GB2188210A - Acoustic noise reduction in ear defenders - Google Patents

Abstract

An ear defender 10 has a shell 21, a baffle 23 dividing the shell into inner and outer compartments 23a, 23b, a speaker 26 incorporating a diaphragm mounted on the baffle and a microphone 27 in close proximity to the speaker diaphragm. A resilient sealing ring 22 engages a wearer during use. Sound absorbing foam 24 fills compartment 23b and engages opposite sides of the baffle 23. An aperture 28 in the baffle equalizes pressure in the compartments 23a, 23b. The baffle 23 may be replaced by one of box-like form and the shell 21 may include an acoustic valve. An active noise reduction arrangement with a speech-in line is described for use with a helmet having two such defenders for a wearer's ears. <IMAGE>

Description

SPECIFICATON Improvements in or relating to acoustic noise reduction This invention relates to acoustic noise reduction, and is concerned with the application of acoustic noise reduction to ear defenders, as used, for example, by crew members offighting vehicles.

According to the invention, an ear defender comprises a shell, baffle means dividing the interior ofthe shell, sound absorption means on opposite sides of the baffle means, a speaker incorporating a diaphragm mounted on the baffle means, and a microphone disposed in close proximity to the speaker diaphragm.

The sound absorption means preferably engages the baffle means.

The baffle means may have inner and outer surfaces both engaged by said sound absorption means throughout their extent.

The microphone may be mounted directly on the speaker.

The microphone may be mounted coaxially on the speaker.

The ear defender may be provided with electro-acoustic circuitry comprising an input line for speech leading to the speaker, and a feedback loop from the microphonetothe input line, so asto provide active noise reduction.

The circuit components may be mounted on or form part of the baffle means.

The baffle means, which may be of box-like form, may have an aperture for equalizing pressure on opposite sides thereof.

The sound absorption means may fill the space between the baffle means and the shell.

As used herein, "close proximity" ofthe microphone to the speaker means not close enough to experience undue near-field effects of the speaker diaphragm, whilst at the same time reducing loop phase to an acceptable level. The distance between the microphone and the speaker diaphragm will vary according to the characteristics of these components and their associated circuitry.

The invention further comprises a helmet having two such defenders for a wearer's ears.

The invention also comprises any novel subject matter or combination including novel subject matter herein disclosed.

The invention may be performed in various ways.

One specific embodiment of the invention, with possible modifications, will now be described by way of example only, with reference to the accompanying drawings, wherein:- Figure 1 is a graph illustrating noise levels, wherein SPL means Sound Pressure Level, Figure2 is an active noise reduction (A.N.R.) system, ) Figure 3 is a graph illustrating attenuation, Figure 4 is a section of an ear defender, Figure 5is a graph of gain (attenuation) and phase, Figure 6 illustrates a more detailed A.N.R. system, Figure 7illustrates a modification of the Figure 6 arrangement, Figures illustrates a modification of the Figure 4 arrangement, and Figure9is an enlarged portion of Figure 8, with added detail.

In the figures, like components and features have like reference numerals.

Active Noise Reduction (A.N.R.) aids voice communication in high noise and provides protection against hearing damage due to noise.

The noise environment, for example in modern strike aircraft, helicopters, and fighting vehicles, is high, often in the range 110-120 dB overall. Trends towards lighter structures and more powerful engines, in the search for higher performance, exacerbate the problem of noise control at source.

The crews of such vehicles who wear conventional protective helmets or headsets are exposed to high noise levels, which, if exposure is prolonged, may lead to hearing damage and operational disability.

Communication between crew members, and communication with the outside world, will be degraded by masking of speech at the ear bythe vehicle noise.

The crew noise environment, in the vast majority of cases, may be characterised broadly as shown in the graph of Figure 1 which shows sound pressure level (SPL) against frequency. The compartment noise environment is usually highest in the low and middle frequency range, 125-250Hz, because of the nature of the noise sources and the structure of the compartment, whether it be an aircraft cockpit, helicopter interior, AFV, aircraft shelter or indeed ship's engine room.

All conventional helmet/headset hearing protectors behave in the same way in that they provide little or no attenuation at low frequencies (below 500 Hz), with a rise in attenuation to a plateau, of 25-35dB, above 2kHz. The noise spectrum atthe crewman's ear, underthe protector, is therefore dominated by low frequency noise, which creates a hearing hazard and masks the reception of speech and warning signals.

A.N.R. is the concept of cancelling the noise within a hearing protector, i.e. ear defender, by the superposition of an anti-phase signal produced bya speaker placed inside the protector.

The basic of the present described arrangement or system is an analogue electro-acousticfeedback circuit incorporating a miniature microphone placed in close proximity to a speaker diaphragm and to the entrance to the user's ear canal, a phase compensation filter and drive amplifier, and the speaker, also mounted inside the ear defender.

Speech signals are introduced at a different point in the circuit, without any nett cancellation.

The overall response of the system, which includestheeffectoftheuser'searitselfonthe acoustics of the protector's internal volume, is designed to ensure high stability in use in vehicles under motion, and a uniform level of attenuation for all users, for the noise environment in question. A simplified schematic diagram of the A.N.R. system is shown in Figure 2, wherein an ear defender 10 includes a microphone 11 and a telephone loudspeaker 12 incorporating a coil and a diaphragm (notshown).Aspeech signal is received on an input line 13, passes through a preamplifier 14,then through an amplifier 15 to be received by the speaker 12. Afeedback line 16from the microphone 11 to a junction 17 between the amplifiers 14,15 includes a feedback filter 18.

The feedback loop creates a pressure null at the microphone 11. In principle, the closerthe microphone 11 to the diaphragm ofthe speaker 12, the less the phase lag at anyfrequency. This helps to increase the frequency band over which A.N.R. will operate. In practice, there are anomalous phase effects closeto the speaker diaphragm, and though good A.N.R. may be created atthe microphone 11, the A.N.R. measured at a distance from the microphone, for example, at the ear, may be poor.

The components have analogue electronic circuitry in order to ensure a rapidity of response needed to deal effectively with variations in the aircraftandvehicle noise during motion.

Typical A.N.R. performance of the system in AFV (armoured fighting vehicle) noise is shown in Figure 3whichshowsaweightedSpLagainstfrequency.

The system provides a maximum of 18 dB attenuation, with a working bandwidth from below 50 Hzto above 800 Hz, and reduces the overall noise level atthe ear by 16 dB(A).

With reference now to Figure 4, an ear defender 20 comprisesa concavecoverorshell 21 having a peripheral, resilient seal 22, and housing baffle means comprising a baffle plate 23 extending across the interior ofthe shell 21 to divide the shell 21 into inner and outer compartments 23a, 23b. The outer compartment 236 is filled with sound absorbing foam plastics material 24. Further material 24 is disposed on the opposite side of the baffle plate 23, i.e. within the inner compartment 23a, leaving space for the wearer's ear. The material 24 engages the inner and outer surfaces of the plate 23 throughout their extent.A speaker 26 incorporating a diaphragm (not shown) is mounted centrally on the inner face of the baffle plate 23 and a microphone 27 is mounted coaxially on the grill of the speaker 26 so as to be disposed in close proximity to the speaker diaphragm.

The baffle plate 23 includes an a perture 28 which provides for inner and outer shell cavity pressure equalization and accommodates rapid changes in atmospheric pressure.

Since a normal diaphragm telephone speaker is a dipole radiator (sound is radiated from front and back of the diaphragm), it must be fitted with an adequate baffle in order not to cancel its own radiation inside the ear defender 20. Unfortunately, this baffle reduces the internal volume ofthe shell 21, as seen by the ear, and therefore reduces the passive attenuation of the device. However, the sensitivity of the telephone speaker 26, and therefore its ability to reproduce the cancelling waveform, is controlled by the compliance of the rear cavity, as determined by its volume,which could be large for greatest sensitivity.

Low frequency passive attenuation is also limited bythe stiffness of the shell structure and by the seal 22. Thus, by careful inspectin of low frequency performance before installation of the components, a compromise baffle location may be selected such that active noise-reduction attenuation outweighs any loss of passive attenuation.

The installed electro-acoustic response ofthe telephone-cavity-microphone combination will generally take the form shown in Figure 5. The microphone signal is amplified and fed back to the telephone speaker. The amplifier gain may be increased provided that the loop gain (i.e. total gain around the loop) remains less than unity. Increasing gain increases the reduction of noise in the muff.

When the loop gain becomes unity atthefrequency forwhichthe loop phase angle is 0"/360", the loop becomes unstable and an oscillatory waveform is generated by the loop. This phenomenon sets a maximum practical attenuation achievable with a given set of components.

At higher frequencies, approximately 1000 the sound-field within the shell 21 becomes nodal and small amplitude/phasevariationsfrom pointtopoint will cause the attenuation achieved to be spatially-dependent.

If the system is assembled as described above, it will generally offer low attenuation before positive feedback is reached. Its performance will vary from person to person, and from ear to ear, (since the ear canal's own acoustic characteristics form part ofthe feedback loop), and temporary instability may arise due to very low frequency transient overload caused by movement of the defender on the wearer's head, which results in large pressure variations intheshell 21. It is desirable thatthe loop gain and phase be controlled down to e.g. 1Hz to avoid creating incipient instabilities, which are activated by movementofthe head and, or, vibrations in the vehicle environment.The large pressure variations are detected by the microphone 27 and fed backto the telephone speaker 26 by way of the feedback loop, which maintains its electrical amplitude response to belowthe 1 Hzin order not to curtail its phase response at audible frequencies.

The system in practice is constructed to accommodate : i) approximately distortion4ree telephone speaker output to match high noise levels (circa 120 dB), ii) shell cavity pressure equalization for rapid changes in atmospheric pressure (e.g. with altitude), iii) active noise reduction attenuation/passive attenuation conflict, iv) dynamic stability, v)variations between different wearers, vi) spatial variations inside the ear defender inner cavity, vii) precompensation of speech inputto offset any cancellation effect as a function offrequency.

In general terms, the described arrangement results in the production of active noise-reduction in an ear defender. The criterion in practice may be good active noise-reduction or wide bandwidth, or some other specific. The optimisation is achieved through a choice of telephone speaker, microphone and defender, and by appropriate acoustic design within the defender.

In particular: Low-frequency (If) considerations 1) Choice of defender sealing (22) to obtain a suitable low-frequency breakpoint for the defender volume used.

2) Choice of microphone low-frequency breakpoint in conjunction with item 1) to maximise the mid-band gain plateau (above the If in-phase gain value).

3) Choice of telephone diaphragm maximum displacement to avoid If saturation.

4)Choice of innerto outer chamber port size (aperture 28) to provide adequate pressure equalization with rapid change of ambient, and without deterioration in item 2).

5) Choice of low-frequency loop compensation in conjunction with items 1), 2) and 4) for good working and stability in dynamic conditions.

Mid-frequencyconsiderations : 6) Choice of telephone speaker to obtain required sensitivity and maximum peak output forthe drive available.

7) Choice of inner-outer shell cavity ratio to obtain desired balance between telephone speaker sensitivity and passive attenuation of the defender.

High-frequency considerations : 8) Choice oftelephone speaker resonance high enough to maximise active noise-reduction bandwidth.

9) Choice of telephone speaker resonance low enough to avoid aggravating loop stability problems at nodal frequencies.

10) Choice of material and siting of acoustic absorption foam (good acoustic impedance characteristic) to control the nodal behaviour of the cavity. A Class 0 open cell polymer foam material of high cell density is preferred.

11) Choice of microphone response to avoid compounding items 9) and 10).

12) Choice of microphone site to reduce loop phase whilst avoiding extreme near-field effects of the telephone speaker, in order to improve the spread of good A.N.R. within the defender 20.

13) Choice of loop compensation in conjunction with items 8) to 12), to give good system operation.

General considerations : 14) Choice of speech equalization to provide any desired response at the ear.

15) Choice of telephone speaker to maximise efficiency in battery-powered systems.

In particular, dynamic stability is provided for by the introduction into the feedback loop of a limiter control which protects the telephone speaker against massive low frequency drive signals arising from dynamic forces on the ear defender. (See Figure 6).

The limiter control 30 operates on the two channels (going respectivelyto the two ear defenders 20) independently but is centrally-controlled for threshold. The common limitercontrol 30 also reduces speech signal level so that during its operation, when the cancellation feedback loop gain is reduced, an artificially high speech signal is not received. The speech signal comes from a boom-mounted microphone carried by the defender or helmet.

Figure 6 also shows the A.N.R. electro-acoustic circuitryto comprise, (in du pl icate - one for each defender 20, certain parts omitted), a loop compensation device 31 which may extend the band-width over which the device may operate, by control of loop phase shift, may increase the maximum achievable stable gain, and maytailorthe inband attenuation to meet a specific noise characteristic. The components may be on a circuit board.

Amicrophonepreamplifier32,atelephone speakeramplifier33and asoftlimiter34are provided. The soft limiter 34 serves to reduce loop gain instantaneously and progressively, with increase of instantaneous signal beyond a preset threshold, thus reducing audible effects created by signals attempting to exceed the system dynamic range.

Thecircuitryalso includes a speech compensation unit 35 which pre-emphasises speech signals to obviate the frequency distortion resulting from feedback loop action at the point of injection, in order to create a desired speech response at the ear.

Variations inside an ear defender 20, and between wearers, are dealt with by the choice of high frequencyfeedbackfilter design which is chosen to tapidly curtail high frequency gain without undesirable consequent phase effects. This filter may comprise, for example, two modified bridge-T network and associated transient lags. The modified bridge-T provides a notch response in which the lead above centre frequency can be controlled to advantage in compensating loop lag. This lead is, however, associated with high frequency gain, and the transient lag network provides a means of controlling this gain. In orderto minimise circuit components, these separate filter stages are functionally integrated.

The acoustic absorption foam material 24 is selected according to desired flow acoustic modes which occur at frequencies above 1000 Hz, due to standing waves within the cavities, and which cause unwanted resonant acoustic loads on the telephone speaker.

Good, ideally uniform, attenuation, throughout the full operating frequency range, in a defender 20, is obtanied by siting the associated microphone 27 in close proximity to the speaker diaphragm; see item 12) above. Unless this is done, the result can be excellent active noise reduction response as measured as the microphone 27 itself, but because of phase/amplitude differences ofthe telephone speaker output between microphone and ear canal entrance, no observable or measurable benefit atthe ear.

To assist portability and integration ofthe system with existing communications installations, and to reduce costs, one form ofthe invention may place the electronic circuit board within the shell 21, in which casethe board itselfmayserveasthe telephone baffle 23, subject to the design criteria described above., Figure 7 illustrates a modification ofthe A.N.R.

circuit of Figure 6, wherein an adaptive controller36 is now provided. The adaptive controller 36 governs loop gain and increases the performance, in bandwidth and attenuation, according to the maximum stable gain achievable for each individual user's ears by continous monitoring of the residual noise level atthe microphone 27.

Figure 8 illustrates a modified ear defender 20a provided with baffle means 40 of box-like form comprising a substantially closed structure with a pressure equalization aperture 28a. The baffle 40 contains sound absorption material 24 and further material 24 substantiallyfills the remainder ofthe shell 21a, leaving space forthe wearer's ear. In this particular example, the baffle 40 is of cylindrical form.

As shown in Figure 9, that part of the shell 21a in the vicinity ofthe baffle 40 incorporates an acoustic valve 45 (not shown in Figure 8). The valve 45 comprisesathin membrane 46which covers apertures 47. The membrane 46 allows nowmal speech to pass through the apertures 47 and into the shell 21a. However, the membrane closes the apertures 47 when exposed to high noise levels such as gunfire. A blanking plate may be provided to keep the valve 45 closed in a vehicle noise environment.

The presence ofthe baffle 40 does not impede sound access for speech when the valve 45 is open.

The modification of Figure 8 assists integration of the invention with existing ear defender shells which make use of acoustic valves such as valve 45.

Claims (14)

1. An ear defender comprising a shell, baffle means dividing the interior of the shell, sound absorption means on opposite sides of the baffle means, a speaker incorporating a diaphragm mounted on the baffle means, and a microphone disposed in close proximity to the speaker diaphragm.

2. An eardefenderaccording to Claim 1,wherein the microphone is mounted directly on the speaker.

3. An ear defender as claimed in Claim 1 or Claim 2, wherein the microphone is mounted coaxially on the speaker.

4. An eardefenderas claimed in any preceding claim, provided with electro-acoustic circuitry comprising an input line for speech leading to the speaker, and a feedbackloop from the microphone to the input line so as to provide active noise reduction.

5. An ear defender as claimed in Claim 4, in which components of the circuitry are mounted on the baffle means.

6. An ear defender as claimed in Claim 4 or Claim Sin which components of the circuitry form part of the baffle means.

7. An ear defender as claimed in any preceding claim, in which the baffle means has an aperturefor equalizing pressure on opposite sides thereof.

8. An ear defender as claimed in any preceding claim, in which the baffle means is of box-like form.

9. An ear defender as claimed in any preceding claim, including an acoustic valve in the shell.

10. An ear defender as claimed in any preceding claim, in which the sound absorption means engages the baffle means.

11. An ear defender as claimed in any preceding claim, in which the baffle means has inner and outer surfaces both engaged by said sound absorption means throughout their extent.

12. An ear defender as claimed in any preceding claim, in which the sound absorption means fills the space between the baffle means and the shell.

13. An ear defender substantially as hereinbefore described with reference to and as shown in Figure 4, or Figures 8 and 9 of the accompanying drawings.

14. A helmet having two such eardefendersfora wearer's ears.