Electrical audio signal processing systems and devices – Acoustical noise or sound cancellation – From appliance
Reexamination Certificate
1999-10-12
2001-02-13
Isen, Forester W. (Department: 2747)
Electrical audio signal processing systems and devices
Acoustical noise or sound cancellation
From appliance
C381S071900, C381S071140, C415S119000
Reexamination Certificate
active
06188770
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to fan noise cancellers and, more particularly, to a fan noise canceller, which is applicable to all fans as sources of noise, such as cooling fans for home electric products and office appliances and also air conditioner fans, and adopts an active system.
Fan noise muffling techniques are roughly classified into those of a passive system, which use sound absorbers and sound insulators, and those of an active system, which positively generate a sound wave in the opposite waveform relation to the fan noise and muffle the fan noise by sound wave interference.
A prior art active noise cancellation system for fan noise canceller is shown in FIG.
8
.
As shown, this fan noise canceller comprises a first microphone
51
disposed in a duct
100
at a position near a fan
50
, a second microphone
52
disposed in the duct
100
and at a predetermined distance from the first microphone
51
, and a muffling loud-speaker
53
disposed mid way between the microphones
51
and
52
.
The fan noise canceller further comprises a controller
54
for controlling the sound wave for cancelling the fan noise outputted from the cancelling loud-speaker
53
according to input signals from the first and second microphones
51
and
52
.
In this fan noise canceller, a sound wave which is generated from the fan
50
as a source of noise and propagated through the duct
100
is detected by the first microphone
51
and coupled to the controller
54
. At this time, a signal from the second microphone
52
which evaluates the cancelling effect is also coupled to the controller
54
.
The second microphone
52
for evaluating the cancelling effect, detects a sound wave that results from the interference of the sound wave generated from the cancelling loud-speaker
53
and the sound wave propagated from the fan
50
. The controller
54
drives the cancelling loud-speaker
53
by generating, in a digital signal processing or like process, a signal for making the signal from the second microphone
52
to be zero. This has an effect of reducing noise at the position, at which the second microphone
52
is disposed.
The prior art fan noise canceller as shown above has an advantage that it can be installed after the installation of the fan
50
. It also has an advantage that it cancels noise on the side of the second microphone
52
(i.e., adjacent the duct end from which air is sent out), and the operation thus is not readily affected by the noise characteristic changes or system changes in long use.
In the above prior art fan noise canceller, however, the first microphone
51
and the cancelling loud-speaker
53
are disposed such as to form a closed loop as an electrical-acoustical system. Therefore, the operation of the controller
54
readily becomes unstable, and sometimes hauling occurs to increase the noise.
In addition, in the prior art fan noise canceller a harmonic wave is generated by detecting the number of rotations of the fan. Therefore, a predetermined time is required for the signal processing that is necessary for generating the opposite waveform sound wave. For this reason, this fan noise canceller is unsuitable for a fan which does not have any duct, although it is suitable for the fan with the duct because a predetermined distance is provided between the fan and the cancelling loud-speaker. This means a disadvantage of the prior art fan noise canceller in that it is necessary to provide the duct or the like.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome the inconveniences in the prior art fan noise canceller and effectively reduce at least the level of the high noise level blade passing frequency, thus providing an efficient and highly reliable fan noise canceller.
According to a first aspect of the present invention, there is provided a fan noise canceller comprising rotation information detecting means for detecting fan rotation information containing the blade passing frequency of noise generated from a fan with the rotation thereof and converting the detected information into an electric signal, a frequency component extracting means for receiving the output of the rotation information detecting means and extracting the blade passing frequency of the noise generated from the fan, an output control means for controlling the amplitude and phase of a blade passing frequency signal of the noise extracted by the frequency component extracting means, and a cancelling loud-speaker for converting an electric signal covering the blade passing frequency signal outputted from the output control means into a sound signal and providing sound of the sound signal for propagation in an interfering relation to noise from the fan, wherein the rotation information detecting means including a rotatable disc coupled to the shaft of the fan and carrying change information corresponding to the number of blades of the fan, and a signal detector for detecting rotation information of the rotatable disc and outputting signals of the blade passing frequency contained in the rotation information and equal to the product of the number of rotations of the fan and the number of blades thereof and harmonics of the blade passing frequency as electric signals.
In the first aspect of the present invention, the rotation information detecting means which is operable with the rotation of the fan, generates an electric signal containing a frequency equal to the product of the “number of rotations per second” and the “number of blades”, and on the basis of this electric signal reference signals for cancelling feature frequency noises of the fan are generated.
Among the reference signals thus generated, a feature frequency signal (here a blade passing frequency signal) is extracted by the frequency component extracting means, which extracts the frequency equal to the product of the “number of rotations per second” and the “number of blades”. The extracted signal and the linear feature frequency noise are strongly correlated to each other, and the output control means for changing the amplitude and phase of the extracted signal, generates an opposite waveform to the linear feature frequency noise. Finally, the loud-speaker generates the opposite waveform sound wave, which is propagated in an inferring relation to the fan noise, thus realizing the cancelling.
The fan noise features that it comprises a wideband component resulting from eddy, separation, etc., and discrete noise components which are attributable to the rotation (i.e., pronounced peak components appearing at harmonics of the rotation frequency). Of the latter components, the one at the frequency equal to the product of the “number of rotations per second” and the “number of blades” of the fan is extremely pronounced. Cancelling this component thus extremely contributes to the reduction of the fan noise. According to the first embodiment of the present invention this is made possible.
According to a second aspect of the present invention, there is provided a fan noise canceller comprising rotation information detecting means for detecting fan rotation information containing the blade passing frequency of noise generated from a fan with the rotation thereof and converting the detected information into an electric signal, a frequency component extracting means for receiving the output of the rotation information detecting means and extracting the blade passing frequency of the noise generated from the fan, an output control means for controlling the amplitude and phase of a blade passing frequency signal of the noise extracted by the frequency component extracting means, and a cancelling loud-speaker for converting an electric signal covering the blade passing frequency signal outputted from the output control means into a sound signal and providing sound of the sound signal for propagation in an interfering relation to noise from the fan, wherein the rotation information detecting means including magnetic members each installed on each of the fan blades, a magnetic sensor disposed in
Isen Forester W.
Mei Xu
NEC Corporation
Sughrue Mion Zinn Macpeak & Seas, PLLC
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