HIGH EFFICIENCY FAN WITH ADAPTIVE NOISE CANCELLATION

Abstract: This invention relates to a new type of compact high efficiency fan utilizing microphones (74, 75), a loudspeaker (69, 70) and a totally adaptive noise cancellation controller (73) whereby the design of the fan eliminates broadband noise and the controller reduces the remaining tonal component thus producing a high efficient silent fan.

Description: High Efficiency Fan with Adaptive Noise Cancellation Area of the invention This invention relates to increasing the efficiency of both centrifugal as well as axial fans while simultaneously cancelling the increase in noise caused by increasing their efficiency. For example, with a reference to Fig. 13 herein, a typical blower or centrifugal fan is shown. The housing geometry of a centrifugal fan is characterized by an expansion angle, β , a development length, 2f 0 , and a housing width, w -,. The expansion angle defines the rate of expansion of the housing scroll. Generally two types of expansion are used; either a linear or exponential expansion. In the case of the linear expansion, the housing scroll radius is described mathematically by r = r 0 (l = / ά^) where: r is the radius at a certain circumferential position, , r 0 is the initial radius of the scroll, and B is the scroll expansion angle. It can be seen that the radius of the scroll increases linearly with the circumferential position, y. The mathematical description of the exponential expansion is: r ~ - r 0 exp (βti " ) The developed length, r 0 , defines the length of the housing expansion. The smallest distance between impeller wheel and blower housing occurs at the housing cutoff and is therefore called the cutoff clearance, cc. The cutoff helps to direct the flow from the impeller wheel into the housing discharge. The cutoff geometry is usually characterized by the cutoff radius, r c . In the instant invention the fan is made more efficient by reducing the cutoff clearance, cc. The tonal component of centrifugal fan noise is generated at the housing cutoff by the interaction of the non-uniform flow leaving the impeller with the cutoff. This phenomenon results in large pressure fluctuations on the cutoff that in turn result in sound radiation at the blade passage frequency, BPF (BPF = jI, where z is the number of impeller blades and N is the rotational speed of the impeller), and its harmonics. It is obvious that the velocity profile of the flow smooths out as the distance between the impeller blades and the cutoff is increased. An enlarged clearance thus results in reduced pressure fluctuations on the cutoff and is, consequently, an appropriate way to reduce the tonal noise component of centrifugal fans but reduces the efficiency of the fan and increases random broadband noise. Another design parameter that affects the tonal blower noise is the radius of curvature of the cutoff. The influence of this parameter has been found to be less significant than the cutoff clearance. However, it has been shown that a noise reduction of up to 6 dB can be achieved by increasing the radius of the cutoff without affecting the blower's performance or efficiency. A conventional blower is characterized by a cutoff edge that is parallel with the impeller blades. Such an arrangement results in pressure fluctuations that are in phase along the full length of the cutoff, thus enhancing the sound radiation. Inclination of either the impeller blades or the cutoff edge or both with respect to the impeller axis results in a progressive phase shift of the fluctuating pressures acting along the length of the cutoff, thereby resulting in some reduction of the radiated sound. By spacing the blades irregularly on the impeller, the blade passage sound energy can be spread over a wider frequency band. This may be desirable from a subjective point of view but the total sound power generated usually remains unchanged. It has been shown that under certain operating conditions, irregularly spaced blades can result in sideband tones having higher levels than the original blade passage tone. Neise and Koopmann were successful in reducing the blade passage frequency tone by applying an acoustic resonator to the cutoff. The resonator was constructed from a series of perforated plates with the same curvature as the original cutoff so as to preserve the original geometry of the housing. The resonator could be tuned by changing its length via a movable plug. By use of the resonator, the blade passage tone could be reduced by as much as 29 dB, thus resulting in an overall sound pressure level reduction of up to 7 dB(A). The aerodynamic performance of the blower was unaffected by the addition of the resonator to the casing. This system only works on single speed fans. Relatively few have explicitly addressed the reduction of broadband blower noise. Neise reports on the application of transition meshes by Petrov, et al. A fine mesh placed at the blade leading edge (i.e., around the inner circumference of the impeller) results in small scale turbulent flow behind the mesh. As a result the blade boundary layer is made turbulent and the point of flow separation is accordingly shifted towards the blade trailing edge. A second transition mesh placed around the outer circumference of the impeller smooths the outlet velocity field and therefore probably further reduces the turbulence scale in the blade wakes; this may also result in a reduced noise generation. The major disadvantage of transition meshes lies in the decreased fan efficiency that was observed, particularly for impellers having forward curved blades. The most comprehensive experimental investigation of the effect of design parameters on centrifugal fan noise that has been conducted to-date was performed by Morinushi. His findings were that there are five (5) important design parameters on the noise generation and aerodynamic performance of a centrifugal fan featuring an impeller having forward curved blades and an outer diameter of 0.18 m. The five design parameters were: (1) the width-inner diameter ratio of the impeller, WID j , (2) the axial inlet clearance, c; (3) the blade setting angle (defined as the angle between the blade chord and a radial line form the impeller axis), (4) the blade pitch-to-chord ratio, and (5) the expansion angle of the scroll. Background pf \]~- Invention The instant invention is a major step forward from prior attempts to quiet fans and blades which are used to move fluids such as air. Initially passive techniques such as baffles and insulating fabrics and walls were employed in an attempt to absorb the noise. While such measures reduced some noise a large amount was left to deal with. Designers, to reduce this noise simply made the noise broadband and, consequently, the fan less efficient Only recently has a third means, active noise cancellation, been employed and these use a linear flow technique, or "ducted" solution. The disadvantages of such a technique is that the duct has to be extremely long and the microphones have to be in the flow itself. The instant invention goes beyond any of the foregoing. The system of this application reduces the random component of the noise by using better fan design. It also cancels the tonal noise using adaptive active controller. The advantages of the instant application are that no long duct is required, there are no microphones required to be directly in the flow thus reducing the expense of microphone construction, the fans become substantially totally quiet and the fans are highly efficient thus reducing the electrical power requirements needed to power them. For best over all performance a fan should be operated near the region of maximum efficiency. A convenient guide to ensure such operation is a parameter known as specific speed, combing the major variables that affect the selection of an air moving device. Q 112 n ^= where N s = specific speed Q = flow rate m cfm n = rotational speed in rpm p s = static pressure in inches of water column The significance of specific speed derives from the fact that for any particular fan design there is a unique attainable value of efficiency for a given value of specific speed regardless of size or rotational speed. The needs of the application determine the specific speed at which the fan must operate, and the optimum fan selection results if a fan is chosen which has high efficiency at the specific speed dictated by the application. Figure 14 shows the ranges of specific speed for which the products in this catalog are suited. As shown, forward curved centrifugals can be applied in the range of specific speeds from 7,000 to 55,000, with best operation being in the range of 20,000 to 35,000; mixed flow impellers can be applied in the range from 20,000 to 72,000, with operation in the range 30,000 to 55,000; while propeller fans can be applied in the range from 90,000 to 215,000, with best operation in the range 100,000 to 150,000. The application engineer usually has some latitude in using specific speed as a selection criterion because the numerical value of specific speed for a given performance is directly proportional to rotational speed. By properly choosing rotational speed for a given requirement specific speed can be made to fall wiώin the efficient operating range for the desired fan type. This approach is especially flexible in dealing with belt driven fans; direct motor drives are necessarily more limited in the values of rotational speed available. Where high efficiencies are needed in the specific speed range between mixed flow fans and axial fans some manufacturers can supply custom-designed vaneaxial fans; for the range below standard centrifugals, custom designed centrifugals can be supplied for specific speeds down to 3000. They can also be supplied for specific speeds as low as 1000 where their simplicity makes them a choice over positive-displacement devices. Li Fig 14 one generally operates on the slopes 200 of the diamond shaped range patterns for quieting and move back toward the apex as at 300 for more efficiency. When one moves off the slopes the noise changes from tonal to random. Prior art fans have been a compromise between operating efficiency and noise acceptability. Often, increasing the efficiency of fans has required smaller clearances between the fan impeller and the housing and while lowering broadband noise levels result in stronger tonal noise. However, these tones have been judged to be less acceptable than broadband "random" noise. Therefore, efficiency has been reduced in order to provide fans with the acceptable broadband "random" noise. However, the broadband noise 'level' is higher than the more efficient fan. Bartenwerfer et. al. -, in "Noise Reduction in Centrifugal Fans by means of an Acoustically Lined Casing" Noise Control Engineering May- June 1977, Vol 8, No 5, pp 100-107, described a method of reducing the overall noise level of fans by means of an acoustically lined casing. However, linings wear away over rime, are not acceptable in some environments and leave substantial tonal noise. Neise and Koopmann in "Active Sources in the Cutoff of Centrifugal Fans to Reduce the Bland Tones at Higher-Order Duct Mode Frequencies Journal of Vibration and Acoustics 19 , Vol , No , pp to , describe the use of tuned resonators located at the cutoff of centrifugal fans to produce a secondary noise source partially cancelling the blade passage frequency tone. Additionally, they describe an electronic system that produces the cancelling noise by filtering a timing signal from the fan and by manually adjusting the phase and amplitude of each frequency component of the cancelling signal. The electronic system utilized costly laboratory test equipment and require manual adjustment making it inappropriate for use in home or related applications. Neither the tuned resonators nor the electronic system adapt to changes in the frequency phase or amplitude of the fan noise. Moreover, simply adding tonal cancellation to an unimproved fan does not increase the efficiency of the fan nor does it decrease the broadband random noise. Neise goes on further in the same vein in "Review of Noise Reduction Methods for Centrifugal Fans" Journal of Engineering for Industry, 104, pp 151-161, 1982. Neise, in U.K. patent no.2,117,046 describes an application of his technology whereby he attempts to overcome the problem of constantly having to tune the length of a resonator. He employs a system whereby the counter sound source is effected by changing the frequency responsive to the speed of the impeller. He phases the counter sound oppositely to the noise he wished to cancel and states that the noises are extinguished at least partly. To accomplish his partial quieting, he uses an adjustable phase shifter circuit utilizing a sound transducer. To activate, the counter sound he utilized a disk and speed pick-up or, in another variant, a microphone in side of the counter-sound speaker box. In the latter embodiment, he uses a phase-shifter circuit which is adjusted so that the amplitude of the audible amplifier is used to change the level of the counter sound. Prior art adaptive noise cancellation methods applied to fan noise have utilized a duct in which an upstream sensor is employed to detect the noise and a downstream actuator produces the cancelling noise. Chaplin, in the article "Anti-noise-the Essex Breakthrough", Chartered Mechanical Engineer, pp 41-48, 1983, together with G.L. Warnaka in U.S. Patent No. 4,473,906 and Ericksson, in "Active Attenuation System with On-line Modeling of Speaker, Error Path and Feedback Path" of U.S. Patent 4,677,676 describe such methods. However, these methods require a length of duct that might not be available in small installations, they require the expense and installation of the upstream sensor that resides with the possibly turbulent air flow and they cancel the noise only in one direction, often the outlet side. A second installation is required to cancel noise at the inlet side of the fan but the interaction makes this difficult Hill, in U.S. Patent 5,010,516 describes an active acoustic attenuation system for attenuating tool blade pass noise using a sinsuoidal rotational speed signal produced by an accelerometer mounted on the fan as a reference signal for a filtered - X least mean square adaptive filter controlling a canceling sound generating speaker. Quinlan, in an article in Proceedings of Inter-noise 89, pp 479-482, entitled, "Active Control of Noise Radiated from Small Axial flow Fans" has described the reduction of noise in axial fans using an electromagnetically produced secondary noise source but this method does not adapt to changes in the noise and does not improve the efficiency of the fan. It is an object of this invention to quiet fans using a totally adaptive controller. Accordingly, it is a further object of this invention to simultaneously quiet a fan while making it more efficient It is another object of this invention to lower broadband noise with a more efficient design and electronically remove tonal component. It's a further object of this invention to reduce the cutoff clearance on centrifugal fans and then sense and electronically cancel the increased noise. It is still a further object of this invention to provide a fast adapting noise controlling system to control the change in frequency, phase and amplitude of the noise of a fan made efficient by reducing clearances. Yet another object is to elin inate the need of resonators in quieting fans. Another object of the invention is to decrease the overall size of the fan and its housing. Still a further object of the invention is to decrease the overall electrical power requirement of the fan. And yet another object of this invention is to reduce the cost of the microphone needed to operate the system. Description of the Invention The present invention provides a more efficient quiet fan via the integration of high efficiency fan designs and adaptive active cancellation of tonal noise. The efficiency of the fan is improved by reducing clearances within the fan such as those between the impeller and the fan casing, between the impeller blades and the struts and between the impeller blades and the fan cutoff. Generally, reducing these clearances results in an increase on the tonal noise at multiples of the blade passage frequency. These tones are cancelled by an active adaptive method that automatically adapts to changes in the noise, frequency, phase and amplitude of the tonal noise components. Moreover, the method of the invention ehminates the duct reqtiired by prior art active cancellation systems and the current invention cancels noise both at the inlet side and the outlet side of the fan. It is well known in the art that active cancellation of noise in free space depends critically on the wavelength of the noise to be cancelled and on the spacing between the noise source of cancelling noise. For example, a simple dipole model shows that the two sources must be within approximately 1/10 wavelength in order to attenuate the noise amplitude by 6 dB. Additional attenuation requires even closer spacing. However, by the use of even a relatively short duct the spacing between the two noise sources can be extended such that the shortest wavelength capable of being attenuated is approximately twice the largest dimension of a rectangular duct or 1.7 times the diameter of a circularduct. For example, noise from a small fan exhausting into a duct with largest dimension six inches could be cancelled up to approximately 1,100 Hz. Presently the most effective application of active noise control for random noise is for linear systems, i.e. if noise is propagating down a duct it is more effectively picked up, analyzed and cancelled although some muffler and headphone applications are exceptions to this. Generally an input microphone can be located near the source of the noise and the cancellation speaker located far enough downstream to allow the controller time to determine the necessary frequency, phase and amplitude of the noise. Another microphone can be placed "upstream" or adjacent the fan to pick up residual noise which is then sent to the controller which makes adjustments for any error. For years, experiments were made with directional transducers so that the cancelling sound wave did not disturb the input microphone signal. This work was fraught with complications, and never resulted in an acceptable solution. Another concept was to penetrate a "trigger" or "synch pulse". This solution will work for repetitive tones, but obviously will not give the controller the proper information to analyze random, broadband, non repetitive propagating oise. Other controllers have been described in many published papers and at various symposia. The approach to create an electronic representation of the ductborne acoustical wave (See Figure 2). The "plant" is the actual propagating sound wave with the "model" being a complex set of transfer functions that allows the controller to understand how the propagating sound wave changes as it travels down the duct The controller contains a high speed digital processor and consists of one or more adaptive filters, including one that compensates for the acoustic feedback. It is critical to realize that the system can achieve global cancellation (across the entire duct cross section) for plane waves only. Testing in the laboratory and in the field indicates higher order modes will continue past a one-channel system, since the controller will direct the speaker to cancel out only the particular phase at the error microphone location. It is clear that the controller must be able to: 1) respond to source changes (amplitude and frequency of tonal noise) and 2) respond to system changes (temperature, air flow rate, transducer characteristics, error plant, and system plant). The length of the sound cancellation system is dictated by: 1) the input microphone probe length and its location out of the most turbulent fan outlet region, and 2) the distance from input microphone to cancellation speaker which allows the controller the required time delay for analyzing the propagating sound. A class of fans used for material handling is radial-bladed centrifugal fans. These "paddle-wheel" fans generate a very strong tone at the blade pass frequency, as well as harmonics, much like vacuum pumps. In this application, material continues in the duct past the active noise control system components as the system cancels tones up to the cutoff frequency. Active systems have an advantage over a conventional silencer which could plug with material in a matter of hours. A very important consideration in applying active noise control is the exact nature of the noise. Conventional absorptive silencers attenuate a certain amount at a fixed frequency. Active systems, on the other hand achieve greater reductions on tones and can adapt to changes in the noise frequency. While passive silencer performance can be quoted in terms of octave band insertion loss, active system performance depends on the actual spectrum. For this reason, as a minimum, 1/3 octave band analyses are required for predicting active or active/passive system requirements. In addition, it is imperative to know the exact frequency of any pure tones near the cut-on frequency for higher order modes for the given duct size, since a one-channel active system will not achieve global cancellation above that. In order to properly apply an active noise control system, the following parameters are required: 1) Sound power of noise source; 2) Upper frequency of required noise reduction; 3) requirement for component protection; 4) 1/3 or narrowband sound measurements. With this information, a system can be designed and performance predicted. In the drawings, Fig 1 shows a side diagrammatic view of a centrifugal fan employing the teachings of this inventions; Fig 2 shows a plan cutaway view of the fan of Fig 1; Fig 3 shows a side diagrammatic view of an axial fan employing the teachings of this invention; Fig 4 is an end view of the fan of Fig 3; Fig 5 is a side view of one embodiment of the invention on a centrifugal fan, Fig 6 is still another embodiment side view of the invention on a centrifugal fan; Fig 7 shows a double transverse speaker arrangement on a centrifugal fan embodiment; Fig 8 shows yet another double speaker embodiment of a centrifugal fan Fig 9 shows an embodiment of a radial fan utilizing the instant invention; Fig 10 illustrates a double speaker embodiment of the instant invention on a radial fan; Fig 11 shows an embodiment of a radial fan utilizing the teachings of this invention; Fig 12 shows another two speaker embodiment of this invention, and Fig 13 shows dimensions on a centrifugal fan. Fig 14 shows a selection criterion graph Figures 1 and 2 Fig 1 shows an example of the invention applied to a centrifugal fan 10. The clearances between the fan impeller and the surrounding housing 12 are much smaller than in conventional fans of today in order to improve the efficiency of the fan 10. As a results of the small clearances, a tonal noise is generated primarily due to the pressure differential at the fan cutoff 13. The cancelling noise source generally noted as transducer 14 is placed or ducted as close as practicable to the cutoff. The cancelling noise source can consist of one/or more loudspeakers, shown as 15 and 16 in Fig 1, or piezo-electrical elements. A short length of tapered duct, such as 17 provides for acoustic coupling at high frequencies. As stated, the efficiency of the fan 10 has been improved by reducing clearances within the fan itself such as those between the impeller and the fan casing, as at 20, between the impeller blades and the struts, as at 51 in Fig 3 and between the impeller blades and the fan cutoff as in Fig 1. The smaller that these clearances are the louder the tonal noise the fan generates. Noise is cancelled both at the inlet portion 22 of the fan as well as at the outlet portion 23. A sound sensor, for example a microphone or piezo-electrical element, 24 is mounted in the fan to provide an error signal that is a measure of the effectiveness of the active cancellation system. The exact location of the sensor can vary as long as it is not within turbulent air and it measures the combined sound level of the fan noise and the cancellation noise source. An active cancellation totally adaptive controller 25 provides a cancellation signal to the transducer 14 producing the cancelling source such that the level of the error signal is reduced. The active noise cancellation controller can be of the synchronous type as described in U.S. Patent 4,878,188 or the single sensor nonsynchronous type as described by Ziegler in a copendingU.S. application, Ser. No. 477,432. Figure 3 and 4 show the invention applied to an axial fan 50. Again the clearances 51 between the fan tip 52 blades and the surrounding housing 53 have been reduced to improve the efficiency of the fan. The resulting tonal noise is generated, primarily, by pressure differentials as the fan blade pass by struts, as at 54, supporting the fan. A short length of tapered duct 55 is used to provide the required acoustic coupling at the frequencies required to be cancelled. Often it is the blade passage frequency and its harmonics although noise at the rotation rate of the fan shaft may also be present. The cancelling noise source, such as speakers 56, 57, is placed, or ducted, into the fan duct. The cancelling noise source can consist of loud speakers or piezo-electrical elements. A sound sensor, as at 58 or 59, such as a microphone, providing an error signal and an active cancellation controller are provided as described with the centrifugal fan. Other variations of the application of this invention to centrifugal fans are shown in Figures 5 through 8. Fig 5 shows a fan housing 60 with an active noise cancelling controller 61 connected to a speaker 62 and tapered duct 63 at the tongue 64 of the fan housing. A microphone 65 sits outside and atop the housing and a speed sensor 66 measures the speed of the bladed fan. In Fig 6, a fan housing 67 and blade 68 have a pair of loudspeakers 69 and 70 connected on opposite sides of the exhaust chamber 71 so as to cancel the noise created by the tight tolerances as at 72 between the blades and the housing. An active noise cancellation controller 73 is connected to the loudspeakers 69, 70 and to speed sensor and microphones 74 and 75. The two microphones signals are merged and analyzed by 73 to maximize elimination of all tonal noise. Fig 7 shows a fan housing 76 and bladed fan 77 and, in general, a configuration similar to that shown in Fig 6, what with two loudspeakers 78,79 mounted on exhaust duct 80, an active noise cancellation controller 81, speed sensor 82 and microphone 83. The major difference in this configuration is that the loudspeakers are mounted transversely. Fig 8 shows a fan housing 84, a bladed fan 85 and two microphones 86, 87 connected to a controller 88 which, in turn, has inputs from sensor 89 and outputs to speakers 90,91, mounted opposite each other on ducts 92, 93 which feed the counter noise to the tongue 94 of the fan to counteract the noise generated by the small clearances as at 95. Fig 9 through 12 show variations of speaker mounts and microphone locations on axial fans. Fig 9 shows axial fan 100 having blades 101 and housing 102 with support members 103. A speed sensor is located as at 104 and a microphone downwind as at 105 which are connected to an active noise controller 106 such as that manufactured by Noise Cancellation Technologies, Inc. and designated the NCT 2000. The controller outputs to a speaker mounted abreast of the fan housing 102 so as to cancel all the tonal noise. Fig 10 shows a fan 110 having blades 111 and housing 112 in a duct 113. A speed sensor 114 and two microphones, 115 and 116, one upstream and one downstream are connected to an active noise cancellation controller 116 which controls loudspeakers 117 and 118. Fig 11 shows an axial fan 120 having a circular housing 121 and blades 122 surrounding motor drive 123. Controller 124 picks up the speed via sensor 125 and the tonal noise via microphone 126. Loudspeaker is 127 located downstream immediately adjacent motor drive 123. Fig 12 shows an axial fan 130 with a circular housing 131 and blades 132. It has a motor drive 133 and has loudspeakers 134, 135 mounted fore and aft. The loudspeakers are controlled by controller which senses the fan speed through sensor 137 and measures the fan noise by microphones 138, 139, mounted upstream and downstream, respectively. While certain selected embodiments of the invention have been described in detail, it will be appreciated by those of ordinary skill in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.

Claims: What is claimed is 1. An active noise cancellation apparatus for cancelling noise created by rotating blades in a fluid moving system and making the system more efficient, said apparatus comprising; means to contain a fluid within a confined and defined space, means to accelerate and compress air within said space so as to reduce broadband noise leaving a tonal component as the dominant noise source means to sense the blade passage frequency means to detect the noise generated by said acceleration and compression means as it forces fluid through said confined and defined space and generate a signal representative thereof, loudspeaker means adjacent said fluid containment means, electronic controller means to sense the blade passage frequency and the signal representative of said generated noise and to produce signals representative of equal and opposite counter noise to said loudspeaker means, whereby said generated increased noise is acoustically reduced by said counter noise and the air moving system is made more efficient and less noisy simultaneously. 2. An apparatus as in claim 1 wherein said fluid moving system is a centrifugal fan having small clearances between a housing thereof and the rotating portion with blades thereon. 3. An apparatus as in claim 2 wherein said noise detection means constitutes at least one microphone means. 4. An apparatus as in claim 3 wherein said microphone means comprises two microphones, one located near the housing inlet and the other located near the outlet of the moving fluid in said system. 5. An apparatus as in claim 2 wherein said loudspeaker means comprises at least one loudspeaker mounted at the tongue of the helix in the centrifugal fan and adapted to produce sound to counter the frequency, phase and amplitude of the tonal noise components of the fan. 6. An apparatus as in claim 1 wherein the fluid moving system is an axial fan mounted in a duct means. 7. An apparatus as in claim 6 wherein said noise detection means comprises a least one microphone means. 8. An apparatus as in claim 7 wherein said microphone means comprises two microphones, one located upstream of said fan and the other located downstream. 9. An apparatus as in claim 6 wherein said loudspeaker means comprises at least one loudspeaker mounted on the center of said axial fan. 10. An apparatus as in claim 6 wherein said loudspeaker means comprises two loudspeakers mounted upstream and downstream, respectively, of said fan. 11. A totally adaptive active noise cancellation system for actively cancelling noise arising from the operation of rotating blade means in an apparatus for moving a gas and simultaneously making the apparatus more efficient, said system comprising; housing means to contain said fan within said boundaries so as to reduce any broadband noises while leaving the tonal noise component, means to move said gas within said boundaries so as to reduce any broadband noises while leaving the tonal noise component, sensing mean adapted to sense the frequency of the gas moving means and generate a signed representative thereof, noise activated means to detect the noise generated by said gas movement through said housing means and generate a signal representative thereof, speaker means near said housing means, and adaptive controller means to take the signals representative of the frequency of the gas moving means and noise activated means and to continually produce noise of opposite and equal amplitude to said fan generated tonal noise to effectively reduce it, whereby said tonal noise is reduced while said apparatus moves gas efficiently. 12. A system as in claim 11 where said apparatus comprises a centrifugal fan and said gas is air. 13. A system as in claim 11 wherein said apparatus is a centrifugal fan and said speaker means is located near the tongue of the helix of said fan and adapted to produce sound to counter the frequency phase and amplitude of the tonal noise. 14. A system as in claim 11 wherein said speaker means is mounted outside of the housing means so as to be out of the path of the moving gas. 15. A system as in claim 14 wherein said housing means includes a short duct. 16. A system as in claim 15 wherein said noise activated means comprises two microphones located adjacent said housing means, each microphone adapted to generate signals representative of noise generated by said gas moving apparatus in its respective locale and said adaptive controller means adapted to integrate said signals to enhance and facilitate its ability to continually and adaptively produce signals to said speaker means to produce noise of equal and opposite amplitude. 17. A system as in claim 16 wherein said apparatus is a centrifugal fan. 18. A system as in claim 11 wherein said apparatus comprises an axial fan mounted in a duct means and said noise detection means comprises at least one microphone means. 19. A system as in claim 18 wherein there are two microphone means located adjacent to said housing means. 20. A system as in claim 18 wherein said speakers means comprised at least one loudspeaker mounted on the center hub of said axial fan.

Inventors: ARNOLD MICHAEL, EGHTESADI KHOSROW, FRIESEN ROBERT, MCLOUGHLIN MICAHEL P, VERSCHUEREN LUC, ZIEGLER ELDON W, ARNOLD, MICHAEL, EGHTESADI, KHOSROW, FRIESEN, ROBERT, MCLOUGHLIN, MICAHEL, P., VERSCHUEREN, LUC, ZIEGLER, ELDON, W.
Applicants: NOISE CANCELLATION TECH, NOISE CANCELLATION TECHNOLOGIES, INC.
Application year: 1991
Main classification: G10K11
IPC codes: F04D29, G10K11