CA1298793C - Active noise control - Google Patents
Active noise controlInfo
- Publication number
- CA1298793C CA1298793C CA000566554A CA566554A CA1298793C CA 1298793 C CA1298793 C CA 1298793C CA 000566554 A CA000566554 A CA 000566554A CA 566554 A CA566554 A CA 566554A CA 1298793 C CA1298793 C CA 1298793C
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- CA
- Canada
- Prior art keywords
- microphone
- duct
- noise
- sound
- propagated
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Abstract
ABSTRACT OF THE DISCLOSURE
Active Noise control An active noise control system for reducing the amount of noise propagated along a duct comprises a microphone mounted in the wall of the duct for detecting the noise, and an active antiphase noise source, such as a loudspeaker, which is mounted substantially in the centre of the cross-section of the duct. A control circuit coupled between the microphone and the antiphase noise source includes an integrator having a specific transfer function, which improves the loop gain of the microphone/source loop at low frequencies, and secures stability by altering the phase.
Active Noise control An active noise control system for reducing the amount of noise propagated along a duct comprises a microphone mounted in the wall of the duct for detecting the noise, and an active antiphase noise source, such as a loudspeaker, which is mounted substantially in the centre of the cross-section of the duct. A control circuit coupled between the microphone and the antiphase noise source includes an integrator having a specific transfer function, which improves the loop gain of the microphone/source loop at low frequencies, and secures stability by altering the phase.
Description
Active Noise Control This invention relates to active control of noise in ducts. The principles of active noise control were established by Paul Lueg in 1936 and basically consist of detecting by a microphone the noise which it is wished to control, and replaying the detected noise in anti-phase via a loudspeaker so that the regenerated noise destructively interferes with the source noise.
Since that time there has been a great deal of research in the field of active noise control. However, the basic configuration for active noise control in a duct has been the pravision of the microphone in the centre of the duct and of the loudspeaker in the duct wall. There are good reasons for this arrangement which will be gone into in greater detail later on in this specification.
However, it has been discovered that this known arrangement has disadvantages when there is a fluid medium flowing through the duct.
An ob~ect of the present invention is to provide an actlve noise control system which reduces these disadvantages.
According to the present invention there is provided an active noise control system for reducing the amount of noise propagated through a duct, comprising, a microphone incorporated in a wall of said duct, and operative for detecting the sound of the propagated noise; a source of anti-sound mounted substantially at the centre of the transverse crocs-section of said duct; and a control circuit responsive to the magnitude of the sound detected by the microphone for driving the anti-sound source to 129~793 substantlally suppress first transverse mode excltatlon ln ~ald duct.
In accordance with the present invention there is provided an active noise control system for reducing noise propagated along a duct through which a fluid medium flows, comprising: ~a) a microphone for detecting the propagated nolse;
(b) means for mounting the microphone in and flush with a wall of the duct at a substantially calm location where the fluid medium flows at a virtually zero velocity; (a) anti-sound means for radiating sound in an anti-phase relationship with the nolse detected by the microphone; (d) means for mounting the anti-sound means substantially at the centre of a tranæverse cross-section of the duct of minimized transverse mode excitation within the duct;
and (e) control means operatively connected to the microphone and the anti-sound means, and operative for driving the anti-sound means in response to the noise detected by the mlcrophone to radlate æound whlch destructively interferes with the propagated noise.
The anti-noise æource may comprise a loudspeaker mounted within the duct, or may comprise an outlet to which the output of a loudspeaker iæ plped, the loudspeaker itself being external of the duct.
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which, Figure 1 is a diagrammatic view of a known active noise control system, h~' lZ98793 2a 20305-1269 Figure 2 is a similar view of a system according to the present invention, Figure 3 is a perspective view of a duct, Figure 4 is a response graph, and Figure 5 is a block diagram of a control circuit.
Referrlng now to the drawings, Flgure 1 shows a known arrangement which is essentially that established by Paul Lueg.
In this arrangement a sensing microphone 1 is positioned in the centre of a duct 2 and a loudspeaker 3 is located in the duct wall. This is the simplest form of an active attenuator for duct-borne sound. The system includes a controller 4 which includes an electrical signal delay to compen~ate for the acoustic propagation delay from the sensing microphone 1 to the anti-source loudspeaker 3. If the microphone is placed directly in front of the loudspeaker piston, this acoustic delay is eliminated and the controller can be a simple inverting amplifier. Since the mlcrophone senses the æound from the loudspeaker in addition to the prlmary noise, a closed loop conflguration exists and there is a danger of instabllity. Substantial nolse attenuatlon can be obtalned provlded the loop gain is high, but stability crlteria llmlt thls. In ducts, the posltions of the sensor and the loudspeaker are important for stablllty and maxlmum obtainable attenuatlon, ao w1ll be shown ln the following paragraph.
:
., .
;
,; :: . :
129~7~93 This can be appreciated by considering the duct shown in Figure 3. A guided propagated wave in this rectangular duct can be described by equation (1);
p(x,y,~,t) = Re.Be~i~Jt~ejbnx. ~n(Y z) (1) where p is the acoustic pressure ~n.B is the pressure amplitude bA = ~k2 - a2 k = the wave number [ ( Y) ( Z) ~ (2) n .~.Y n7.~r. z rn(Y,2) = D(ny~nz)~cos ~ .cos--I (3) with the constant D(ny,nz) determined from the identity for the orthogonality of eigenfunctions:
1/A ~ ~(y,z). ~,(y,z)dA = ~n' where A is the duct cross-sectional area.
A microphone placed in this duct will sense the pressure as described in equation (1) and measures both plane and transverse waves. The latter cannot be cancelled with a simple monopole antisource and the contribution of these waves to the total pressure, and consequently to the overall loop gain, does not contribute to the cancellation of plane waves. The phase shift caused by these transverse modes, especially at resonance frequencies, is also detrimental to the noise reduction which is obtainable. This is due to the reduction in the open loop gain necessary to maintain stability. It can be shown, however, that the ratio between total acoustic pressure and pressure due to plane waves is minimal when the microphone is placed in the 12S~793 duct centre. Ir the loudspeaker is mounted in the duct wall, as in Figure 1, most transverse m~des can be generated in addition to the plane wave mode, and the plane waves and even-numbered (ny and nz are even numbers) transverse modes are sensed by the centre mounted m~cro-phone. Positioning the ~icrophone in the centre Or the duct has, however, the disadvantage that airrlow in the duct causes turbulence at the microphone resulting in a locally generated noise field. This gives rise to the electrical output of the microphone no longer being directly related to the acoustic field propagating down the duct. This severely restricts the obtainable attenuation and some form of microph~ne wind screening is essential.
Accordingly the present invention proposes that the microphone should be located flush with the duct wall, as is shown in Figure 2 of the accompanying drawings.
In this position the microphone no longer generates any flow noise because the air flow velocity at the duct surface is zero. However, there is limitation Or attenu-ation because the microphone is no longer at a position where the contribution of transverse modes to the total acoustic pressure is minimal.
This problem can be alleviated by placing the antisource loudspeaker in the centre of the duct. In this way a minimum number of transverse modes are generated. Thus if a point source (xO,yO~zO~ is placed in a duct the pressure amplitude can be written as ptx,y,z) = S~X~(x). ~ (y,z) (4) -2~S. ~ (YO-zO) jb ~
with S the monopole pressure amplitude. Fro~ equat~on (3) it can be shown that yO = ~ and zO 3~ then ~n~Yo,zO) ls aonzero ~
only if ny and nz are even integers. Hence, only one quarter of all transverse ~aves will be generated.
.
:
, : :
, ~
1298~93 It has been found that an acti~e noise control system with the conriguration Or a wall-mounted microphone and a centre-placed antisource yields satisfactory r attenuation when there is airflow in the duct.
It will be appreciated that an antisource placed in the duct rather than in the duct wall will generally occupy a larger volume than a microphone and will therefore provide a larger obstruction to the airflow. In most practical applications, however, the active system will be integrated with a passive absorber, such as a splitter silencer. In such a case there would not be a significant increase in the overall air resistance.
Another important consideration is system stability.
The active noise control system operates in a closed loop configuration due to the acoustic signal path from the loudspeaker back to the sensing microphone, and consequently the system could become unstable. To prevent this, stability criteria must be met and gain and phase need to be controlled. Since the amplitude-frequency response of a loudspeaker rolls of r at low frequencies (i.e. a decreasing output with decreasing frequency), the open loop gain in this frequency re,gion will decrease as well. ', The effect on the closed loop transfer function is that the loop phase goes through zero, which could lead to instability.
To meet this problem the system according to the invention incorporates an integration circuit. This is shown at 10 in Figure S from which figure it can be seen that the control circuitry leading from microphone 1 to loudspeaker 3 comprises a microphone preamplifier 9, the integrator 10 and an inverting power amplifier 11.
The inverting amplifier 11 provides the necessary phase shift to ensure that the output of the loudspeaker 3 interferes destructively with the noise detected bythe microphone 1.
lZ98793 The integrator circuit 10 is intended not only to improve the loop gain at low frequencies thereby increasing the achievable attenuation, but also to modify the phase shift around the loop to secure operational stability. The integrator circuit 10 has therefore been given the amplitude-frequency response shown in the graph of Figure 4. To produce this response the circuit 10 has a transfer function H(s) =
s~ + 1 where s = j. ~ , j = Ir-~ is the frequency in rads and ~ the circuit time constant. High frequency stability can be ensured by reduction of gain by means of passive absorptive material placed on the walls of the duct.
Since that time there has been a great deal of research in the field of active noise control. However, the basic configuration for active noise control in a duct has been the pravision of the microphone in the centre of the duct and of the loudspeaker in the duct wall. There are good reasons for this arrangement which will be gone into in greater detail later on in this specification.
However, it has been discovered that this known arrangement has disadvantages when there is a fluid medium flowing through the duct.
An ob~ect of the present invention is to provide an actlve noise control system which reduces these disadvantages.
According to the present invention there is provided an active noise control system for reducing the amount of noise propagated through a duct, comprising, a microphone incorporated in a wall of said duct, and operative for detecting the sound of the propagated noise; a source of anti-sound mounted substantially at the centre of the transverse crocs-section of said duct; and a control circuit responsive to the magnitude of the sound detected by the microphone for driving the anti-sound source to 129~793 substantlally suppress first transverse mode excltatlon ln ~ald duct.
In accordance with the present invention there is provided an active noise control system for reducing noise propagated along a duct through which a fluid medium flows, comprising: ~a) a microphone for detecting the propagated nolse;
(b) means for mounting the microphone in and flush with a wall of the duct at a substantially calm location where the fluid medium flows at a virtually zero velocity; (a) anti-sound means for radiating sound in an anti-phase relationship with the nolse detected by the microphone; (d) means for mounting the anti-sound means substantially at the centre of a tranæverse cross-section of the duct of minimized transverse mode excitation within the duct;
and (e) control means operatively connected to the microphone and the anti-sound means, and operative for driving the anti-sound means in response to the noise detected by the mlcrophone to radlate æound whlch destructively interferes with the propagated noise.
The anti-noise æource may comprise a loudspeaker mounted within the duct, or may comprise an outlet to which the output of a loudspeaker iæ plped, the loudspeaker itself being external of the duct.
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which, Figure 1 is a diagrammatic view of a known active noise control system, h~' lZ98793 2a 20305-1269 Figure 2 is a similar view of a system according to the present invention, Figure 3 is a perspective view of a duct, Figure 4 is a response graph, and Figure 5 is a block diagram of a control circuit.
Referrlng now to the drawings, Flgure 1 shows a known arrangement which is essentially that established by Paul Lueg.
In this arrangement a sensing microphone 1 is positioned in the centre of a duct 2 and a loudspeaker 3 is located in the duct wall. This is the simplest form of an active attenuator for duct-borne sound. The system includes a controller 4 which includes an electrical signal delay to compen~ate for the acoustic propagation delay from the sensing microphone 1 to the anti-source loudspeaker 3. If the microphone is placed directly in front of the loudspeaker piston, this acoustic delay is eliminated and the controller can be a simple inverting amplifier. Since the mlcrophone senses the æound from the loudspeaker in addition to the prlmary noise, a closed loop conflguration exists and there is a danger of instabllity. Substantial nolse attenuatlon can be obtalned provlded the loop gain is high, but stability crlteria llmlt thls. In ducts, the posltions of the sensor and the loudspeaker are important for stablllty and maxlmum obtainable attenuatlon, ao w1ll be shown ln the following paragraph.
:
., .
;
,; :: . :
129~7~93 This can be appreciated by considering the duct shown in Figure 3. A guided propagated wave in this rectangular duct can be described by equation (1);
p(x,y,~,t) = Re.Be~i~Jt~ejbnx. ~n(Y z) (1) where p is the acoustic pressure ~n.B is the pressure amplitude bA = ~k2 - a2 k = the wave number [ ( Y) ( Z) ~ (2) n .~.Y n7.~r. z rn(Y,2) = D(ny~nz)~cos ~ .cos--I (3) with the constant D(ny,nz) determined from the identity for the orthogonality of eigenfunctions:
1/A ~ ~(y,z). ~,(y,z)dA = ~n' where A is the duct cross-sectional area.
A microphone placed in this duct will sense the pressure as described in equation (1) and measures both plane and transverse waves. The latter cannot be cancelled with a simple monopole antisource and the contribution of these waves to the total pressure, and consequently to the overall loop gain, does not contribute to the cancellation of plane waves. The phase shift caused by these transverse modes, especially at resonance frequencies, is also detrimental to the noise reduction which is obtainable. This is due to the reduction in the open loop gain necessary to maintain stability. It can be shown, however, that the ratio between total acoustic pressure and pressure due to plane waves is minimal when the microphone is placed in the 12S~793 duct centre. Ir the loudspeaker is mounted in the duct wall, as in Figure 1, most transverse m~des can be generated in addition to the plane wave mode, and the plane waves and even-numbered (ny and nz are even numbers) transverse modes are sensed by the centre mounted m~cro-phone. Positioning the ~icrophone in the centre Or the duct has, however, the disadvantage that airrlow in the duct causes turbulence at the microphone resulting in a locally generated noise field. This gives rise to the electrical output of the microphone no longer being directly related to the acoustic field propagating down the duct. This severely restricts the obtainable attenuation and some form of microph~ne wind screening is essential.
Accordingly the present invention proposes that the microphone should be located flush with the duct wall, as is shown in Figure 2 of the accompanying drawings.
In this position the microphone no longer generates any flow noise because the air flow velocity at the duct surface is zero. However, there is limitation Or attenu-ation because the microphone is no longer at a position where the contribution of transverse modes to the total acoustic pressure is minimal.
This problem can be alleviated by placing the antisource loudspeaker in the centre of the duct. In this way a minimum number of transverse modes are generated. Thus if a point source (xO,yO~zO~ is placed in a duct the pressure amplitude can be written as ptx,y,z) = S~X~(x). ~ (y,z) (4) -2~S. ~ (YO-zO) jb ~
with S the monopole pressure amplitude. Fro~ equat~on (3) it can be shown that yO = ~ and zO 3~ then ~n~Yo,zO) ls aonzero ~
only if ny and nz are even integers. Hence, only one quarter of all transverse ~aves will be generated.
.
:
, : :
, ~
1298~93 It has been found that an acti~e noise control system with the conriguration Or a wall-mounted microphone and a centre-placed antisource yields satisfactory r attenuation when there is airflow in the duct.
It will be appreciated that an antisource placed in the duct rather than in the duct wall will generally occupy a larger volume than a microphone and will therefore provide a larger obstruction to the airflow. In most practical applications, however, the active system will be integrated with a passive absorber, such as a splitter silencer. In such a case there would not be a significant increase in the overall air resistance.
Another important consideration is system stability.
The active noise control system operates in a closed loop configuration due to the acoustic signal path from the loudspeaker back to the sensing microphone, and consequently the system could become unstable. To prevent this, stability criteria must be met and gain and phase need to be controlled. Since the amplitude-frequency response of a loudspeaker rolls of r at low frequencies (i.e. a decreasing output with decreasing frequency), the open loop gain in this frequency re,gion will decrease as well. ', The effect on the closed loop transfer function is that the loop phase goes through zero, which could lead to instability.
To meet this problem the system according to the invention incorporates an integration circuit. This is shown at 10 in Figure S from which figure it can be seen that the control circuitry leading from microphone 1 to loudspeaker 3 comprises a microphone preamplifier 9, the integrator 10 and an inverting power amplifier 11.
The inverting amplifier 11 provides the necessary phase shift to ensure that the output of the loudspeaker 3 interferes destructively with the noise detected bythe microphone 1.
lZ98793 The integrator circuit 10 is intended not only to improve the loop gain at low frequencies thereby increasing the achievable attenuation, but also to modify the phase shift around the loop to secure operational stability. The integrator circuit 10 has therefore been given the amplitude-frequency response shown in the graph of Figure 4. To produce this response the circuit 10 has a transfer function H(s) =
s~ + 1 where s = j. ~ , j = Ir-~ is the frequency in rads and ~ the circuit time constant. High frequency stability can be ensured by reduction of gain by means of passive absorptive material placed on the walls of the duct.
Claims (8)
1. An active noise control system for reducing the amount of noise propagated through a duct, comprising: a microphone incorporated in a wall of said duct, and operative for detecting the sound of the propagated noise; a source of anti-sound mounted substantially at the centre of the transverse cross-section of said duct; and a control circuit responsive to the magnitude of the sound detected by the microphone for driving the anti-sound source to substantially suppress first transverse mode excitation in said duct.
2. A system as claimed in claim 1, wherein said anti-sound source comprises a loudspeaker.
3. A system as claimed in claim 1, wherein said control circuit includes integrating circuit means for improving loop gain and aiding stability of a loop comprising said microphone and said anti-sound source by changing the phase of a microphone signal output.
4. A system as claimed in claim 3, wherein said control circuit further comprises microphone preamplifier means coupled between said microphone and said integrating circuit means, and inverting power amplifier means coupled between said integrating circuit means and said anti-sound source.
5. A system as claimed in claim 4, wherein said integrating circuit means has a transfer function H(S) which is equal to 1/s ?
+1, where s=j.omega. where j is ,.omega. is the frequency in rads and ?
the circuit time constant .
+1, where s=j.omega. where j is ,.omega. is the frequency in rads and ?
the circuit time constant .
6. A system as claimed in claim 5, wherein passive absorptive material is disposed at the inner surface of the walls of said duct.
7. An active noise control system for reducing noise propagated along a duct through which a fluid medium flows, comprising:
(a) a microphone for detecting the propagated noise;
(b) means for mounting the microphone in and flush with a wall of the duct at a substantially calm location where the fluid medium flows at a virtually zero velocity;
(c) anti-sound means for radiating sound in an anti-phase relationship with the noise detected by the microphone;
(d) means for mounting the anti-sound means substantially at the centre of a transverse cross-section of the duct of minimized transverse mode excitation within the duct; and (e) control means operatively connected to the microphone and the anti-sound means, and operative for driving the anti-sound means in response to the noise detected by the microphone to radiate sound which destructively interferes with the propagated noise.
(a) a microphone for detecting the propagated noise;
(b) means for mounting the microphone in and flush with a wall of the duct at a substantially calm location where the fluid medium flows at a virtually zero velocity;
(c) anti-sound means for radiating sound in an anti-phase relationship with the noise detected by the microphone;
(d) means for mounting the anti-sound means substantially at the centre of a transverse cross-section of the duct of minimized transverse mode excitation within the duct; and (e) control means operatively connected to the microphone and the anti-sound means, and operative for driving the anti-sound means in response to the noise detected by the microphone to radiate sound which destructively interferes with the propagated noise.
8. A system as claimed in claim 7, wherein the control means includes an integrator having an amplitude versus frequency transmission characteristic wherein low frequencies are attenuated to a greater extent than high frequencies.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000566554A CA1298793C (en) | 1988-05-12 | 1988-05-12 | Active noise control |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000566554A CA1298793C (en) | 1988-05-12 | 1988-05-12 | Active noise control |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1298793C true CA1298793C (en) | 1992-04-14 |
Family
ID=4138011
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000566554A Expired - Fee Related CA1298793C (en) | 1988-05-12 | 1988-05-12 | Active noise control |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1298793C (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112901887A (en) * | 2021-01-14 | 2021-06-04 | 哈尔滨工程大学 | Pipeline low-frequency noise control device based on electroacoustic coupling |
-
1988
- 1988-05-12 CA CA000566554A patent/CA1298793C/en not_active Expired - Fee Related
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112901887A (en) * | 2021-01-14 | 2021-06-04 | 哈尔滨工程大学 | Pipeline low-frequency noise control device based on electroacoustic coupling |
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| Date | Code | Title | Description |
|---|---|---|---|
| MKLA | Lapsed |