NOISE REDUCTION IN VEHICLE CABINS

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 (12) which has as inputs (10, 14), 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 (14) positioned in the cabin and has as outputs, signals to loudspeakers (16). The loudspeakers (16) produce a noise cancellation field to reduce noise in the cabin. The invention may be employed in combination with a public address system in an environment, such as an aircraft cabin, where noise control is required.

Description: - 1 - Title Noise Reduction in Vehicle Cabins Field of the invention This invention relates generally to the reduction of nois in the cabin area of a passenger carrying vehicle, especially an aircraft. More specifically, this inventio concerns the reduction, by active rather than by passive means, of vehicle-cabin noise that is periodic due to 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 turbin driven aircraft, where the cabin is the enclosure and the unwanted noise is that arising from the rotating machiner e.g. due to rotation of the propeller or propellers. Loudspeakers or other transducers can be positioned in th 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>e 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 an active noise-reduction system for reducing noise inside the cabin of a vehicle driven by rotating machinery, especially a propeller driven, unducted fan driven or gas turbine driven aircraft, comprising means for producing an angular position, or angular position and speed, dependent signal associated with at least one rotary component of the rotating machinery and for which noise control is required; a relatively large plurality of microphones positioned in the cabin to monitor the noise field therein and produce noise dependent signals: - an electronic controller which has as inputs both the angular position dependent signal(s) and the noise dependent signals and produces output signals dependent on detected parameters of the input signals? and - a relatively large plurality of loudspeakers positioned in the cabin to produce a noise cancellation field which reduces noise in the cabin at least at those regions thereof where noise reduction is required. Thus, the invention provides 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, including 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 and has as outputs, signals to loudspeakers. The total number of monitoring microphones may be between 0.2 and 3 times the total number of passenger seats in the cabin, and the total number of loudspeakerβ may be between 0.2 and 3 times the total number of passenger seats in the cabin, the number of microphones not necessarily being the same as the number of loudspeakers. Desirably, the fundamental noise tone of the rotary component at frequency F and at approximate harmonics up to the fourth harmonic, approximately at frequency 4F, are controlled if necessary, whilst the microphones and loudspeakers are positioned to give the required noise- reduction for seated passengers, and possibly also at standing head-height in the aisle(s). It is to be noted that the controller is adapted to take into account the fact that, for example due to small variations in propeller speed, the fundamental frequency and its overtones may not always correspond exactly to F, 2F, 3F, etc. Microphones may be mounted in seat headrests or backs, overhead stowage bins or cabin trim ? loudspeakers may be mounted in or under seats, in overhead stowage bins or cabin trim or the floor. Preferably, a digital electronic controller which comprises part of the ANRS is adaptive and is able to accommodate variations in the speed of the rotary component, (e.g. the propeller) and/or in cabin acoustics. Preferably, microphones mounted in regions of high vibration are specially selected to have low vibration sensitivity. Additionally, the microphones preferably have a frequency-response especially tailored to the application in order to reduce or eliminate the requirements for anti-aliasing filters on the signals from the microphones. For this purpose, the sensitivity of at least some of the microphones is designed to begin to decrease within 1/2 octave of the highest propeller-noise harmonic being controlled, and the rate of reduction in sensitivity above that frequency is at least 12dB/oct. The loudspeaker size is preferably varied according to its position in order to minimize weight, whilst some or all of the loudspeakers may also be used for the aircraft public address systems. In this case, frequency-response shaping of the loudspeakers ' outputs may be used to improve broadcast speech/music quality. The acoustic outputs from at least some loudspeakers may be ducted into the cabin so that the loudspeakers can be stowed in available spaces, not necessarily coincident with the desired locations of acoustic outputs. The invention may be employed in combination with a public address system which has been modified so that signals correlated with the rotary component-noise at the controlled harmonics are attenuated when an on-board microphone is used as the public-address source. More generally, according to a further aspect of the invention, there is provided an active noise reduction system in combination with a public address system in an environment where noise control is required, having noise- monitoring microphones in the area where the public address system is operative, and an electronic controller responsive to the outputs of the noise monitoring microphones and providing control signals to the source of the public address system to assist in reduction of noise in said area where the public address sys.J-.em is operative. Power amplifiers are preferably mounted close to, or on, their respective loudspeakers in order to reduce cabling mass, for example being mounted on the loudspeakers' chassis so that the chassis act as heat sinks for the power amplifiers. The controller preferably continually or periodically monitors loudspeaker-to-microphone transfer functions in order to diagnose transducer performance changes or failure. 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 filter characteristics; Figure 4 shows a complete system in more detail; and Figure 5 shows a loudspeaker arrangement. Description of embodiment The illustrated active noise reduction system can be split into three different sub-systems 1) Sensors - 7 - 2) Loudspeakers 3) Electronic Controller The overall structure of the system is as illustrated in Figure 1. A sensor 10A, 10B, etc. for each propeller (o analogous rotary component) measures the propeller angula 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 electronic controller 12 outputs signals via power amplifiers 18 to loudspeakers 16A, 16B, etc. distributed in the cabin. Th 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. 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-reduction 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 jς>r other piezoelectric devices generally are not. Before the monitoring microphone 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, V. is the input voltage modulus, V Q the output voltage modulus, f the frequency of the highest controlled harmonic of the propeller noise (f = m x F) , and f- 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. 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/or b) 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 field 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 may 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: 20 - public address cabin microphone 22 - filter for removing noise correlated to the propeller noise at the controlled harmonics 24 - optional frequency-response shaping filters 26 - noise cancelling loudspeakers also used for public address 28 - loudspeakers solely for noise cancelling 30 - loudspeakers solely for public address 32 - noise-cancellation system controller 34 - propeller speed/position sensors 36 - cabin-noise monitoring microphones 38 - optional pre-amplifiers 40 - 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 from 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 correct outputs to feed to the noise-cancelling loudspeakers via power amplifiers from inputs from the monitoring microphones 36 and propeller position 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 comparing them with previously stored values. Should one or more loudspeaker or microphone change significantly, this procedure will detect the change, issue 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 simultaneously 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. 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: - 15 - Claims 1. An active noise-reduction system for reducing nois inside the cabin of a vehicle driven by rotating machinery, especially a propeller driven, unducted fan driven or gas turbine driven aircraft, comprising - means for producing an angular position, or angula position and speed, dependent signal or signals associat with at least one rotary component of the rotating machinery and for which noise control is required; a relatively large plurality of monitoring microphones positioned in the cabin to monitor the noise field therein and produce noise dependent signals; - an electronic controller which has as inputs both the angular position dependent signal or signals and the noise dependent signals and produces output signals dependent on detected parameters of the input signals; and a relatively large plurality of loudspeakers positioned in the cabin to produce a noise cancellation field which reduces noise in the cabin at least at those regions thereof where noise reduction is required. 2. A system according to Claim 1, wherein the total number of monitoring microphones is between 0.2 and 3 times the total number of passenger seats in the cabin. 3. A system according to Claim 1 or Claim 2, wherein the total number of loudspeakers is between 0.2 and 3 times the total number of passenger seats in the cabin. times the total number of passenger seats in the cabin, 4. A system according to any one of the preceding claims, wherein the fundamental noise tone of the rotary- component, at a frequency designated F and at approximate harmonics up to and including the fourth harmonic at a frequency of approximately 4F, is controlled if necessary. 5. A system according to any one of the preceding claims, wherein the microphones and loudspeakers are positioned so as to give the required noise reduction for seated passengers. 6. A system according to any one of the preceding claims, wherein the microphones and loudspeakers are positioned so as to give the required noise reduction at standing head-height in the aisle or aisles between seating areas in the cabin. 7. A system according to any one of the preceding claims, wherein the microphones are mounted in seat headrests or backs, or in overhead stowage bins, or in cabin trim, in the cabin. 8. A system according to any one of the preceding claims, Wherein the loudspeakers are mounted in or under seats, or in overhead stowage bins, or in cabin trim, or in the floor, in the cabin. 9. A system according to any one of the preceding claims, wherein a digital electronic controller comprises part of the system, which controller is adaptive and is able to accommodate variations in the speed of the rotary component and/or in cabin acoustics. 10. A system according to any one of the preceding claims, wherein those microphones mounted in regions of high vibration are selected to have low vibration sensitivity. 11. A system according to any one of the preceding claims, wherein the microphones have a frequency response tailored so as to reduce or eliminate requirements for anti-aliasing filters on the signals from the microphones. 12. A system according to Claim 11, wherein the sensitivity of at least some of the microphones is designed to begin to decrease within of an octave of the frequency of the highest harmonic of the rotary component being controlled, and the rate of reduction in sensitivity above that frequency is at least 12dB per octave. 13. A system according to any one of the preceding claims, wherein some or all of the loudspeakers are also used for a public address system used in the vehicle . 14. A system according to Claim 13, 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 public address system. 15. A system according to any one of the preceding claims, wherein the acoustic outputs from at least some of the loudspeakers are ducted into the cabin. 16. A system according to any one of the preceding claims, wherein power amplifiers are mounted close to, or on, their respective loudspeakers in order to reduce cabling mass. 17. A system according to any one of the preceding claims, wherein the controller continually or periodically monitors loudspeaker-to-microphone transfer functions in order to diagnose transducer performance changes or failure. 18. An active noise-reduction system in combination with a public address system in an environment where noise control is required, having noise-monitoring microphones in the area where the public address system is operative, and an electronic controller responsive to the outputs of the noise monitoring microphones and providing control signals to the source of the public address system to assist in reduction of noise in the area where the public address system is operative.

Inventors: DORLING CHRISTOPHER MARK, EATWELL GRAHAM PAUL, JOHNSTON JOHN ANDREW, LANGLEY ANDREW JOHN, ROSS COLIN FRASER, SUTCLIFFE SEAN GEORGE CRONIN, DORLING, CHRISTOPHER, MARK, EATWELL, GRAHAM, PAUL, JOHNSTON, JOHN, ANDREW, LANGLEY, ANDREW, JOHN, ROSS, COLIN, FRASER, SUTCLIFFE, SEAN, GEORGE, CRONIN
Applicants: TOPEXPRESS LTD, TOPEXPRESS LIMITED
Application year: 1989
Main classification: G10K11
IPC codes: B64C1, B64D47, G10K11