DE3688624T2 - Interference limitation arrangements. - Google Patents

Abstract

An active noise reduction (ANR) arrangement for reducing 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 measured 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 fields within the interiors or enclosures of so-called ear protection devices or earphone arrangements when they are worn by people (for example pilots, vehicle drivers, industrial workers) in environments with a high noise level.

Known active noise reduction (ANR) systems for reducing the aforementioned acoustic noise field in ear protectors include small noise-pickup microphones and noise-cancelling transducers mounted within the interiors or enclosures of the respective ear protectors. The noise-pickup microphones produce electrical output signals in response to the acoustic noise field within said spaces and these output signals are phase-inverted, filtered and amplified in a feedback loop to produce a noise-cancelling signal to be fed to a noise-cancelling transducer which produces corresponding noise-cancelling acoustic signals of substantially equal amplitude but opposite phase to the waveforms of the acoustic noise field.

An example of an ANR arrangement is described in the publication DE 3133107 A1, which shows the use of two microphones and an automatic gain control circuit which provides a variable feedback gain depending on the variable noise reduction produced by the operation of the ANR arrangement.

In the present description, the term "ambient noise" is to be understood as noise that occurs as a result of external sources within the space of an ear protector. A reduced noise level will be achieved within the space by Analog Noise Reduction (ANR). It can be shown that such ANR arrangements produce an an acoustic noise reduction at a certain frequency, which is given by:

(1 + 2GcosΦ + G²)1/2

where G is the total gain of the feedback arrangement and Φ is the total loop phase change at a certain frequency under consideration. From this formula it is immediately apparent that the degree of noise reduction achieved is highly dependent on the total feedback loop gain. Due to the imperfect transfer functions of the sound-pickup microphones and noise-suppressing transducers, the acoustic noise reduction arrangement will, at certain frequencies, feed in-phase signals (i.e., positive feedback) instead of anti-phase signals (i.e., negative feedback) to the noise-reducing transducers. To prevent the ANR system from becoming unstable, the feedback gain of the entire system must be kept less than one at the frequencies in question, otherwise the noise levels in the earphone array rooms will actually be increased rather than reduced by the positive feedback signals fed to the noise-reducing transducers. Although the feedback gain must be kept below one at the above frequencies to maintain stability, the feedback gain of the ANR must still be sufficiently high to achieve optimal acoustic noise reduction.

Fixed feedback gain control techniques could be used if there were no changes likely to occur in the characteristics of the components of the ANR over time. However, such fixed feedback gain techniques would not provide the necessary compensations for changes in the sensitivity of the noise-reducing transducers resulting from changes in the volume of the earphone cavities that occur when an earphone assembly is worn by different persons or from small changes in the position of the earphone assembly that are likely to be caused by normal movements of the wearer's head.

Automatic feedback gain control technique would be suitable to provide the aforementioned required compensation, but the conventional approach has been to use the output of the sound-pickup microphones of the ANR array for automatic gain control purposes.

Changes in microphone output may result from a change in feedback gain (e.g. due to headphone movement) which requires the automatic gain control arrangement to react to adjust the gain, and may also result from a change in the external noise spectrum/level. However, the cause of these changes in feedback gain cannot be distinguished in an active noise cancelling system which uses sound-pickup microphone outputs only for gain control purposes. It follows that the automatic gain control produces a gain which continuously oscillates around the required value.

An object of the present invention is to provide an arrangement for noise reduction which has means for automatically adjusting the feedback gain in response to the level of the measured received noise.

According to the present invention there is provided an earphone assembly for active noise reduction with an earphone device, a first and second microphone and an automatic gain control circuit arranged to effect a variable feedback gain in dependence on a variable noise reduction produced by the operation of the active noise reduction assembly, characterized in that the first and second microphones are arranged respectively within front and rear interior spaces of the earphone device, the front space being open to come into contact with the wearer's head and the rear space being a closed space within the earphone device, and the microphones are arranged on the opposite sides of a noise reducing transducer diaphragm extending between these two spaces, in which the first microphone detects the noise level within the front space and produces an output signal which is amplified and inverted by a variable amplifier before being transmitted to the noise reduction transducer, and in which the second microphone senses the noise level within the rear room and produces an output signal which is processed together with the output signal of the first microphone in the automatic gain control circuit to produce a gain control signal which is transmitted to the variable amplifier and which serves to ensure that a level of the measured noise reduction is kept constant.

Preferably, the automatic gain control circuit comprises an adder circuit arranged to receive signals in dependence on the output signals of the first and second microphones; a first converter for providing a direct current signal in dependence on the output signal of the first microphone, a second converter for providing a direct current signal in dependence on the output signal of the adder circuit, and a comparator circuit for comparing the signals in dependence on the direct current signals provided by the first and second converters; arranged to provide a gain control signal for controlling the gain of the noise-reducing arrangement in dependence on this comparison.

An embodiment of the invention is explained in more detail below with reference to the single drawing, in which a schematic diagram of an arrangement for active noise reduction, which includes an automatic gain control circuit, is shown.

The noise reduction arrangement shown in the figure comprises a generally cup-shaped, circumaural earphone arrangement 1 arranged to enclose the ears 2 of the wearer. The edge of the arrangement 1 is padded against the side of the head 3 of the wearer by means of a flexible ring pad 4.

As with known arrangements for active noise reduction, the earphone arrangement 1 encloses a small microphone 5, which records the sound volume inside the earphone next to the wearer's ear 2 and provides an electrical output signal depending on the recorded sound. This output signal of the microphone is amplified by an amplifier 6 and the amplified signal is then inverted and filtered by a phase inverter/filter 7 before it reaches a noise suppressing transducer 8. This transducer 8 includes a movable membrane 9 which is attached to an opening of a rigid wall element 10. The membrane 9 and the wall element 10 divide the interior of the earphone assembly 1 into a front space 11 which houses the sound-receiving microphone 5 and a closed rear space 12.

The provision of means which divides the earphone assembly into two separate compartments allows the earphone to produce high level low frequency sounds. The background pressure behind the diaphragm 9 is maintained in the rear compartment 12 of the earphone and is thereby prevented from mixing with the pressure in the front compartment. Typically the noise level at frequencies up to about 500 Hz is consistent in both earphone compartments 11 and 12. The noise cancelling signals applied to the transducers 8 cause vibrations of the diaphragm 9 and produce acoustic signals of equal amplitude but opposite phase to the noise signals picked up by the microphone 5.

It is clearly evident that the sound-recording microphone 5 only records the noise level within the earphone space 11 after noise reduction has occurred.

For the purpose of varying the feedback gain of the feedback circuit in accordance with changes in the measured noise, the illustrated arrangement according to the invention includes a further noise-receiving microphone 13, identical to the microphone 5, which is arranged within the closed rear space 12 of the earphone structure 1.

It should be noted here that the noise in both earphone compartments 11 and 12 has the same amplitude and phase at low frequencies because the earphone is essentially transparent to low frequency noise. Movement of the transducer diaphragm 9 toward the front compartment 11 increases the pressure in the front compartment but decreases the pressure in the rear compartment 12. As a result, noises generated in the rear compartment 12 are 180º out of phase with those generated in the front compartment 11. Accordingly, microphone 5 picks up ambient noise plus antiphase noise while microphone 13 picks up ambient noise minus antiphase noise. The resulting noise-representing output signals obtained from microphones 5 and 13 are fed to corresponding bandpass filters 14 and 15, which provide an output in the low frequency range (typically 300-400 Hz). At these low frequencies, the noise-suppressing signals fed to transducer 8 are 180º out of phase with the noise picked up by microphone 5 arranged in room 11. The outputs of filters 14 and 15 are fed to a signal adder 16, which provides output signals that are related only to the ambient noise, with the antiphase noise-reducing components picked up by microphones 5 and 13 being mutually cancelled out by the addition process.

The output signal of the adder 16 and the output signal of the filter 14 are converted to DC levels by respective basic means of square wave voltage to DC converters 17 and 18, with the signals obtained from the microphone 5 having received a predetermined gain value by an amplifier 19 to correspond to the required level of noise reduction. The outputs of the converters 17 and 18 (after amplification by the amplifier 19) are fed to 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.

It will be appreciated that a further active noise reduction arrangement with automatic feedback amplification, as shown in the drawing, is in practice provided for the other ear of the wearer.

It will also be appreciated that although the particular embodiment described here is directed to a circumaural earphone structure, the invention may also be applied to alternative dual-chamber earphone structures, such as those of the SUPRA-AURAL type.

Claims (4)

1. An earphone assembly for active noise reduction comprising an earphone device, a first (5) and second (13) microphone and an automatic gain control circuit (14-20) arranged to effect a variable feedback gain in dependence on a variable noise reduction produced by the operation of the active noise reduction assembly, characterized in that the first and second microphones (5, 13) are arranged respectively within front and rear interior spaces (11, 12) of the earphone device (1), the front space (11) being open to come into contact with the wearer's head (3) and the rear space (12) being a closed space within the earphone device, and the microphones on the opposite sides of a space extending between these two spaces, noise reducing transducer diaphragm (9), in which the first microphone (5) detects the noise level within the front room (11) and generates an output signal which is amplified and inverted (7) by a variable amplifier (6) before being transmitted to the noise reducing transducer (8), and in which the second microphone (13) detects the noise level within the rear room (12) and generates an output signal which is processed together with the output signal of the first microphone (5) in the automatic gain control circuit (14-20) to generate a gain control signal which is transmitted to the variable amplifier (6) and which serves to ensure that a level of the measured noise reduction is kept constant. 2. A noise reduction arrangement according to claim 1, wherein the automatic gain control circuit an adding circuit (16) arranged to receive signals in dependence on the output signals of the first and second microphones (5, 13); a first converter (18) to provide a direct current signal in dependence on the output signal of the first microphone (5), a second converter (17) to provide a direct current signal in dependence on the output signal of the adding circuit (16), and a comparator circuit (20) to compare the signals in dependence on the direct current signals provided by the first and second converters (18, 17); arranged to provide a gain control signal for regulating the gain of the noise-reducing arrangement in dependence on this comparison. 3. Arrangement for noise reduction according to claim 1 or 2, which further comprises filter means (14, 15) for filtering the output signals of the microphones (5, 13). 4. Arrangement for noise reduction according to one of claims 1 to 3, which further comprises an amplifier (19) between the comparator circuit (20) and the first converter (18) in order to amplify the signals received by the comparator circuit (20) from the first converter (18).