ACTIVE NOISE CONTROL OF AN ENCLOSURE WITH MULTIPLE TRANSDUCERS

Abstract: An active noise (''NOISE SOURCE'') control system for an enclosure having at least two transducers (''SECOND SOURCES'') and multiple channels in which there is interaction between the channels provides for attenuation of the desired regions within the enclosure.

Description: ACTIVE NOISE CONTROL OF AN ENCLOSURE WITH MULTIPLE TRANSDUCERS Baf.kcfrm.nr. of the Tr.vf.nt-jnn The present invention relates to the development of an improved arrangement for controlling repetitive phenomena cancellation in an arrangement wherein a plurality of residual repetitive phenomena sensors and a plurality of cancelling actuators are provided. The repetitive phenomena being cancelled in certain cases may be unwanted noise, with microphone sensors and loudspeaker as the repetitive phenomena sensors and cancelling actuators, respectively. The repetitive phenomena being cancelled in certain other cases may be unwanted physical vibrations, with vibration sensors and counter vibration actuators as the repetitive phenomena sensors and cancelling actuators, respectively. A time domain approach to the noise cancellation problem is presented in a paper by S. J. Elliott, I.M Strothers, and P.A. Nelson, "A Multiple Error LMS Algorithm and Its Application to the Active control of Sound and Vibration, " IEEE Transactions on Acoustics, Speech, and Signal Processing, VOL. ASSP-35, No. 10, October 1987, pp. 1423-1434. The approach taught in the above paper generates cancellation actuator signals by passing a single reference signal derived from the noise signal through Na FIR filters whose taps are adjusted by a modified version of the LMS algorithm. The assumption that the signals are sampled synchronously with the noise period is not required. In fact, the above approach does not assume that the noise signal has to be periodic in the first part of the paper. However, the above approach does assume that the matrix impulse responses relating the actuator and sensor signals is known. No suggestions on how to estimate the impulse responses are made. The frequency domain approach to the interpretation of the problem is presented as follows, as shown in Figure 5 herein which is a block diagram of the system: The system consists of a set of Na actuators driven by a controller that produces a signal C which is a Na x 1 column vector of complex numbers. A set of Ns sensors measures the sum of the actuator signals and undesired noise. The sensor output is the Ns x 1 residual vector R which at each harmonic has the form R = V + HC where V is a Ns x 1 column vector of noise components and H is the Ns x Na transfer function matrix between the actuators and sensors at the harmonic of interest. The problem addressed by the present invention is to choose the actuator signals to minimize the sum of the squared magnitudes of the residual components. Suppose that the actuator signals are currently set to the value C which is not necessarily optimum and that the optimum value is Copt = C + dC. The residual with Copt would be Ro = H (C + dC) + V = (HC + V) + H dC = R + H dC The problem is to find dC to minimize the sum squared residual Ro @ Ro where @ denotes conjugate transpose. An equivalent statement of the problem is: Find dC so that H dC is the least squares approximation to -R. This problem will be represented by the notation -R == H dC The solution to the least squares problem has been studied extensively. One approach is to set the derivatives of the sum squared error with respect to the real and imaginary parts of the components of dC equal to 0. This leads to the "normal equations" H @ H dC = -H @ R If the columns of H are linearly independent, the closed form solution for the required change in C is The present invention provides methods and arrangements for accommodating the interaction between the respective actuators and sensors without requiring a specific pairing of the sensors and actuators as in prior art single point cancellation techniques such as exemplified by U.S. Patent 4,473,906 to Warnaka r U.S. Patents 4,677,676 and 4,677,677 to Eriksson r and U.S. Patents 4,153,815, 4,417,098 and 4,490,841 to Chaplin. The present invention is also a departure from prior art techniques such as described in the above-mentioned Elliott et al. article and U.S. Patent 4,562,589 to Warnaka which handle interactions between multiple sensors and actuators by using time domain filters which to not provide means to cancel selected harmonics of a repetitive phenomena. Active noise control has been shown to be effective in reducing low frequency noise in applications such as mufflers, headsets, engine mounts, fans, etc. Adaptive control has proved to be an effective technique for the implementation of active noise attenuation. Most applications have focused on single channel ada. _ve control (one sensor and actuator pair) or multiple channels when the interactions between channels is negligible. Applications such as cabin quieting and active enclosures have made apparent the need for multiple channel control algorithms. In these applications beca- e of the complexity of the noise source, a single transduce- will not be able to provide attenuation at the required regions. Additionally, interactions on the multiple transducers can cause adaptive algorithms to become unstable unless the interactions are accounted for in the control process. University of Maryland has developed the MISACT algorithm. This invention considers the problem of controlling noise radiating from a enclosure with multiple openings and multiple transducers. Performance of the MISACT Algorithm will be shown both experimentally and using a simulation model. In the past active noise control has been used to reduce low frequency noise in applications such as mufflers, headsets, etc. These attempts were noted in the article by G. Eatwell, M.J. Burke, Kh. Eghtesadi and W. E. Gossman entitled "The Application of Active Cancellation to Vehicle Noise and Vibration" presented at the 1990 International Conference on Quiet Revolutions. The use of adaptive control has been shown to be effective in implementing active noise attenuation. Various papers on this phenomena have been presented including B Widrow, "Adaptive Noise Cancelling: Principles and Applications", Proceedings of the IEEE, 63(12), 1962- 1716, 1975 and, S.J. Elliott, I Stother and P. Nelson, "A Multiple Error LMS Algorithm and Its Application to the Control of Sound and Vibration", IEEE Transactions on Acoustics, Speech and Signal Processing, 35(10), 1423- 1434, 1987. The use of the MISACT algorithm [Multiple Interacting Actuators as Sensors] is discussed by Kh. Eghtesadi, M.P. McLoughlin and E.W. Ziegler, Jr., "Development of the Simulation Model of the Multiple Interacting Sensors and Actuators (MISACT) for an Active Control System", Proceedings of the Conference on Recent Advances in Active control of Sound and Vibration, 246-257, 1991. The algorithm is also discussed in co-pending Application, PCT/GB90/2021 which is hereby incorporated by reference herein. Others have devised single channel systems such as in U.S. Patent No. 4,989,252 to Toshiba in which the enclosure must have a dimensional correlation with itself and a standing wave of sound to be attenuated. However, these systems do not allow for "cross talk" between multiple units operating at the same frequency and allows for two or more holes to be located in the enclosure walls. The aforementioned Toshiba patent allows for only one hole. In the Toshiba system the limitation to one opening means that the opening must be relatively large to provide adequate airflow and therefore limiting the frequency range that can be cancelled. The Toshiba system also has trouble operating in the presence of loud interfering noise. The basic active noise cancellation employed here is described in U.S. Patent No. 4,417,098. Summary of the Invention A primary object of this invention is to provide an enclosure having multiple means to quiet sounds therein. Another object of this invention is the provision of multiple transducers within an enclosure to allow for multiple openings in the enclosure so as to facilitate fluid flow. Still another object of this invention is the application of a multiple interacting actuators and sensors algorithm to the task of quieting an enclosure. Yet another object of this invention is the use of a multi-channel active noise cancellation system in the quieting by a noise cancellation system of an enclosure. These and other objects will become apparent when reference is had to the accompanying specification and drawings in which: Fig 1 is a diagrammatic view of a multi-channel active noise cancellation system showing the overall system, and Fig 2 shows a noise spectrum plot of frequency versus sound level, and Fig 3 shows a similar noise spectrum plot of frequency versus sound level at the residual microphone location, and Fig 4 shows a specific application of the multiple channel approach to silencing a refrigerator compressor, and Fig 5 is a block diagram of the algorithm frequency time domain approach to controlling the instant system. The application of active noise quieting suggests itself to the area of home appliances. Home appliances do not produce noise levels that are dangerous or very obnoxious. The purpose of quieting appliances falls under the goal of providing a kitchen with an overall sound power level not greater than 40 dBA or to provide an appliance quieter than the competition. In a kitchen, for example, refrigerators, microwaves, rangehoods and dishwashers are all candidates for quieting techniques. Fans, motors and fluid noise are all present in the appliances mentioned above. Quieting techniques will have to address all the noise sources in a particular appliance. Quieting one source may well make a previously masked source annoying. A typical kitchen refrigerator has two main sources of noise, the compressor and the freezer compartment fan. The compressor consists of an electric motor and compressor device such as a piston. The fan is usually an axial type mounted in the freezer compartment. The enclosure referred to in this application can be either of these compartments. To develop torque, the compressor motor turns at a rate slightly slower than the line frequency, e.g. 58.5 Hz instead of 60 Hz. This frequency is the fundamental rate of the noise heard from the compressor. Harmonics of this fundamental at varying amplitudes are then heard. In addition, there is a low pressure inlet and high pressure outlet valve that open and close each cycle that produce the "clicking" type noise heard from the compressor. The refrigeration system has other noise sources also. There is a tone around 1500 Hz that varies in amplitude as a function of time that is produced in the piston part of the compressor and is fluid borne into the cooling coils where it can be heard. The expansion valve in the freezer compartment produces noisy turbulent fluid flow in the return line to the compressor. This noise varies with time and is proportional to the amount of Freon in the system that is being moved by the compressor (much less noticeable if the Freon level is low) . A natural question at this point is why active control of noise? Why not just a foam lined box around the noise? The problem is heat. A refrigerator collects heat from the freezer compartment and dumps it into the room by way of the cooling coils. Enclosing the compressor and coils will necessitate an inlet and outlet and fan possibility for heat transfer and you are back where you started. Enclosing the .compressor only is a better idea because the heat transfer to the room is not compromised and the source of the most noise is directly addressed. An inlet and outlet for heat is necessary because the compressor does get hot; however, no fan is needed because the flow of Freon through the compressor helps to cool it. Enclosing the compressor and adding active control is good for several reasons. Passive materials lower higher frequencies where active alone is not as effective. Active control of lower frequencies is very effective where passive materials are not. The low frequency noise that can be heard all through the house is cancelled at the source giving good global cancellation. Heat transfer from the coils is not interfered with. This technology allows all the active components to fit within the enclosure and in high volumes is very cost effective. Specific Description A simulation of the MISACT algorithm was developed to assist in predicting the performance of the attenuator system. The simulator uses a model of the operating environment to reproduce the interactions between actuators and sensors and can either be user defined or experimentally measured. The simulator will accept up to four actuators and four sensors, along with user inputs for the noise type and frequency range, transfer functions type (measured or user defined) and sample rates. For this paper the transfer functions between all speakers and microphones were measured in real time by the NCT 2010 controller. Simulation runs were made with noise frequencies of 228 and 456 Hz. To help illustrate the importance of interactions simulations were also run for the case of a diagonal transfer function matrix (no interactions) . Results are tabulated in Table 1 for both noise frequencies. The simulation results show that when full interaction between sensors is used, the overall noise level of both noise frequencies is reduced. However, when the interactions are not used the algorithm becomes unstable at around 228 Hz. For experimental measurements an enclosure as in Figure 1 was constructed. The sides of the enclosure were constructed of plywood. A speaker inside the enclosure served as the noise source. Two 6 inch ports were used as outlets. Speakers were mounted to inject the cancelling signal at each port. Microphone elements were mounted at the top of the cabinet to provide the feedback signal. Both microphones and speakers were connected to a NCT 2010 MISACT controller. A synthetic noise source was connected to the speaker inside the enclosure and provided a speed signal to the controller. A B&K type 2230 sound level meter was used to measure the overall noise reduction along with a Tektronix 2630 spectrum analyzer. The controller performed a calibration to determine the system response between the full matrix of speakers and microphones. A noise signal consisting of 228 and 456 Hz was then generated in the enclosure. The controller was then enabled and allowed to reach steady state operating condition. The noise spectrum measured at the monitor microphone is shown in Figure 2 for the controller both on and off. The overall noise level was reduced from 59.3 dBA to 45.5 dBA. To illustrate the importance of accounting for the interactions, the control was implemented with two independent channels. The response at the two error microphones is shown in Figure 3. While the 456 Hz tone is reduced, the level of the 228 Hz tone is increased. It should be noted that the signal level was prevented from increasing further by limiting logic in the software. Table 1, following, shows the simulation results for interacting versus independent control. Noise Frft ienπy Control Mod Sensor 1 .Sensor 2 Overall 228 Hz interacting -3.8 dB -8.1 dB -5.4 dB 228 Hz Non-Interacting 15.8 dB 20.0 dB 18.4 dB 456 Hz Interacting -4.2 dB -12.7 dB -5.6 dB 456 Hz Non-Interacting -11.7 dB -3.1 dB -5.6 dB Table 1. Simulation results for interacting cs. independent control. The instant invention solution to refrigerator quieting is seen in the attached drawings, Fig 4. It is a combination active and passive approach. A shell was constructed around the compressor compartment to take out the high frequency tones and random noise. This shell also had an input slit and output port to allow heat from the compressor to escape. The shell and port were designed to resonate at approximately 60 Hz. Testing showed that the compressor shell temperature stayed under manufacturer's guidelines even when subjected to an ambient temperature of 110 degrees F. An accelerometer placed on the compressor shell se - as the noise sync signal. The antinoise actuator was a speaker inside the shell. The speaker cabinet was optimized for the low frequency fundamental, 58.7 Hz, of the compressor. A microphone on the outside of the shell served as the residual sensor. The NCT control algorithm then calculated the impulse response from the speaker to the residual microphone and produced the correct cancelling signal. A reduction of 10 dBA rear and 5 dBA front over an identical unmodified refrigerator was obtained. The uniqueness of the solution is as follows: 1) Combination passive and active approach for a system solution. 2) Tuning the shell at the fundamental frequency of interest. 3) Heat transfer of the compressor taken into account. 4) Flexible residual microphone location for optimum cancellation when installed. 5) Speaker location flexible inside the shell. 6) Very low (milliwatts) power needed for speaker. 7) Non-sensitive to loud transients. This invention shows control of low frequency noise from an enclosure with multiple transducers. It was shown that simulation can provide insight about performance of an active control system. The simulation model has been used as an analytical tool on electronic mufflers and vibration mounts. Furthermore it was demonstrated both through simulation and experiment that the MISACT algorithm will remain stable in situations where independent control channel may become unstable.

Claims: What is claimed Is: 1. A system for actively controlling and quieting repetitive noise arising from one or more noise sources comprising an enclosure means surrounding said one or more noise sources, multiple opening in said enclosure means, speaker means mounted in near proximity to said multiple openings and adapted to cause deadening of sound generated from said noise sources, residual microphone means adapted to receive noise and to generate electrical signals in response thereto, controller means connected to said speaker means and adapted to receive signals from said residual microphone means to thereby generate counter sound in the proximity of said multiple opening means to thereby deaden the sound emanating from said noise sources within said enclosure. 2. A system as in claim 1 wherein said speaker means are transducers located adjacent said multiple openings. 3. A system as in claim 1 wherein there are two openings in said enclosure and two speaker means adjacent said openings. 4. A system as in claim 3 wherein said openings are approximately circular and said speaker means are transducers. 5. A system as in claim 1 and including a synchronous signal means whereby an electronic signal is fed from said noise source to said controller means. 6. A system as in claim 5 wherein said noise source is a compressor and said synchronous signal means generates a signal related to the motor of said compressor. 7. A system as in claim 1 wherein said noise source includes a motor and said synchronous signal means generates a signal in response to the motor's revolutions. 8. A system as in claim 1 wherein said noise source is a fan and said synchronous signal means generates a signal in response to the fans revolutions. 9. An active noise cancellation system for actively cancelling noise emitted by an enclosure with multiple openings containing at least one noise source, said system comprising multiple speaker means affixed adjacent said multiple openings and adapted to generate sound of opposite polarity to that emanating from said noise source so as to cancel it, microphone means mounted in the proximity of said enclosure means and to receive sound from said noise source and enclosure and transmit an electrical impulse in response thereto, and controller means operatively connected to said speaker means and said microphone means so as to cause signals to be generated to said speaker means to generate said opposite polarity sound. 10. A system as in claim 9 and including a synchronous signal means connecting said noise source and said controller means. 11. A system as in claim 10 wherein said speaker means are transducers. 12. A system as in claim 9 wherein said noise source includes a compressor and said system is adapted to cancel its noise. 13. A system as in claim 12 wherein said noise source also includes an electric motor. 14. A system as in claim 9 wherein there are two approximately circular openings in said enclosure and said speaker means are mounted in said openings. 15. A system as in claim 15 and including a synchronous signal means operatively connected between said noise source and controller means.

Inventors: EGHTESADI KHOSCROW, MC LOUGHLIN MICHAEL P, SMITH DEXTER, ZIEGLER ELDON W JR, EGHTESADI, KHOSCROW, MC LOUGHLIN, MICHAEL, P., SMITH, DEXTER, ZIEGLER, ELDON, W., JR.
Applicants: NOISE CANCELLATION TECH, NOISE CANCELLATION TECHNOLOGIES, INC.
Application year: 1991
Main classification: G10K11
IPC codes: G10K11