Title:
Method for reducing noise of a cooking hood and a cooking hood based on such method
Kind Code:
A1


Abstract:
A method and system for reducing noise of a cooking hood having a body containing a motor and a muffler mounted on an upper section of the body so as to be in fluid connection therewith. A motor active noise reduction unit produces motor reduction noise for reducing motor noise. Likewise, a muffler active noise reduction unit produces a muffler noise reduction signal for reducing muffler noise.



Inventors:
Amir, Nehemia (Rishon Lezion, IL)
Application Number:
10/405619
Publication Date:
10/07/2004
Filing Date:
04/03/2003
Assignee:
SILENTIUM LTD. (Rehovot, IL)
Primary Class:
Other Classes:
126/299D
International Classes:
F24C15/20; (IPC1-7): F24C15/20
View Patent Images:
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Primary Examiner:
BASICHAS, ALFRED
Attorney, Agent or Firm:
EITAN, PEARL. LATZER & COHEN ZEDEK, LLP (NEW YORK, NY, US)
Claims:
1. A method for reducing noise of a cooking hood having a body containing a motor and a muffler mounted on an upper section of the body so as to be in fluid connection therewith, the method comprising: providing or using in association with the motor a motor active noise reduction unit capable of producing a motor reduction noise for reducing motor noise; and providing or using in association with the muffler a muffler active noise reduction unit capable of producing a muffler reduction noise for reducing muffler noise.

2. The method according to claim 1, wherein the motor reduction noise is in the same amplitude and opposite phase to the motor noise.

3. The method according to claim 1, wherein the muffler reduction noise is in the same amplitude and opposite phase to the muffler noise.

4. The method according to claim 1, wherein the muffler active noise reduction unit is adapted to produce the muffler reduction noise by: determining a component of the motor reduction noise reaching the muffler active noise reduction unit; and adjusting the muffler active noise reduction unit so as to reduce an effective muffler noise corresponding to noise sensed by the muffler active noise reduction unit less said component of the motor reduction noise.

5. The method according to claim 4, wherein the component of the motor reduction noise is predetermined.

6. The method according to claim 4, wherein the component of the motor reduction noise is determined by a motor-muffler transfer function.

7. The method according to claim 5, wherein the predetermined component of the motor reduction noise is measured offline.

8. The method according to claim 1, wherein the motor active noise reduction unit and the muffler active noise reduction unit are mounted with sufficient mutual separation that interference between the motor active noise reduction unit and the muffler active noise reduction unit is negligible.

9. The method according to claim 1, wherein the motor active noise reduction unit is adapted to produce the motor reduction noise by: determining a component of the muffler reduction noise reaching the motor active noise reduction unit; and adjusting the motor active noise reduction unit so as to reduce an effective motor noise corresponding to noise sensed by the motor active noise reduction unit less said component of the muffler reduction noise.

10. The method according to claim 9, wherein the component of the muffler reduction noise is predetermined.

11. The method according to claim 9, wherein the component of the muffler reduction noise is determined by a muffler-motor transfer function.

12. The method according to claim 10, wherein the predetermined component of the muffler reduction noise is measured offline.

13. The method according to claim 1, wherein the motor active noise reduction unit is adapted to produce the motor reduction noise by: determining an emulated motor noise at an error point; and adjusting the motor reduction noise to yield emulated motor reduction noise so as to reduce the emulated motor noise.

14. The method according to claim 13, wherein the emulated motor noise is determined by a microphone transfer function.

15. The method according to claim 13, wherein the emulated motor reduction noise is adjusted by a speaker transfer function.

16. The method according to claim 1, wherein the muffler active noise reduction unit is adapted to produce the muffler reduction noise by: determining an emulated muffler noise at an error point; and adjusting the muffler reduction noise to yield emulated muffler reduction noise so as to reduce the emulated muffler noise.

17. The method according to claim 16, wherein the emulated muffler noise is determined by a microphone transfer function.

18. The method according to claim 16, wherein the emulated muffler reduction noise is adjusted by a speaker transfer function.

19. A cooking hood having a body containing a motor and further containing a muffler mounted on an upper section of the body so as to be in fluid connection therewith, the cooking hood comprising: a motor active noise reduction unit mounted in association with the motor; and a muffler active noise reduction unit mounted in association with the muffler.

20. The cooking hood according to claim 19, wherein the motor is mounted on a substantially hollow base and the motor active noise reduction unit is mounted in the base.

21. The cooking hood according to claim 19, wherein the muffler includes a hollow space and the muffler active noise reduction unit is distributed in the hollow space.

22. A motor unit for a cooking hood, said motor unit containing a motor and a motor active noise reduction unit integral therewith.

23. The motor unit according to claim 22, wherein the motor active noise reduction unit comprises: at least one microphone for receiving motor noise; a speaker capable of producing a motor reduction noise for reducing the motor noise; and an electronic circuit connecting the speaker and the at least one microphone, for adjusting the motor reduction noise in accordance with the motor noise.

24. The motor unit according to claim 23, wherein the motor active noise reduction unit further comprises: an emulation circuit having respective inputs connected to the speaker and to each microphone and having an output connected to the electronic circuit, for reducing noise at a predetermined error point.

25. A muffler unit for a cooking hood, said muffler unit containing a muffler and a muffler active noise reduction unit integral therewith.

26. The muffler unit according to claim 25, wherein the muffler active noise reduction unit comprises: at least one microphone for receiving muffler noise; at least one speaker capable of producing a muffler reduction noise for reducing the muffler noise; and an electronic circuit connecting the at least one speaker and the at least one microphone, for adjusting the muffler reduction noise in accordance with the muffler noise.

27. The muffler unit according to claim 26, wherein the muffler active noise reduction unit further comprises: an emulation circuit having respective inputs connected to the at least one speaker and to the at least one microphone and having an output connected to the electronic circuit, for reducing noise at a predetermined error point.

28. The muffler unit according to claim 26, wherein the at least one speaker is mounted on an internal surface of a muffler's wall.

29. The muffler unit according to claim 26, wherein one of the at least one speaker is centrally disposed inside the muffler.

30. The muffler unit according to claim 27, wherein the at least one speaker is mounted on an internal surface of a wall of the muffler.

31. The muffler unit according to claim 27, having a single speaker disposed centrally inside the muffler.

32. A motor active noise reduction unit for reducing motor noise for use in a cooking hood, said motor active noise reduction unit comprising: at least one microphone for receiving the motor noise; a speaker capable of producing motor reduction noise for reducing the motor noise; and an electronic circuit connecting the speaker and the at least one microphone, for adjusting the motor reduction noise in accordance with the motor noise.

33. The motor active noise reduction unit according to claim 32, wherein the motor active noise reduction unit further comprises: an emulation circuit having respective inputs connected to the speaker and to the at least one microphone and having an output connected to the electronic circuit, for reducing noise at a predetermined error point.

34. A muffler active noise reduction unit for reducing muffler noise for use in a cooking hood, said muffler active noise reduction unit comprising: at least one microphone for receiving the muffler noise; at least one speaker capable of producing muffler reduction noise for reducing the muffler noise; and an electronic circuit connecting the at least one speaker and the at least one microphone, for adjusting the muffler reduction noise in accordance with the muffler noise.

35. The muffler active noise reduction unit according to claim 34, wherein the muffler active noise reduction unit further comprises: an emulation circuit having respective inputs connected to the at least one speaker and to the at least one microphone and having an output connected to the electronic circuit, for reducing noise at a predetermined error point.

Description:

FIELD OF THE INVENTION

[0001] This invention relates to reducing noise of cooking hoods.

BACKGROUND OF THE INVENTION

[0002] Cooking hoods are usually noisy appliances in the kitchen. The prior art describes efforts to reduce the noise.

[0003] U.S. Pat. No. 5,720,274 entitled “Low-noise vapor exhaust hood” (1998, Brunner et al., in the name of Gaggenau-Werke Haus-und Lufttechnik GmbH) discloses a vapor exhaust hood with a housing which features at least one intake opening equipped at a minimum with one filter, and features at least one outlet opening, and the inside surfaces of which are in part lined with a sound-deadening material, with a fan arranged in the housing. Arranged in the housing, at least opposite each inlet opening of the fan, is sound-absorbing material which from the opposite inlet opening is spaced at least the radius of the fan wheel. The sound-absorbing material has a surface which in size corresponds at least to a circular surface whose radius matches that of the fan wheel. Further measures for noise reduction consist in the arrangement of a vane in the area of the outlet opening, in the application of sound-deadening and/or sound-damping material as well as in the elastic mounting of fan and/or fan motor. The invention disclosed in U.S. Pat. No. 5,720,274 thus creates a vapor exhaust hood in which, for one, the number of noise generators is reduced and, for another, inevitable noise is damped and deadened.

[0004] U.S. Pat. No. 5,890,484 entitled “Exhaust system for kitchens” (1999, Yamada Yoshihiro) discloses an exhaust device for a kitchen exhaust system incorporating a range hood, which is capable of substantially reducing generation of noise while efficiently evacuating fumes from the kitchen without a suction loss. The exhaust device comprises a vent box communicating with an exhaust duct, which may be housed in the hood or in a housing box provided above the hood. The vent box includes a connection enclosure in its upper portion, which converges toward the duct. The vent box contains a pair of partitions each having a tilted plate, which provide a pair of vent routes. Between the partitions is provided a drive motor and in each vent route is provided a sirocco fan driven by the motor whose blades are converged toward the motor.

[0005] U.S. Pat. No. 6,439,839 entitled “Blower” (2002, Song Sung Bae et al., in the name of LG Electronics Inc.) discloses a blower. The blower comprises an impeller and a scroll housing. The impeller is provided with a plurality of blades and rotated. The scroll housing guides and discharges air sucked by the impeller to the outside, and surrounds the impeller. The expansion angle of the curvature radius of the contour of the scroll housing is designed to be less than an expansion angle in conformity with an Archimedic curve in a suction region ranging from a cutoff start angle to 160-200° from a reference angle. Additionally, the expansion angle of the curvature radius of the contour of the scroll housing is designed to be greater than the expansion angle in conformity with the Archimedean curve in a discharge region ranging exceeding 160-200°.

[0006] U.S. Pat. No. 6,368,062 entitled “Turbo fan for range hood and range hood storing turbo fan” (2002, Yagami Mototake and Sato Kiyohiko, in the name of Fuji Industrial Co., Ltd) describes that the spring-back state of the blade press formed from the metallic thin plate is restricted to form the blade of wing sectional shape strictly in accordance with the design disclosed in U.S. Pat. No. 6,368,062. The blade is formed into the wing sectional shape having a hollow inner part with both sides fixed to the upper plate and the lower plate being released under application of the press forming of the metallic thin plate. The blade is made such that the metallic thin plate having a rectangular shape as seen from its top plan view with one side being a wing width size is applied with a coining work, a number of linear deformation segments in parallel with the side of the wing width size are properly spaced apart along a side crossing at right angle with the side of the wing width size in side-by-side relation, the direction crossing at a right angle with the side of the wing width size of the metallic thin plate is formed into the curved surface of predetermined curvature and then a transfer of the recovering force generated at each of the belt-like plates between the linear deformation segments is shut off at the linear deformation segments so as to restrict influence against the entire metallic thin plate.

[0007] U.S. Pat. No. 5,983,888 entitled “Low noise cooker hood” (1999, Anselmino Jeffery John and Wu Guolian, in the name of Whirlpool Corporation) discloses a low noise hood having a housing with an air inlet and an air outlet. A first modular device in the form of an intake muffler with a first air duct passage extending therethrough is mounted within the housing near the air inlet. A second modular device in the form of a discharge muffler with a second air duct passage extending therethrough is mounted within the housing near the air outlet. The second air duct passage is shaped to prevent a straight unobstructed passage for airflow through the second passage. A third device comprising an air moving device is secured by a mounting arrangement within the housing. A first vibration isolator such as a plastic saddle is provided in the mounting arrangement for the air moving device for absorbing vibrations and a second vibration isolator in the form of a flexible connector is positioned between the air moving device and the second air duct passage. A third vibration isolator in a mounting arrangement for the discharge muffler may also be used.

[0008] The above-referenced disclosures employ passive methods for noise reduction. Such passive noise reduction is mainly effective for frequencies above about 1000 Hz, and may affect airflow in the cooking hood. Also, the passive noise reduction methods and systems disclosed therein do not reduce noise coming out from the top of the hood.

[0009] JP 6,185,778 entitled “Range Hood” (1994, Takeyama Hiroaki et al., in the name of Matsushita Electric Works Ltd.) describes that the noise of a sirrocco fan or a discharge duct mounted at a rear of the fan is propagated toward the vicinity of a cooking range. The propagated noise is sensed by a sensor microphone, subjected to inversion processing or adaptive signal processing by a signal processor amplifier to direct an added sound, the added sound is radiated by a loudspeaker, and waves of the sound are interfered with those of the noise to be silenced. A voice signal of a sound source input via an audio jack is sent by the loudspeaker via the amplifier to allow a person near a hood body to listen to music or broadcasting.

[0010] There is a need in the art to provide for an improved method for reducing noise of a cooking hood and a cooking hood based on such method.

SUMMARY OF THE INVENTION

[0011] It is the object of the invention to provide a method for reducing noise of a cooking hood having a body containing a motor and a muffler mounted on an upper section of the body so as to be in fluid connection therewith, the method comprising:

[0012] providing or using in association with the motor a motor active noise reduction unit capable of producing a motor reduction noise for reducing motor noise; and

[0013] providing or using in association with the muffler a muffler active noise reduction unit capable of producing a muffler reduction noise for reducing muffler noise.

[0014] It is another object of the invention to provide for a cooking hood having a body containing a motor and further containing a muffler mounted on an upper section of the body portion so as to be in fluid connection therewith, the cooking hood comprising:

[0015] a motor active noise reduction unit mounted in association with the motor; and

[0016] a muffler active noise reduction unit mounted in association with the muffler.

[0017] Still further, it is the object of the invention to provide for a motor unit for a cooking hood, said motor unit containing a motor and a motor active noise reduction unit integral therewith.

[0018] Yet further, it is the object of the invention to provide for a muffler unit for a cooking hood, said muffler unit containing a muffler and a muffler active noise reduction unit integral therewith.

[0019] Still further, it is the object of the invention to provide for a motor active noise reduction unit for reducing motor noise for use in a cooking hood, said motor active noise reduction unit comprising:

[0020] at least one microphone for receiving the motor noise;

[0021] a speaker capable of producing motor reduction noise for reducing the motor noise; and

[0022] an electronic circuit connecting the speaker and the at least one microphone, for adjusting the motor reduction noise in accordance with the motor noise.

[0023] It is also the object of the invention to provide for a muffler active noise reduction unit for reducing muffler noise for use in a cooking hood, said active noise reduction unit comprising:

[0024] at least one microphone for receiving the muffler noise;

[0025] at least one speaker capable of producing muffler reduction noise for reducing the muffler noise; and

[0026] an electronic circuit connecting the at least one speaker and the at least one microphone, for adjusting the muffler reduction noise in accordance with the muffler noise.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0028] FIG. 1 illustrates a cooking hood according to one embodiment of the invention.

[0029] FIG. 2 illustrates the motor active noise reduction unit according to one embodiment of the invention.

[0030] FIG. 3 illustrates the motor active noise reduction unit according to another embodiment of the invention, employing at least two microphones.

[0031] FIG. 4 illustrates the muffler according to one embodiment of the invention.

[0032] FIG. 5 illustrates an interference-preventing electronic circuit according to one embodiment of the invention.

[0033] FIG. 6 illustrates an emulation circuit emulating the noise at the far field, according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0034] In the following description, components that are common to different embodiments are referenced by identical reference numerals.

[0035] FIG. 1 illustrates a cooking hood 101 according to one embodiment of the invention. The cooking hood is composed of a body 102 and a muffler 103 mounted on an upper section of the body 102 and in fluid connection therewith. The body 102 is surrounded by body walls. The muffler 103 has a wall surrounding a hollow space of the muffler, from which the air blows out of the cooking hood. A motor 104 within the body 102 moves air from an air inlet 105 at the bottom of the body 102, to an air outlet 106 at the top of the body. At least one air opening 107 formed in a casing of the motor 104 allows air to enter into the motor 104. The motor 104 circulates air entering through the openings 107 to the air outlet 106, creating air turbulence constituting a dominant noise source within the cooking hood 101. The air is then circulated out of the cooking hood through the muffler 103. The noise propagates out of the cooking hood, for example through the air inlet 105, through the cooking hood's body 102 or through the air outlet 106 and muffler 103. The noise carried out through the air inlet 105 is referred to herein as motor noise, and it has a typical dominant spectrum of up to about 5000 Hz. The noise carried out through the air outlet 106 and muffler 103 is referred to herein as muffler noise.

[0036] In the cooking hood 101 there is a combination of passive noise reduction mechanisms, which taken in isolation are familiar to those versed in the art. For example, the air inlet 105 is padded with layers of absorbing material 108 and 109, which together constitute air inlet padding for reducing noise by passively absorbing high frequencies, e.g. frequencies higher than about 1000 Hz. Passive noise absorbing materials are well known in the art, suitable examples being polyester and melamine. However, known passive noise reduction methods show poor performance when reducing low frequency noise, and the invention proposes a combination of active and passive noise reduction techniques, in order to improve the performance at both low and high frequencies. Active noise reduction, according to different embodiments of the invention, is effected by generating a reduction noise, which interacts with the noise and reduces its intensity. The reduction noise is most preferably of opposite phase and equal amplitude to the noise created by the cooking hood, thereby reducing the noise signal.

[0037] As seen in FIG. 1, a motor active noise reduction unit 110 is placed within the cooking hood, in association with the motor 104. The motor active noise reduction unit 110 is positioned close to the motor in order to detect and reduce the motor noise generated therein. Thus, in the figure the motor active noise reduction unit 110 is located beneath the motor 104 and it is padded with the noise absorbing material 108. The motor active noise reduction unit 110 is placed so as not to interfere with airflow through the air inlet 105, and is used to reduce low frequency noise created by the air turbulence at the motor and carried out of the cooking hood through the air-inlet 105.

[0038] As further seen in FIG. 1, a muffler active noise reduction unit 111 is placed in association with the muffler 103, and is used to reduce low frequency muffler noise carried through the air outlet 106 and the muffler 103. However, the muffler active noise reduction unit 111 need not necessarily be confined inside a housing, and can be distributed in the muffler's hollow space, as will be described later with reference to FIG. 4.

[0039] There can exist a physical connection between the cooking hood's body 102 and the motor 104. It is at least partly because of this physical connection that motor vibrations create vibrations of the body 102. The body vibrations are converted to acoustical noise. This noise may become dominant when all other noise sources in the cooking hood are reduced. In order to reduce the noise created by the motor vibrations, dampers 112 that are suitable for the motor speed and weight can be used.

[0040] Besides the air inlet 105, the air outlet 106 and the muffler 103, noise created by air turbulence in the motor 104 can emanate through the cooking hood's body 102. This noise can lower the total effect of noise reduction performed passively and/or actively by the motor active noise reduction unit 110, by the muffler active noise reduction unit 111 and by the passive mechanisms described above. In order to reduce the noise emanating from the cooking hood's body 102, the body 102 can be built of acoustically insulating material. However, normally the weight and the size of the passive materials needed for effective insulation increase as the insulation requirement increases, and optimization is required in order to keep the cooking hood's body as light-weight and small as possible while reducing the noise emanating therefrom to negligible intensities.

[0041] The optimally reduced noise emanating from the walls of the body 102 is expected to be lower by about 10 dB or more than the reduced noise that emanates from the air-inlet 105. It is also expected to be lower by about 10 dB or more than the reduced noise that emanates from the air-outlet 106 and the muffler 103. By “reduced” noise is meant that noise that emanates from the body 102, the air-inlet 105 and the air-outlet 106 when passive and active noise reduction mechanisms are activated. It is possible to measure the reduced intensity of the noise emanating from different locations of the cooking hood 101 using known methods for acoustic power measurement. Several of such methods are described by acoustic standards such as ECMA-160 (second edition December 1992).

[0042] FIG. 2 illustrates the motor active noise reduction unit 110 according to one embodiment of the invention. The outer walls of the motor active noise reduction unit 110 are made of material transparent to sound, such as a dense grid of metal, enabling sound waves to pass through. According to the described embodiment, the motor active noise reduction unit 110 includes a hollow space, enclosed within inner walls, forming an enclosure 201. A speaker 202 is located inside the enclosure 201 and produces a motor reduction noise for reducing noise generated by the motor. According to the described embodiment, a microphone 203 is located at the center top, close to the motor 104 (see also FIG. 1), in order to receive the motor noise. The microphone 203 produces then an electric signal referred to as a “motor noise signal” that corresponds to the motor noise. An electronic circuit 204 connects the microphone 203 to the speaker 202 in order to calculate a motor noise reduction signal to be fed to the speaker 202, based on the motor noise signal picked up by the microphone 203. The speaker 202 generates the motor reduction noise in accordance to the motor noise reduction signal it receives.

[0043] The enclosure 201 has at least one noise-transparent inner-wall, such as the bottom wall. The other inner-walls are noise opaque. Speakers manufactured by currently available technology typically generate acoustical waves having opposite phases from opposite sides of the speaker's membrane. According to the described embodiment, the noise opaque inner-walls acoustically insulate the speaker, preventing the signals with the opposite phases from propagating out of the speaker enclosure.

[0044] As already noted with regard to FIG. 1, the walls of the motor active noise reduction unit (i.e. the outer walls) are padded with noise absorbing material 108. The noise absorbing material padding 108, together with the noise absorbing material padding 109 passively absorb high frequency noise created by the motor and leaving the cooking hood through the air inlet.

[0045] As known in the art, air turbulence may develop around objects positioned in the airflow route. This air turbulence is referred to as local turbulence. Local turbulence evolving around the motor active noise reduction unit 110 may affect the acoustic noise received by the microphone 203.

[0046] FIG. 3 illustrates an alternative embodiment that copes with local turbulence, reducing its effect on the acoustic noise received by the motor active noise reduction unit 110. The motor active noise reduction unit 110 employs at least two microphones 301a, . . . , 301n. The microphones 301a, . . . , 301n together form a microphone section 302. The microphones are positioned as pairs disposed symmetrically on opposite sides of the motor active noise reduction unit 110. For example, there are shown four microphones designated 301a, 301b, 301c and 301d, where 301a and 301c form a first pair, and are disposed symmetrically on opposite sides of the motor active noise reduction unit 110. The same applies with regard to the microphones 301b and 301d, which form a second pair. Electronic circuits 303a, . . . , 303n electronically connect the at least two microphones 301a, . . . , 301n, whereupon they all summed at a summation point 304. Electronic circuit 204 exists also between the at least two microphones 301a, . . . , 301n and the speaker 202 in order to calculate the motor noise reduction signal and therefore the motor reduction noise generated by the speaker 202 with the motor noise signal picked up by the microphones 301a, . . . , 301n.

[0047] The microphones are gain- and phase-calibrated by means of the electronic circuits 303a, . . . , 303n, to have the same gain and phase. According to the described embodiment, the microphones are selected to have matched phase within a frequency range up to about 1000 Hz. The electronic circuits 303a, . . . , 303n are used to calibrate the sensitivity of all microphones 301a, . . . , 301n so as to be identical, by changing the analog gain of each microphone. Using multiple (at least two) microphones increases the signal to noise ratio of microphone section 302 as is known in the art. This way the motor active noise reduction unit can detect noise even when high-speed airflow exists, reducing the acoustic effect of local turbulence. Gain and phase calibration per se is well known in the art. See for example U.S. Pat. No. 4,454,597, U.S. Pat. No. 4,956,867 and U.S. Pat. No. 5,574,824, disclosing methods for gain and phase calibration as well as methods to increase the signal to noise ratio in the presence of a number of microphones.

[0048] FIG. 4 illustrates the muffler 103 according to a non-limiting embodiment of the invention shown with a round cross-section although it should be notified that the muffler's cross-section can be of any form. Airflow through the muffler 103 is directed from the muffler's inlet 402 to a muffler outlet 403, therefore being asymmetrical. As explained with reference to FIG. 1, the muffler 103 is mounted on an upper section of the body 102, and air blows out of the cooking hood blows through the muffler's hollow space. It was also explained that the air blows out of the cooking hood's body through the air outlet 106. Therefore, the air outlet 106 and the muffler's inlet 402 can be considered as equivalents.

[0049] As also explained previously, the muffler noise emanates from the cooking hood's air outlet 106 through the muffler 103. The muffler performs both passive and active noise reduction, in order to reduce the noise emanating therefrom. In order to perform passive noise reduction, the muffler's walls are padded or are built of acoustically absorbing material 401, reducing frequencies above about 1000 Hz. Frequencies below about 1000 Hz are reduced by the muffler active noise reduction unit 111 described below.

[0050] According to the asymmetrical muffler airflow, the muffler active noise reduction unit 111 is also asymmetrically distributed along the muffler 103. At least two microphones 404a, . . . , 404n are disposed around the muffler's walls internal surface near the muffler inlet 402, forming a muffler microphone section 405. Each of the microphones 404a, . . . , 404n is connected by means of respective micro-phone electronic circuits 406a, . . . , 406n to a summation point 407, summing the respective outputs of the microphones 404a, . . . , 404n. The muffler 103 also includes a speaker 408 mounted on the internal surface of the muffler's wall, and connected by an electronic circuit 409 to the muffler microphone section 405 via the summation point 407. The speaker 408 receives a muffler noise reduction signal and produces a corresponding muffler reduction noise, such as noise of opposite phase and equal amplitude to the noise emanated through the muffler, for reducing noise generated thereby.

[0051] In variations of the above embodiment multiple speakers may be located around the internal periphery of the muffler and a single microphone may likewise be employed. In the case where a single speaker is used, it may be displaced horizontally from the position shown in FIG. 4 so that it is located at the center of the muffler's cross-section instead of on the internal surface of the muffler's wall. Likewise, when a single microphone is used it, too, may be displaced horizontally from the position of the muffler microphone section 405 shown in FIG. 4 so that it is located at the center of the muffler's cross-section instead of on the internal surface of the muffler's wall.

[0052] As is known in the art, as the number of microphones 404a, . . . , 404n increases, the signal to noise ratio of the microphone section 405 increases. An exemplary embodiment uses 8-12 microphones, but this is not limiting and other numbers of microphones may be employed. This way the muffler active noise reduction unit 111 can detect noise even when high-speed airflow exists, reducing the acoustical effect of the local turbulence at the microphone section 405. The at least two microphones 404a, . . . , 404n are symmetrically arranged around an internal surface of the muffler wall. The microphone electronic circuits 406a, . . . , 406n perform gain- and phase-calibration of the microphones 404a, . . . , 404n, in order for them to have the same gain and phase. According to this embodiment, the microphones are selected to have matched phase within a frequency range up to about 1000 Hz. The microphone electronic circuits 406a, . . . , 406n are used to calibrate the microphones 404a, . . . , 404n, by changing the analog gain of the microphones so that they are identical.

[0053] The speaker 408 normally generates a muffler reduction noise oriented towards the interior and exterior of muffler 103. In order to prevent the reduction noise oriented towards the muffler exterior from emanating outside the muffler's wall, the speaker's posterior side is made of or is padded with insulating material.

[0054] In any of the embodiments so far described, if the two active noise reduction units (the motor active noise reduction unit 110 and the muffler active noise reduction unit 111) are located close to each other, interference may occur. A reduction noise generated by one unit may affect the other, reducing its performance of active noise reduction. Thus, when the microphone 203 for example measures the motor noise, it measures also a component of the muffler reduction noise reaching the motor active noise reduction unit, i.e. the microphone 203 measures a cumulative characteristic of motor noise together with a component of the muffler reduction noise. In response to measuring the cumulative characteristic of motor noise together with a component of the muffler reduction noise, the microphone 203 produces a signal referred to as a cumulative motor noise signal. In the same manner, microphones 404a, . . . , 404n measure a cumulative characteristic of muffler noise together with a component of the motor reduction noise, and produce a signal referred to as a cumulative muffler noise signal.

[0055] Therefore, in the case that interference exists, it is desirable to reduce the component of the muffler reduction noise from the cumulative characteristic of motor noise together with a component of the muffler reduction noise measured by the microphone 203 of the motor active noise reduction unit 110. It is likewise desirable to reduce the component of the motor reduction noise from the cumulative characteristic of muffler noise together with a component of the motor reduction noise measured by the microphones 404a, . . . , 404n of the muffler active noise reduction unit 111. This correction must be done notwithstanding that the motor reduction noise and the muffler reduction noise both change during the operation of the cooking hood.

[0056] FIG. 5 illustrates an interference-preventing electronic circuit 501 according to one embodiment of the invention. The microphone 203 of the motor active noise reduction unit 110 is positioned a constant distance from the speaker 408 generating the muffler reduction noise. Thus, for the purpose of interference preventing the medium through which the muffler reduction noise passes on its way to the microphone 203 can also be considered constant. Therefore, the way this constant medium affects the passing muffler reduction noise is also constant. This constant effect can be measured offline (i.e. when the cooking hood's motor is switched off) and represented by a muffler-motor transfer function 502 in ways known to those versed in the art. The muffler-motor transfer function 502 is coupled between the speaker 408 and the microphone 203 via a muffler-motor subtraction point 504.

[0057] The muffler-motor transfer function 502 receives the muffler noise signal fed into the speaker 408 of the muffler noise reduction unit 111. Representing the constant medium effect on the component of muffler reduction noise, the muffler-motor transfer function 502 calculates the component of muffler noise reduction signal corresponding to this component of muffler reduction noise.

[0058] The muffler-motor subtraction point 504 subtracts the component of muffler noise reduction signal from the cumulative motor noise signal produced by the microphone 203. Thus the subtraction yields an effective motor noise signal component that correspond to the effective motor noise. This effective motor noise signal component is used, instead of the cumulative motor noise signal, for the generation of the motor noise reduction signal, and its corresponding motor reduction noise, therefore reducing the effective motor noise and not the cumulative motor noise.

[0059] Likewise, the effect on the motor reduction noise generated by the speaker 202 and reaching microphones 404a, . . . , 404n may also be considered constant, and can be measured and represented by a motor-muffler transfer function 503. The motor-muffler transfer function 503 connects the speaker 202 to the microphones 404a, . . . , 404n via a motor-muffler subtraction point 505. The motor-muffler subtraction point 505 subtracts the component of motor noise reduction signal from the cumulative muffler noise signal produced by the microphones 404a, . . . , 404n. Thus the subtraction yields an effective muffler noise signal component corresponding to the effective muffler noise. This effective muffler noise signal component is used, instead of the cumulative muffler noise signal, for the generation of the muffler noise reduction signal, and its corresponding muffler reduction noise, therefore reducing the effective muffler noise and not the cumulative muffler noise.

[0060] If the muffler-motor transfer function 502 is known, and the changing muffler reduction noise is measured, it is possible to calculate the component of the muffler noise reaching the microphone 203. In the same manner, if the motor-muffler transfer function 503 is known, and the changing motor reduction noise is measured, it is possible to calculate the component of the motor reduction noise reaching the microphones 404a, . . . , 404n of the muffler active noise reduction unit 111.

[0061] As explained previously with reference to the muffler-motor transfer function 502 and the motor-muffler transfer function 503, it should be noted that in a non-limiting manner the components of the muffler and motor reduction noise can be measured offline to constitute a predetermined component of muffler reduction noise and a predetermined component of motor reduction noise. The predetermined muffler and motor reduction noise can then be subtracted to yield the effective muffler and motor signal components. This can be done by means of using transfer functions, as previously described with reference to FIG. 5.

[0062] Further to what has been described so far, also known to those versed in the art are active noise reduction units having microphones, speakers and electrical cancellation circuits, designed to reduce noise at the microphone position. Noise at the microphone location is referred to as the “near field”. However, noise reaches also to locations remote from the motor, referred to as the “far field”. Known active noise reduction units, which are designed to reduce noise at the microphone location, normally cannot reduce the far field noise, which is located far from the motor, and therefore far from the microphone.

[0063] FIG. 6 illustrates an emulation circuit 600 emulating the noise at the far field, according to one embodiment of the invention. The emulation circuit 600 includes a microphone 601 connected via a microphone electronic connection 602 to a microphone transfer function 603. The emulation circuit includes also a speaker 604 connected via a speaker electronic connection 605 to a speaker transfer function 606. The microphone transfer function 603 and the speaker transfer function 606 are connected by a summation point 607 where the emulation circuit's output is computed. The emulation circuit's output is directed through the output electronic connection 608.

[0064] The emulation circuit is suitable for both the motor noise reduction unit 110 and the muffler active noise reduction unit 111. Therefore, in the motor noise reduction unit 110, where the emulation circuit is part of the electronic circuit 204, the microphone 601 is either analogous to the microphone 203 as referenced in FIG. 2, or to microphones 301a, . . . , 301b as referenced in FIG. 3, and the speaker 604 is analogous to the speaker 202. In the muffler noise reduction unit 111, where the emulation circuit is part of the electronic circuit 409, the microphone 601 is analogous to the microphones 404a, . . . , 404n and the speaker 604 is analogous to the speaker 408.

[0065] It should also be noted that when the emulation circuit is applied to the motor active noise reduction unit 110 and with reference to FIG. 6, the term “noise” refers to “motor noise”, the term “noise signal” to “motor noise signal”, the term “noise reduction signal” refers to “motor noise reduction signal” and the term “reduction noise” refers to “motor reduction noise”. When applying to the muffler noise reduction unit 111, the terms refer to “muffler noise”, “muffler noise signal”, muffler noise reduction signal” and “muffler reduction noise” respectively.

[0066] The microphone 601 has to process the received acoustic signals (the noise) as if it were positioned in the far field at a far point where the noise is expected to be reduced to its minimal level. This far point is known to those versed in the art as an error point. The processing is carried out in order to tune the speaker 604 accordingly, so as to reduce the noise in the far field. The microphone transfer function 603 represents the environment between the microphone and the error point. Thus, applying the microphone transfer function 603 to a noise received by the microphone 601 emulates the noise that would have been received by a microphone positioned at the error point, if such a microphone would have been positioned thereat.

[0067] The speaker 604 produces noise (reduction noise according to the described embodiment) which reach the error point reducing noise thereat. A speaker transfer function 606 describes the environment between the speaker and the error point. By applying the speaker transfer function 606 to the noise reduction signal that is fed into speaker 604, it is possible to estimate the reduction noise that would have been received by a microphone positioned at the error point, if such a microphone would have been positioned thereat.

[0068] It should be noted that the term “emulated noise” (corresponding both to “emulated motor noise” and to “emulated muffler noise”) is used with reference to the noise corresponding to the noise signal received by the microphone 601 after being processed by the microphone transfer function 603. The noise signal, processed by the microphone transfer function 603 is referred to as an “emulated noise signal”. If the noise reduction signal is fed to the speaker transfer function 606 before being fed to the speaker 604 it is referred to as an “emulated noise reduction signal”. The noise corresponding to the emulated noise reduction signal generated by the speaker is referred to as “emulated reduction noise” (corresponding to “emulated motor reduction noise” and to “emulated muffler reduction noise”).

[0069] The noise at the error point is reduced to its minimal level when the sum (as computed by a summation point 607) of the emulated noise and the emulated reduction noise (i.e. when the sum of the emulated noise signal and the emulated noise reduction signal) is minimal. Therefore, in order to receive a minimal noise level at the error point when the two transfer functions and the noise are known, the emulation circuit computes a required noise reduction signal (i.e. the emulated noise reduction signal), and feeds this emulated noise reduction signal to the speaker 604, which generates the corresponding emulated reduction noise. Therefore, in the muffler noise reduction unit 111 (see FIG. 4), for example, the output signal of the emulation circuit is fed by the output electronic connection 608 to the electronic circuit 409, effecting adjustments as computed at the summation point 608.

[0070] The microphone transfer function 603 and the speaker transfer function 606 can be measured by many methods known in the art such as, for example, the Least Mean Square (LMS) method described in U.S. Pat. No. 6,389,440, U.S. Pat. No. 5,909,425 and U.S. Pat. No. 4,977,591 whose contents are incorporated herein by reference.

[0071] According to one embodiment of the invention, the microphone transfer function 603 and the speaker transfer function 606 can be measured offline (i.e. when switched off) and at any location, e.g. at a factory where the cooking hood is manufactured. As was explained above, the transfer functions 603 and 606 both represent the environment of the cooking hood. The environment is specific to the site and time of measurement. However, when moving the cooking hood to a different location, such as a kitchen, the environment changes. The microphone 601, the speaker 604 and the motor 104 are positioned close to each other. Because of this close position the environment's changing effect on the reduction noise transmitted from speaker 604 to the error point is approximately equal to its effect on the expected noise at the error point when measured by the microphone 601. Therefore, when the cooking hood is moved to a different position or location, the environment changes but as the microphone 601, the speaker 604 and the cooking hood (being a noise source) are positioned close to each other, the microphone transfer function 603 and the speaker transfer function 606 may be expected to change equally, and represent the environment's effect on the noise. That is, the noise arriving to the error point from the cooking hood changes in the same manner as does the reduction noise, thus compensating for the change. Thus, effecting noise reduction at the error point is expected be effective regardless of the environment where the cooking hood is located. This allows measuring the acoustical transfer functions offline and at any location and even moving the cooking hood to different locations with a minimal effect on the far field active noise reduction.