but where wind noise is still a problem) the device may be provided with a temporary clip which fits over the ear of a user to hold the device accurately in position. Likewise in a further modification, the device may be designed to fit within the ear canal of the wearer in a similar fashion to digital hearing aids. Although in many instances the main function of the device is to select and substantially cancel the wind noise component, the device may include an input to allow music, speech, audible alert etc. to be played to the wearer. Also the device may be used to reduce other forms of noise instead of or in addition to wind, such as mechanical noise.