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between the primary and secondary waves, of the same frequency, reaching the observation point. Thus it will provide a basis for measuring (unknown) separation between the primary and secondary sources. The family of cancellation curves in Fig. 10 was veri- fied using a fixed frequency and varying the value of the power ratio for each distance difference value. The results of such an experiment using a fixed frequency of 100 Hz agree with Fig. 10 to within experimental accuracy.

Figure 10 between the primary and secondary waves, of the same frequency, reaching the observation point. Thus it will provide a basis for measuring (unknown) separation between the primary and secondary sources. The family of cancellation curves in Fig. 10 was veri- fied using a fixed frequency and varying the value of the power ratio for each distance difference value. The results of such an experiment using a fixed frequency of 100 Hz agree with Fig. 10 to within experimental accuracy.

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Active noise control was one of the earliest applications of electronics to the control of physical systems. Lueg filed for a patent in Germany in 1933 and in the USA in 1934 and was granted US Patent 2043416 in 1936 [8]. In his patent, Lueg made use of the two basic principles of realising an ANC system: interference and absorption. The interference principle results from the arbitrary physical mixing of acoustic waves resulting in construc- tive and destructive interference which, in turn, cause intensification and weakening of the sound field, respec- tively. He attempts to manipulate this principle of super- position so that the destructive interference of sound waves could be used to eliminate unwanted noise. He introduced the concept of active attenuation of sound by using artifically produced acoustic waves mixed with the unwanted sound so that the waves were in antiphase, and destructive interference resulted by design. Lueg also uses the absorption principle by synchronising the movements of a loudspeaker’s diaphragm in antiphase to the unwanted noise so that the noise energy is absorbed by the loudspeaker. Lueg’s proposals for obtaining destruc- tive interference and sound energy absorption are shown in Fig. 1, which is taken from the illustration page of his patent. Fig.1 — [llustrations from Lueg’s patent
Active noise control was one of the earliest applications of electronics to the control of physical systems. Lueg filed for a patent in Germany in 1933 and in the USA in 1934 and was granted US Patent 2043416 in 1936 [8]. In his patent, Lueg made use of the two basic principles of realising an ANC system: interference and absorption. The interference principle results from the arbitrary physical mixing of acoustic waves resulting in construc- tive and destructive interference which, in turn, cause intensification and weakening of the sound field, respec- tively. He attempts to manipulate this principle of super- position so that the destructive interference of sound waves could be used to eliminate unwanted noise. He introduced the concept of active attenuation of sound by using artifically produced acoustic waves mixed with the unwanted sound so that the waves were in antiphase, and destructive interference resulted by design. Lueg also uses the absorption principle by synchronising the movements of a loudspeaker’s diaphragm in antiphase to the unwanted noise so that the noise energy is absorbed by the loudspeaker. Lueg’s proposals for obtaining destruc- tive interference and sound energy absorption are shown in Fig. 1, which is taken from the illustration page of his patent. Fig.1 — [llustrations from Lueg’s patent
Fig. 3. Performance of Olson’s electronic sound absorber at micro- phone location The performance of the Olson device as a ‘sound pres- sure reducer’ at the detector location is shown in Fig. 3.
Fig. 3. Performance of Olson’s electronic sound absorber at micro- phone location The performance of the Olson device as a ‘sound pres- sure reducer’ at the detector location is shown in Fig. 3.
Olson’s absorber is shown in Fig. 2. The microphone detects the unwanted noise and feeds it to the amplifier. The amplifier, in turn, drives the loudspeaker such that the sound pressure at the microphone location is reduced. This effectively creates a ‘zone of cancellation’ in front of the absorber. As might be expected from the advance of technology in the 1950s, Olson’s device shows considerable sophistication and ingenuity. In fact, it was possible to control the phase with good accuracy over a reasonably broad frequency range. However, the basic feedback structure of the system, along with other system errors, limits the utility of Olson’s device.
Olson’s absorber is shown in Fig. 2. The microphone detects the unwanted noise and feeds it to the amplifier. The amplifier, in turn, drives the loudspeaker such that the sound pressure at the microphone location is reduced. This effectively creates a ‘zone of cancellation’ in front of the absorber. As might be expected from the advance of technology in the 1950s, Olson’s device shows considerable sophistication and ingenuity. In fact, it was possible to control the phase with good accuracy over a reasonably broad frequency range. However, the basic feedback structure of the system, along with other system errors, limits the utility of Olson’s device.
Fig. 4 Performance of Olson's device at various distances relative to microphone a 10 cm (4 in) 6 25 cm (10 in) pet ney f SAK The high-frequency performance of the system is limited by the feedback control structure of the system and the limited electronic capabilities, whereas the low-frequency performance limitation is due to the response of the loud- speaker in that range. Nevertheless, the performance of the device as measured at the microphone location is good. Unfortunately, as shown in Fig. 4, both the
Fig. 4 Performance of Olson's device at various distances relative to microphone a 10 cm (4 in) 6 25 cm (10 in) pet ney f SAK The high-frequency performance of the system is limited by the feedback control structure of the system and the limited electronic capabilities, whereas the low-frequency performance limitation is due to the response of the loud- speaker in that range. Nevertheless, the performance of the device as measured at the microphone location is good. Unfortunately, as shown in Fig. 4, both the
There has been considerable effort devoted to duct noise since Lueg’s invention and primarily since Jessel’s first work [11, 12, 25, 26, 30-72]. These efforts have resulted in the development of various systems, depicted in Fig. 6, for duct noise. The acoustic monopole system is Fig.6 Types of active noise attenuators for duct noise a Monopole (Lueg) b Tripole (Jessel) c Dipole (Swinbanks) d Dual monopole (Chelsea system)
There has been considerable effort devoted to duct noise since Lueg’s invention and primarily since Jessel’s first work [11, 12, 25, 26, 30-72]. These efforts have resulted in the development of various systems, depicted in Fig. 6, for duct noise. The acoustic monopole system is Fig.6 Types of active noise attenuators for duct noise a Monopole (Lueg) b Tripole (Jessel) c Dipole (Swinbanks) d Dual monopole (Chelsea system)
where c is the velocity of sound in the medium. The attenuating factors H, and H,, on the other hand, are inversely proportional, respectively, to the square of the distances r, and r,. and the autopower spectral densities can be written as (108, 109]
where c is the velocity of sound in the medium. The attenuating factors H, and H,, on the other hand, are inversely proportional, respectively, to the square of the distances r, and r,. and the autopower spectral densities can be written as (108, 109]
Fig. 7 Primary and secondary sources emit acoustic waves to produce cancellation at point O Gcco(@) < Gppo(w) For the purpose of a quantitative description of the phe- nomenon of cancellation we shall define the field cancel- ation factor K as the ratio of the cancelled spectrum Gppo(w) — Gcco(@) to the primary spectrum Gppo() which existed at the given point O prior to the super- position of the secondary field s,(t); i-e.
Fig. 7 Primary and secondary sources emit acoustic waves to produce cancellation at point O Gcco(@) < Gppo(w) For the purpose of a quantitative description of the phe- nomenon of cancellation we shall define the field cancel- ation factor K as the ratio of the cancelled spectrum Gppo(w) — Gcco(@) to the primary spectrum Gppo() which existed at the given point O prior to the super- position of the secondary field s,(t); i-e.
Fig. 10 Field cancellation factor as related to the power ratio and cross-spectral density factor of primary and secondary waves If B is treated as a parameter in eqn. 13, the field can- cellation factor K as a function of power ratio « can be represented as a family of curves, each curve correspond- ing to a particular value of B, as shown in Fig. 10.
Fig. 10 Field cancellation factor as related to the power ratio and cross-spectral density factor of primary and secondary waves If B is treated as a parameter in eqn. 13, the field can- cellation factor K as a function of power ratio « can be represented as a family of curves, each curve correspond- ing to a particular value of B, as shown in Fig. 10.
Fig. 9 Shaded region represents —B values for which cancellation occurs for a certain This is the phase condition for cancellation and is shown in Fig. 9. The amplitude condition for cancellation can also be obtained from eqn. 14. As shown in Fig. 9, —f lies between the functions U and L. However, from the defi- nition of 8, the cross-spectral density factor is bounded by one; i.e.
Fig. 9 Shaded region represents —B values for which cancellation occurs for a certain This is the phase condition for cancellation and is shown in Fig. 9. The amplitude condition for cancellation can also be obtained from eqn. 14. As shown in Fig. 9, —f lies between the functions U and L. However, from the defi- nition of 8, the cross-spectral density factor is bounded by one; i.e.
from which it is clear that wt — 6(w) represents an angle either in the second or third quadrant, including the x axis. Solution of eqn. 28 for wt — 6(w) yields that for the secondary field to cancel the primary field the phase dif- ference wt — @(w) between the same-frequency com- ponents of the waves is required to be Fig. 11 Range of difference in phase between primary and secondary waves for cancellation Using the relationship between the velocity of sound, the signal wavelength and the signal frequency, eqn. 27 can be written as
from which it is clear that wt — 6(w) represents an angle either in the second or third quadrant, including the x axis. Solution of eqn. 28 for wt — 6(w) yields that for the secondary field to cancel the primary field the phase dif- ference wt — @(w) between the same-frequency com- ponents of the waves is required to be Fig. 11 Range of difference in phase between primary and secondary waves for cancellation Using the relationship between the velocity of sound, the signal wavelength and the signal frequency, eqn. 27 can be written as
Fig.12 Experimental set up for verification of general conditions of active noise cancellation tiometers, one NAIM power amplifier and a condenser- type Cirrus microphone. the amplifier used to amplify the microphone’s output. The amplifier does not have a significant effect on the overall transfer characteristics of Fig. 14, whereas the effect of the acoustic path on the phase is clear from Fig. 14. Within the range 60-900 Hz the loudspeaker- microphone combination has an almost flat transfer characteristic. In the range 900-2800 Hz the amplitude characteristic varies between —3.5 and +3.5 dB. This variation reflects on the phase characteristic as a variable phase with an average equal to the linear phase charac- teristic resulting from the time delay of the acoustic path. The loudspeaker response diminishes above 4.2 kHz. facilities for single- and dual-channel measurements. The analytical procedure of the device is based on the fast Fourier transform (FFT) through which the normal frequency-domain analysis functions, such as autopower, synchronous spectrum, crosspower, transfer function, coherence and power cepstrum with a maximum fre- quency of 30 kHz, are provided. The time-domain analysis functions provided by the instrument, via inverse FFT, are the time waveform, impulse response, autocor- relation and crosscorrelation functions. The instrument generates three types of test signals, namely PRBS (pseudo-random binary sequence) signal, periodic multi- sine signal and impulse signal. Among the input/output facilities of the instrument are two analogue input chan- nels, digital plotter, analogue plotter and video signal output. The instrument’s interface unit is the IEEE Stan- dard 488 (1978) with full remote control.
Fig.12 Experimental set up for verification of general conditions of active noise cancellation tiometers, one NAIM power amplifier and a condenser- type Cirrus microphone. the amplifier used to amplify the microphone’s output. The amplifier does not have a significant effect on the overall transfer characteristics of Fig. 14, whereas the effect of the acoustic path on the phase is clear from Fig. 14. Within the range 60-900 Hz the loudspeaker- microphone combination has an almost flat transfer characteristic. In the range 900-2800 Hz the amplitude characteristic varies between —3.5 and +3.5 dB. This variation reflects on the phase characteristic as a variable phase with an average equal to the linear phase charac- teristic resulting from the time delay of the acoustic path. The loudspeaker response diminishes above 4.2 kHz. facilities for single- and dual-channel measurements. The analytical procedure of the device is based on the fast Fourier transform (FFT) through which the normal frequency-domain analysis functions, such as autopower, synchronous spectrum, crosspower, transfer function, coherence and power cepstrum with a maximum fre- quency of 30 kHz, are provided. The time-domain analysis functions provided by the instrument, via inverse FFT, are the time waveform, impulse response, autocor- relation and crosscorrelation functions. The instrument generates three types of test signals, namely PRBS (pseudo-random binary sequence) signal, periodic multi- sine signal and impulse signal. Among the input/output facilities of the instrument are two analogue input chan- nels, digital plotter, analogue plotter and video signal output. The instrument’s interface unit is the IEEE Stan- dard 488 (1978) with full remote control.
Fig.14 Transfer characteristics of secondary source plus microphone a Amplitude h Phace
Fig.14 Transfer characteristics of secondary source plus microphone a Amplitude h Phace
To verify eqns. 39a and b the same experimental arrangement as in Fig. 12 was used. The observer micro- phone was placed opposite to the primary source with r, = 2m and the separation between the sources was varied so that Ar = 0.385m was obtained at d = 1.3 m. The sources were driven by a PRBS signal and adjusted so that a power ratio a of —6.0 dB was obtained over a range of 0-5 kHz at the observation point. Substituting these parameters and c = 340 m/s in eqns. 39 yields the frequencies of the limit of cancellation as
To verify eqns. 39a and b the same experimental arrangement as in Fig. 12 was used. The observer micro- phone was placed opposite to the primary source with r, = 2m and the separation between the sources was varied so that Ar = 0.385m was obtained at d = 1.3 m. The sources were driven by a PRBS signal and adjusted so that a power ratio a of —6.0 dB was obtained over a range of 0-5 kHz at the observation point. Substituting these parameters and c = 340 m/s in eqns. 39 yields the frequencies of the limit of cancellation as
Fig.18 Schematic diagram of feedback control structure The basic feedback control structure (FBCS) is shown schematically in Fig. 18. The primary source emits an
Fig.18 Schematic diagram of feedback control structure The basic feedback control structure (FBCS) is shown schematically in Fig. 18. The primary source emits an
Fig. 20 Schematic diagram of feedforward control structure ondary source to be superimposed on the unwanted noise. The aim is to reduce the sound pressure level to zero at an observation point which is at a distance of r, relative to the primary source and a distance r, relative to the secondary source.
Fig. 20 Schematic diagram of feedforward control structure ondary source to be superimposed on the unwanted noise. The aim is to reduce the sound pressure level to zero at an observation point which is at a distance of r, relative to the primary source and a distance r, relative to the secondary source.
Fig. 25 = Transfer characteristics of DFC compared to H(z) a Amplitude b Phase constant-length transmission line. Thus the maximum cancellation condition is satisfied for only one frequency J, (163 Hz). Noise components at frequencies smaller or
Fig. 25 = Transfer characteristics of DFC compared to H(z) a Amplitude b Phase constant-length transmission line. Thus the maximum cancellation condition is satisfied for only one frequency J, (163 Hz). Noise components at frequencies smaller or
Fig. 24 Offset in controller characteristics from required values a Amplitude error b Phase error processor using 192 instructions and setting the cutoff frequencies of the lowpass antialiasing and reconstruction filters at 16 kHz. The amplitude and phase characteristics
Fig. 24 Offset in controller characteristics from required values a Amplitude error b Phase error processor using 192 instructions and setting the cutoff frequencies of the lowpass antialiasing and reconstruction filters at 16 kHz. The amplitude and phase characteristics
that by changing the gain level in the controller path the characteristic |H(j@)| can very closely approximate | C(j@) | such that relation 55 is satisfied over a broad fre-
that by changing the gain level in the controller path the characteristic |H(j@)| can very closely approximate | C(j@) | such that relation 55 is satisfied over a broad fre-
in an increase in the cancellation of the primary noise for frequencies below 300 Hz. However, as Fig. 24 implies, this will deteriorate the performance of the DFC for fre-
in an increase in the cancellation of the primary noise for frequencies below 300 Hz. However, as Fig. 24 implies, this will deteriorate the performance of the DFC for fre-
Using the DFC the performance of the system, as shown in Fig. 30b, is significantly improved. Cancellation
Using the DFC the performance of the system, as shown in Fig. 30b, is significantly improved. Cancellation
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Active noise controlNoise Abatement
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