Noise attenuation system

Abstract: An active noise control (ANC) system (26) for attenuating engine generated noise contains at least one sinusoidal noise component having an amplitude and frequency which vary with changes in engine rotational speed. The ANC system (26) includes means (42-150,52) for generating an input signal that contains one or more sinusoidal signal components, where each signal component has a frequency corresponding to that of a respective one of the noise components. The input signal is filtered with an adaptive filter (206-210) having adjustable filtering characteristics to produce a filter output signal. Noise cancelling waves are generated by a noise cancelling actuator (28,32,36) in response to the filter output signal, and the cancelling waves are superimposed with the undesirable noise generated by the engine. The level of residual noise resulting from this superposition is sensed and a corresponding error signal is developed. The filtering characteristics of the adaptive filter (206-210) are adaptively adjusted in accordance with the error signal and sensed indications for the engine rotational speed and acceleration. The characteristics of the adaptive filter (206-210) are adjusted according to an algorithm having a convergence factor and a leakage factor, where the value of the convergence factor is scaled as a function of engine acceleration and the value of the leakage factor is scaled as a function of engine speed and acceleration.

Description: This invention relates to a noise attenuation system, for example to an active noise control (ANC) system used for attenuating undesirable noise produced by an internal combustion engine, where the noise may contain multiple closely spaced sinusoidal frequency components having amplitudes and frequencies varying in relationship with the rotational speed of the engine. Conventional active noise control (ANC) systems attenuate undesirable noise by producing and superimposing noise cancelling waves, which are substantially equal in amplitude and frequency content, but shifted 180 degrees in phase with respect to the undesirable noise. As used in the present specification and the appended claims, the term noise is intended to include both acoustic waves and mechanical vibrations propagating from a noise source. Recently, active noise control has been accomplished by employing modern digital signal processing and adaptive filtering techniques. Typically, an input sensor is utilized to derive a signal representative of the undesirable noise generated by a source. This signal is then fed to the input of an adaptive filter and is transformed by the filter characteristics into an output signal used for driving a cancellation transducer or actuator such as an acoustic speaker or electromechanical vibrator. The speaker or vibrator produces cancelling waves or vibrations which are superimposed on the undesirable noise generated by the source. The observed or residual noise level resulting from the superposition of the cancelling waves on the undesirable noise is then measured with an error sensor, which develops a corresponding error feedback signal. This feedback signal provides the basis for modifying the characteristics of the adaptive filter to minimize the overall level of the observed or residual noise. Such systems have been successfully applied to attenuate, for example, repetitive noise generated by fans or electric motors and random noise propagating down heating and air conditioning ducts. The nature of acoustic and vibrational noise generated by an internal combustion engine differs quite significantly from the nature of the repetitive or random noise encountered in the past. Engine generated noise generally contains a large number sinusoidal noise components having amplitudes and frequencies functionally related to the rotational speed of the engine. These frequency components have been found to be the even and odd harmonics of the fundamental frequency of engine rotation (in revolutions per second), as well as half-order multiples or sub-harmonics interposed between the even and odd noise harmonics. Consequently, at low engine speeds, the difference in frequency between adjacent noise components (that is, those noise components immediately preceding or following each other in the frequency domain) can become quite small, for example, as little as 5 Hz at engine idle. In addition, the amplitude, frequency, and phase of the engine generated noise components can vary quite rapidly in response to changes in engine rotational speed brought about by acceleration or deceleration of the engine. Also, engines having differing numbers of cylinders generate noise characterized by different dominant frequency components due to the difference in their firing frequencies. Finally, engine generated noise can have different amplitude and frequency characteristics depending upon the particular type of noise, for example, acoustic noise propagating from the engine intake or exhaust system, or mechanical vibrations produced by operation of the engine, which are transmitted to a vehicle frame. The present invention seeks to provide an improved noise attenuation system. According to an aspect of the present invention, there is provided a noise attenuation system for attenuating noise generated by an internal combustion engine as specified in claim 1. The invention can provide a flexible active noise control system which can be tailored to attenuate effectively undesirable noise containing multiple sinusoidal frequency components, for example in applications where the difference in frequency separating noise components is small in comparison with the values of their individual frequencies and where the amplitude, frequency and phase of the sinusoidal noise components can change quite abruptly, such as in noise generated by an internal combustion engine during periods of rapid engine acceleration or deceleration. A preferred embodiment provides an active noise control (ANC) system for attenuating engine generated noise, where the noise contains at least one sinusoidal noise component having an amplitude and frequency which vary in relation to changes in engine rotational speed. The system of this embodiment includes means for generating at least one input signal which contains one or more sinusoidal signals each having a frequency corresponding to that of a respective one of the noise components. Each input signal is filtered by a respective adaptive filter having adjustable filtering characteristics to produce a corresponding filter output signal. Noise cancelling waves are generated by an actuator in response to at least one filter output signal, and the cancelling waves are superimposed with the undesirable noise generated by the engine. The level of residual noise resulting from this superposition is sensed and an error signal indicative of the residual noise level is developed. The response of the ANC system to variations in engine noise is improved by adaptively adjusting the filtering characteristics of at least one adaptive filter to reduce the residual noise level based upon either the error signal and sensed rotational acceleration of the engine, or the error signal and both the sensed rotational speed and acceleration of the engine. Preferably, the filtering characteristics of at least one adaptive filter are adaptively adjusted in accordance with an algorithm having a convergence factor and a leakage factor, where the value for the convergence factor is scaled as a function of the sensed rotational acceleration of the engine and/or the value of the leakage factor is scaled as a function of the sensed engine rotational speed and acceleration. Scaling the convergence factor of a filter in this fashion has been found to improve the ability of the system to track and respond to variations in the amplitude and frequency of noise components induced by changes in engine rotational speed. Scaling the leakage factor of an adaptive filter as a function of sensed engine rotational speed and acceleration can enable the system to respond more quickly to the disappearance of engine noise components that occurs when the engine is rapidly decelerated at lower engine rotational speeds. When the engine noise contains multiple noise components, it is also preferable that the system includes signal generating means for producing a plurality of generator output signals; a plurality of adaptive filters, each filtering a respective one of the generator output signals to produce a corresponding filter output signal; and summing means for adding all of the filter output signals to obtain a cancelling signal for driving the system noise cancellation actuator. With this configuration, the signal components can be partitioned among the generator output signals according to the amplitude behavior of the corresponding noise components with engine rotational speed changes. By partitioning the signal components in this fashion, the adaptive adjustment of the filtering characteristics of each adaptive filter can be tailored in accordance with the amplitude behaviour of those engine noise components that correspond to the signal components in the generator output signal for the respective adaptive filter to improve the performance of the system. An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an active noise control system incorporating an embodiment of multi-channel electronic noise controller for attenuating different forms of undesirable noise generated by an engine; Figure 2 is a block diagram representative of the electronic components employed in the noise controller shown in Figure 1; Figure 3 is a mathematical model containing a parallel configuration of signal generator and adaptive filter pairs representing signal processing steps programmed into and carried out by the digital signal processor of Figure 2 for active noise control; Figure 4 is a block diagram representing the modelling components contained within each of adaptive filters AF j of the model shown in Figure 3; Figure 5 is a block diagram illustrating an off-line calibration process for a compensation E filter included within each adaptive filter AF j of the model illustrated in Figure 4; Figure 6 is a mathematical model programmed into a digital signal processor of the controller, which includes a filter controller for use in conjunction with the parallel configuration of signal generator and adaptive filter pairs employed for attenuating engine generated exhaust noise; Figures 7A-C illustrate typical values for control signals produced by the filter controller in the model shown in Figure 6 as a function of the rotational speed and/or acceleration of the engine when cancelling exhaust noise; and Figure 8 is a flow chart representative of the steps executed by a routine programmed into the digital signal processor to perform the signal generating, adaptive filtering, and control functions of the mathematical model shown in Figure 6. Referring to Figure 1, there is shown schematically an internal combustion engine 10, with its associated air intake system 12 and exhaust system 14. A rotatable throttle valve 16 is included within the air intake system 12 for regulating air flow to the engine 10. Also shown are two sensors generally associated with the electronic control of engine performance. The first is a standard throttle position sensor 18, such as a potentiometer, which is connected to throttle valve 16 for developing an electrical signal TP related to the degree or percent of throttle valve opening. The second is a conventional engine rotation sensor, in this case shown as a toothed wheel 42 mounted on the engine crankshaft, and an electromagnetic sensor 44 which produces a SPEED signal having pulses corresponding to the movement of teeth on wheel 42 past sensor 44. As shown, toothed wheel 42 has six symmetrically spaced teeth, which produce six equally spaced pulses in the engine SPEED signal for every complete revolution of the engine 10. This particular toothed wheel is merely exemplary, and wheels having different numbers of teeth can just as easily be used or, alternatively, any other known type of sensor or transducer capable of producing outputs pulses in response to the rotation of the engine can be employed. During the operation of engine 10, acoustic pressure waves are generated, which propagate away from the engine through the ducts and tubes forming the air intake and exhaust systems. Eventually, these pressure waves propagate from openings in the intake and exhaust systems as observable engine induction noise 20 and exhaust noise 22. In addition, the engine generates undesirable noise in the form of mechanical vibrations 24, which are transferred to a mounting frame 40 used to support engine 10. The system includes an active noise controller, the general components of which are shown in Figure 1. For illustrating a few of the many different applications that are possible, electronic noise controller 26 is shown as a multi-channel device having three separate channels, with each channel operating to attenuate one of the different forms of engine noise discussed above; that is, intake induction noise, exhaust noise and vibrational noise. One channel of the noise controller 26 is utilized to attenuate the engine generated induction noise propagating inside the air intake system 12. As will be described, the electronic noise controller 26 generates a cancelling OUTPUT 1 waveform based upon the input engine SPEED signal. This OUTPUT 1 signal drives a cancelling actuator 28, which in this case is an acoustic speaker, which produces cancelling acoustic waves which are superimposed on the engine generated induction noise. Sensor or transducer 30, in this case an acoustic microphone, is positioned in the air intake system 12 to measure the level of the residual or attenuated induction noise remaining in the air intake system 12 after the superposition of the cancelling acoustic waves. Sensor 30 develops an ERROR 1 signal representing the level of the residual induction noise, which is fed back to the induction noise channel of the electronic noise controller 26. This ERROR 1 signal provides the basis for minimizing the observed or residual induction noise 20 propagating out of engine intake system 12. A second channel of the noise controller 26 is employed to cancel exhaust noise. The operations described above for the induction noise application are duplicated, except that a noise cancelling signal OUTPUT 2 is produced to drive the exhaust noise cancelling actuator 32 (in this case an acoustic speaker) positioned to generate and propagate acoustic waves in the exhaust system, and an error sensor 34 (in this case an acoustic microphone) for developing an ERROR 2 signal representing the level of residual exhaust noise propagating from engine 10. Similarly, for cancelling engine generated vibrational noise 24, a third channel of the noise controller 26 produces noise cancelling signal OUTPUT 3 to drive an electromechanical vibrator 36, shown here disposed between engine 10 and mounting frame 40. Electromechanical vibrator 36 may be any suitable type which is capable of producing the required out-of-phase cancelling vibrations for superposition with the engine generated vibrations transmitted to mounting frame 40. For example, a commercially available Model LAV 2-3/5-6 actuator manufactured by Aura Inc could be used as shown in Figure 1 or, alternatively, a Model 203B Shaker supplied by Ling Electronics Inc could be mounted on frame 40 for producing the required out-of-phase cancelling vibrations. For this channel, an error feedback signal ERROR 3 representing the residual vibrations transferred to the mounting frame 40 is developed by an error sensor 38, which in this case is a standard accelerometer attached to the mounting frame 40. Referring now to Figure 2, the electronic circuitry within the noise controller 26 will now be described in terms of a block diagram containing standard well known electronic components present in the second channel 46 in the noise controller. The first and third channels, 48 and 50 respectively, contain the same components adapted to provide the appropriate input and output levels for their particular cancellation actuators and error sensors and, accordingly, only the components within the second channel will be described. It will be recognized that the implementation shown in Figure 2 is merely exemplary and other variations in the hardware are possible, as evident in the numerous patents, texts, and publications directed towards active noise control, see for example "Hardware and Software Considerations for Active Noise Control", M. C. Allie, C. D. Bremigan, L. J. Eriksson, and R. A. Greiner, 1988, IEEE, CH 2561-9/88/0000-2598, pp. 2598-2601. One of the principal electronic components in the preferred embodiment of noise controller 26 is a digital signal processor (DSP) designated by numeral 52. Digital signal processors are commercially available, such as the Motorola 56000, and typically include a central processing unit (CPU) for carrying out instructions and arithmetic operations, random access memory (RAM) for storing data, a programmable read only memory (PROM) for storing program instructions, and clock or timing circuitry used, for example, to establish the data sampling rate at which the digital signal processor operates. For the multiple channel operation illustrated in Figures 1 and 2, the digital signal processor 52 is programmed to function as one or more adaptive filters for each channel and it operates sequentially to perform the necessary steps or operations for each channel within the established data sampling rate (2.5 KHz in the present embodiment). As described above, an indication of the angular rotational position of the engine is preferably provided to the electronic noise controller 26 by the SPEED signal developed by the electromagnetic speed sensor 44. The SPEED signal contains pulses generated by the movement of toothed wheel 42 past electromagnetic sensor 44. After entering the noise controller 26, the SPEED signal is passed to standard conditioning circuitry 146, where the pulses are shaped or squared up into a format compatible with the digital circuitry that follows. These formatted digital pulses represent a measure of the angular rotation of the crankshaft and are passed to a standard frequency multiplier/divider circuit 148 which generates a fixed or predetermined number of pulses during one complete rotational cycle of the engine. The pulses from the frequency multiplier/divider 148 are then counted by a conventional modulo counter 150 to provide a digital output signal designated as COUNT. This digital COUNT signal is then used as a reference input signal to the digital signal processor 52 representing the time-varying degree of engine rotation through a complete engine cycle. As such, it will be recognized that the value of the COUNT signal will be functionally related to the frequencies of sinusoidal noise components generated by the engine. In general, the number of teeth on wheel 42, the frequency multiplier/divider, and the modulo counter are selected to provide an integer count ranging in value from 0 to a maximum value of MAX each time the engine completes a cycle, a complete cycle in a four-stroke engine being two full revolutions of the engine crankshaft. The value of COUNT then represents the time-varying angular rotational position of the engine in an operating cycle or the fractional portion of an engine cycle completed at any given time (the cycle position). Based upon the value of the COUNT reference input signal, the digital signal processor 52 is able to generate signals containing different sinusoidal components having frequencies which correspond to those of the sinusoidal noise components generated by the engine. In addition to the SPEED signal, the other analogue signals directed to the noise controller 26 are sampled at the rate established by digital signal processor 52 and digitized for further use within the digital signal processor 52. Sets of sample values for the digitized input signals are retained in the random access memory of digital signal processor 52 for use in computing sample values for digital output signals in accordance with the programmed adaptive filters in each channel. The computed digital output signal samples from digital signal processor 52 are then converted into analogue form and appropriately amplified to drive the channel cancellation actuators. With regard to analogue input signals directed to the electronic noise controller 26, the analogue throttle position signal TP from sensor 18 is first passed through amplifier 152 and then converted into a digital input signal TP(n) for the digital signal processor 52 by the action of sample and hold circuit 154 and analogue-to-digital converter 156. TP(n) then represents the nth or most recent digitized sample value for the analogue throttle position signal TP, TP(n-1) represents the digitized sample value for TP obtained during the previous sampling period and, likewise, for earlier sample values of the throttle position signal. Although not required in this embodiment, the digitized throttle position signal TP is shown as an input to the digital signal processor 52 for completeness, since it provides an indication of engine loading and may be used to improve the performance of the noise controller, as described in EP-A-0,470,656, the disclosure in which is incorporated herein by reference. The analogue ERROR 2 signal developed by microphone sensor 34 is first amplified by a variable gain amplifier 158 and then passed through a bandpass filter 160 having, for example, a passband from approximately 20 to 700 Hz in this embodiment. Bandpass filter 160 acts as an anti-aliasing filter and removes any direct current from the amplified ERROR 2 signal. The filtered ERROR 2 signal is then fed to sample and hold circuit 162, which acts in conjunction with analogue-to-digital converter 164 to provide a digitized sample ER(n) of the analogue ERROR 2 signal to the digital signal processor 52, where, as stated previously, n represents the nth or most recently sampled value. Based upon the value of the digitized ER(n) sample, the digital signal processor 52 supplies a digital GAIN signal to digital-to-analogue converter 166, which in turn controls the gain of amplifier 158 to maintain the amplitude of the amplified analogue ERROR 2 signal within upper and lower limits determined by the input capability of sample and hold circuit 162 and analogue-to-digital converter 164. This form of automatic gain control is well known in the art and is commonly used in digital signal processor and microprocessor interfacing circuitry when digitizing an analogue signal having an amplitude which can vary over a large dynamic range, such as the ERROR2 signal in the present embodiment. Sequential digital sample values for an output noise cancelling signal ..., Y T (n-2), Y T (n-1), and Y T (n) are computed by the digital signal processor 52 in accordance with the above described input signals and the characteristics of the adaptive filters programmed into the digital signal processor 52 for the second channel. These digital output samples are directed to digital-to-analogue converter 168, where a corresponding analogue waveform is produced. The analogue waveform is then passed through lowpass filter 170, which has an upper cutoff frequency of approximately 700 Hz in this embodiment. The lowpass filter acts as a smoothing filter to remove any high frequency components introduced by the digital-to-analogue conversion process. Next, the filtered analogue waveform is amplified by power amplifier 172 to produce the final output noise cancelling waveform designated as OUTPUT 2 . The OUTPUT 2 signal drives the cancellation actuator (speaker) 32 to produce the noise cancelling waves which are superimposed with and attenuate the undesirable engine exhaust noise. Depending upon the amplitude of the noise being attenuated, it may be desirable to prevent the amplitude of the noise cancelling waveform from becoming saturated or clipped at an upper limit related to the physical size of the cancellation actuator. An approach which may be used to prevent such clipping or saturation of the output noise cancelling waveform is described in EP-A-0,492,680 and EP-A-0,470,656, the disclosures in which are incorporated herein by reference. Referring now to Figure 3, there is shown a mathematical model for a generalized parallel configuration of signal generator and adaptive filter pairs which represents signal processing steps programmed into and carried out by the digital signal processor 52 for the second channel of the noise controller 26. It will be recognized that the other channels of noise controller 26 can be programmed to have similar configurations and signal processing steps. In general, the parallel configuration of Figure 3 is shown to include a total of J signal generators SG 1 , SG 2 ,... and SG J , designated by the respective numerals 200, 202 and 204, and J correspondingly paired adaptive filters AF 1 , AF 2 ,... and AF J , designated respectively as 206, 208 and 210. Based upon the value of the common input reference signal COUNT, each signal generator SG j synthesizes a sampled output signal X j (n), which is then used as input for the corresponding paired adaptive filter AF j , where j = 1, 2,... and J. Each adaptive filter AF j operates on its respective sampled input signal X j (n) to produce a sampled digital filter output signal Y j (n). All of the sampled filter output signals Y j (n) from the adaptive filters AF j are directed to a signal summer 211, where they are added together to produce digital samples for the final output noise cancelling signal Y T (n) generated by the digital signal processor 52. Each adaptive filter AF j is also provided with digital sample values ER(n) representing the second channel feedback error signal ERROR 2 (see Figure 2), so that the characteristics of the J adaptive filters can be adapted to reduce the magnitude of the ERROR 2 signal and the corresponding level of residual exhaust noise 22 propagating from the engine 10 (see Figure 1). The operation of each of the signal generators will now be described in terms of a jth signal generator SG j , where j can have the values of j = 1, 2,... or J. In general, the jth generator SG j produces a value for its sampled output signal X j (n) based on the current value of the COUNT reference input signal in accordance with the following equation: where the B mj term represents the amplitude of the mth sinusoidal signal component in a summation that can contain a total of M such components, and q is a conversion constant given by q = 2π/(MAX+1), where MAX is the maximum count reached by the COUNT signal from the modulo counter 150, before it resets to zero to restart counting. It will be recognized that one or more sinusoidal signal components can be removed from the above summation by setting their respective amplitudes to zero, that is the mth sinusoidal component can be removed from the signal X j (n) produced by the jth signal generator SG j by merely setting B mj = 0 to eliminate any contribution from that component. It will also be recognized that since COUNT ranges in value from 0 to MAX for each complete engine cycle (two revolutions in a four stroke engine), the frequencies of the sinusoidal components in above equation (1) will be functionally related to and synchronized with the rotational frequency of the engine (in revolutions per second). For example, if the engine rotates at a frequency of f 0 revolutions per second (engine RPM/60), then the frequency of the mth sinusoidal component in the summation of equation (1) is given by f m = mf 0 /2. Consequently, by setting the appropriate amplitude terms B mj equal to zero, the sampled generator output signal X j (n) produced by the jth signal generator SG j can be easily adjusted to contain sinusoidal components having frequencies at selected even, odd and/or half-order multiples of the fundamental frequency of rotation of the engine f 0 . In what follows, it will be understood that when a set of sequential sample values for a digitized signal, such as the output signal for the jth signal generator X j (n-N j +1),..., X j (n-2), X j (n-1) and X j (n), are retained in the random access memory of the digital signal processor 52 prior to storing a newly generated sample value in the memory location corresponding to the nth sample value X j (n), the previous stored value for X j (n) will be shifted downwardly and stored as the next earlier sample value X j (n-1) and, likewise, for all earlier sample values down to the last retained sample in the sequence X j (n-N j +1). The preferred form for each of the adaptive filters in Figure 3 will now be described in terms of a general model representing the jth adaptive filter AF j as shown in Figure 4, which is commonly known as the filtered-X least-mean-square (LMS) configuration. The components within each adaptive filter AF j include a transversal digital filter A j 212 which filters or transforms the filter input signal, in this case the sampled signal X j (n) synthesized by the jth signal generator SG j , to produce a sampled filter output signal Y j (n) according to the equation: where the set of W ij (n) terms represents the most recently computed adaptive filter weighting coefficients for the transversal filter A j 212, and N j represents the number of taps or size of transversal filter A j , and also the number of samples of the jth signal generator output X j (n) retained in memory for computing the current sample value for the filter output signal Y j (n). In general, the transversal filter A j 212 in each adaptive filter AF j can have a different number of taps N j , however, the number of taps for a particular filter should be equal to at least twice the number of sinusoidal signal components selected to be synthesized by that filter's corresponding signal generator SG j so that the transversal filter will be capable of forming a separate passband for each synthesized sinusoidal signal component. After each new sample value for the filter output signal Y j (n) is computed according to equation (2), the weighting coefficients W ij (n) for each transversal filter A j are updated, as indicated by the UPDATE A j block 214, to minimize the value of the sampled error signal ER(n). This updating is preferably accomplished using the leaky least-mean-square (LMS) algorithm, although any other known algorithm for adapting filter weighting coefficients to minimize the error signal could be used. The UPDATE A j block has two inputs, the first being the current sample value for the error signal ER(n), and the second being a filtered sequence of sample values designated as FX j (n), which are derived by passing samples of the filter input signal (here the jth signal generator output X j (n)) through an auxiliary compensation E filter 216. This auxiliary filtering process gives rise to the filtered-X nomenclature associated with this type of adaptive filter. The preferred leaky LMS algorithm for updating each of the filter weighting coefficients from the present value W ij (n) to the value W ij (n+1) for use during the next sampling interval is given by: W ij (n+1) = g j W ij (n) - µ j ER(n) FX j (n) , where g j is known as the filter leakage factor, generally having a value in the range of 0 << g j < 1, and µ j is known as the filter convergence factor, generally having a value in the range of 0 < µ j << 1. The convergence factor µ j is related to the rate at which the filter output signal samples represented by Y j (n) converge to values which minimize the sampled error signal ER(n). The leakage factor g j prevents the accumulation of digital quantization error that typically occurs when using a digital signal processor having fixed point arithmetic capabilities, such as the Motorola 56000 DSP. In applications where the frequency and amplitude of the noise are stationary or slowly varying with respect to time, g j and µ j are conventionally fixed at constant values. The sequence of sample values for the FX j (n) signal required by equation (4) are obtained by filtering the sequence of sample output values X j (n) from the jth signal generator SG j with an auxiliary compensation E filter 216 according to the following equation: where the E i terms for i = 0, 1,..., and N E -1 represents the fixed weighting coefficients for the transversal compensation E filter 216, with N E representing the number of E filter taps. As described in "An Analysis of Multiple Correlation Cancellation Loops with a Filter in the Auxiliary Path", D. R. Morgan, IEEE Transactions on Acoustic Speech Signal Processing, Vol. ASSP-28, No. 4, 1980, pp.454-467, a compensation filter such as the E filter 216 is typically used to compensate for the delay and distortion introduced by components in the error path of the active noise control (ANC) system. This error path typically includes the channel cancellation actuator and the associated output circuitry within noise controller 26; the error sensor and the associated error input circuitry within noise controller 26; and the characteristics of the physical path between the channel cancellation actuator and error sensor, over which the engine noise propagates. Referring now to Figure 5, there is shown a schematic diagram representing a process which can be used for off-line calibration of each compensation E filter 216. In this process, the E filter is calibrated (that is, its weighting coefficients are adjusted) to have a transfer function equivalent to the combined electrical components in the error path of the second channel between the the digital signal processor (DSP) digital output signal Y T (n) and digital input error signal ER(n). When calibrating the E filter, the noise cancellation actuator 32 and the error sensor 34 are made to remain at the same locations as when they are used for attenuating noise, to assure that the characteristics of the propagation path between the cancellation actuator 32 and error sensor 34 remain constant. During the calibration process, a conventional random noise generator 218 is used to generate a sequence of random signal values designated as IN(n). The random signal samples are directed as an input to the auxiliary compensation E filter 216, and are also passed through the components of the error path to produce a corresponding sequence of samples designated as D(n). In passing over the error path, the IN(n) samples are subjected to the same electronic components as are output Y T (R) samples and the resulting ER(n) samples of the second channel 46 of the noise controller 26 of Figure 2. For the calibration configuration shown in Figure 5, the expression associated with operation of the digital compensation E filter 216 and the algorithm associated with its UPDATE E adaptation block 220 are given by: E i (n+1) = g E E i (n) + µ E ERR(n) IN(n) , where g E and µ E are set at approximately g E = 0.99998 and µ E = 0.001, OUT(n) represents sample values output by the digital E filter 216, and the ERR(n) samples are produced as output from signal summer 222 and are given by: ERR(n) = D(n) - OUT(n), where D(n) represents the sample values derived from the channel error sensor 34. With this off-line calibration process, the weighting coefficients of the digital E filter 216 are adaptively updated to minimize the ERR(n) values. The adaptive modelling procedure is complete when the variable weighting coefficients E i (n) sufficiently converge to fixed values. These fixed values then correspond to the fixed weighting coefficients E i used in implementing the E filter 216. When the adaptive modelling procedure is complete, the transfer function of the digital E filter 216 duplicates that of the combined components in the channel error path and it is used for filtering the sampled X j (n) signal to produce filtered samples for the FX j (n) signal input to the UPDATE A j block 214 in Figure 4. Filtering the samples of the X j (n) signal in this manner compensates for distortion and delay introduced by the error path components and improves the stability and rate of convergence of the adaptive filter AF j . As indicated in Figure 4, the same E filter 216 is preferably used with each adaptive filter AF j , with j= 1, 2,...J, since it provides a broad band characterization of the error path over a range in frequency which embraces the expected variations in frequency of the engine noise components. Also, use of the same compensation E filter for each adaptive filter AF j provides advantages in that the implementation is simplified and RAM storage space is conserved; however, different E filters could be used with each adaptive filter if desirable or advantageous for the particular application. When employed in active noise control (ANC) systems, the above-described parallel configuration of signal generators SG j and paired adaptive digital filters AF j provides distinct advantages over conventional adaptive filtering approaches, particularly when the difference in frequency between adjacent noise components (that is, those noise components immediately preceding or following each other in the frequency domain) become relatively small. First, the required number of parallel signal generator and adaptive filter pairs in the configuration can be less than the total number of noise frequency components being attenuated, since each signal generator is capable of producing or synthesizing more than one of the sinusoidal signal components corresponding in frequency with the noise components. More importantly, each signal generator in the parallel configuration can be programmed to produce a set of sinusoidal signal components having frequencies corresponding to non-adjacent noise frequency components. This aspect is significant because the difference in frequency between successive sinusoidal signal components filtered by any one adaptive filter is effectively increased, which enables a reduction in the number of filter taps without adversely affecting filtering performance. As will be recognized by those skilled in the art, the filter convergence factor can be enlarged to increase the rate of convergence as the number of filter taps decreases without degrading filter stability. Also, as the number of filter taps is reduced, quantities known as the LMS algorithm excess means square error and computational round off error decrease to improve filter accuracy. Consequently, significant improvements in the filter accuracy and the rate of response to fluctuation in noise amplitude, frequency and phase can be realized by employing the above-described parallel configuration of signal generators and correspondingly paired adaptive filters in active noise control (ANC) systems in the above-described manner. Referring now to Figure 6, there is shown a block diagram for a model representing signal processing steps programmed into the digital signal processor 52 of the electronic noise controller 26 (see Figure 2) for attenuating engine generated exhaust noise. The model includes a filter controller, which operates in conjunction with the above described configuration of parallel signal generator and adaptive filter pairs. For this particular application and type of noise, it has been found that three parallel signal generator and adaptive filter pairs (that is, J = 3) are sufficient to achieve an acceptable degree of attenuation of the engine exhaust noise. It has also been found that each transversal filter A j and each compensation filter E provides satisfactory performance with sixty-four taps, that is N j = N E = 64; however, it will be understood that a larger or smaller number of filter taps could be used for any of the filters depending upon the required degree of noise attenuation and any limitations on the amount of memory available for storage in the digital signal processor 52. In selecting the frequency make-up of the set of sinusoidal signal components synthesized by any one signal generator, the measured variations in amplitude and frequency of the exhaust noise components with changes in engine speed was considered in conjunction with separating those sinusoidal signal components which correspond to exhaust noise components having adjacent frequencies. The following predominant characteristics of the engine exhaust noise were identified during the operation of engine 10, which for the present embodiment was a 4.9 litre, 8 cylinder engine: (1) as the engine was operated over its range of possible rotational speeds, the dominant noise components in the exhaust noise (those having significant amplitudes) were found to have the frequencies of mf 0 /2, where m has integer values ranging from 2 to 16 and f 0 is the fundamental frequency of rotation of the engine in Hz (that is, revolutions per second or RPM/60); (2) the exhaust noise component having a frequency of 4f 0 was found to have a significant amplitude at rotational speeds between 600 and 2200 RPM; (3) engine noise components having frequencies of f 0 , 2f 0 , 3f 0 , 5f 0 , 6f 0 , 7f 0 and 8f 0 were found to have significant amplitudes when the engine was operated above 1400 RPM; and (4) engine noise components having frequencies of 3f 0 /2, 5f 0 /2, 7f 0 /2, 9f 0 /2, 11f 0 /2, 13f 0 /2 and 15f 0 /2 were found to have significant amplitudes when the engine was operated above 2500 RPM. In addition to identifying the above characteristics, it was also found that the amplitude and frequency of the exhaust noise components can vary quite rapidly in response to abrupt changes in engine rotational speed during rapid acceleration or deceleration of the engine. As is explained below, additional improvements in performance of the active noise control system can be realized by programming each signal generator to synthesize a distinct set of signal components having frequencies corresponding to those noise components that behave in the same manner over a particular range of engine rotational speeds. As will be explained, additional improvements in the performance of the active noise control system can be realized by programming each signal generator to synthesize sinusoidal signal components corresponding in frequency with engine noise components having similar amplitude behavior at different engine rotational speeds. In the block diagram model illustrated in Figure 6, signal generator SG 1 was programmed to synthesize the sinusoidal signal component having a frequency corresponding to a noise component frequency of 4f 0 . This was done by assigning the value of zero to all B m1 amplitude terms in equation (1) for the SG 1 signal generator, except for the B 81 amplitude term associated with the sinusoidal signal component having the frequency of 4f 0 . The signal generator SG 2 was programmed to synthesize sinusoidal signal components corresponding to engine noise components having the non-adjacent frequencies of f 0 , 2f 0 , 3f 0 , 5f 0 , 6f 0 , 7f 0 and 8f 0 that were found to have significant amplitudes when the engine 10 was operated above 1400 RPM. This was accomplished by assigning values of zero to those amplitude terms B m2 corresponding to frequencies not produced by the second signal generator SG 2 , that is B m2 = 0 for m = 1, 3, 5, 7, 8, 9, 11, 13 and 15. The third signal generator SG 3 was programmed to synthesize sinusoidal signal components corresponding to engine noise components having the non-adjacent frequencies of 3f 0 /2, 5f 0 /2, 7f 0 /2, 9f 0 /2, 11f 0 /2, 13f 0 /2 and 15f 0 /2 that were found to have significant amplitudes when the engine was operated above 2500 RPM. Again, this was accomplished by assigning values of zero to those amplitude terms B m3 corresponding to those frequencies not produced by the third signal generator SG 3 , that is B m3 = 0 for m = 2, 4, 6, 8, 10, 12, 14 and 16. Those amplitude terms B mj corresponding to the frequencies of sinusoidal components that are produced by the signal generators can be assigned values of 1.0 or they can be assigned relative values obtained by averaging the measured amplitudes of the corresponding engine noise frequency components over the range of possible engine rotational speeds. The above manner of partitioning not only separates the sinusoidal components so that no one signal generator is required to synthesize any sinusoidal components which correspond to engine noise components having adjacent frequencies but it also enables the implementation of a control strategy, whereby the operation of the configuration of parallel signal generator and adaptive filter pairs can be regulated to compensate for the rapid variations in the amplitude and frequency of engine generated noise components when engine rotational speed changes. As indicated in Figure 6, the control of each signal generator and paired adaptive filter in the parallel configuration is carried out by a filter controller 250 and three signal multipliers 252, 254 and 256, each being interposed between one of the signal generator outputs and the input of the corresponding paired adaptive filter. During each new sampling interval, filter controller 250 develops sets of digital values for the output control signals S j , U j and G j which are directed to control the operation of each signal generator SG j and corresponding paired adaptive filter AF j (where j = 1, 2 and 3). These output control signals are derived in the filter controller 250 based upon information provided by digital COUNT signal. The control signals S j are fed to the respective signal multipliers 252, 254 and 256 to effect amplitude scaling of each signal generator output signal X j (n), thereby producing a new amplitude scaled input signal S j X j (n) for each adaptive filter AF j . It will be understood that the above-described equations (2) and (4) will continue to apply, except that each occurrence of the expression X j (n) will be replaced by its corresponding amplitude scaled counterpart S j X j (n). The control signals U j and G j are fed directly to each of the respective adaptive filters AF j to modify the updating or adaptation process. This is accomplished by adjusting or scaling the values of the leakage factor g j and convergence factor µ j in the algorithm used to update each adaptive filter, based upon the current values of the control signals U j and G j . Consequently, the leakage and convergence factors are allowed to have variable values according to the following expressions: g j = G j g 0 , and µ j = U j µ 0j , where g 0 and µ 0j are set at constant values and the control signals G j and U j effectuate a scaling of these constant values to provide the scaled filter adaptation factors g j and µ j . For example, g 0 = 0.99998, µ 01 = 0.001 and µ 02 = µ 03 = 0.01, provided stable filter operation in the present embodiment, but it will be recognized that the values selected for the µ 0j terms will differ for different applications depending upon factors such as the average power contained in the filter input signals, the number of adaptive filter taps and the electrical characteristics of hardware components such as the actuators and transducers outside the digital signal processor. The values for the control signals S j , U j and G j are determined by filter controller 250 as a function of the engine rotational speed in RPM and/or the engine rotational acceleration expressed in RPM/second (or RPM/s). Engine rotational speed for the current sampling interval can be computed by filter controller 250 using the expression: SP(n) = K1 (COUNT - COUNT -1 ) , where K1 is a scaling constant, COUNT represents the value of the modulo counter 150 during the current sampling interval and COUNT -1 represents the stored value of the modulo counter during the previous sampling interval. Although equation (10) provides an acceptable estimate for the actual engine rotational speed, it has been found that a more accurate representation is obtained by averaging a few of the most recently computed sample values for SP(n). Consequently, the preferred expression for computing engine rotational speed for the current or nth sampling interval is given by: where N represents the number of samples of SP(n) which are averaged, for example, N = 32 in the present embodiment. The rotational acceleration of the engine for the current sampling interval is computed by use of the expression: ACCEL(n) = K2 (SPEED(n) - SPEED(n-1)) , where K2 represents an additional scaling constant, SPEED(n) represents the rotational speed of the engine for the current sampling interval computed in accordance with equation (10) and SPEED(n-1) represents the stored rotational speed of the engine found during the previous sampling interval. It will be recognized that the scaling constants K1 and K2 are selected so that equations (10), (11) and (12) provide the correct values for the engine rotational speed and acceleration for the sampling interval associated with the sampling rate established by the digital signal processor 52. Referring now to Figures 7(A)-(C), values for the output control signals S j , U j and G j provided by filter controller 250 are illustrated in graphical form as a function of the computed engine SPEED(n) and/or ACCEL(n), with j = 1, 2 and 3 as used in the present embodiment. The values for each control signals S j shown in Figure 7A were selected to correlate with the behaviour of the engine noise frequency components which correspond in frequency with the sinusoidal signal components synthesized by each respective signal generator SG j . For example, the engine noise component having a frequency of 4f 0 was generally found to have a significant amplitude when the engine was operated at rotational speeds in the range from 600 to 2200 RPM. Since the control signal S 1 is used to scale the amplitude of the output sinusoidal signal from signal generator SG 1 , which corresponds to this engine noise component at the frequency of 4f 0 , S 1 is given a value of 1.0 for rotational speeds in the range from 600 to 2200 RPM and a value of zero outside this range, except for narrow transitional bands of a few hundred RPM on either side of the 600-2200 RPM range to prevent S 1 from shifting too abruptly between the values of zero and one. Likewise, the values for the control signals S 2 and S 3 in Figure 7A are selected to follow the behavior of the engine noise components corresponding in frequency with the sinusoidal signal components being synthesized by the signal generators SG2 and SG3, respectively. This amplitude scaling of the output signals synthesized by the signal generators based upon the rotational speed of the engine was found to improve significantly the ability of the active noise control system to attenuate engine noise components having substantial changes in amplitude with variations in engine rotational speed. This is because the scaled amplitudes of the synthesized sinusoidal signal components entering the adaptive filters from the signal generators now more closely match the amplitude behavior of the actual noise components generated by the engine as it operates at different rotational speeds. As a consequence, the adaptive filters require less time to converge or adapt the filter output signals to achieve a satisfactory attenuation of the engine generated noise. When engine 10 is operating at rotational speeds above 1200 RPM and the engine rotation is rapidly decelerated at rates ≥ 1000 RPM/second to speeds below 1200 RPM (for example, during closed throttle, heavy vehicle braking), it has been found that the amplitudes of engine noise components having frequencies other than 4f 0 approach zero very rapidly (in less than one second). Conventional adaptive filters can not respond quickly enough to remove the filter passbands corresponding to these frequencies and the filter output signals Y 2 (n) and Y 3 (n) will contain signal components having significant amplitudes at these passband frequencies. This will occur even if the actual input signals directed to adaptive filters AF 1 and AF 2 are scaled to zero by control signals S 1 and S 2 as described above, due to the existence of noise which passes through the adaptive filters at the passband frequencies. As a result, the observed noise level 22 will actually increase due to the presence of these unnecessary signal components in the cancelling waves, since the corresponding engine noise components will have disappeared. It was recognized that the values of the leakage factor g j used in the least-mean-square (LMS) filter adaptation algorithm (see equation (3)) determine the degree to which values of the updated filter weighting coefficients W ij (n+1) are influenced by past values of the weighting coefficients W ij (n) determined during the previous sampling interval. The larger the value of the leakage factor g j , the larger will be the contribution from the past value of a filter weighting coefficient in determining its new updated value and the more prone the filter is to retaining or remembering its past filtering characteristics. Consequently, by decreasing the leakage factor of adaptive filters AF 2 and AF 3 during periods of rapid engine deceleration where engine rotational speed drops below 1200 RPM, it was found that the filter passbands associated with the disappearing engine noise components can be eliminated more quickly during the adaptation process. As indicated in equation (8) and Figure 6, the adaptive filter leakage factors in the present embodiment are modified by scaling their values in accordance with the control signals G j produced by the filter controller 250. During normal engine operation, all of the control signals G j have values of 1.0. However, when filter controller 250 computes an ACCEL(n) ≤ -1000 RPM/s, the two control signals G 2 and G 3 are determined as a function of the computed engine SPEED(n) as shown in Figure 7B. The value of G 1 remains equal to 1.0, since the engine noise component at the frequency 4f 0 does not disappear during rapid engine deceleration and, consequently, adaptive filter AF 1 must retain a passband at the frequency 4f 0 . By determining the values of G 2 and G 3 in this fashion and scaling the leakage factors of adaptive filters AF 2 and AF 3 according to equation (8), it has been found that the adaptive filters can be made to respond more quickly to the disappearance of engine noise components during periods of rapid engine deceleration, thereby significantly enhancing the performance of the active noise control system. It will also be recognized that the above approach can easily be adapted to scale adaptive filter leakage factors in situations where engine noise components are found to disappear quickly with rapid engine acceleration rather than deceleration, as described above. In addition to the above-described amplitude fluctuations the engine noise components and hence the sinusoidal signal components synthesized by the signal generators, have frequencies which shift in response to variations in engine rotational speeds during abrupt engine acceleration and deceleration. Conventional adaptive filters having fixed convergence factors will not generally be able accurately to track filter input signal components which have rapid frequency fluctuations. This is because filter convergence factors are conventionally fixed at relatively small values, for example µ = 0.01, to reduce the magnitude of system residual error (the residual noise level represented by the error signal ER(n) in the present embodiment). If the filter convergence factor is increased, for example to µ = 0.03, the ability of the adaptive filter to track rapid fluctuations in the frequency of components in the filter input signal will be enhanced but the magnitude of the system residual error will increase for constant engine speed operation. It was recognized that the above difficulties could be overcome by providing each adaptive filter with a variable rather than a fixed convergence factor. Since the rate at which signal component frequencies vary depends directly upon the rate of change of the engine rotational speed, that is the engine rotational acceleration, the values of the filter convergence factors are made to depend upon the magnitude of the engine rotational acceleration. This is accomplished in the embodiment illustrated in Figure 6 by the filter controller 250, which generates control signals U j for scaling the convergence factors of the adaptive filters AF j . Referring now to Figure 7C, values for the control signals U 1 , U 2 and U 3 are illustrated in graphical form as a function of the engine acceleration ACCEL(n) computed by the filter controller 250 based upon equation (12). For the present embodiment, all of the U j control signals were made to vary in the same way with changes in ACCEL(n), however, it will be understood that the control signal for each adaptive filter can be made to vary differently with changing engine acceleration and that the values presented in Figure 7C are merely exemplary for the chosen exhaust noise application. When the engine operates at constant rotational speeds (ACCEL(n) = 0), the frequencies of the engine noise components remain stationary, so it is generally more important to assure that system residual error is a small as possible, rather than to achieve a rapid rate of filter convergence. On the other hand, when the engine is accelerating or decelerating, the frequencies of the engine noise components change rapidly and it is more important to assure that the filter has a rapid rate of convergence. It will be noted from equation (9) that the adaptive filter convergence factors in the present embodiment are given by µ j = U j µ 0j , where the control signals U j effect a scaling of the corresponding fixed values for µ 0j , which in the present embodiment are given by µ 01 = 0.001 and µ 02 = µ 03 = 0.01. In view of the foregoing, the U j control signals are given values of 1.0 (see Figure 7C) to provide the adaptive filters with relatively small values for the convergence factors when the rotational speed of the engine is stationary (ACCEL(n) = 0). For rapid engine acceleration or deceleration where ACCEL(n) ≥ 1000 RPM/s or ACCEL(n) ≤ -1000 RPM/s, the control signals U j are given values of 3.0 to provide the adaptive filters with relatively larger convergence factors and increased rates of convergence. As shown, the values of the control signals U j increase linearly from 1.0 to 3.0 as engine acceleration or deceleration increases from zero up to 1000 RPM/s. Scaling of the adaptive filter convergence factors so that they vary in value as a function of engine acceleration in the above-described fashion significantly enhances the ability of each adaptive filter to track frequency fluctuations in filter input signal components caused by variations in engine rotational speed. The control signals S j , G j and U j in Figure 7A-C were shown to vary as piecewise linear functions of either engine speed or acceleration. It will be recognized that these representations for the control signals were merely exemplary and that other linear or non-linear representations could just as easily be used. It will also be understood that different forms of noise and/or noise generated by different types of engines will generally have characteristics which differ from those described above but the control techniques can still be applied to these applications by selecting the appropriate values for the control signals based upon the principles set forth above. Referring now to Figure 8, there is shown a flow chart representative of the program steps executed by the digital signal processor 52 in performing the parallel signal generating, adaptive filtering and in controlling functions indicated by the block diagram of Figure 6. The active noise control (ANC) routine is entered at point 300, after each system interrupt associated with the sampling rate of the digital signal processor 52. It will be understood that prior to the first pass through the ANC routine, the appropriate variables, timers, counters and so on will have been initialized to the proper starting values. From point 300, the program proceeds to step 302, where the values of the input signal COUNT and digitized error signal ER(n) are read. Next, at step 304, values for the current engine speed SPEED(n) and acceleration ACCEL(n) are computed according to equations (11) and (12) described previously. The routine then proceeds to step 306 where values for the control signals S j , G j and U j for j = 1, 2 and 3 are determined based upon the computed values for the engine speed SPEED(n) and/or acceleration ACCEL(n) found at step 304. This is accomplished either by computing values for the control signals based upon stored sets of coefficients for empirical equations representing the piecewise linear functional relationships shown in Figure 7A-C or by providing stored look-up tables containing values for the control signals based upon engine speed and/or acceleration, as indicated in Figure 7A-C. It will be noted that the values of the control signals G 2 and G 3 are less than 1.0 and vary with engine speed as shown in Figure 7B, only when the computed engine acceleration ACCEL(n) ≤ -1000 RPM/s. Consequently, the control signals G 1 , G 2 and G 3 are given the values of 1.0 and the routine reduces these values for G 2 and G 3 in accordance with SPEED(n) as shown in Figure 7B, only after first determining that the computed ACCEL(n) ≤ -1000 RPM/s. Once the appropriate values for the control signals are either computed or looked up at step 306, the routine proceeds to step 308 where the sampled amplitude value X j (n) for each generator output signal (j = 1, 2 and 3) is derived (or looked up) in a respective predetermined schedule of stored values based upon the current value of the reference input signal COUNT found previously at step 302. The stored values for each signal generator schedule are determined from equation (1) by summing the appropriate sinusoidal terms selected to be synthesized by each signal generator over the range of different values for COUNT. It will be recalled that the particular sinusoidal signal components synthesized by each signal generator in the present embodiment were selected as described above. After determining a set of values for a particular generator in this fashion, the values are usually normalized to range between -1 and 1 prior to storage in their respective look-up schedule. This is achieved by dividing each value in the set by the magnitude of the value found to be largest in the set. After completing step 308, the routine passes to step 310 where the amplitude of each signal generator output signal is scaled. This is accomplished by multiplying the digital amplitude values produced by each signal generator, represented here as X j (n) OLD , by the value for the corresponding control signal S j determined at step 306. The product S j X j (n) OLD then replaces the previous value for X j (n) stored in memory. Next, at step 312, the filtered output signal Y j (n) for each adaptive filter AF j (j = 1, 2, and 3) is computed according to equation (2), with the filter input signal being the sequence of digital values stored in the designated memory locations for jth signal generator output X j (n-i), where in the present embodiment i = 0, 1, ... 63, since the adaptive filters have sixty-four taps. Note that each value in the stored sequence represented by X j (n-i) will have been amplitude-scaled due to step 310. At step 314, the leakage and convergence factors g j and µ j , employed in the adaptation algorithm of each adaptive filter, are modified by scaling their values according to equations (8) and (9) using the values of the control signals G j and U j found previously at step 306. Then, at step 316, updated values for the weighting coefficients W ij (n+1) for each adaptive filter AF j (for j = 1, 2 and 3) are computed according to equation (3) using the modified values for the leakage and convergence factors g j and µ j found at step 314 above. These updated values for the filter weighting coefficients are the ones which will be used at step 312 during the next pass through the routine (next sampling interval) to compute new values for the digital filter output signals. From step 316, the routine proceeds to step 318, where the individual values for the filter output signals Y j are summed or added together to produce a digital sample for the final output noise cancelling signal Y T (n). After completing step 318, the routine is exited at step 320. It will be understood that the above ANC ROUTINE is repeatedly executed by the digital signal processor 52 after each sampling interrupt to generate successive sample values for the digital noise cancelling signal Y T (n) for output by the digital signal processor 52. As described above, the ANC ROUTINE implements the particular configuration shown in Figure 6, which comprises three signal generators and their corresponding paired adaptive filters connected in parallel to the signal summer, with the operation of each signal generator and adaptive filter under the direct control of the filter controller. It will be understood that the flow chart for the ANC ROUTINE is equally applicable when implementing the general parallel signal generator and paired adaptive filter configuration of Figure 3, which does not include the filter controller or its scaling functions. In this case, the integer variable j would be allowed to have integer values ranging from 1 to J, the leakage and convergence factors for each adaptive filter would be assigned fixed values and steps 304, 306, 310 and 314 would be removed from the ANC ROUTINE. Thus, those advantages provided by the parallel filtering configuration and the separation of sinusoidal components having frequencies corresponding to those of adjacent noise frequencies for synthesis by different signal generators can be realized without implementing the filter controller functions. It will also be recognized that the filter controller 250 provides three different types of control signals S j , G j and U j to each signal generator and corresponding paired adaptive filter in the parallel configuration shown in Figure 6. Each type of control signal performs a separate and distinct function which can provide advantages or benefits different and apart from those provided by the other types of control signals. Consequently, it will be understood that each control function can be implemented either individually or in combination with any of the other control functions in the ANC ROUTINE by simply removing or adding the appropriate scaling operations at steps 310 and 314. It will also be understood that these control functions can be applied to a single signal generator and its correspondingly paired adaptive filter, by setting j = 1 or to any number of parallel signal generators and correspondingly paired adaptive filters, merely by permitting j to have larger integer values. In the foregoing description, the preferred form for each adaptive filter in the active noise control system has been the filtered-X configuration, with the adaptation or updating of filter weighting coefficients achieved by use of the leaky least-mean-square algorithm. Other types of adaptive filter configurations and updating algorithms are known and used by those skilled in the art of active noise control, and it will be recognized that the principles underlying the above embodiments will be equally applicable to other types of active noise control systems having different adaptive filter configurations and updating algorithms. For example, the equivalent leakage and/or convergence factors in any type of filter updating algorithm can be scaled in accordance with changes in engine rotational speed and/or acceleration to improve the adaptive filter response to such changes. Also, the amplitude of any input signal representing the engine noise to be cancelled by an active noise control system can be scaled as a function of engine rotational speed to make the input signal more representative of the behaviour of the amplitude of the noise as engine rotational speed changes.

Claims: A noise attenuation system for attenuating noise generated by an internal combustion engine, which noise contains at least one sinusoidal noise component having an amplitude and frequency varying in dependence upon engine rotational speed, the system comprising signal generating means (42-150) for generating an input signal containing at least one sinusoidal signal component having a frequency substantially corresponding to the frequency of a respective sinusoidal noise component; acceleration sensing means (250) for sensing engine rotational acceleration; at least one adaptive filter (206-210) having adjustable filtering characteristics for filtering the input signal to produce a filter output signal; actuator means (28,32,36) for generating noise cancelling waves in response to the filter output signal and for superimposing the generated noise cancelling waves on the noise; residual noise sensing means (30,34,38) for sensing a residual noise level following superposition of the noise cancelling waves on the noise and for developing an error signal representative of the residual noise level; and adjusting means (52,206,208) for adjusting the filtering characteristics of the or each adaptive filter on the basis of the error signal and the sensed engine rotational acceleration to reduce the sensed residual noise level. A noise attenuation system according to claim 1, wherein the signal generating means (42-150) is adapted to produce a plurality of input signals, each containing at least one sinusoidal signal component having a frequency corresponding to a respective one of the sinusoidal noise components; the system comprising a plurality of adaptive filters (206-210), each having adjustable filtering characteristics for filtering a respective input signal to produce a corresponding filter output signal; and summing means (211) for adding each filter output signal to produce the output cancelling signal, the actuator means being responsive to the output cancelling signal, the adjusting means being adapted to adjust the filtering characteristics of each of the adaptive filters. A noise attenuation system according to claim 1 or 2, wherein the adjusting means is adapted to adjust the filtering characteristics of the or each adaptive filter on the basis of an algorithm having a convergence factor, and to scale the value of the convergence factor as a function of the sensed engine rotational acceleration. A noise attenuation system according to any preceding claim, comprising speed sensing means (52,250) for sensing engine rotational speed, the adjusting means being adapted to adjust the filtering characteristics of the or each adaptive filter on the basis of the error signal and the sensed engine rotational speed and acceleration. A noise attenuation system according to claim 4, wherein the adjusting means is adapted to adjust the filtering characteristics of the or each adaptive filter on the basis of an algorithm having a leakage factor and to scale the value of the leakage factor as a function of the sensed engine rotational speed and acceleration. A noise attenuation system according to claim 4 or 5, wherein the adjusting means is adapted to adjust the filtering characteristics of the or each adaptive filter on the basis of an algorithm having a convergence factor and a leakage factor and to scale the value of the convergence factor as a function of the sensed engine rotational acceleration and the value of the leakage factor as a function of the sensed engine rotational speed and acceleration. A noise attenuation system according to claim 4, 5 or 6, wherein the sinusoidal signal component contained in the or each input signal is partitioned on the basis of the behaviour of the amplitudes of the or the corresponding sinusoidal noise component with changes in engine rotational speed.

Inventors: YUAN YI, YUAN, YI
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1993
Main classification: G10K11
IPC codes: G10K11, F01N1, F16L55