Title: ACTIVE CONTROL OF SOUND Professor Mike Brennan Institute of Sound and Vibration Research University of Southampton, UK
1ACTIVE CONTROL OF SOUNDProfessor Mike
Brennan Institute of Sound and Vibration
ResearchUniversity of Southampton, UK
2Active control of sound
- Active control of sound in ducts
- Single secondary source
- Two secondary sources
- Where does the power go?
- Control of harmonic disturbances
- Control of random disturbances
- Single channel feedforward control
- Constraint of Causality
- Active control of sound in enclosures
- Cars
- Aircraft
- Active head sets
- Vibroacoustic control
3Passive Control of Sound
?
Sound source
Observer
Passive control relies on barriers, absorption
and damping. It works well when the acoustic
wavelength is short compared with typical
dimensions ? Higher frequency solution.
4Active Control of Sound
?
Sound source
Observer
Acoustic or structural actuators are driven to
cancel waves It works well when the acoustic
wavelength is long compared with typical
dimensions ? Lower frequency solution.
5Patent for Active Control of sound by Paul Lueg
1936
Active Control of Duct-Borne Sound
6Loudspeaker source in a duct
If the frequency of interest is such that the
acoustic wavelength is greater than twice the
dust cross-section then it can be modelled as a
pair of massless pistons forced to oscillate
apart with a fluctuating volume velocity q(t)
between them.
7Loudspeaker source in a duct
For x gt 0 the complex pressure and particle
velocity fluctuations can be written as
8The plane monopole source
9The plane monopole source
We define the source strength as
So
10Cancellation of downstream radiation using a
single secondary source
Secondary source
Primary source
11Cancellation of downstream radiation
Secondary source
Primary source
This requirement is
that is the secondary source is a delayed
inverted form of the primary source.
12The net sound field in the duct
The field between the primary and secondary
sources is give by
Upstream of the primary source it is given by
Downstream of the secondary source it is given by
13The net sound field in the duct
Note that when Ln?/2 the pressure upstream of
the primary source 0
14Time domain interpretation
Secondary source
Primary source
15Cancellation of downstream radiation using a pair
of sources
Primary source
Secondary sources
16The net sound field in the duct
The field upstream of the secondary sources is
given by
Between the secondary sources it is given by
Downstream of the secondary sources it is given by
17The net sound field in the duct
18Time domain interpretation
19Time domain interpretation
Secondary sources
Primary source
20Sound absorption by real sources
Electrical power supplied
The acoustical power can be negative in such
cases less electrical power will be required to
sustain a given piston velocity u
21The influence of reflections from the primary
source
Absorbing surface having a complex
reflection coefficient R
Secondary source
Primary source
22The influence of reflections from the primary
source
Secondary source
Primary source
23Adaptation in Feedforward Control
Active Control of Transformer Noise, Conover 1956
An error microphone is introduced to monitor the
performance. Changes in the disturbance and plant
response, from loudspeaker to the microphone,
require adaptation of the feedforward controller.
24Single channel feedforward control
Periodic Primary source
Error sensor
Secondary source
Electrical reference signal
Electronic controller
(Unaffected by secondary source)
25Single channel feedforward control
26Control of random noise in a duct
Sound from Primary source
Error sensor
Secondary source
Detection sensor
Electronic controller
There are two main differences between the
control of random and harmonic disturbances
- The detected signal x(t) is generally influenced
by the - electroacoustics of the feedback path
2. There is a constraint of causality on the
controller
27Control of random noise in a duct
Measurement noise at detection sensor
Primary path
Signal at detection sensor
Signal to secondary source
Controller
Error signal
Error path
Signal due to primary source
Feedback path
Measurement noise at detection sensor
28Optimal controller
disturbance and measurement noise
The block diagram becomes
Primary and measurement noise
Error signal
Controller and feedback path
Error path
29Optimal controller
Power spectral density of the error signal is
where E is the expectation operator and
denote complex conjugation
30Optimal controller
The power spectral density of the error signal
can be written as
31Optimal controller
To find minimum error substitute
into
32Optimal controller
Controller
Error signal
Error path
Feedback path
33Digital implementation of the controller
Sound from Primary source
Secondary source
Error sensor
Detection sensor
Electronic controller
34Digital implementation of the controller
The overall frequency response of the controller
is
Sampling time
Frequency response of filters and data converters
Digital filter
Causality condition
Approximate delay through an analogue filter is
roughly due to 45 phase lag or 1/8 cycle of
delay at its cut-off frequency, fc Total delay
through two filters which have a total of n poles
is n/8fc The cut-off frequency is typically 1/3
the sampling frequency (fs1/T), so that
fcfs/31/(3T) Allowing 1 sample delay for the
data converters and the digital filter means the
total delay is given by
35Causality condition - example
Sound from Primary source
Secondary source
Error sensor
Detection sensor
Electronic controller
Rectangular duct with largest dimension D0.5m
single channel control can only be achieved
below about 300 Hz
Sampling frequency 1kHz (T1ms) Two 4th order
analogue filters (n8) Delay in analogue path is
about 4ms
36Active control of sound in a duct experimental
work (Roure 1985)
37Active control of sound in a duct experimental
work (Roure 1985)
Amplitude spectra of the fan noise at the error
microphone with a mean duct velocity of 9m/s
Active control off
dB
Active control on
Frequency (Hz)
38Active control of sound in enclosures
Electronic Sound Absorber H.F. Olson and E.G.
May, Journal of the Acoustical Society of
America, pp. 1130-1136, 1953
39Active Control of Sound inside Cars
Low-frequency engine noise in the car cabin can
be controlled with 4 loudspeakers, also used for
audio, and 8 microphones, also used for
hands-free communication (Elliott et al. 1986).
40Initial Demonstration Vehicle
41Measured Results in a Demonstration Vehicle
A-weighted sound pressure level at engine firing
frequency
42Active Sound Control in Propeller Aircraft
System is standard fit on Dash 8 Q400 (Stothers
et al. 2002)
43Active Sound Control in Propeller Aircraft
www.bombardier.com Periodic excitation generates
intense harmonic soundfield inside cabin
44Active Sound Control in Propeller Aircraft
Spectrum of Pressure Inside Propeller Aircraft
45Active Sound Control in Propeller Aircraft
Control System for Propeller Aircraft Active
Noise System
Centralised digital system made by Ultra
Electronics controls 5 harmonics with 48
structural actuators at 72 acoustic sensors,
distributed throughout cabin.
46Active Sound Control in Propeller Aircraft
Typical Performance of an Active Aircraft System
Single multichannel centralised digital
controller used with 48 actuators and 72 sensors
distributed throughout the cabin
47Feedback control of Sound
Active Headset using Feedback Control
If no external reference signal is available,
conventional feedback control can be used to
control sound at low frequencies.
48Feedback control of Sound
Active Headset using Feedback Control
Active control off
dB
Active control on
Frequency (Hz)
49Feedback control of Sound
Active Headset using Feedback Control
www.Bose.com
50Active headrest
51Active headrest zones of quiet
kL0.2
KL0.5
10dB
20dB
KL1
KL2
52Active Vibroacoustic Control
53The Problem
baffle
Incident sound power
Transmitted sound power
Simply supported panel
Objective To minimise the transmitted sound power
54The Active Control System
Panel
Accelerometer
Piezoceramic actuator
Analogue controller
55Piezoceramic Actuators
56Active Control Performance (simulations)
57What Happens to the Panel Vibration?
Integrated from 0-1kHz
Piezoceramic Actuators
Kinetic energy (dB)
Force Actuators
Feedback gain
58Experimental Result (after Bianchi et al)
Pressure (dB re arbitrary units)
- Gain limited by accelerometer resonance
- Compensator used in feedback circuit
59 Concluding Remarks
- Active sound control is being used as an
alternative to passive - control in many different applications
especially at low - frequencies
- ducts
- aircraft
- automobile
- Combination of acoustic and vibration control
maybe seen in - the future