CA1337178C

CA1337178C - Noise reduction in vehicle cabins

Info

Publication number: CA1337178C
Application number: CA000609306A
Authority: CA
Inventors: Colin Fraser Ross; Christopher Mark Dorling; Graham Paul Eatwell; Andrew John Langley; Sean George Cronin Sutcliffe; John Andrew Johnston
Original assignee: Noise Cancellation Technologies Inc
Current assignee: Noise Cancellation Technologies Inc
Priority date: 1988-09-06
Filing date: 1989-08-24
Publication date: 1995-10-03
Anticipated expiration: 2012-10-03
Also published as: GB8820922D0; WO1990003026A1; JPH04500566A; EP0433331A1; AU4189989A

Abstract

An active noise-reduction system (ANRS) designed to reduce cabin noise in a passenger vehicle, especially to reduce propeller-noise inside an aircraft which is driven by a propeller, unducted fan or gas turbine, includes an electronic controller which has as inputs, firstly an angular position-dependent signal from the or each propeller or analogous rotary component for which noise control is required and, secondly, signals from monitoring microphones positioned in the cabin and has as outputs, signals to loudspeakers. The loudspeakers produce a noise cancellation field to reduce noise in the cabin.

The invention may be in combination with a public address system in an environment, such as an aircraft cabin, where noise control is required.

Description

Title Noise Reduction in Vehicle Cabins Field of the invention This invention relates generally to the reduction of r.oise in the cabin area of a passenger carrying vehicle, especially an aircraft. More specifically/ this invention concerns the reduction, by active rather than by p2ssive means, of vehicle-cabin noise that is periodic due 'o the operation of rotating machinery on board the vehicle, e.g.
noise which is linked to the propeller-blade passing rate or whine emanating from a gas turbine engine.

Prior art It has been known for some time that, in principle, it is possible at least partially to cancel an unwanted sound field inside an enclosure by sources that produce an inverted replica of the unwanted noise. This principle can be applied directly to propeller-driven or gas turbine driven aircraft, where the cabin is the enclosure and the unwanted noise is that arising from the rotating machinery e.g. due to rotation of the propeller or propellers.
Loudspeakers or other transducers can be positioned in the cabin to generate a sound field opposed to unwanted turbomachinery noise, and hence in effect reduce the magnitude of the latter.

The broad principle of aircraft-cabin nois~ reduction was clearly enunciated in the article "Anti-noise - the Essex ~' Breakthrough", by B. Chaplin, published in Chartered Mechanical Engineer, January 1983. The article also refers to other applications for active noise reduction in an enclosure, when the unwanted noise is periodic or approximately periodic.

U.K. Specification No. 2149614B relates to procedures for active noise reduction in an enclosure when the noise source is periodic and lying outside the enclosure, and gives some details of methods which might be adopted to effect the control.

U.K. Specification No. 2132053B also relates to the concept of active noise reduction in an enclosure, and outlines methods that might be adopted and constraints which should be fulfilled.
However, this proposal does not exploit the fundamentally periodic nature of propeller or gas turbine noise, and the type of controller described is probably not practical for use in suppressing cabin noise in an aircraft or other passenger vehicle driven by turbomachinery.

At the present time, therefore, it appears that an active control system practical for suppression of cabin noise in a passenger vehicle is unknown in the prior art.

The invention According to one aspect of the invention, there is provided in a region subject to noise, a system comprising: first and second loudspeaker means for transmitting audio signals into the region;
means electrically coupling said loudspeaker means to a common channel; a sound system for feeding electrical audio signals to said common channel, such that the loudspeaker means both produce in unison perceptible broadcast information in the region; and a noise reduction system including sensing means responsive to ambient noise in the region, electronic controller means responsive to said sensing means to produce separate _ 3 _ 1337 178 electrical noise cancellation signals for said loudspeaker means respectively, and means for feeding the separate noise cancellation signals to said loudspeaker means individually, whereby said loudspeaker means in use produce respective noise cancellation signals and both produce broadcast information for the region.

Brief description of drawings An active noise reduction system for an aircraft cabin, in accordance with the invention, is exemplified hereinafter with reference to the accompanying drawings, in which:-Figure 1 shows the exemplary system in block diagram form;

Figures 2 and 3 are diagrams indicative of microphone filtercharacteristics;

Figure 4 shows a complete system in more detail; and Figure S shows a loudspeaker arrangement.

Description of embodiment The illustrated active noise reduction system can be split into three different sub-systems 1) Sensors ~, 2) Loudspeakers 3) Electronic Controller The overall structure of the system is as illustrated in Figure 1. A sensor lOA, lOB, etc. for each propeller (or analogous rotary component) measures the propeller angular position (or rotation speed and angular position), and this information forms a set of inputs to the controller 12. Another set of inputs is obtained from monitoring microphones 14A, 14B, etc. which are distributed in the cabin.

From data provided by the sensors 10, 14, the electroni~
controller 12 outputs signals via power amplifiers 18 to loudspea~ers 16A, 16B, etc. distributed in the cabin. ~ne outputs of the controller are computed to effect the desired propeller-noise reduction at the monitoring microphones 14. Variations in the operating conditions of the propeller(s), or the noise field in the cabin, are compensated by adapting the outputs to ensure that the desired noise-reduction is maintained.

If the fundamental frequency of the propeller blade passing-rate is F, for example, then the propeller noise will comprise noise at frequencies close to F and its harmonics 2F, 3F, etc. Generally, not all of the harmonics will be considered to be a nuisance, and the controller is programmed to effect noise reduction upon some or all of the first few harmonics, e.g. F, 2F, 3F, 4F.

The sub-systems are described in more detail below.

. - .......
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1. Sensors For each propeller, a sensor measures at least the angular position of each propeller for which noise reduction is required, and the outputs of these sensors form a set of inputs to the controller. The angular position signal may be in the form of a once-per-revolution pulse.

Monitoring microphones, equal in number to between 0.2 and 3 times the total number of passenger seats in the cabin, are distributed in the cabin. At least one microphone may be attached close to each seat at which significant noise-reduction is required. Other microphones may be attached to overhead stowage bins or in the ceiling of the cabin if noise reduction is also required at standing head-height in the aisle(s) or if microphones cannot be fitted into the seats. The precise location of each microphone is chosen to optimize the noise-reductio~
performance according to design criteria and within the constraints of acceptable mounting positions. In particular, microphones are concentrated in areas close to the unwanted source(s) of noise, for example, close to the plane of the propellers. Fewer if any microphones may be needed where noise levels are already acceptable, for example, towards the rear of the cabin.

The type of microphone is selected to be relatively insensitive to vibration. This vibration insensitivity is important for microphones which might be attached to a vibrating trim, for example. In this respect, a normal electrodynamic microphone is considered to be relatively sensitive to vibration, whereas electret ~or other piezoelectric devices generally are not.

Before the monitoring micropn~le signals are converted to digital signals by the analogue-to-digital converters of the controller, these signals are normally filtered by a low-pass filter in order to avoid aliasing errors. The anti-aliasing filters are typically analogue low-pass filters with a pass-band which extends to just beyond the frequency of the highest harmonic (mxF) at which noise reduction is being performed. For example, if the first three harmonics of the propeller noise are being attenuated, the filters' passbands might extend to 1/8 of an octave above 3F. The stop-band of the filters will be typically -40dB relative to the passband, and this degree of attenuation is reached typically within 1.5 octaves of the pass-band (see Figure 2 for an example). In this figure, as also in Figure 3, P is the modulus of the pressure at the microphone, Vl is the input voltage modulus, V0 the output voltage modulus, fO the frequency of the highest controlled harmonic of the propeller noise (fO = m x F), and fl the frequency marking the edge of the passband of the filter. The anti-aliasing filters can be simplified, or even eliminated (therefore saving on electronics size and weight) if the microphones' mechanical design is such that the sensitivities of the microphones begin to reduce at frequencies within 1/2 octave of mxF and approach zero sensitivity above that frequency at a rate of preferably at least 12dB/oct (Figure 3). This type of response is quite different from that of normal microphones whose bandwidths are designed to be as wide as possible, and whose rate of sensitivity-reduction at high frequencies is not an important design feature.
~1 2. Loudspeakers Loudspeakers, equal in number to between 0.2 and 3 times the number of passenger seats in the cabin (but not necessarily the same number as the number of microphones) are mounted:-a) attached to overhead stowage bins, and/orb) in the cabin interior-trim, and/or c) in or under passenger seats.

The loudspeaker positions are chosen to optimize their efficiency at producing the desired cancelling sound fielZ
in those areas of the cabin where noise reduction is required. In particular, loudspeakers may be concentrated close to the apparent source(s) of the propeller-noise, for example, close to the plane of the propellers; and few, if any, may need to be placed towards the rear of the cabin.

In order to save weight, the sizes of the loudspeakers m~y be different, with larger units being used where the propeller-noise, in the absence of active cancellation, appears relatively louder.

Again in order to save weight, and as shown in Figure 4, some or all of the loudspeakers of the active cancellation system and their respective power amplifiers can also be used for the aircraft's public address system. In Figure 4, the following references are employed for the respective component parts of the combined public address/noise reduction system:

- public address cabin microphone ~

22 - filter for removing noise correlated to the propeller noise at th~ controlled harmonics 24 - optional frequency-response shaping filters 26 - noise cancelling loudspeakers also used for public address 28 - loudspeakers solely for noise cancelling - loudspeakers solely for public address 32 - noise-cancellation system controller 34 - propeller speed/position sensors 36 - cabin-noise monitoring microphones 38 - optional pre-amplifiers - power amplifiers 42 - signal summing devices 44 - non-microphonic public address source (e.g.
tape-recorder) Frequency-response-shaping filters 24 may be introduced at some or all of the locations indicated in Figure 4 in order to improve the quality of speech or music broadcast over loudspeakers mounted in unusual positions (e.g. under seats).

In order to avoid potential controller-instabilities, public address messages ~rom the on-board microphone 20 may be filtered to attenuate signals that are correlated with the controlled propeller-noise harmonics. This filtering may be achieved by an analogue filter with fixed notches centred on the normal values F, 2F etc. up to mxF
(the highest controlled harmonic), which notches are just wide enough to accommodate usual variations in propeller speed. Alternatively, the filtering may be performed by a similar tracking notch filter whose notches are varied in response to a signal of propeller speed, so that the centres of the notches always lie close to F....mF. Such notch filters will not seriously degrade the quality of broadcast speech.

Where space is restricted, signals from any of loudspeakers 26, 28 or 30 may be ducted to the desired location by a duct 50, so that the loudspeaker can be placed remotely in an available space, as shown in Figure 5.

Moreover, in order to save weight and packaging volume, it may be advantageous to build the power-amplifiers of the noise-reduction loudspeakers close to, or on, the loudspeakers' chassis. The loudspeaker chassis may then be used as a heatsink for the power-amplifier. Further, since the input impedance of the power-amplifier is typically much greater than the resistance of the loudspeaker-coil, much lighter cables can be used to feed the signal from the controller to the power-amplifiers compared to that which would be required to feed a signal from a power-amplifier to a loudspeaker. The amplifiers may be powered by tapping into one of the supply busses running along the aircraft.

3. The Electronic Controller The controller is a signal processor for computing the correctoutputs to feed to the noise-cancelling loudspeakers via power amplifiers from inputs from the monitoring microphones 36 and propeller position sensors 34. The controller operates by digital processing and includes analogue to digital converters that receive the analogue signals from microphones 36 and sensors 34. Noise reduction is performed for the propeller-noise fundamental frequency F, and possibly for harmonics 2F, 3F up to 4F. Up to 64 microphones and 64 loudspeakers are used in the system. Computations are performed by one or more microprocessors in the controller. The controller is programmed to adapt the output signals as operating conditions (e.g.
propeller speed, cabin acoustics) change, in order to maintain the desired noise cancellation performance.

In addition, the controller monitors the state of each loudspeaker and microphone by periodically re-calculating the transfer functions between the loudspeakers and the microphones and compairing them with previously stored values. Should one or more loudspeaker or microphone change significantly, this procedure will detect the change, isssue or store a warning of which unit(s) have changed, and the controller will adapt its outputs to minimise the degrading effect of a transducer failure or performance-change on the noise cancellation performance.

One method to achieve this is for the controller to output signals uncorrelated to the cabin-noise. These signals might be sent sequentially to each loudspeaker separately, or simul-taneously to all loudspeakers, in which case any loudspeaker signal should also be uncorrelated with any other loudspeaker signal. By measuring the monitoring microphone responses to these outputs it is possible to compute the loudspeaker-to-microphone transfer functions.

- tl- 1337178 In order that these additional outputs signals should not significantly affect existing cabin noise, they should be at such a low level that they are masked by the existing cabin-noise. Standard signal processing techniques can be used to remove the effect of the masking cabin-noise on such transfer-function measurements.

Various modifications of the above-described and illustrated embodiment are possible within the scope of the invention hereinbefore defined.

Claims

1. In a region subject to noise, a system comprising:
first and second loudspeaker means for transmitting audio signals into the region;
means electrically coupling said loudspeaker means to a common channel;
a sound system for feeding electrical audio signals to said common channel, such that the first and second loudspeaker means both produce, in unison, perceptible broadcast information in the region; and a noise reduction system, including sensing means responsive to ambient noise in the region, electronic controller means responsive to said sensing means to produce separate electrical noise cancellation signals for said first and second loudspeaker means respectively, and means for feeding the separate noise cancellation signals to said first and second loudspeaker means individually, whereby said first and second loudspeaker means, in use, produce respective noise cancellation signals and both produce broadcast information for the region.

2. The system according to claim 1 in a vehicle cabin, the vehicle being driven by rotary drive means.

3. The system according to claim 2, wherein said sensing means includes means for producing an angular position, or angular position and speed, dependent signal or signals associated with said rotary drive means, and a plurality of microphones positioned in the cabin to monitor the ambient noise therein and produce noise dependent signals, and said electronic controller including analog to digital converters receiving as inputs both the angular position dependent signal(s) and the noise dependent signals and which digitally processes the converted signals to produce output signals dependent on detected parameters of the input signals, said converters being adaptive to accommodate variations in speed of the rotary drive means and/or in cabin acoustics.

4. The system according to claim 3, wherein the electronic controller means monitors loudspeaker-to-microphone transfer functions.

5. The system according to claim 3, wherein the total number of monitoring microphones is between 0.2 and 3 times the total number of passenger seats in the cabin.

6. The system according to claim 2, wherein the total number of loudspeakers is between 0.2 and 3 times the total number of passenger seats in the cabin.

7. The system according to claim 2, wherein the fundamental noise tone of the rotary drive means, at a frequency designated F and at approximate harmonics up to and including the fourth harmonic at a frequency of approximately 4F, are processed by said electronic controller means.

8. The system according to claim 3, wherein the microphones and loudspeakers are positioned so as to produce a noise reduction for seated passengers.

9. The system according to claim 3, wherein the microphones and loudspeakers are positioned so as to give a noise reduction at standing head-height in the aisle or aisles between seating areas in the cabin.

10. The system according to claim 3, wherein those microphones mounted in regions of high vibration are selected to have low vibration sensitivity.

11. The system according to claim 3, wherein the sensitivity of at least one of the microphones is designed to begin to decrease within 1/2 of an octave of the frequency of the highest harmonic of the rotary drive means, and the rate of reduction in sensitivity above that frequency is at least 12dB
per octave.

12. The system according to claim 2, wherein said sound system and said loudspeakers provide a public address system for the vehicle.

13. The system according to claim 12, wherein the frequency-response shaping of the output of the loudspeakers is such as to improve the quality of speech or music broadcast through the sound system.

14. The system according to claim 2, wherein the acoustic outputs from at least some of the loudspeakers are ducted into the cabin.

15. The system according to claim 1, wherein power amplifiers for said loudspeakers are mounted close to, or on, respective ones of said loudspeakers in order to reduce cabling mass.

16. The system according to claim 1, wherein said sound system includes means for playing music through said loudspeakers.

17. The system according to claim 16, wherein said sound system includes player means for playing pre-recorded audio signals.