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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to active acoustic attenuation systems which reduce
noise by generating canceling sound substantially equal in magnitude but
opposite in phase to the noise, and in particular to such systems for
reducing tonal noise produced by small cooling fans for electronic
equipment such as computers
2. Background Information
Many computers and other electronic equipment use forced ventilation as a
means of cooling the electronic components. The primary complaint from
computer operators is that the ventilation fans produce annoying tones.
One solution places the operators in a separate room from the computer.
However, with the advent of the personal computer and the workstation, the
operator must be near the device
Fan noise originates from two sources. The first source is the turbulent
flow of air as it is exhausted by the fan. This noise is random and can be
reduced by proper fan and grill design. The second source of noise is
called blade pass noise. As the fan blade passes a nearby support, a
pressure wave is produced. Since the fan rotates at a constant speed, a
periodic sequence of pressure waves produces a fundamental tone plus
higher order harmonics.
Common solutions to fan noise control involve the use of sound absorption
devices and repositioning of the fans. Intake and exhaust silencers reduce
the noise produced by fans; however, these solutions sacrifice airflow,
i.e., reduce cooling, for noise reduction.
Active control of noise in enclosed systems such as ducts has been known
for sometime. In this type of noise control, a destructive interference
pattern is generated by a speaker positioned near the noise source, which
radiates a signal that is 180 degrees out of phase with the noise. The
signal from the speaker cancels the noise. Often, the source noise is
sampled by an input microphone to generate an input reference and the
resultant sound produced by the combination of the canceling wave from the
speaker with the noise is sampled by an error microphone. The difference
between the two sampled sounds is used to generate the signal driving the
speaker. Typically, the control system which generates the speaker signal
utilizes an adaptive filter which accommodates for the time required for
sound to propagate from the source and from the speaker to the error
microphone. A common type of adaptive filter used in this application is
the LMS (least means square) adaptive filter. An example of such a filter
is described in U.S. Pat. No. 4,473,906. A modification of this filter is
the filtered-X LMS adaptive filter which overcomes difficulties in
obtaining convergence of the adaptive filter to control tonal noise in the
presence of broad band noise. Such a filtered-X LMS adaptive filter is
described by Bernard Widrow and Samuel D Stearns in Adaptive Signal
Processing, Prentice-Hall, Inc., 1985, pp 288-292.
Active control systems which use the source noise as an input reference
signal must contend with feedback from the speaker to the input
microphone. Where fan blade pass noise is to be attenuated, this problem
can be avoided by using a signal representative of the rotational speed of
the fan as the input reference signal for the control system, as noted in
U.S. Pat. No. 4,677,677. An experimental system which uses a toothed wheel
and an optical detector to generate a square wave signal from which the
blade passing frequency can be derived is described by G. H. Koopmann and
D. J. Fox, in Active Source Cancellation of the Blade Tone Fundamental and
Harmonics in Centrifugal Fans Journal of Sound and Vibration, Academic
Press Limited, 1988, pp 209-220. That system, however, uses two 2-channel
phase shifting phase locked loops to generate signals for two speakers
used to attenuate the fundamental frequency and only one selected harmonic
of the blade pass noise in a centrifugal fan. Microphones are used only to
monitor the resultant sound and not to control the phase locked loops
which must be manually adjusted.
There is a need for an improved active acoustic attenuation system for
attenuating tonal noise generated by rotating equipment.
There is a further need for such a system which attenuates the fundamental
and all harmonics of tonal noise generated by the fans.
There also is a need for such a system which can be implemented
economically.
SUMMARY OF THE INVENTION
These and other needs are satisfied by the invention which is directed to
an active acoustic attenuation system for attenuating tonal noise
associated with rotating apparatus including means generating a sinusoidal
reference signal having a frequency representative of the rotational speed
of the apparatus. The invention is particularly applicable to fans having
multiple blades which produce blade pass tonal noise having a fundamental
frequency which is a multiple of the fan rotational speed, and harmonics
thereof.
In accordance with one aspect of the invention, the sinusoidal reference
signal is generated by an accelerometer which detects imbalance in the
rotating fan. Preferably, the sinusoidal signal is passed through a low
pass filter having a cut-off frequency only slightly higher than the
frequency of the sinusoidal signal. The filtered sinusoidal signal
eliminates the effects of broadband noise such as that generated by
turbulent flow and other noise such as bearing noise. Since a fan has
several blades, the filter cutoff frequency is below the fundamental
frequency of the blade pass noise, let alone the harmonics. However, a
signal containing the fundamental frequency and harmonics of the blade
pass noise is generated by clipping the sinusoidal signal. Preferably, the
clipping is effected by a zero crossing detector which produces a
rectangular wave signal having a fundamental frequency equal to the
rotational speed of the fan. A rectangular wave with other than a 50% duty
cycle is needed so that the harmonics of the clipped sinusoidal speed
signal include the fundamental frequency of the tonal noise and its
harmonics. Such an unbalanced rectangular wave can be generated by the
zero crossing detector by delaying the change of state of the rectangular
signal for zero crossings in one direction.
Canceling means in the form of a speaker spaced from the fan generates
canceling sound which combines with and attenuates the fundamental
frequency and harmonics of the tonal noise. The clipped sinusoidal signal
is applied, together with an error signal produced by a microphone which
picks up the combined sound of the fan and the speaker, to an electronic
controller which drives the speaker. An analog to digital converter
digitizes the sinusoidal signal for input to the electronic controller in
the form of a digital computer which is programmed to implement the zero
crossing detector which clips the sinusoidal signal and to implement the
adaptive filter. In the exemplary system the adaptive filter is a
filtered-X least means square (LMS) adaptive filter.
As applied to an axial flow fan with a central hub, the acoustic
attenuation system of the invention includes mounting the speaker
coaxially with the fan hub. In the case of a fan with a coaxial discharge
conduit, the speaker is mounted in the conduit spaced from, but coaxially
with and facing, the fan. Where the fan has a discharge chamber with a
lateral opening, the speaker is mounted in a rear laterally extending wall
spaced from the fan by the lateral opening, with the speaker coaxially
aligned with and facing the fan. In this latter configuration, the speaker
can be larger in lateral dimension than the hub without at all restricting
the flow of air discharged by the fan.
BRIEF DESCRIPTION OF THE DRAWINGS
A full understanding of the invention can be gained from the following
description of the preferred embodiments when read in conjunction with the
accompanying drawings in which:
FIG. 1 is a schematic diagram in block form of an active acoustic
attenuation system for a fan in accordance with the invention.
FIG. 2 is a diagram in block form of the program for the electronic
controller which forms a part of the system of FIG. 1.
FIG. 3A is a plot of the sinusoidal reference signal produced by an
accelerometer used by the system of the invention.
FIG. 3B is a plot of a rectangular wave signal produced by clipping the
sinusoidal signal of FIG. 3A in accordance with the invention so that the
rectangular wave signal does not have a 50% duty cycle.
FIG. 4 is a schematic diagram illustrating one configuration of an
acoustics attenuation system of the invention as applied to an axial flow
fan.
FIG. 5 is a schematic diagram illustrating another configuration of an
acoustics attenuation system of the invention as applied to an axial flow
fan.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention will be described as applied to an axial flow fan 1 as shown
in FIG. 1, however, those skilled in the art will realize that the
invention has application to other types of rotating apparatus which
generate tonal noise. The fan 1 has a plurality of blades 3 which can
produce blade pass noise as the blades successively pass a nearby support.
The periodic sequence of pressure waves produced as the successive fan
blades 3 pass a support produces a fundamental tone and higher order
harmonics.
The exemplary fan 1 is an axial flow fan driven by an induction motor
energized by 60 Hz ac power. Due to slip inherent in the induction motor,
the fan has a rotational speed of 3180 rpm or 53 Hz. The fan 1 has five
blades 3 such that the fundamental tone of the blade pass noise has a
frequency of 265 Hz with second through fifth harmonics occurring at a 530
Hz, 795 Hz, 1060 Hz, and 1325 Hz, respectively.
The active acoustic attenuation system 5 of the invention includes an
accelerometer 7 mounted on the fan 1. The accelerometer responds to the
inevitable imbalance of the rotating parts of the fan 1 to generate a
sinusoidal signal having a frequency equal to the rotational speed of the
fan. In the case of the exemplary fan this is 3180 rpm or 53 Hz. This
sinusoidal signal is amplified in a charge amplifier 9 and the amplified
sinusoidal signal is passed through a low pass filter 11. The low pass
filter 11 has a 60 Hz cutoff frequency to attenuate noise in the
accelerometer signal. The filtered sinusoidal signal is applied to an
analog to digital converter 13 which digitizes the analog sinusoidal
signal for input into a digital signal processor 15. The digital signal
processor 15 is a single chip microcomputer such as a Texas Instruments,
Inc. TMS320C14 digital signal processor.
The digital signal processor 15 responds to the digitized sinusoidal
reference signal, X, and a feedback error signal, E.sub.m, to generate a
digital signal, Y. The signal Y is converted to an analog signal in
digital to analog converter 17, amplified in a power amplifier 19, passed
through a low pass filter 20 having a cut-off frequency above a desired
harmonic of the tonal noise (for example 2000 Hz) and applied to a speaker
21. As will be discussed more fully later, the speaker 21 is positioned to
generate a canceling sound which combines with the sound generated by the
fan 1. The canceling sound produced by the speaker contains the
fundamental frequency and selected lower harmonics of the tonal noise
produced by the fan properly phase shifted and adjusted in magnitude by
the digital signal processor 15 to produce a destructive interference
pattern with the tonal noise. The combined noise is picked up by an error
microphone 23. The error signal generated by the error microphone 23 is
amplified in a microphone amplifier 25 and passed through a low pass
filter 27. The low pass filter 27 has a cutoff frequency of 2000 Hz such
that the fundamental frequency and the major harmonic frequencies of the
tonal noise are passed. This filtered signal is digitized in an analog to
digital converter 29 to produce the digital error signal E.sub.m.
The digital signal processor 15 serves as an electronic controller for
generating the cancellation signal, Y, which drives the speaker 21. A
microcomputer which serves as the digital signal processor 15 is
programmed first to clip the reference signal, X. Clipping of this
digitized sinusoidal signal is effected by a zero crossing detector
algorithm 33 which converts the sinusoidal signal to a rectangular wave
signal having the same fundamental frequency as the sinusoidal signal. As
is well known, the rectangular wave signal also contains harmonics of the
sinusoidal signal. As will be recalled, the frequency of the sinusoidal
signal, and therefore of the rectangular wave signal also, is equal to the
frequency of rotation of the fan. However, the fundamental frequency of
the tonal noise is a multiple of this frequency determined by the number
of fan blades, and hence, corresponds to one of the harmonics of the
rectangular wave reference signal. In the exemplary system where the fan
has five blades, the fundamental frequency of the tonal noise corresponds
to the fifth harmonic of the rectangular wave reference signal It follows
then, that the harmonics of the tonal noise are higher order harmonics of
the rectangular wave reference signal. Thus, in accordance with the
invention, a reference signal having a fundamental frequency of the tonal
noise and its significant harmonics but without significant extraneous
noise is simply and economically generated using an accelerometer and a
zero crossing detector algorithm
It should be noted that the rectangular wave signal generated by clipping
the sinusoidal speed signal should not be a square wave, that is a
rectangular wave with a 50 percent duty cycle, since a square wave only
contains odd harmonics of the fundamental square wave signal. A
rectangular signal with other than a 50 percent duty cycle contains both
odd and even harmonics which are required to assure that the fundamental
of the tonal noise and its significant harmonics are present. The zero
crossing detector can be adjusted to produce such an unbalanced
rectangular wave from a sinusoidal signal by delaying the change of state
of the rectangular signal for zero crossings in one direction. This
technique as applied to the digitized sinusoidal signal, X, is illustrated
by FIGS. 3A and 3B. With the sinusoidal signal, S, going positive, the
rectangular wave R goes high at the first sample providing an indication
of a zero crossing. However, when the sinusoidal signal is going negative,
the rectangular wave signal R does not go low until several samples after
the zero crossing. Selection of the delay can be made to adjust the
contribution of selected harmonics, however, as will be discussed, the
adaptive filter can adjust the gains for the various harmonics as long as
they are present.
Returning to FIG. 2, the digital signal processor 15 in addition to
clipping the sinusoidal accelerometer signal using the zero crossing
detector algorithm 33, also implements the adaptive filter 39, which in
the exemplary system is a filtered-X LMS (least means square) filter. Such
a filter is described in Adaptive Signal Processing referred to above.
This filter includes an active control adaptive filter 41 having a
transfer function A(z), an LMS algorithm 43 and an error plant 45 with a
transfer function C(z). The filter 41 is a transversal filter which uses
the reference signal X as clipped by the zero crossing detector 33 to
generate the speaker signal Y which drives the speaker 21 to produce sound
with the appropriate frequencies and phase shifts to cancel the tonal
noise produced by the fan. The LMS algorithm 43 is a least means square
algorithm which monitors the error signal, E.sub.m, generated by the error
microphone 23, and the clipped reference signal, X, and adjusts
coefficients in the transversal filter 41 in a manner which produces the
least means square error between the fan tonal noise and the output of the
speaker 21. The combination of the adaptive transversal filter 41 and the
LMS algorithm is known as an LMS adaptive filter. Such a filter is
described for instance in U.S. Pat. No. 4,473,906.
The error plant 45 applies the transfer function C(z) to the clipped
reference signal, X, to accommodate for the acoustic delays between the
speaker 21 and the error microphone 23. Actually, the error plant models
the system from the digital to analog converter 17 through the power
amplifier 19, the low pass filter 20, the speaker 21, the error microphone
23, the acoustic path between the speaker 21 and the error microphone 23,
the amplifier 25, and the low pass filter 27 back to the analog to digital
converter 29.
The error plant 45 used is another LMS adaptive filter in which the
coefficients are set by generating random noise into the digital to analog
converter 17 and into the LMS filter of the error plant. The output of the
error plant is compared with the error signal E.sub.m, and the difference
is used to adapt the filter of the error plant. The error plant filter
coefficients then become set at the adapted values.
The error plant 45 shifts the phase of (delays) the clipped reference
signal, X, generated from the accelerometer into the same time domain as
the error microphone signal. The gradient (the cross-correlation between
the output of the error plant and the error signal E.sub.m) is then used
to update the coefficients of the active control adaptive filter 41. The
addition of the error plant 45, makes the LMS adaptive filter comprising
41 and 43 the filtered-X LMS adaptive filter 39.
It may be noted that the clipped reference signal, X, produced by the zero
crossing detector 33, contains, in addition to the fundamental of the
tonal noise and its harmonics, the fundamental of the sinusoidal
accelerometer signal and its other harmonics which do not correspond to
the tonal noise fundamental and its harmonics. However, since those
frequencies are not in the noise picked up by the microphone, the gains of
the active filter 41 for these frequencies will be driven to zero, and
thus the speaker will not generate sound at those frequencies. It is the
nature of these adaptive filters that the reference signal must contain
all of the frequencies that are to be canceled, and other frequencies are
attenuate to zero by the filter which adjusts the coefficients for those
frequencies to provide a gain of zero. It should also be noted that the
relative magnitudes of the fundamental and harmonic frequencies in the
reference signal do not have to be the same as in the actual noise, since
the coefficients of the filter will adjust to provide the appropriate
gain.
Another aspect of the invention is directed to the placement of the speaker
and error microphone. As shown in FIG. 4, the typical small fan used for
cooling electronic equipment, is an axial flow fan 47 having a central hub
49 which houses an electric motor (not shown) driving the fan. In the
arrangement shown in FIG. 4, a discharge conduit 51 is axially aligned
with the fan so that airflow produced by the rotating blades 53 of the fan
47 passes through a grill 55 into the atmosphere. In accordance with this
embodiment of the invention, the speaker 57 generating the canceling sound
is mounted in the discharge conduit 51 in axial alignment with and facing
the hub 49 of the fan 47. It will be noted that the speaker 57 is no
larger in diameter than the hub 49 so that it does not interfere with the
flow of cooling air through the discharge duct 51. The error microphone 59
is mounted in the discharge conduit 51 where it can pick up the
combination of the noise from the fan and the canceling sound from the
speaker 57.
FIG. 5 illustrates another embodiment of the invention in which an axial
flow fan 61 having blades 63 and an axial hub 65 discharges cooling air
into a discharge chamber 67 having a lateral opening 69 covered by a grill
71. In this arrangement, the discharge chamber 67 has a rear wall 73
spaced from the fan 61 by the lateral opening 69. The speaker 75 is
mounted in this rear wall 73 facing the fan 61 and axially aligned with
the hub 65. This arrangement allows a speaker of any size to be used,
including a speaker which is larger in lateral dimension than the hub 65
of the fan, without interferring at all with air flow. The error
microphone 77 is placed in the chamber 67 at a location where it can pick
up both the fan noise and the canceling sound generated by the speaker.
While specific embodiments of the invention have been described in detail,
it will be appreciated by those skilled 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
* * * * *
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