GB2234882A

GB2234882A - Noise reduction system

Info

Publication number: GB2234882A
Application number: GB8917742A
Authority: GB
Inventors: Anthony John Salloway
Original assignee: GEC Marconi Ltd; Plessey Co Ltd; Marconi Co Ltd
Current assignee: Plessey Co Ltd; BAE Systems Electronics Ltd
Priority date: 1989-08-03
Filing date: 1989-08-03
Publication date: 1991-02-13
Anticipated expiration: 2009-08-03
Also published as: GB8917742D0; GB2234882B

Abstract

An active noise reduction system for an earphone 1 or ear defender, the system comprising a noise reduction sound generator 14, a microphone 5 acoustically coupled to said generator, a feedback loop connected between the microphone and generator, and including loop stabilisation means 6, 7 for filtering and inverting the phase of a microphone signal and means 12 for amplifying the signal, the sound generator 14 being mounted within an earphone or oar defender on a partition wall 16 between a rear cavity which is closed and a front cavity which is closed only when the structure is supported against a user's ear, in which the partition wall 16 is provided with an air vent hole 9 effective to equalise air pressure across the wall such that the effect of a large acoustic pressure pulse in the front cavity will be reduced in amplitude and thereby prevented from causing significant displacement of a diaphragm of sound generator 14, the diaphragm thus being able to be driven solely by the applied electrical signals. This restrains the effects of large external pressure pulses or buffets from causing damage to the diaphragm. <IMAGE>

Description

NOISE REDUCTION SYSTEM This invention relates to a noise reduction system. It relates particularly to a system for reducing the level of an acoustic noise field within an ear defender or earphone structure intended to be worn by a person (for example, a pilot, vehicle driver or factory worker) in a high noise environment.

A renown active noise reduction (ANR) system for reducing the acoustic noise field in an ear defender comprises a pick-up microphone and a noise-reduction sound generator (which usually takes the form of a loudspeaker) mounted within the internal cavity or enclosure of the respective ear defender. The noise pick-up microphone produces an electrical signal output in response to the acoustic noise field within the cavity and the signal output is phase inverted, filtered and amplified in a feedback loop. The output is then fed to the noise-reduction sound generator which produces noise-reduction acoustic signals of substantially the same amplitude but of opposite phase to the acoustic noise field waveform.An outline of such a system is given in 'Some transducer design considerations for earphone active noise reduction systems', Tainey et al, Vol 7, part 2, pp95-102, Proc Spring Conference, 1985, York, Institute of Acoustics.

One problem which can occur is that of large pressure pulses (buffets) which may arise inside an ear defender or earphone structure due to relative motion between the human head and the earphone or which may propagate from a device that causes large, low frequency pressure changes, for example, a gun discharge. a helicopter rotor, vehicle engine or an explosive device. These pressure pulses cause the loudspeaker diaphragm to move towards the extreme of its possible displacement. Also, the pulses create large signals in the feedback loop as a result of a high system loop gain. Thus, a diaphragm that is already considerably displaced can receive a large electrical 'anti-buffet' signal requiring appreciable diaphragm displacement in the same direction.The result is that the acoustic output will be very low and active noise reduction across the noise reduction bandwidth is reduced because the diaphragm will reach the limit of its possible physical displacement. Additionally, the feeder wire of the loudspeaker speech coil may be caused to break through metal fatigue or excessive displacement.

It is an object of the present invention to reduce the above problem by permitting the diaphragm to move only as a result of the applied electrical signals and in such a way that it is substantially unaffected by the influence of the pressure pulse.

According to the present invention, there is provided an active noise reduction system for an earphone or ear defender, the system comprising a noise reduction sound generator, a microphone acoustically coupled to said generator, a feedback loop connected between the microphone and generator, the feedback loop including loop stabilisation means for filtering and inverting the phase of a microphone signal and means for amplifying the microphone signal, the said sound generator being mounted within an earphone or ear defender structure on a partition wall between a rear cavity which is closed and a front cavity which is normally open but which is closed only when the structure is supported against a user's ear, in which the said partition wall is provided with an air vent hole effective to equalise air pressure across the wall such that the effect of a large acoustic pressure pulse in the said front cavity will be reduced in amplitude and thereby prevented from causing significant displacement of a diaphragm of said sound generator, the said diaphragm thus being able to be driven solely by the applied electrical signals.

The air vent hole may be of such a size as to pass buffet pulses having a frequency in a range up to one hundred hertz. In a different embodiment, the air vent hole may be replaced by ts o or more holes of smaller diameter arranged such that a laminar fluid flow through said holes will be achieved for a given sound pressure level.

The invention also comprises an active noise reduction headset comprising an ear defender or earphone incorporating the abovementioned noise reduction system.

By way of example, a particular embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a block diagram of the components of the noise reduction system, Figure 2 is a graph showing the sound pressure level reduction against frequency for different numbers of inter-cavity vent holes, and, Figure 3 is a graph showing enhancement against frequency for different numbers of vent holes.

The arrangement depicted in Figure 1 shows an earphone shell 1 supported against the ear 2 of a user with the user's head 3 being in contact with a sealing pad 4 of the earphone shell. The internal construction of the shell 1 includes a partition wall 16 which divides the volume enclosed into a rear and a front cavity. A loudspeaker 14 is mounted in an opening in the partition wall 16.

The active noise reduction system comprises the loudspeaker 14, a microphone 5 acoustically coupled to said loudspeaker, a feedback loop connected between the microphone and the loudspeaker, the feedback loop including a low frequency filter 6, a loop filter 7 for stabilisation, an inverter 8 for inverting the phase of the microphone signal and a power amplifier 12 for amplifying the microphone signal. A speech signal may be fed into the loop along a line 18.

To maintain extended low frequency sound output (and an active noise reduction response to low frequencies) it might be thought that a perfect seal between the two cavities would be required. However, a small leak would normally be provided to allow air to flow between the cavities to allow for environmental (temperature and atmospheric pressure) changes.

The present invention requires a greater size of inter-cavity leak than this environmental leak in order to couple the cavities for possible buffet frequencies. The spectral content of vibration or rapid head movement buffet pulses can extend to frequencies as high as one hundred hertz. The air vent hole of the present invention is provided by forming a small number of vent holes 9 in the partition wall 16 supporting the loudspeaker 14. The invention thus permits the high pressure buffet pulses to be present on both sides of the loudspeaker diaphragm by the effect of inter-cavity coupling in the earphone shell.

Figure 2 is a graph which shows the sound pressure level reduction S on the vertical axis against frequency F (in hertz) on the horizontal axis. The sound pressure level reduction is significant above the marked value of zero shown as a horizontal line, whilst for frequency a value of one hundred hertz has been indicated.

The curves of Figure 2 show the results of tests made on noise reduction headsets using different numbers of holes of 3.2 millimetres in length and 0.5 millimetre diameter. The Curve A shows the result with five holes, Curve B applies to seven holes whilst Curve C applies to ten holes.

Figure 2 shows that, in this example, a small number of holes through the transducer partition wall is required in order to couple the cavities to a frequency of about one hundred hertz. A number of small holes is used in preference to a single hole in order to maintain laminar flow through the holes up to the highest sound pressure levels expected. Thus a degree of cavity coupling occurs up to the highest expected buffet amplitude levels (about 140dB sound pressure level).

Additionally, the low frequency amplitude and phase responses are maintained to the highest sound pressure levels. Thus, a consistent active noise reduction effect is achieved at low frequency irrespective of sound pressure level.

The buffet amplitude level is reduced when the earphone's front and rear cavities are coupled. The sound pressure level at buffet frequencies is inversely proportional to the cavity volume.

Consequently, the buffet pressure level without cavity coupling is inversely proportional to the front cavity volume (VF). When the cavities are coupled, the sound pressure level is inversely proportional to the front (VF) and rear (VR) cavity volumes.

Therefore the reduction in buffet amplitude is proportional to the front cavity volume and inversely proportional to the total volume VF VF + VR This reduction in buffet levels can be very significant (of the order of 10dB) which is extremely important in preventing the electrical buffet signals from exceeding the dynamic range of the active noise reduction system, thus resulting in intermittent noise reduction operation.

The use of one or more holes between headphone cavities, to maintain laminar flow and hence to maintain invariant amplitude and phase responses to high sound pressure levels has been investigated previously and it has been disclosed partly in the pending European Patent Application No. 87300610.0. The required dimensions (length and radius) of each hole depend on the maximum sound pressure level to be encountered in the system.To maintain laminar flow, Reynolds condition must be satisfied: r3 pp < < Reynolds number 4n 21 where r is the radius of the hole p is the pressure difference across the hole n is the viscosity of the fluid medium 1 is the length of the hole p is the density of the fluid From Reynolds condition, it can be seen that to maintain laminar flow up to a high sound pressure level, a long length, small radius hole is required.

The number of holes required to couple the cavities depends on the cavity volumes, thickness of the transducer plate (hole length) and the hole radius.

A fluid leak through a hole will obey Poiseuille's law: sr4 p V= 8 1 where V is the volume rate of fluid flow.

It can be seen that once the required hole size for laminar flow has been determined, one or more holes are introduced to obtain the required fluid flow and hence a degree of inter-cavity coupling up to a frequency of about one hundred hertz.

The number of holes introduced and hence the degree of cavity coupling provided has to be weighed against the effect on two less beneficial factors: a) Enhancement (the generation of sound which constructively interferes with the original noise to produce a higher sound pressure level than with the system inactive) should be at a low frequency. As the dimensions of the inter-cavity leak are increased, the enhancement frequency increases due to the change in the loudspeaker's amplitude and phase responses.

b) Sound generation, which occurs during doffing and donning of the headset due to the loudspeaker's change in amplitude and phase responses, should be kept to a minimum.

The above factors depend not only on the acoustic design of the headset, but also on the characteristics of the electronic filters of the noise reduction system.

With the appropriate number (one or more) of intercavity holes, the cavities can be coupled at buffet frequencies resulting in lower amplitude buffet pulses. The loudspeaker diaphragm will remain central with a high degree of coupling or it will remain closer to the central position with a lower degree of cavity coupling for the duration of the pressure pulse and it will respond to the applied electrical signal. As a consequence of the reduced diaphragm displacement, active noise reduction across the noise reduction bandwidth is maintained and the expected lifetime of the loudspeaker is able to be extended due to a reduced tendency to straining of the feeder wires for the loudspeaker speech coil. The linearity of the system can be maintained to very low frequencies and to the maximum generated sound pressure level capability.

Enhancement can be arranged to occur at a low frequency where its effect will not be significant due to the low sensitivity of the human ear. However, if enhancement occurs at too high a frequency, active noise reduction will be reduced at higher frequencies where the noise reduction is required to minimise the dB(A) value in a high noise environment (for example, use in a noisy vehicle).

Figure 3 is a graph which shows the enhancement value E in dB on the vertical axis against frequency F (in hertz) on the horizontal axis. An enhancement value of zero has been indicated by a horizontal line.

For the graph of Figure 3, the dimensions of the holes present are the same as those of Figure 2, and the references A to C for the relevant numbers of holes also correspond. The curves show that as the volume of fluid flow through the vent hole is allowed to increase, the enhancement effect similarly increases in frequency.

The preferred vent arrangement between the cavities is one of between one and ten holes, each hole being of 3.2 millimetres in length and 0.5 millimetres diameter. The number of holes required for a particular headset application will depend on the front and rear cavity volumes and the characteristics of the electronic filters in the noise reduction system and of other acoustic components within the system The foregoing description of an embodiment of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, it is not essential that the sound generator should be a moving-coil electromagnetic loudspeaker, in a different embodiment the sound generator could be a piezoelectric or electrostatic loudspeaker.

Claims

CLAIMS:

1. An active noise reduction system for an earphone or ear defender, the system comprising a noise reduction sound generator, a microphone acoustically coupled to said generator, a feedback loop connected between the microphone and generator, the feedback loop including loop stabilisation means for filtering and inverting the phase of a microphone signal and means for amplifying the microphone signal, the said sound generator being mounted within an earphone or ear defender structure on a partition wall between a rear cavity which is closed and a front cavity which is normally open but which is closed only when the structure is supported against a user's ear, in which the said partition wall is provided with an air vent hole effective to equalise air pressure across the wall such that the effect of a large acoustic pressure pulse in the said front cavity will be reduced in amplitude and thereby prevented from causing significant displacement of a diaphragm of said sound generator, the said diaphragm thus being able to be driven solely by the applied electrical signals.

2. A system as claimed in Claim 1, in which the said air vent hole is of such size as to pass buffet pulses having a frequency in a range up to one hundred hertz.

3. A system as claimed in Claim 1 or 2, in which the said air vent hole is replaced by two or more holes of smaller diameter arranged such that a laminar fluid flow through said holes will be achieved for a given sound pressure level.

4. An active noise reduction system substantially as hereinbefore described, with reference to any one of the accompanying drawings.

5. An active noise reduction headset, comprising a ear defender or earphone and a noise reduction system according to any one of Claims 1 to 4.

6. An active noise reduction headset substantially as hereinbefore described, with reference to Figure 1 of the accompanying drawings.