Active Noise Control - Introduction

• What is Active noise control?

It works on the principle of destructive interference between soundfeelds generated by the original (primary) source and the secondary sources, which can be controlled.

• Why active control?

Convential methods of suppressing acoustic noise using sound absorbers do not work well at low frequencies(wavelength >> thickness of a typical absorber, 100 Hz � 3.4m, using velocity of acoustic waves in air = 333 m/s)

Active Noise Control – Historic background

• 1936: Paul Lueg describes basic ideas of active control

He considered sound to be travelling as SODQH�ZDYH�LQ�D GXFW caused by SULPDU\�VRXUFH�$. A PLFURSKRQH�0 detects the sound wave and supplies the HOHFWURQLF�FRQWUROOHU�9, which drives the ORXGVSHDNHU�/ (see Figure 1, Diagram 1) � GHVWUXFWLYH�LQWHUIHUHQFH.Diagrams 2, 3 and 4 show his intentions of mirror waveforms for nonsinusoidal signals and signals extending in 3 dimensions

Figure 1: Diagrams from Paul Lueg‘s patent 1936

Active Noise Control – Acoustical Principles (1)

• Acoustic Objectives– Power output minimisation

– Quiet zone– Power absorption

• Control Strategies– Feedback control

– Feedforward control

All of these principles rely on VXSHUSRVLWLRQ, which applies in all linear systems.

The propagation of an DFRXVWLF�ZDYH is a OLQHDU�SURFHVV.

1RQOLQHDULW\ usually occurs in the ORXGVSHDNHU�acting as the secondary source

Active Noise Control – Acoustical Principles (2)

• How can destructive interference be achieved?

3KDVH�DQG�DPSOLWXGH of a signal driving one loudspeaker have to be adjusted UHODWLYH to that driving another loudspeaker � DFRXVWLF�SUHVVXUH at a single point can be driven to zero.

On the other hand, FRQVWUXFWLYH�LQWHUIHUHQFH can occur in other points �Problem.

• How to solve this problem?

Position PLFURSKRQH�DQG�VHFRQGDU\�VRXUFH FORVH�WRJHWKHU

� 6HFRQGDU\�VRXUFH is ZHOO�FRXSOHG WR the PLFURSKRQH, and a low voltage is needed to drive the loudspeaker

� $FRXVWLF�SUHVVXUH at other points will QRW�EH�DIIHFWHG by the secondarysource

� 4XLHW�]RQH around the microphone. ���G%�UHGXFWLRQ in a zone around the microphone within �����RI�WKH�ZDYHOHQJWK is achieved. (0.34m at 100 Hz, 3.4 mm at 10 kHz)

Active Noise Control – Acoustical Principles (3)

• How to achieve global cancellation?

• An Example:

– 2 loudspeakers work at ORZ�IUHTXHQFLHV�LQ�D�IUHH�ILHOG. If the GLYHUJLQJ�ZDYHIURQWV are FORVHO\�VSDFHG compared with the acoustic wavelength, and the two sources are of the VDPH�DPSOLWXGH, but RXW�RI�SKDVH, then global destructive interference can be achieved. (See figure 2(a)).

– If the IUHTXHQF\�LV�LQFUHDVHG, the interference will be GHVWUXFWLYH in some points, but FRQVWUXFWLYH�in others, and global control will not be achieved.(See figure 2(b)).

Figure 2: Wavefronts from 2 acoustic sources at frequencies, where the space between the diverging wavefronts is (a) large and (b) nearly the same compared with the acoustic wavelength

Active Noise Control – Acoustical Principles (4)

• How to achieve global cancellation?

Another way to describe global cancellation is to use the QHW�DFRXVWLF�SRZHUoutput of two equal, but out of phase acoustic sources, as shown in figure 3(c).

– At large separation distances compared with the wavelength, the two sources generate a total output which is twice that of only one operating alone.

– If the distance between the two sources is reduced, the interference becomes much stronger.

Figure 3: WTD describes the net acoustic power(dB) of two sources of the sameamplitude, but out of phase.WTO is the net acoustic power(DB) if amplitude and phase of the secondary source are optimally adjusted to reduce the power output.

Active Noise Control – Sound in Enclosures (1)

• Standing Waves

– In an enclosure, for example the interior of a car, The VRXQG�ILHOG is UHIOHFWHG and causes LQWHUQDO�VWDQGLQJ�ZDYHV at certain frequencies.

– These 3-dimensional acoustic waves are the PRGHV�of the enclosure.

– They can be used to GHVFULEH the DFRXVWLF�SUHVVXUH efficiently.

• Acoustic potential energy

– In enclosures, the quantity to be controlled is the DFRXVWLF�SRWHQWLDO�HQHUJ\. It is proportional to the sum of the mean square amplitude of each of the acoustic modes.

– It can be controlled similar to the Example mentioned before, bychanging amplitude and phase of the secondary source.

– 0LQLPLVLQJ�WKH�DFRXVWLF�SRWHQWLDO�HQHUJ\�LV�HTXLYDOHQW�WR�

PLQLPLVLQJ�WKH�QHW�DFRXVWLF�SRZHU�

Active Noise Control – Sound in Enclosures (2)

• An Example:

– Figure 4 (solid line) shows the DFRXVWLF�SRWHQWLDO�HQHUJ\ generated in a UHFWDQJXODU�HQFORVXUH of dimensions 1.9 m x 1.1 m x 1.0 m, which is equivalent to the interior of a small car, by a SXUH�PRQRSROH�DFRXVWLF�

VRXUFH in one corner of the enclosure.

– A VHFRQGDU\�DFRXVWLF�VRXUFH is placed in the opposite corner and driven at the same discrete frequency as the primary source. Its DPSOLWXGH�DQG�

SKDVH are adjusted to PLQLPLVH the WRWDO�DFRXVWLF�SRWHQWLDO�HQHUJ\.The UHVXOWLQJ�WRWDO�DFRXVWLF�SRWHQWLDO�HQHUJ\ is plotted as the dashed curve.

Figure 4: Total acoustic potantial energy in an enclosure equivalent to a small car.

Active Noise Control – Sound in Enclosures (3)

• Conclusions:

– The secondary source FDQ�QRW�FRQWURO PRGHV�EHWZHHQ WKRVH�RI�WKH�

SULPDU\�VRXUFH without increasing the excitation of other modes. Therefore the optimum secondary source strength is reduced and only OLWWOH�UHGXFWLRQ at these frequencies is achieved.

– In practice, it is QRW�SRVVLEOH WR�GHWHFW�WKH�WRWDO�DFRXVWLF�SRWHQWLDO�

HQHUJ\ in an enclosure, because an LQILQLWH�QXPEHU�RI PLFURSKRQHV

would be required.

– The microphones need to be positioned such that they are affected by all the dominant acoustic modes.The secondary sources need to be positioned that they can excite these modes.

• How many microphones are reasonable?

In practice, using WZLFH�DV�PDQ\�PLFURSKRQHV�DV�VHFRQGDU\�VRXUFHV is a good compromise between complexity and performance.

Active Noise Control – Feedback Control (1)

• How it works

The effect of the IHHGEDFN�ORRS forcing H to be VPDOO�FRPSDUHG�WR�G, will be to FDQFHO the DFRXVWLF�SUHVVXUH at the monitor PLFURSKRQH, as required for active control. Figure 5 a) shows an active noise system using feedback control.In 5 b) the electrical configuration can be seen:

d ... Disturbancee ... microphone signalH ... electrical filterC ... secondary source.

Figure 5: (a) Active noise system using feedback control. (b) Equivalent electrical configuration.

Active Noise Control – Feedback Control (2)

• Applications:

Feedback systems have been used for EURDGEDQG�QRLVH�FRQWURO�LQ�KHDGVHWVand ear defenders. Several commercial systems achieve ���± ���G%�UHGXFWLRQ

of the acoustic pressure at frequencies from 30 Hz to 500 Hz.

• Problems in this application

– Although the PLFURSKRQH DQG�WKH�VHFRQGDU\�VRXUFH�DUH�SODFHG�YHU\�

FORVH together, the KLJK�IUHTXHQF\�OLPLW is set by the DFFXPXODWLRQ�RI�

SKDVH�VKLIW�ZLWK�IUHTXHQF\, causing instability unless the gain is reduced.

– The DFRXVWLF�SDWK between the secondary loudspeaker and themicrophone FKDQJHV as the headset is worn by different people, or in different positions of the same person, or even if it is lifted on and off the head.

Active Noise Control – Feedforward Control (1)

• How it works

In the case of feedforward control, a separate UHIHUHQFH�VLJQDO�is used to GULYHthe VHFRQGDU\�VRXUFH YLD the HOHFWULFDO�FRQWUROOHU.This reference signal must be ZHOO�FRUUHODWHG ZLWK the signal from the SULPDU\�

VRXUFH.The reference signal can often be REWDLQHG GLUHFWO\ from the PHFKDQLFDO�

RSHUDWLRQ RI�WKH�SULPDU\�VRXUFH (tachometer, ...) and is FRPSOHWHO\�

XQDIIHFWHG E\�WKH�VHFRQGDU\�VRXUFH.

� WKH�FRQWURO�LV�SXUHO\�IHHGIRUZDUG. (See figure 6)

The UHIHUHQFH�VLJQDO SURYLGHV�DGYDQFHG�LQIRUPDWLRQ about the SULPDU\�

QRLVH before it reaches the microphone. This HQDEOHV the FRQWUROOHU�WR�HIIHFW�

FDQFHOODWLRQ.

When the noise VLJQDO is GHWHUPLQLVWLF (harmonic tones, ...), this information has OLWWOH�PHDQLQJ. The controller only has to implement the necessary JDLQ�DQG�SKDVH�VKLIW.

Active Noise Control – Feedforward Control (2)

• How it works

Because the SURSHUWLHV�RI the SULPDU\�QRLVH and the FKDUDFWHULVWLFV RI the DFRXVWLF�SDWK between the secondary source and the microphone can FKDQJH�ZLWK�WLPH, the FRQWUROOHU in active feedforward systems is often PDGH�

DGDSWLYH.Therefore, GLJLWDO�V\VWHPV are used.

Figure 6 (a) Active noise system using feedforward control.(b) Equivalent electrical configuration:P ... Primary sourceW ... Electrical filterC ... Secondary sourcex ... Reference signald ... Disturbancee ... Microphone signal

Active Noise Control – Practical applications (1)

• Goals and problems in practical solutions

– Active control is OLPLWHG�WR�VLWXDWLRQV in which the VHSDUDWLRQ between the SULPDU\�DQG�VHFRQGDU\�VRXUFHV is DW�PRVW�of the VDPH�RUGHU as the DFRXVWLF�ZDYHOHQJWK.� In HQFORVXUHV, whose smallest GLPHQVLRQV are of the order of a IHZPHWHUV, the XSSHU�IUHTXHQF\ is restricted to a IHZ�KXQGUHG�KHUW].

– At these frequencies, DFWLYH�QRLVH�FRQWURO is PRUH�HIIHFWLYH WKDQ SDVVLYH�

QRLVH�FRQWURO techniques, because of the higher acoustic wavelengths.

– At low frequencies, passive control systems need a KLJK�DPRXQW�RI�

ZHLJKW. Therefore, active systems are especially used in DLUFUDIWV and OLJKWZHLJKW�FDUV.

Active Noise Control – Practical applications (2)

• Active headsets

They are designed to reduce any external noise, deterministic and random. They use a IHHGEDFN�V\VWHP. The typical performance is shown in figure 7:

8SSHU�FXUYH� Spectrum of cockpit noise in a jet aircraft0LGGOH�FXUYH� Noise at the ears of the pilot when wearing a

conventional headset%RWWRP�FXUYH� Noise at the pilot‘s ear using the headset with active control

Figure 7

Active Noise Control – Practical applications (3)

• Active control in air conditions

This system operates on the IHHGIRUZDUG�SULQFLSOH. Figure 8 shows the spectrum of the resulting pressure signal in an DLU�FRQGLWLRQ�GXFW with DQ�DLUIORZ�RI�DERXW����P�V.Below 40 hertz, the performance is limited by high levels of turbulence.

Figure 8

Active Noise Control – Practical applications (4)

• Active control in a propeller plane

On a ���VHDW�SURSHOOHU�SODQH, an active noise control system with PXOWLSOH�

ORXGVSHDNHUV�DQG�PLFURSKRQHV was implemented.���ORXGVSHDNHUV and ���PLFURSKRQHV where used in a IHHGIRUZDUG V\VWHP

to minimise the sum of the squared pressure of the blade passing frequency and its first two harmonics.A UHGXFWLRQ RI����± ���G% was achieved. The sound pressure in the passenger cabin is shown in figure 9 (a) without and (b) with active noise control.

Figure 9

Active Noise Control – Practical applications (5)

• Active control in cars

At higher engine speeds, many FDUV�H[KLELW some kind of ERRP, caused by the engine. Figure 10 shows an active noise control systems developed for reduction of engine noise in cars.A IHHGIRUZDUG�FRQWUROOHU uses a reference signal taken from the FLUFXLW�RI�WKH�HQJLQH to drive ��ORXGVSHDNHUV (often those already installed in the car).��PLFURSKRQHV are used to provide an error signal.

Figure 10

Active Noise Control – Practical applications (6)

• Active control in cars - Results

The UHVXOWV of using a ��ORXGVSHDNHU����PLFURSKRQH active control system in a VPDOO�����OLWUH���� F\OLQGHU�FDU are shown in figure 11.The car was accelerated at full load from 1500 to 6000 rpm. This corresponds to QRLVH�IUHTXHQFLHV from ���WR�����+].

)LJXUH���

6ROLG�OLQH� Active control system off'DVKHG�OLQH� Active control system on

Active Noise Control – The future of active control

• Future applications of active control, which are now developed

– Active control of structural vibration

– Adaptive sound reproduction systems– Active control of fluid flow

– Active control of electromagnetic fields