Ear defenders employing active noise control

Details Ear defenders employing active noise control Ear defenders employing active noise control

1995-08-22

1997-02-04

Isen, Forester W.

Electrical audio signal processing systems and devices

Acoustical noise or sound cancellation

Adjacent ear

381 72, G10K 1116

Patent

active

056007299

DESCRIPTION:

BRIEF SUMMARY
This invention relates to noise reduction techniques, and in particular to the application of Active Noise Reduction (ANR) in ear defenders.
The application has application to ear defenders of the type required simply to reduce the ambient level of noise to enable the wearer to concentrate better on his task or to reduce or avoid physical or psychological damage caused by an excessive noisy environment, as well as to ear defenders incorporating means for communicating audible data (such as speech or warning signals) to the wearer. In the latter case, the invention is intended to provide means for attenuating ambient noise while leaving the data signal unaffected.
Conventional ear defenders of the type at present contained in the helmets of aircrew can only provide a certain amount of noise protection for the wearer. The amount of protection provided by these conventional, passive means is primarily dictated by the mass, volume and seal stiffness of the hearing defender and the spectral characteristics of the noise field. Thee is a requirement to provide greater hearing protection (with little or no increase in helmet mass) and this requirement will grow as new aircraft with more demanding noise fields come into service, and it will become ever more desirable to provide means for adapting the noise reduction action of the defender to changes in the cavity configuration caused by its movement over the wearer's head or high "G" forces.
ANR has been successfully demonstrated in the past and a basic analogue technique is described in UK patent application no 2188210A. This technique is based upon a conventional circumaural ear defender and means are provided for sensing the noise field at the wearer's ear using a miniature microphone. The information from the microphone is then processed and used to generate, via a speaker inside the ear defender, an acoustic field which is in anti-phase with respect to the noise at the wearer's ear. The effect is to acoustically cancel some of the noise reaching the wearer's ear and thus provide noise reduction which is additional to that provided by the passive defender.
Present, conventional ANR systems are implemented using an analogue control strategy.
FIG. 1 illustrates the overall system, in which the output from a miniature microphone 1 located close to the wearer's ear 2 is applied to a feedback controller 3. The output of the feedback controller is added to the input signal (when present) to a speaker 4 to increase the stability of the system and to reduce noise enhancement in certain frequency bands. Because the controller 3 employs an analogue filter, the filter is fixed and therefore retains the same characteristics over time. Since the same filter is used in all implementations, the filter's characteristics in both amplitude and phase are arranged to be approximations of the average requirements.
In the illustration, the microphone and speaker are depicted as single components. In practice, a number of separate microphones or speakers may be deployed within the earshell and may be weighted and distributed in any required manner.
FIG. 2 shows the system applied to a simple ear defender and reduced to a block diagram.
Referring to FIG. 2, Vm is the voltage at the output terminals of the sense microphone and the two inputs to the summation point are the noise at the microphone's diaphragm and the ANR signal arriving at the microphone's diaphragm. The block F represents the operator relating the input to the speaker 4 to the corresponding output of the microphone 1 and is governed by the electrical transfer functions of the speaker and microphone and the acoustic transfer function of the earshell itself. This operator will be referred to hereinafter as the electroacoustic transfer function of the system.
The block W represents an electronic controller whose task it is to operate on the microphone voltage and produce a signal which, when operated on by the aforementioned electroacoustic transfer function, will result in a reduction of acoustic noise at the microphone.

REFERENCES:
patent: 4566118 (1986-01-01), Chaplin et al.
patent: 4985925 (1991-01-01), Langberg et al.
patent: 5138664 (1992-08-01), Kimura et al.
patent: 5172416 (1992-12-01), Allie et al.
patent: 5251263 (1993-10-01), Andrea et al.
patent: 5267321 (1993-11-01), Langberg
patent: 5305387 (1994-04-01), Sapiejewski

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