Note: Descriptions are shown in the official language in which they were submitted.
ACTIVE ACOUSTIC TRANSl\~ISSION LOSS BOX
Back~round of the Invention
The present invention relates generally to noise or sound control and more
particularly to the control of radiated sound from vibrating machinery by enclosing the
5 machinery in what is termed an "active box or container". The purpose of the active box is
to markedly reduce the radiation of the sound from the machine to observation points in the
surrounding field, with a very lightweight, compact, non-airtight structure.
Discussion of Related Art
In many applications the radiation of sound from vibrating machines is an
10 annoying noise problem. One technique which has been used in the past is to enclose the
machine in a high transmission loss (TL) box in order to reduce the radiated sound (as
described, for example, in U.S. Patent No. 4,715,599 and in "Noise and Vibration Control"
by L. Beranek, 1988). These conventional boxes attenuate the sound transmitted through
their walls by passive means. In order that the container be effective, i.e. strongly reduce
15 the sound, it has to be both airtight and constructed from material which has a high density
and thickness. These t~,vo conditions have a number of practical disadvantages. For
example, the airtight condition implies that it would be extremely difficult to build an
effective high TL container for applications which require air flow (e.g. a.c. units,
compressors, etc.) or piping and wiring connections or ventilation for cooling. These
20 requirements would imply significant holes through which the acoustic energy could leak.
The high density material condition of course would imply that the box be extremely heavy
and large in size, a problem which is exacerbated as the frequency of sound becomes lower.
Previous work has shown the extremely high potential of using active
vibration inputs to structures to reduce the radiated sound from the structural vibration.
25 Such work is described in "Apparatus and Method for Global Noise Control", U.S. Patent
No. 4,715,599, 1987, by C.R. Fuller and "Control of Sound Radiation with Adaptive
Structures", Journal of Intelligent Material Systems and Structures, Vol. 2, pp. 431-452,
1991, by R.L. Clark and C.R. Fuller. The control inputs can be in the form of point force
shakers or surface strain devices, such as piezoelectric elements, bonded to the surface of
30 the structure. In order that the control approach be efficient and effective, the variable to
be minimi7~d has to be the radiated sound from the panel, measured, for example, by error
microphones located in the radiated sound field as in Fuller. The controller format can be
any control approach which adjusts the oscillating voltage inputs to the piezoelectric inputs,
for example, in order to minimize the radiated sound observed at the error microphones.
35 Polyvinylidene fluoride (PVDF) piezoelectric distributed
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~nsors on the surface of a panel have been used in place of mi~;l~hones to sense modes
of the panel which are r~ ting effir~ently to the far field such as that described in
"Modal ~nsing of ~rr- ;~ ." ~o~ ;c r~ol~ with polyv~lidene flnl~nde ~ trih~ltrd
sensors in active ~lluclul~l ~eo~ control approaches", J. Aeol~ti~l Society of
~m~( ~, pp. 3321-3329, June 1992,' by Clark and Fuller. The work of Clark and Fuller,
for ~ ,lf, d~ n~l~at4s 5i~ AI;f~ of the order of 20 dB of sound r~ t~d from
panels in the low ~u~ l~cies ff 5 600 Hz) with only one or two active ~cl~Ato) inputs.
Ob~t~ of thr Tnv~minn
o It is ~coldillgly an object of the present invention to achieve high ~tten~l~tion of
;Alf~l sound from a vih~ting n..~r.l-;,u. by enclosing it with an "active acoustic
."~".;~:,.n loss box".
It is another object of the invention to achieve very high global (here global
means th~oughout an ~;~ t. .l~led area of volume) of sound with the above box constructed
15 from very lightweight thin m~t~i~l, or to use the sides of the sound source itself to
reduce .q~ tr~ noise.
It is another object of the invention to achieve ve~y high global sound ~t.. l;on
with a cn..-A;n. . that is not airtight, rather it has c~ ;r~c~nl air gaps or holes located in
the walls of thc co~
These and other objects will become a~)a~ when l~.f.,~ ce is had to the
accompanying drawings in which
Figure 1 is a scl~ of a typical box (in this case r~ctA~g,.l~r) SW1UU1~ding a
noisy ~-~ e The active inputs, e~ror mi~ hone,s and PVDF film as rliscu~c~d above
are shown. Also 13e...~ t~.t~l iS an air gap in the box sidewall.
Figure 2 is a typical general controller h--i ~ U.~ 1 used to derive the correctactive control signaL using mi~ù~ s as error sPnc~
Figure 3 is a typical gener~al controller ~rr~ng~ t used to derive the correct
active control signal using PVDF film as an error sensor.
Flgu1re 4 is a scl-e-~-~l;r of the use of panels to s.,l~ ,d a noisy sllu~wc.
Figure S is an ~ ull plot of typical noise radiation from an c~ Gs~ with and
witi~Oul active control.
Pigure 6 shows a typical noise ~cllwll at a s~ t~ error mi.ilo~llone with and
WillWUI controL This result shows control of br~ nll or mul~le L~u~ cies
~,;",..1~ ouCly.
~ 7 ~
Summary of the Invention
The machine to be quieted is surrounded by an active enclosure. Arrays of
vibration inputs (for example, shakers, piezoceramic, etc.) are attached to the walls of the
active enclosure, or loudspeakers located inside the enclosure can be used to excite the sites of
the enclosure. An array of error microphones are located in the radiated acoustic field or
PVDF strips are positioned on the wall. A controller senses the levels of sound observed at the
error microphones or PVDF film and adjusts the oscillating inputs (in terms of frequency,
content, phase and magnitude) to the active vibration inputs in order to minimi7~ the radiated
sound. On minimi~ing the sound at the error microphones or PVDF film the radiated sound
from the machine is globally attenuated. Note that the container can be of any shape and
material, and can have significant air gaps through the walls.
Description of Preferred Embodiments
Referring now to the drawings, an example configuration of the "Active
Acoustic Transmission Loss Box" is shown in Figure 1 as 10. A machine 11 is operating and
radiating unwanted noise inside the box. The machine requires some air flow for cooling etc.
as well as piping and electrical connections and an air gap 23 can be provided. In order to
control the sound radiation the machine is surrounded by an enclosure, in this case a
rectangular box 12. In the example of Figure 1, the box 12 is resting on the machine support
base 13 but also could totally surround it. Damping or absorptive materials can also be added
to the box to attenuate high frequency noise and improve the structural response of the
enclosure. The box can be constructed from a variety of materials such as thin steel,
aluminum, etc. In the case shown, the box is manufactured from 6.35 mm plexiglass and has
dimensions 304.8 x 304.8 x 406.4 mm. Piezoceramic control actuators such as 13, 14, 15
(type G1195 of thickness 0.19 mm and dimensions 38.1 x 63.5 mm) are bonded to the center
of each panel. Each actuator consists of a piezoceramic element bonded onto each side, co-
located and wired in parallel with 180~ phase shift. Such a configuration produces high
vibration of the panels. These elements can be positioned in various arrays and also embedded
in the material if required.
In order to sense the radiated noise field, a number of error microphones such
as 16, 17, 18 are positioned in the radiated noise field. The number and location of the error
microphones is dependent upon the modal contribution (from the panel vibration) and radiation
directivity of the noise. Hydrophones may be used in place of error microphones 16, 17, 18.
A controller 19 is employed which measures the output of the error microphones and then
constructs an oscillating control signal of the correct frequency content and phase which, when
fed to the control actuators 13, 14, 15, etc. causes the sound to be markedly reduced at the
error microphones and other locations. An alternative to microphones is PVDF thin film which
can be placed on the
L~
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walls in such a way that energy in the r?~livsting modes is sensed. One l~ossible
configurstiQn for the PVDF strips such as 20, 21, ~ is shown in Figure 1. Another
s.1t~rnative would be to use accell.u... t - ~ to sense the motion of ~ir~c points on the
çn~lo~ ; walls.
One paTticular control strsngt m~nt embodies the Filtered-X adaptive LMS
slg~rithm J;s~ l by FuUer. An osG1lls~ing lef~,~.lce signal which has the Ll~u~,ncy
content ~f the noise to be c~ W is taken from machine 50. This .~f~ nce signal 51 is
also highly cohere.ll with the output of the error cl~hol~s. The reference signal is
passed through an analog to digital (A/D) converter 52 and fed through a number of
adaptive filters 53. The ~ of adaptive filters is equal to the n~ ~. of control
~ t.~lo.~ used. The arr.sn~-m~.nt of the adaptive filter is d~,~e~-de~lt upon the frequency
content of the noise. The outputs of Ihe adaptive filters is then passed through D/A
converters 54 and smoothing filters 55. For p;e,~ ~c actuators 57, this control
signal is typically passed through a high voltage power ~ ;r~e~ and then connect~l to
15 the elec~,des of each nctllst~r. The e~ror signals from the ~;lophoncs 56 are s~mpled
using A/D converters and then used in conillnction with the ~,Ç~.r~,Ace signal and a
filtered-X update equation in the controller 61 in order to adapt or change the
coP,ffi~ent~ of the adaptive filters so as to minimi7~ the C11U1 signals from the
mi.i.u~hn~-es as far as possible.
In an e p~ g,~ .nl to test the ~. r5,.. ~ of such a system the
noisy ...~1.;. ~ is replaced with a 165.1 mm speaker 58 po~ in a 184.2 m x 184.2 m
x 114.3 m reflex box. Various test L~u~ cies are then fed to the speaker to ~n~-~dle
noise. The lef~ ce signal 51a in this case is taken direcdy ~om dle signal 59 dIiving
the s,l,e~L~ - . For this test the control actuators on Ai~ .h ;I ~ily o~J~sil~ panels were
25 wired in phase, creating in cûn~ .. with a top n~ OI 60, dlree i~ d~t controlçh~nnPl~ and hence three ~ , filters. Three error u~ Jpl.o..~.s such as 56 were
at a ~ ce~ of a~ u~ately 2 m from the box. In this ~n~em~nt the air
gap 23 shown in Figure 1 is a~ t~.d by raising the box of 254 mm blocks at each
corner thus leaving a total air gap of 361.2 cm2, giving a pc..,~,nlage open area in the box
30 of 6.5%.
Figure S shows a typical l- A;A~ directivity pattern ~ d around t,he box at
mid plane and a ~ ce of 1.7m. The curve 90 labeled "cont~ol off" gives the r~ te l
noise field wit,hûut any enClo~lJ~;. The curve 91 "control on" gives the radiated noise
field when the box is in place but the control is not activated. It is al)pal~,nl that the box
3s only provides a small ~ I;Qn of the sound. When the control is tumed on, the results
of ~igure 6, labeled "with control" show high sound re~uctinn~ of the order of 20 dB at
all angles (i.e. global control).
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.~_
As .1;~C~S~1 by Fuller in Patent No. 4,715,599 the active a~n~ n is achieved
as follows. The noise source inside the box radiates sound which strikes the e~-clr~sl~e
walls and causes it to vibrate (at the same frequency content as the noise source). The
vil~ting walls then radiate sound away to the exterior free field of the box where it
s appears as ullwalllcd noise. The active inputs work as follows. The structural ~rn~atnrs
cause anti-vibratinn in the walls of the enclosllre. When the inputs to the structural
Slu~ a are adjusl~d cc~l~lly these anti-vibrations cancd out those vibrations in the
box which were previously ra-liatin~ sound, thus leading to glob~ sound reduction. As
in Fuller's patent, not all vibrations (or modes) in the enrlosllre will radiate sound and
10 thus the active inputs need only cancel those vibr~tiQn~ (or modes) that are efficient
l9~ c.~ ~ rather than controlling all the vibratiQn This al,pl~ach leads to a very low
nu~. of control actuators as opposed to totally c~ce~ling the box vibration, and is the
key to the success of dhe a~ &ch.
An alt~rn~tive, shown in Figure 4, is to enclose dhe noisy structure 80 with close
15 fitting panels 85 instead of a free st~nding enclos~lre. In dlis case dle enrlos~lre panels
are ~tt~hed direcdy to dhe sides of dle noise source. If dhe regions gel~cl ~ing noise are
locali7~1 or if noise control is needed in certain d~cliOlla, an advantage to this method is
dlat the need to enclose dhe entire allU~lUI~ is el~ In addition, in many cases a
mo~e colni~n~ enclosllre can be conahll~t~ wi~ ul ltsll;cling airflow needed for20 cooling. An e-;.n .ple of an applirptiQn of this method would be for the lrJ~Iul;Ol- of
"hum"fromek rUlnh,~. T~.,f~ noiseis~ 1~from
m~ tos~ e forces in the coil and are prop~g,~t~ l to the l~ ,fol~ skin through the
oil field and coil ~...,~l}-;nn
Figure 4 shows a c~ncell~tion system 80 for ~ - Iclos; -g a noisy ~7llu~;lul~ with close
2s fitting panels. Controller 81 receives a reference signal 82 from the structure and inputs
83, from error micr~hone 84. Ac~ualc,l~. 86 are located on close fitting panels 85.
Still another ~lt~rn~tive shown in Figure 3 is to place the ~ ol directly on thesurface of the noise source.
Flgure 3 shows noise re~uction system 70 with active S~ClU~;J1 control provided
30 with a Noise ~nce-ll~1ion Te~hnolc~s, Inc. controller 71 and power ~mplifier 72 having
outputs to p P7~C~ such as 73, 74 and inputs from PVDF sensor film
strips such as 75, 76, 77.
An ~lt~fn~tive to using structural ~ v~. to and-vibrate the enClos~lre walls is
to use lnu~ e~k~rs to g~ te a yl~,ssul~, field inside the box that will p~duce the anti-
3s vibl~ >n~ (~mlJin~l;ol~.; of diff~ ll sensors such as speakers and mic.~ phones can alsobe used.
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Having ~les~ibe~l the ill~,.,~n in detail it will be obvious to those of o¢dinary
skill in the art that ch~n~es can be made without deyal~g ~om the scope of the
ay~.lded claims in which ~ .
