Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
W092/08223 ~ J ~3 PCT/GB91/01849
ACTIVE VIBRATION CONTROL SYSTEM
Field of Invention
The invention relates to active control systems for
vibration which reduce vibration in a region where it
is undesirable or i~,possible to mount residual sensors
in order to monitor the reduced level of vibration.
Background to the invention
, .
The term vibration, when u~ed in this document, should
be taken to include sound and other small amplitude
linear disturbances.
The use of simple active techniques to control sound
or vibration is well known, and the technology is
progressing fairly rapidly. The first work that was
reported, related to the control of sound in a duct
(Leuq US2043416). The system described used an
'upstream' microphone to act as an input sensor. This
input sensor generated a signal related to the
amplitude and phase of the sound wave progressing down
the duct (the primary sound wave). The signal from the
input sensor was used in a feedforward controller to '
generate another signal which was used to drive the
loudspeaker positioned further down the duct. (Here up
and down refer to the direction of propagation of the
primary sound wave.) The secondary sound field
generated by the loudspeaker destructively interfered
with the primary sound field to create a guieter
region downstream of the loudspeaker. The transfer
function of the feedforward controller (sometimes
referred to as the transfer characteristic, or
frequency response, or impulse response), which
relates the output signal of the controller to its
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W092/08223 ~J ~ j ~ PCT/GB91/01849
input signal, ~as adjusted during a set-up phase and
then left fixed. Since Lueg there were sirllar
systems described which accom21ished the same task but
used more ~odern technology.
-
Lawson-Tancred (GBl492963) described a system for a
duct but he also began to rec~gnize that the input
sensor would also respond to the cancelling field
(secondary field) generated by the loudspeaker (or
actuator). This made the controller more difficult to
set-up as the degree of attenuation was potentially
very sensitive to small changes in the feedforward
transfer function. Consequently, small changes in the
acoustics of the system being controlled (here a duct)
l~ or changes in the feedforward transfer function of the
controller due to the 'drift' in analogue components
resulted in poor sound reduction performance. One
feature of his invention was to incorporate a passive
attenuator between the loudspeaker and the input
sensor. Sw;nh~n~c (GB1456018) made further
improvements by usinq an input sensor which only
responded to and an actuator which only generated
sound in one direction, thus acoustically decoupling
the input sensor and the actuator. Chaplin (GB1555760)
recognized that this 'acoustic feedbacX' could be
dealt with by electronic subtraction once digital
technology was available with its more constant
characteristic. This move to digital technology and
the use of signal subtraction was a key aspect in the
30 development of the technolQgy.
Later Swinh~n~ (GB2154830 and GB2054999) devised a i-
scheme for adjusting the feedforward transfer function
of the controller in response to the output from a
35 second microphone positioned in the region where quiet
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W092/08223 PCT/GB91/01849
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was required (a residual sensor). The initial work
envisaged manual adjustment of the feedfor~ard
transfer function, but recognized that the process
could be automated. In 1981 Ross, one of the current
- inventors, (GB2097629/US4480333) showed ho~ the
transfer function of the feedforward part of the
controller could be automatically adjusted using
information from a residual sensor in the region where
the sound attenuation was reguired. The term
lC controller now being used to include the feedforward
path and the mechanism which adjusts the transfer
function of that feedforward path. One key aspect of
that invention was the recognition of the fact that
the feedforward transfer function was at its optimum
l_ when the correlation between the input sensor and the
residual sensor was at a minim~m, The automatic
proceedure inimized the correlation, and thus
described a self-calibrating system which could be
used to reduce the sound in a region around the
residual sensor. When a system of this type is used a
duct, for example, because the primary field is
propagating in one direction the region where quiet is
produced is everywhere downstream of the actuator, and
the residual sensor is at a point which represents the
acoustic excitation in that whole region due to waves
emanating upstream in the duct. Thus even though the
region is large, and goes beyond the locality of the
residual sensor, the residual sensor is in the region
and the cor~Loller is functioning by m;n1~izing the
sound at the residual sensor.
In 1983 Chaplin (PCT/GB82/00299 or GB2107960)
described the workings of a controller which was for
controlling 'periodic' sounds made by rotating or
repetitive sources. This controller used a tachometer
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W092/08223 PCT/G891/01849
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as an input sensor. An input se~sor of this type is
Gnly responsive to the phase of the (primary)
vibration field as it is not responsive to the
amplitude of the vibration. The system required
- sensors (microphones or accelerometers) in the region
where quiet was required, and the operation of the
system was to minimize the residual sensor signal's
correlation with the input sensor signal. There have
been further developments of such systems which
envisage using multiple actuators to produce the
secondary vibration field and multiple residual
sensors to measure the combination of the primary and
secondary vibration fields in the region where guiet
is required. Nelson and Elliott (GB2149614) described
a system of this type in which a few algorithms are
used to reduce the sound in a cabin. The system
minimizes the sum of the squares of the amplitude of
the residual sensor signals.
All systems of this type will give the best vibration
attenuation at or near to the residual sensors as they
are designed to reduce the sound at those points. One
fnn~ ~ntal problem has been that when an active
vibration control system is installed in a passenger
vehicle, for example, it is not convenient to position
the residual sensors in the region where the sound
reduction is required. Typically it is only
acceptable to mount the microphones in the roof trim
and yet the region in which the sound reduction is
required is the region where the passengers' ears
move. In the past, part of the skill in designing an
act~ve control system for such a cabin was in
positioning the microphones so that when the sound was
reduced at the microphones it also tended to be
.
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W092~08223 ~ J PCT/GB91/01849
reduced in the region required. There have been ~,any
papers pu~lished which describe an internal scund
field in terms of its 'acoustic modes' and how to
position the microphones and loudspeakers best to
reduce the modal amplitudes and thus the sound in the
cabin generally (Nelson, Curtis, Elliott & Bullmore,
Active minimization of harmonic enclosed sound fields,
Journal of Sound and Vibration, 117(1),1-~3).
However, all of these papers have envisaged using
10 control systems which minimize the sum of the squares
of the amplitudes of the residual sensor signals.
Further publications by Nelson & Elliott
(PCT/GB87/00706 and a paper which predates the
l; application: Elliot and Sothers, A multichannel
adaptive algorithm for the active control of start-up
transients, EUROMECH 213, September 1986, Marseille)
describe more control system algorithms for the
rin; ;zation of the residual sensor signals. However,
20 since the desire has been to reduce the sound in the
cabin generally the publication suggests that the
algorithms described achieve this, and do not
specifically mention that they are limited to
~;n; ;zing the sum of the squares of the residual
25 sensor signals and that any attenuation of the primary
sound field more generally is due to the careful
positioning of the actuators and residual sensors.
A system for controlling the vibration in a helicopter
30 8tructure generally i6 described by King (US4819182
and G~2160840) which uses multiple (12) residual
sensors, however, the system again envisages that by
reducing the vibration at the accelerometers the
vibration o~ the whole structure will be reduced, but
35 it does not describe a way of achieving this.
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W092/08223 PCT/GB91/01~9
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An earlier patent application by the cLrrent inventors
(PCT/GB89/00964) discusses the provision of an active
control system inside a cabin which achieves
attenuation of the sound at passengers' head
positions.
Summary of the Invention
According to the present invention an active vibration
control system comprises:
at least one input sensor which generates first
signals related to the phase and/or the amplitude of a
primary vibration field, : -
a plurality of actuators driven by second signals
which produce a secon~Ary vibration field,
a plurality of monitoring sensors positioned in a
first region where they are excited by the combination
of the said primary and second~ry vibration fields and
which produce third signals,
a controller including a feedforward path which is
responsive to the said first signals and generates the
said second signals so that the vibration in a second
region, which is excited by the said primary and
se-on~ry vibration fields, tends to be reduced,
30 and wherein the controller adapts the transfer ~:~
function of the said fee-dforward path, with reference
to the said first and third signals, so that the
vibration in the second region is maintained at a
reduced level.
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WO 92/08223 ~ PCT/G~91/01849
The controller's transfer function adaption m2y ~,a~e
use of the relationship between the primary vibration
field in the first and second regions.
The controller's transfer function adaption may ~ake
use of the measured relationship between the third
signals, and fourth signals obtained from calibration
sensors positioned in the second region during
calibration.
In any of the foregoing the controller's transfer
function adaption may make use of the relationship
between the secondary vibration field in the first
region and the controller output.
1'
The controller's transfer function adaption may make
use of the relationship between the secondary
vibration field in the second region and the
controller output.
In any of the foregoing the controller's transfer
function adaption makes use of the relationship
between the primary vibration field in the first and
second regions and the relationship between the
secondary vibration field in the first and second
regions.
In any of the foregoing the controller's transfer
function adaption may be mad~ so as to m;n; ; ze the
weighted sum of products of calculated signals which
represent the expected vibration in the second region.
The CGn~ oller's transfer function adaption may be
made so as to minimize the weighted sum of products of
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W092/082Z3 PCT/GB91/01849
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calculated signals which represent the expected
vibration ln the second region and the weighted sum of
products of the transfer function elements.
The controller's transfer function adaption may be
made so as to minimize the weighted sum of products of
the third signals where the weighted sum is chosen so
that the vibration in the second region is reduced.
lQ The controller may be calibrated using calibration
sensors positioned in the second region which produce
the said fourth signals representative of the
vibration in the said second region
The relationships A, B, C and D (as hereinafter
defined) may be stored as required in the controller.
The relationships A, C or D may be adjusted in order
to take account of changes.
The first region is close to and/or bounded by the
surface of a cabin interior to be occupied by at least
one person and the second region is that region within
the said cabin interior which will normally be
occupied by the person's head.
The actuators may be positioned close to or on the .
interior surface of the said cabin interior.
'
The actuators may be incorporated in the structure
s~L.ou~ding the said cabin.
As applied to a motor vehicle such as a car, the
primary vibration field is that produced by the engine
or tyres of the vehicle, the cabin interior is at
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W092/08223 ~ ~ ~ ,u~,-, PCT/GB91/01849
least part of the p2ssenger compartment of the vehicle
and the actuators are incorporated in the engine or
suspension mounting systems of the vehicle.
The first region may be close to and/or bounded by the
surface of a body and the second region may be the
surrounding space, and the actuators may be positioned
on or close to the surface of the said body, or
incorporated into the structure of the body.
The first and second reqions may of course partly
overlap.
The invention will now be described by way of example
with reference to the accompanying drawings, in which
Figure l shows the layout of prior art systems; and
Figure 2 shows an embody ?nt of the invention.
In the prior art system shown in Figure l the residual
sensors, 8, are positioned in the region where guiet
is desired, 9. The input sensor, 2, is a single
tachometer. The actuators, l, are loudspeakers. The
feedforward part of the controller, 7, has its
transfer function adjusted in response to the signals
from the residual sensor signals.
The present invention provides an active vibration
cont~ol system which reduces the vibrztion in a
(s~c:n~) region without the need to position residual
~ensors in that region. The invention does require
sensors (monitoring sensors) but these can be outside
the second reqion, where the vibration is to be
r~u~e~. To recognize the fact that these monitoring
senQors are not required to be in the second region
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W092/08223 J J PCl/GB91/01849
they are described as being in a first region. When
the system is operating, and the vibration in the
second region is reduced (by the tendency of the
primary and secondar~ fields to cancel each other),
the vibration at the monitoring sensors (ie in the
first region), which will also be a combination of the
primary and secondary fields, will not necessarily
have a lower amplitude than the primary field alone.
The third signals (from the monitoring sensors) are
used, in conjunction with the first (input sensor)
signals, to adapt the transfer function of the
feedforward part of the controller in order to
maintain the attenuation in the second region.
1; In one embodyment of the invention this operation can
be thought of as calculating the amplitude of the
vibration in the second region from the values of the
third signals and the first signals and adjusting the
feedforward transfer function to minimize this
calculated value.
It can thus be seen that the present invention differs
from previously disclosed systems in that the
additional sensors (in this case called monitoring
sensors to distinguish them from the 'residual'
sensors of previous systems) are not necessarily in
the quiet region and thus the adaption of the
feedforward transfer function of the controller of the
present invention does not minimize the sum of the
8quares of the amplitude of these additional sensor
~ignals.
'
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One way in which the present invention operates to
achieve this is now described with reference to Figure
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W092/08223 PCT/GB91/01849
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2. The active vibration control system has actuators,
l, driven by second signals which produce a secondary
vibration field, input sensors, 2, which generate
first signals related to the phase and/or amplitude of
the primary vibratio~ field, monitoring sensors, 3,
positioned in a first region, ~, which produce third
signals, and a controller, 5, which generates the
second signals in response to the first and third
signals. The primary and secondary fields meet ln a
second region, 6, and the transfer function of the
part of the controller between the first signals and
the second signals (the feedforward part), 7, is
adjusted in response to the first and third signals in
order to maintain a reduction in the vibration in the
second region.
The functioning of the controller will now be
described in the frequency domain. However, this
should not be considered as limiting the invention to
systems which operate in the frequency domain. There
are many equivalent time-domain systems (and other
frequency-domain systems) follow the following
formulation and which those skilled in the art wilI
see the correspondence. Examples of the
correspondence between different types of time and
frequency domain algorithms are given in Widrow &
Stearns (Adaptive Signal Processing, Prentice-Hall,
USA, 1985), and in Nelson & Elliott (PCT/GB87/00706).
During a calibration phase, before control is
required, calibration sensors are mounted in the
second region in order to give information about the
acoustics of the system under control and the
statistics of the signals. These calibration sensors
produce fourth signals. The calibration sensors can be
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W092/08223 PCT/GB91/01~9
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12
removed after the calibration proceedure has been
finished.
The signals from the input sensors (the first signals)
are represented by u (where u is a vector of i complex
numbers for a single frequency, with each element
corresponding to one of the i first signals). The n
second signals (which are used to drive the n
actuators) are represented by an n-long vector x. The
third signals (from the monitoring sensors) due to the
primary field alone are represented by an r-long
vector Ym~ and Pm when the primary and secondary
fields are present. The fourth signals (from the
calibration sensors) due to the primary field alone
are represented by a t-long vector Yc~ and Pc when the
primary and secondary fields are present.
The transfer function of that part of the controller
between the first and second signals is represented by
a complex matrix Q (which has n rows corresponding to
the n second signals, and i columns corresponding to
the i first signals). Thus:
_ = Q!l- (1)
:
The transfer function relating the second signals to
the third signals is represented by the complex matrix
A (which has r rows and n columns) and the transfer
function relating the second signals to the fourth
5ignals i6 represented by the complex matrix D (which
has t rows and n columns). Thus:
Pc ~ Yc + DQ~, and (2)
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W092/08223 ~ ~ ~ rJ ~' ~ b PCT/GB91/01849
Pm = Y~, + AQu.
It is a prlmary aim of the system is to minimize the
vibration field in the second region. This is ac~ieved
by positioning the calibration sensors in the second
region at a set of points which are representative of
the vibration characteristics of the said second
region. The output of the calibration sensors is
described above as the fourth signals. In one
embodiment of the invention the active vibration
control system operates by calculating the expected
value of the fourth signals from the third signals and
the first signals and adjusts the feedforward transfer
function to minimize the weighted sum of the squares
lS of these calculated signals.
During part of an initial calibration phase the
relationship between the parts of the fourth and third
signals that are correlated with the input signals and
which are due to the-primary field only can be
measured, and can be expressed in the following way:
.
{ ~ u*?{uu*}-l = C{Ym_*}{uu*~-l + e- (4)
Where C is a complex matrix with t rows and r columns
which is chosen to minimize the norm of e, and {..} is
the expectation operator.
Thus when the control system is operating the expected
value of the fourth signals is approximated by
- C ( Pm ~ AQ~ )- (5)
.
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W092/08223 PCT/GB91/01~9
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If the fourth signals were available and the aim of
the system were to minimize the weighted sum of t~e
products of these signals, Pc GPC (where G is a full,
t-by-t, complex, Hermitian matrix), then the optimum
feedforward transfer function Qopt is:
Qopt = ~ tD GD) lD G{ycu }{uu } 1 ~6)
If, additionally, the variance of the second signals
were to be minimized so that the function to minimize
was Pc GPC + ax Hx ~where H is a complex Hermitian
matrix) then the optimum feedforward transfer function
would be:
Qopt = ~ (D GD+aH~ lD G{YC_ }{uu } 1. ~7)
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During the calibration phase no attempt is made to
minimize the vibration in the second region but the
following quantities can be measured:
D (the relationship between the fourth signals and
the second signals) this is either measured with
the primary field off by supplying suitable second
signals during this first part of the calibration,
xt, to each actuator, which excite a secondary
vibration field, and recording the resulting
fourth signal, Ec~ in which case the signal to
noise ratio is good, or, alternatively, with the
primary field on when the signal to noise ratio can
be improved by increasinq the level of the second
l; signal during this first part of the calibration.
The estimate of D is
{~cxt }{Xt-t } ~ (8)
A (the relationship between the third signals and
the second signals) this is either measured with
the primary field off by supplying other suitable
second signals during this second part of the
calibration, xr, to each actuator, which excite a
secondary vibration field, and recording the
resulting third signal, ~m~ in which case the
signal to noise ratio is good, or, alternatively,
with the primary field on when the signal to noise
ratio can be improved by increasing the level of
the calibration second signals during this second
part of the calibration. The estimate of A is
{~m~r }{xrxr*}~l. t9)
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W092/08223 PCT/GB91/OlX49
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C (the relationship bet~een the part of the fourth
and third sign21s which is correlated with the
input signals and which is due to the primary field
alone varying over whole the range of expected
primary fields) this is measured during the third
part of the calibration by allowing the primary
field to vary over the whole range of expected
fields and recording Yc, Ym~ u during the
variation. The estimate of C is then calculated as
lC
{ycu*}{uu*}-l{uu }~l{uYm }[{Ymu }{uu } l{uu } ~ ,,
uYm } ] 1-
( 10 ~
1-- When the system is first switched on, the controller's -
feedforward transfer function can be set equal to the
value given in eguation (6) or (7). However, once the
system has been operating for some time the
vibrational characteristics may have changed and it
will be desirable to adapt the controller's
feedforward transfer function in order to compensate
for the changes. This now requires the calculated
value of the fourth signal to be used as the
calibration sensors are no longer available.
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W092/08223 ~ PCT/GB91/01849
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The new value o the optimur, feedforward transfer
function is:
l Qopt~ = ~ (D GD-aH) lD G{C(Pm - AQoptYi)U }iUU }
(11)
It may also be updated in a gradient descent algorithm
of the form:
Qopt' = (l-f)Qopt - cD G{CPmu ~ (D-
CA! QoptUU } {UU } 1 ~
(12)
:
where c is the step size and f is a small factor to
allow the size of the second signals to be limited
whilst minimising the vibration in the second region.
During operation of the controller it is desirable not
only to adapt the feedforward transfer function but
also to adjust the stored values of any parameters on
which the adaption process depends and which may have
changeid. It is possible to re-estimate the value of A,
on line, directly, using 'system identification', such
as described by Chaplin (GB2107960). It is more
difficult, on the other hand, to maintain an accurate
value of either C or D.
Modi~ication of D. The transfer function D represents
the ~ignal path from the second signals, through the
actuators, through the vibration bearing medium, to
the seCQn~ region. The transfer function A represents
the signal path from the second signals, through the
actuators, through the vibration bearing medium, to
the first region, and ~inally through the monitoring
.:
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W092/08223 PCT/GB9l/01849
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18
microphones. It will thus be seen that some elements
of the two transfer functions are similar and so it
may be appropriate to model the transfer functio~ D as
the product BA, where B is chosen so that D=BA.
Changes in A can then be used to update D by assuming
that B remains constant.
Choice of C. The initial choice of C, which satisfies
equation (4), is given by equation (10). ~owever, it
is likely that the matrix [~Ymu }{uu*} 1{uu } 1{uYm }]
which covers the range of expected primary vibration
fields will be ill-conditioned and thus computation of
equation (10) will be inaccurate. This indicates that
there is some flexibility in the choice of C for
excitations which do not fall in the sub-space
[{Ym~*}{ _ *} 1{uu*} 1{uYm*}]. When the primary field
falls outside that sub-space it is desirable that the
performance of the system is maintained. This could
be, for example, that the system minimizes the
weighted sum of the products of the monitoring sensor
signals (third signals) for fields outside the class,
and this can be accomplished by ensuring that the
matrix C tends to be such that D-CA=0 tor C tends to
be equal to B) for the class of excitations outside
the known sub-space. In this situation the system
operates to minimise the sound in the second region by
minimising a weighted sum of products of the third
signals, the weights being selected so that the
vibration in the second region is minimised.
An alternative method of finding C would extend the
range of primary vibration f ields measured during the
third part of the calibration by using additional
~ourc-s, driven by additional test signals, and close
to the positions of the real sources. This would
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W092/08223 l)J JiJ~,/ PCT/CB91/n1~9
19
ensure that the system would continue to rhinimise the
weighted su~ of products of the expected fourth
signals for all classes of expected primary fields and
those created by the additional test signals which
would be chosen to represent likely departures of the
primary field from its normal state.
The subject invention is particularly useful for
controlling sound in a cabin or passenger compartment
of a passenger vehicle. In that case the monitoring
sensors could be placed in or close to the trim of the
cabin walls, or in or close to the seats and the
second region would be the region in the cabin which
1, would be occupied by the heads of the passengers. The
actuators for producing the secondary vibration field
could be loudspeakers positioned in or close to the
trim of the cabin or in or close to the seats.
Alternatively the actuators could form part of the
structure of the cabin walls. For example, they could
be electro or magneto strictive materials attached to
the cabin walls which cause the cabin walls to
vibrate. A further alternative form of actuators
could be positioned in or close to the suspension
bushings or engine mounts or any other part of the
vibration path from a source of sound in a vehicle
cabin to the cabin interior. In this case the
monitoring sensors may be better positioned close to
the actuators and not in the trim as first described.
The invention will also be seen to be useful in
controlling the noise radiated from an object when it
is difficult or impractical to position residual
sensors in the far field. This would be the case in
controlling the radiated noise from a transformer
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ca---rg, a ship, or the tyre of a road ~?hicle. Ir ~ry
cf _h?S? Caâ?S the monitoring senscrs ~-ould likely ~e
p,-itioned close enough to the surface of the
radi~ting body to be in the near field of both the
actuators and the body itself. Consecluently, whilct
the primary and secondary fields will combine they
will not combine in a way to cancel each other when
the cancellation is at its optimum in the far field.
~he present invention is thus particularly useful in
controlling noise from such objects.
It is understood that the invention is not limited to
situations where the first and second regions do not
- o~erlap.
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