CA2145862C

CA2145862C - Active acoustic transmission loss box

Info

Publication number: CA2145862C
Application number: CA002145862A
Authority: CA
Inventors: Christopher R. Fuller; Michael Mcloughlin; Stephen Hildebrand
Original assignee: Noise Cancellation Technologies Inc
Current assignee: Noise Cancellation Technologies Inc
Priority date: 1992-10-08
Filing date: 1992-10-08
Publication date: 1999-03-09
Anticipated expiration: 2012-10-08
Also published as: CA2145862A1; WO1994009484A1

Abstract

The invention relates to noise or sound control achieved by enclosing the noise source in an active enclosure (10). Arrays of vibration inputs such as shakers, piezoceramic, etc. (14, 15) are attached to th e walls of the active enclosure, or loudspeakers located inside the enclosure can be used to excite the sites of the enclosure. A n array of error microphones are located in the radiated acoustic field or PVDF strips (20, 21, 22) are positioned on the wall. A control ler (19) senses the levels of sound observed at the error microphones (16, 17, 18) or PVDF film (20, 21, 22) and adjusts the osc illating inputs (in terms of frequency, content, phase and magnitude) to the active vibration inputs in order to minimize the rad iated sound.

Description

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 W094/09484 2 ~ PCr/USg2/0840 ~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~

WO 94/09484 PCr/USg2/08401 _ 4 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).

~145862 WO 94/09484 PCr/US92~08401 .~_ 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.

214586~
WO 94/09484 PCr/US92/08401 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 ~ .

Claims

1. An active noise reduction system for canceling a noise disturbance, said system comprising a structural container means surrounding a noise disturbance with actuator meansdirectly attached thereto to generate anti-vibrations in said structural container means, and a plurality of error sensing means in the radiated noise field and sensing noiseradiation external to said structural container means and providing error signals, and a reference signal generator means for providing a reference signal containing frequency and temporal information on the noise disturbance, and a controller means comprising circuit means for independently controlling each actuator, in response to said error sensing means and said reference signal to drive said error signals to minimum values simultaneously.

2. The system of Claim 1 wherein said actuator means are embedded piezoceramic actuators.

3. The system of Claim 1 wherein said actuator means are electrodynamic shakers.

4. The system of Claim 1 wherein said actuator means are surface mounted piezoceramic actuators.

5. The system of Claim 1 wherein said actuator means are loudspeaker means.

6. The system of Claim 1 wherein said error sensing means are PVDF film.

7. The system of Claim 1 wherein said error sensing means are microphones.

8. The system of Claim 1 wherein said error sensing means are hydrophones.

9. The system of Claim 1 wherein said structural container means has an air gap therein.

10. The system of Claim 9 wherein said actuator means are piezoceramic actuator means.

11. The system of Claim 9 wherein said actuator means are loudspeaker means.

12. The system of Claim 9 wherein said error sensing means are PVDF film.

13. The system of Claim 1 wherein said container means has wall means adapted to be close fitting to said noise disturbance.

14. The system of Claim 13 wherein said actuator means comprise piezoceramic actuator means.

15. The system of Claim 13 wherein said error sensing means comprises PVDF film.

16. A method for controlling sound radiation of a noise disturbance by active control of a structural transmission loss container, comprising the steps of:
(1) surrounding said noise disturbance with a structural transmission loss container;
(2) sensing a respective error signal indicative of the noise field external to said sound radiation;
(3) generating a reference signal containing frequency and temporal content of the noise disturbance;
(4) actively vibrating the structural transmission loss container with active inputs in the form of vibration inputs directly attached or injected into said structural transmission loss container via active actuators;
(5) controlling the sound radiation at the error signals by adjusting oscillating inputs to the active actuators by a suitable control law.

17. The system of claim 1 wherein the actuator means generate anti-vibrations only for vibrations of the structural container that are efficient radiators.