IMPROVEMENTS RELATING TO NOISE REDUCTION ARRANGEMENTS

Abstract: An active noise reduction (ANR) arrangement for reduc­ ing acoustic noise in an earphone structure (1) includes an automatic gain control arrangement (14-20) providing a variable loop gain dependent upon the variable noise reduction which is produced in the earphone (1) by the active noise reduction arrangement and which is effectively mea­ sured by noise pick-up microphones (5 and 13) located, respectively, in earphone structure front and rear internal cavities (11 and 12) positioned in front of and at the rear of a noise-cancelling transducer diaphragm (8,9) and providing a signal related to the measured noise reduction.

Description: The invention relates to arrangements for reducing the level of acoustic noise field within the internal cavities or enclosures of so-called ear-defenders or earphone structures when being worn by personnel (for example, pilots, vehicle drivers, industrial workers) in high noise environments. Known active noise reduction (ANR) arrangements for reducing the aforesaid acoustic noise field in ear-defenders comprise small noise pick-up microphones and noise-cancelling transducers mounted within the internal cavities or enclosures of the respective ear-defenders. The noise pick-up microphones produce electrical signal outputs in response to the acoustic noise fields within the aforesaid cavities and these signal outputs are phase inverted, filtered and amplified in a feedback loop arrangement for the production of noise-cancelling signals fed to the noise-cancelling transducers which accordingly produce noise-cancelling acoustic signals of substantially the same amplitude but of opposite phase to the acoustic noise field waveforms. An example of an ANR arrangement is disclosed in offenlegungsschrift DE 3133107 A1 which shows the use of two microphones and an automatic gain control circuit providing a variable loop gain which is dependant upon the variable noise reduction produced in operation by the ANR arrangement. In the present specification, it will be understood that the term 'ambient noise' refers to noise, due to external sources, present within the cavity of an ear-defender. There will be a reduced noise level within the cavity following acoustic noise reduction (ANR). It can be shown that such ANR arrangements produce a reduction in noise at a particular frequency given by:- (1 + 2 G cos  + G²) 1/2 where G is the total gain of the feedback loop arrangement and  is the total loop phase change at the particular frequency concerned. From this expression it can readily be appreciated that the scale of noise reduction achieved is highly dependent upon the total loop gain. Due to the imperfect transfer functions of the noise pick-up microphones and noise-cancelling transducers the acoustic noise reduction arrangements will, at certain frequencies, be feeding inphase (that is, positive feedback) signals rather than anti-phase (that is, negative feedback) signals to the noise-cancelling transducers. To prevent the ANR system becoming unstable the overall loop gain of the system must be kept at less than unity at the frequencies concerned otherwise the noise levels in the cavities of the earphone structures will actually be increased rather than reduced by the positive feedback signals fed to the noise-cancelling transducers. However, although the loop gain must be kept below unity at the aforesaid frequencies in order to maintain stability the loop gain of the ANR must be sufficiently high to provide the optimum acoustic noise reduction. Fixed loop gain control techniques could be used were it not for changes that would be likely to occur in the characteristics of the components of the ANR system with the passage of time. However, such fixed loop gain techniques would not provide the requisite compensation for changes in sensitivity of the noise-cancelling transducers resulting from changes in the volume of the earphone structure cavities which occur when the earphone structures are worn by different persons or from small changes in earphone structure position likely to be caused by normal movements of the wearer's head. Automatic loop gain control techniques would be capable of providing the requisite aforesaid compensation but the conventional procedure has hitherto been to utilise the output signal from the noise pick-up microphone of the ANR arrangement for automatic gain control purposes. Changes in the microphone output can result from a change in loop gain (for example, due to earphone movement) which requires the automatic gain control arrangement to act to adjust the gain and it can also result from a change in external noise spectrum/level. However, the cause of these changes in loop gain cannot be distinguished in an active noise reduction system utilising noise pick-up microphone outputs only for gain control purposes. Consequently, the automatic gain control may provide a gain which continuously oscillates about the requisite value. An aim of the present invention is to provide a noise reduction arrangement having means for automatically adjusting the loop gain in response to the level of measured noise detected. According to the present invention there is provided an earphone active noise reduction arrangement comprising an earphone structure, first and second microphones and an automatic gain control circuit arranged to provide a variable loop gain dependent upon a variable noise reduction which is produced in operation by the active noise reduction arrangement, characterised in that said first and second microphones are located, respectively within front and rear internal cavities of said earphone structure, the said front cavity being open for contact with the wearer's head and the said rear cavity being a closed space within the earphone structure, the microphones being positioned on the opposite sides of a noise-cancelling transducer diaphragm extending between said cavities, in which the first microphone detects the noise level within the front cavity and produces an output signal which is amplified by a variable gain amplifier and inverted before being applied to the noise-cancelling transducer, and in which the second microphone detects the noise level within the rear cavity and produces an output signal which together with the output signal from the first microphone is processed in the automatic gain control circuit for the generation of a gain control signal which is applied to the variable gain amplifier and which serves to ensure that the level of measured noise reduction is maintained constant. Preferably, the automatic gain control circuit comprises an adder circuit arranged to receive signals dependent upon the output signals from the first and second microphones, a first converter for providing a DC signal dependent upon the output signal from the first microphone, a second converter for providing a DC signal dependent upon the output signal from the adder circuit, and a comparator circuit, for comparing signals dependent upon the DC signals provided by the first and second converters, arranged to provide, in dependence upon this comparison, a gain control signal for controlling the gain of the noise reduction arrangement. By way of example, a particular embodiment of the present invention will now be described with reference to the accompanying single-figure drawing which shows a schematic diagram of an active noise reduction arrangement incorporating an automatic gain control circuit. Referring to the drawing the active noise reduction arrangement illustrated comprises a generally cup-shaped circumaural earphone structure 1 arranged to enclose the wearer's ear 2. The rim of the structure 1 is cushioned against the side of the wearer's head 3 by means of a compliant ring cushion 4. As with known active noise reduction arrangements the earphone structure 1 embodies a small noise pick-up microphone 5 which detects the volume of noise within the earphone adjacent the wearer's ear 2 and provides an electrical output dependent upon the detected noise. This output signal from the microphone is amplified by an amplifier 6 and the amplified signal is then inverted and filtered by a phase inverter/filter 7 before being applied to a noise-cancelling transducer 8. This transducer 8 includes a movable diaphragm 9 attached to an opening in a rigid wall structure 10. The diaphragm 9 and wall 10 divide the interior of the earphone structure 1 into a front cavity 11 containing the noise pick-up microphone 5 and a closed rear cavity 12. The provision of means dividing the earphone structure into two separate cavities allows the earphone to produce high levels of low frequency sound. The back pressure from the rear of the transducer diaphragm 9 is contained in the earphone rear cavity 12 and prevented from mixing with the front cavity pressure. Typically, at frequencies up to about 500 Hz the noise level is coherent in both earphone cavities 11 and 12. The noise-cancelling signals applied to the transducer 8 cause vibration of the diaphragm 9 and generation of acoustic signals of the same amplitude but of opposite phase to the noise signals picked up by the microphone 5. It will be appreciated that the noise pick-up microphone 5 only detects the noise level within the earphone cavity 11 after noise reduction has occurred. For the purpose of varying the loop gain of the feedback loop arrangement in accordance with variations in measured noise the depicted arrangement according to the invention includes a further noise pick-up microphone 13 identical to the microphone 5, the microphone 13 being located within the closed rear cavity 12 of the earphone structure 1. It may here be mentioned that the noise in both of the earphone cavities 11 and 12 will share the same amplitude and phase at low frequencies since the earphone is substantially transparent to low frequency sounds. Movement of the transducer diaphragm 9 towards the front cavity 11 will increase the front cavity pressure but decrease the pressure in the rear cavity 12. Consequently, the sound produced in the rear cavity 12 by the transducer diaphragm will be 180° out of phase with that produced in the front cavity 11. Accordingly, the microphone 5 senses ambient noise plus anti-phase noise while the microphone 13 senses ambient noise minus anti-phase noise. The consequential noise-representative output signals derived from the microphone 5 and 13 are applied to respective band-pass filters 14 and 15 which provide output in the low frequency range (typically 300-400 Hz). At these low frequencies noise-canceling signals fed to the transducer 8 will be 180° out of phase with the noise sensed by the microphone 5 located in cavity 11. The outputs from the filters 14 and 15 are applied to a signal adder 16 which will provide an output signal related solely to the ambient noise level, the anti-phase noise-reducing components sensed by the microphones 5 and 13 being mutually cancelling by the process of addition. The output signal from the adder 16 and the output signal from the filter 14 are converted to DC levels by respective root mean square voltage to DC converters 17 and 18 with the signal derived from the microphone 5 being given a predetermined value of gain by an amplifier 19 in order to correspond with the required level of noise reduction. The output from the converters 17 and 18 (after amplification by amplifier 19) are fed into a voltage comparator 20 which compares the DC levels of the outputs and accordingly generates a gain control signal which is fed to the amplifier 6, the control signal having a value which ensures that the measured noise reduction of the arrangement is kept constant. As will be appreciated another active noise reduction arrangement with automatic loop gain as shown in the drawing will in practice be provided in respect of the other ear of the wearer. It will also be appreciated that although the particular embodiment specifically described is applied to a circumaural earphone structure the invention may also be applied to alternative two cavity earphone structures such as those of the supra-aural type.

Claims: An earphone active noise reduction arrangement comprising an earphone structure (1), first (5) and second (13) microphones and an automatic gain control circuit (14-20) arranged to provide a variable loop gain dependent upon a variable noise reduction which is produced in operation by the active noise reduction arrangement, characterised in that said first and second microphones (5, 13) are located, respectively within front and rear internal cavities (11, 12) of said earphone structure (1), the said front cavity (11) being open for contact with the wearer's head (3) and the said rear cavity (12) being a closed space within the earphone structure, the microphones being positioned on the opposite sides of a noise-cancelling transducer diaphragm (9) extending between said cavities, in which the first microphone (5) detects the noise level within the front cavity (11) and produces an output signal which is amplified by a variable gain amplifier (6) and inverted (7) before being applied to the noise-cancelling transducer (8), and in which the second microphone (13) detects the noise level within the rear cavity (12) and produces an output signal which together with the output signal from the first microphone (5) is processed in the automatic gain control circuit (14-20) for the generation of a gain control signal which is applied to the variable gain amplifier (6) and which serves to ensure that a level of measured noise reduction is maintained constant. A noise reduction arrangement according to claim 1, wherein the automatic gain control circuit comprises and adder circuit (16) arranged to receive signals dependent upon the output signals from the first and second microphones (5, 13), a first converter (18) for providing a DC signal dependent upon the output signal from the first microphone (5), a second converter (17) for providing a DC signal dependent upon the output signal from the adder circuit (16), and a comparator circuit (20), for comparing signals dependent upon the DC signals provided by the first and second converters (18, 17), arranged to provide, in dependence upon this comparison, a gain control signal for controlling the gain of the noise reduction arrangement. A noise reduction arrangement according to claim 1 or claim 2, further comprising filter means (14, 15) for filtering the output signals from the microphones (5, 13). A noise reduction arrangement according to any one of claims 1 to 3, further comprising between the comparator circuit (20) and the first converter (18) an amplifier (19) for amplifying the signal received by the comparator circuit (20) from the first converter (18).

Inventors: SALLOWAY, ANTHONY JOHN, TWINEY, ROBERT CHRISTOPHER
Applicants: SIEMENS PLESSEY ELECTRONIC SYSTEMS LIMITED
Application year: 1986
Main classification: G10K11
IPC codes: G10K11