JPH07334168A - Active noise control system - Google Patents

Abstract

PURPOSE:To provide a steep frequency cut-off characteristic by using a frequency cut-off filter by a digital signal process for interrupting signals in medium/high frequency areas being out of an object of muffling CONSTITUTION:Noise information in a duct 21 is detected by a first microphone 22, and is digitized by an analog-digital converter 28 to be made the signal only in a low frequency area with the frequency cut-off filter 31. An operation part 36 consisting of a muffle signal generating filter 34 and an adaptive control algorithm 35 forms a muffling signal with a sound pressure same as and a phase opposite to the noise, and makes it analog to radiate a muffling sound wave from a speaker 23 into the duct 21. A second microphone 24 existing on the lower reaches of the muffling speaker 23 detects a muffle condition in the duct 21, and when no noise is made to zero, the analog signal of the noise is digitized, and the operation part 36 forms the muffling signal with the sound pressure same as and the phase opposite to the noise, and the muffling sound wave is radiated from the muffling speaker 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active noise control system for canceling noise by radiating sound waves having the same sound pressure and anti-phase in noise in a duct for propagating noise in a duct.

[0002]

2. Description of the Related Art Heretofore, as one method for muffling noise propagating in a duct, a passive muffling method has mainly been adopted, such as a method of absorbing noise by a sound absorbing material stuck inside the duct. There are problems such as size and size.

On the other hand, active noise control systems have been actively researched in which sound waves having the same sound pressure and opposite phase to each other are simultaneously radiated into the duct, and the sound waves are silenced by the interference of both sound waves. . However, many problems still remain.

FIG. 2 is a structural explanatory view showing a conventional active noise control system. That is, when the noise propagating in the duct 1 is actively silenced, the signal of the noise information in the duct 1 from the first microphone 2 and the signal of the silence condition information from the second microphone 3 are taken in Various processing is performed on one signal to create a muffling signal for driving the muffling speaker 4. As an important process among the various processes, there is a process of “cutting off signals in the frequency domain that is not subject to noise reduction in order to process only signals in the frequency domain that are subject to noise reduction”.

In order to reproduce only the frequency range to be silenced, even when the silencer speaker 4 reproduces the silencer signal generated by the arithmetic unit 7 including the silencer signal generation filter 5 and the adaptive control algorithm 6, It is necessary to perform a process of cutting off a signal in a frequency region that is not the target of muffling.

The frequency cutoff filters 8, 9 and 10 perform the processing. The frequency cutoff filters 8 and 9 are provided for the signals taken in from the first microphone 2 and the second microphone 3 and the signal created by the arithmetic unit 7. I have to go through 9 and 10.

The information on the noise propagating through the duct 1 obtained by the first microphone 2 and the information on the muffling situation obtained by the second microphone 3 are analog electric signals. Analog signal processing is performed on the analog electric signals that are taken in by the microphones 2 and 3 and amplified by the corresponding microphone amplifiers 11 and 12,
Signals in the frequency domain that are not subject to noise reduction have been blocked.

The analog electric signals passed through the frequency cutoff filters 8 and 9 are converted into analog-digital converters 14 and 15.
Is converted into a digital electric signal and input to the arithmetic unit 7, and the arithmetic unit 7 processes the digital signal.

Further, in order to reproduce only the sound wave in the frequency range to be muted by the muffling speaker 4, it is necessary to cut off the signal in the frequency range outside the muffling target. At that time, the arithmetic unit 7 is also used. The mute digital electric signal generated by the above is converted into a mute analog electric signal by the digital-analog converter 16 and then subjected to analog signal processing.

That is, only the frequency cutoff filters 8, 9 and 10 based on the analog signal processing cut off the frequencies that are not the target of the noise reduction. By the way, the conventional passive noise reduction method exerts a sufficient noise reduction effect on noise in the middle and high frequency regions, but does not achieve the same sufficient noise reduction effect on noise in the low frequency region. It was difficult. (Excessive use of the muffling duct using glass wool increases the air resistance in the duct, which increases the load on the air conditioner. Also, unless the muffling duct is quite large, it is not possible to effectively absorb the sound in the low frequency range. Therefore, it is considered that the particularly effective frequency range for actively muting noise is the low frequency range.

When the signal in the middle / high frequency region is cut off by analog signal processing (when only the signal in the low frequency region is taken in), "time required for processing = time delay" and "precision of frequency cutoff = frequency cutoff characteristic". There are conflicting relationships between them. As shown in FIG. 6, the frequency cutoff characteristic is made steep to f ′.
If the signal of Hz or more is cut off accurately, the time delay in the frequency cutoff filters 8, 9 and 10 becomes large, and conversely, if the time delay in the frequency cutoff filters 8, 9 and 10 is made small, the frequency becomes as shown in FIG. The cutoff characteristic is gentle and f '
Signals of Hz or more are also included, and the accuracy of frequency cutoff deteriorates.

After the sound wave of noise is detected by the first microphone 2, the sound wave further propagates in the duct 1 to the installation position of the muffling speaker 4. Since the sound waves are silenced, the distance between the first microphone 2 and the silencing speaker 4 must be increased as the time required for signal processing in the system increases (the delay time increases). (It does not make sense if the sound wave to be silenced passes through the installation position of the silencing speaker 4 during the signal processing.) Therefore, the signal in the low frequency region only is obtained by the conventional analog signal processing method. In order to extract the signal with high precision, the frequency cutoff characteristic must be made steep and the time delay becomes large. Therefore, the distance between the first microphone 2 and the speaker 4 must be increased, and the system becomes large.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances and provides accurate frequency cutoff with a small time delay, highly accurate noise control, and silence with the first sensor. An object of the present invention is to provide an active noise control system capable of reducing the distance of a speaker for use and making it compact.

[0014]

In order to achieve the above object, the active noise control system of the present invention uses digital rather than analog signal processing as means for cutting off signals in the middle and high frequency regions that are not targets of noise reduction. By using a frequency cutoff filter by signal processing, a sharp frequency cutoff characteristic is obtained with a small time delay.

[0015]

With the above-mentioned means, the present invention is applicable to
When a frequency cutoff filter by digital signal processing is used to cut off a signal in the high frequency range, a steep frequency cutoff characteristic is obtained with a time delay smaller than that of the frequency cutoff filter by analog signal processing, and the low frequency range to be silenced is obtained. The signal of can be taken out with high accuracy.

[0016]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a configuration explanatory view showing an embodiment of the present invention. That is, the information of noise propagating in the duct 21 is taken in as an analog electric signal by the first microphone (first sensor) 22 and further amplified by the microphone amplifier 25.

Since the present system targets the noise in the low frequency range to be silenced, it is necessary to pass the frequency cutoff filter 31 for blocking the signals in the middle and high frequency ranges which are not the target.

Therefore, in this embodiment, the analog electric signal amplified by the microphone amplifier 25 is first converted into a digital electric signal through the analog-digital converter 28. The frequency cutoff filter 31 performs digital signal processing on the digital electric signal to obtain a digital electric signal containing only the signal in the low frequency region to be silenced.

The frequency cutoff filter 31 based on digital signal processing stores an input signal from a past to the present for a certain fixed time in a memory, convolves a frequency cutoff coefficient with respect to the input signal, and performs middle / high frequency cutoff processing. Since the latest signal is calculated, a steep frequency cutoff characteristic can be obtained without much time delay.

Further, the muffling condition in the duct 21 due to the operation of the system indicates that the second microphone (second muffler) installed downstream of the muffling speaker 23 from the noise source.
Sensor) 24. This second microphone 24
Similarly to the analog electric signal obtained from the first microphone 22, the analog electric signal which is the information on the muffling condition in the duct 21 obtained from the microphone amplifier 27
Then, the analog-to-digital converter 30 and the frequency cut-off filter 33 based on the digital signal processing are used to obtain only the digital electric signal of the signal in the low frequency region to be silenced.

The frequency cutoff filter 33 based on digital signal processing stores an input signal from a past to the present for a certain period of time in a memory, and convolves a frequency cutoff coefficient with respect to the input signal to perform middle / high frequency cutoff processing. Since the latest signal is calculated, a steep frequency cutoff characteristic can be obtained without much time delay.

In the calculation unit 36 including the noise reduction signal generation filter 34 and the adaptive control algorithm 35, the noise reduction speaker 23 creates a noise reduction signal that radiates a sound wave having the same sound pressure and opposite phase to the noise propagated in the duct 21. However, the noise in the tug 21 cannot be silenced simply by making the noise signal obtained from the first microphone 22 have the same sound pressure and opposite phase. Therefore, in the calculation unit 36, the “silence-reduction speaker 23
However, by radiating sound waves of the same sound pressure and opposite phase to the sound waves of the noise inside the duct 21 at the location where it is installed, the noise inside the duct 21 after sound wave interference becomes zero. " The following calculation is performed so as to generate a signal for use.

In the system, as shown in FIG. 3, when a sound wave propagates in the duct 21 or a signal passes through the circuit,
They are affected by their own transfer functions such as time delay and changes in frequency characteristics. Therefore, it is necessary to correct the transfer function that particularly adversely affects the noise control. Therefore, those transfer functions are measured in advance before the system is operated, and the coefficient for correcting the transfer function is calculated by the calculation unit 36 by convolution with the digital electric signal of noise obtained from the first microphone 22.

From the second microphone 24, information on how much the noise propagated in the duct 21 has been silenced by the operation of the system is input. The signal approaches zero if it is effectively muted. Therefore, the calculation unit 3
In 6, the information is taken in and the optimum coefficient that the signal always approaches zero is calculated based on the adaptive control algorithm 35, and the convolution operation is performed on the input signal from the first microphone 22 as the filter coefficient of the silence signal generation filter 34. Do.

In this way, the arithmetic unit 36 calculates various coefficients as the first coefficient.
A signal for silencing is created by performing a convolution operation on the input signal from the microphone 22. After that, the muffling speaker 23 is driven by this muffling signal to generate a muffling sound wave in the duct 21
However, in order to radiate only the sound wave in the low frequency range, which is the target of noise reduction, into the duct 21, it is necessary to block the signal in the frequency range outside the target of noise reduction. Therefore, the noise-eliminating digital electric signal is finally passed through the frequency cutoff filter 32 by digital signal processing once again.

The frequency cutoff filter 32 based on digital signal processing stores an input signal from a past to the present for a certain fixed time in a memory, and convolves a coefficient of frequency cutoff with respect to the input signal to perform middle / high frequency cutoff processing. Since the latest signal is calculated, a steep frequency cutoff characteristic can be obtained without much time delay.

The sound-deadening signal, which has been converted to a digital electric signal containing only the signal in the low frequency range to be muted through the frequency cut-off filter 32, is converted into a sound-deadening analog electric signal through the digital-analog converter 29.

This muffling analog electric signal is amplified by the power amplifier 26 to drive the muffling speaker 23, and the muffling sound wave is radiated into the duct 21. In this way, various signal processing is performed, and the sound wave for silencing having the same sound pressure and the opposite phase to the noise propagating in the duct 21 is radiated into the duct 21. The emitted sound wave for silencing interferes with the noise sound wave propagating in the duct 21 and cancels each other, and a highly accurate sound deadening effect is obtained.

Next, the in-system transfer function will be described in more detail. A transfer function that needs special consideration in an active noise control system, a transfer function "D, E" in a circuit through which a signal obtained from the first microphone 22 passes before being emitted from the muffling speaker 23, a muffling speaker A transfer function "B" in the duct 21 through which the sound wave radiated from the sound wave radiated from the sound source 23 passes until it reaches the first microphone 22, and a duct through which the sound wave radiated from the sound-deadening speaker 23 reaches the second microphone 24. The transfer function "C" in 21 will be described with reference to FIG.

First, the function "D, E" will be described. The information on the noise in the duct 21 includes the first microphone 22, the microphone amplifier 25, and the analog-digital converter 2.
8, processed by the frequency cutoff filter 31, the calculation unit 36, the frequency cutoff filter 32, the digital-analog converter 29, the power amplifier 26, and the sound deadening speaker 23, and radiated into the duct 21 as sound deadening sound waves.

Since processing time is required when processing the signal, there is a time delay until the signal detected by the first microphone 22 is emitted from the muffling speaker 23. If the sound wave to be silenced passes through the noise elimination speaker 23 during the delay time generated by this processing, the noise elimination effect is not obtained. Therefore, this transfer function "D, E"
Is important in determining the distance between the first microphone 22 and the muffling speaker 23.

Next, regarding the transfer function "B". The muffling sound wave radiated from the muffling speaker 23 propagates not only in the downstream direction (the second microphone 24 direction) of the duct 21 but also in the upstream direction (the first microphone 22 direction). Therefore, the first microphone 22 detects not only the sound wave of the noise information in the duct 21 but also the sound wave for silencing emitted by the silencing speaker 23. This causes howling and measurement errors, which adversely affects the convergence accuracy in coefficient calculation.

Finally, regarding the transfer function "C". The sound wave for muffling emitted from the muffling speaker 23 interferes with the noise in the duct 21 and propagates in the downstream direction of the duct 21. The interference sound wave is detected by the second microphone 24, and the calculation unit 3
The coefficient is calculated in 6. Therefore, unless the time delay generated in this path is taken into consideration, the convergence accuracy in the coefficient calculation is adversely affected.

The transfer functions "D, E" can be avoided by considering the distance between the first microphone 22 and the muffling speaker 23. Further, the transfer function “B” and the transfer function “C” can be avoided by electrically processing them in the system.

To electrically process the transfer functions "B" and "C", specifically, the following processes are performed as "H" and "I" in FIG. Regarding the transfer function "B",
The muffling speaker 23 to the first microphone 2 in advance.
Measure the transfer function up to 2. It is convoluted with the muffling signal created by the calculation unit 36 to artificially create a signal as if the muffling signal propagated from the muffling speaker 23 to the first microphone 22. Process "H" of subtracting this artificially created signal from the signal of the propagation noise in the duct 21 obtained from the first microphone 22.
Give.

Regarding the transfer function "C", the transfer function from the muffling speaker 23 to the second microphone 24 is measured in advance, and the delay time in this path is read.
This delay time is adjusted to the signal of the propagation noise in the duct 21 obtained from the first microphone 22, and the coefficient calculation unit 3
Enter in 6. In other words, the process "I" is performed in which the coefficient update in the calculation unit 36 is made to wait for the time required from the emission of the sound wave for silencing to the detection of the sound wave in the silencing state.

[0037]

As described above, according to the present invention, when only the signal in the low frequency region of interest is extracted by using the frequency cutoff filter by digital signal processing, the time delay at the time of processing becomes short, Moreover, the processing can be performed with a sharp frequency cutoff characteristic. In addition, since the time delay can be shortened while performing highly accurate silencing, the distance between the first microphone and the silencing speaker is shortened, and the system itself is compact.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing a conventional active noise control system.

FIG. 3 is an explanatory diagram showing an example of a transfer function of each part in the active noise control system of the present invention.

FIG. 4 is an explanatory diagram showing a process for correcting a transfer function that needs special consideration among the transfer functions of the respective parts in FIG.

FIG. 5 is a characteristic diagram showing an example of a gentle frequency cutoff characteristic of a frequency cutoff filter.

FIG. 6 is a characteristic diagram showing an example of steep frequency cutoff characteristics of a frequency cutoff filter.

[Explanation of symbols]

21 ... Duct, 22 ... First microphone, 23 ... Silent speaker, 24 ... Second microphone, 25 ... Microphone amplifier, 26 ... Power amplifier, 27 ... Microphone amplifier, 28 ... Analog-digital converter, 29 ... Digital-analog Converter, 30 ... Analog-digital converter, 31, 32, 33 ... Frequency cutoff filter, 34
... Silencer signal generation filter, 35 ... Adaptive control algorithm, 36 ... Operation part.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hiroshi Asayama 2-4-10 Higashi Nakanobu, Shinagawa-ku, Tokyo Inside Timewear Co., Ltd.

Claims (1)

[Claims] 1. An active noise control system for simultaneously radiating sound waves having the same sound pressure and antiphase to the sound wave of noise propagating in the duct into the duct, and suppressing the sound propagating in the duct by the interference of both sound waves. A first sensor that detects propagating noise and converts an acoustic signal into an analog electric signal; a second sensor that detects a mute state in the duct and converts an acoustic signal into an analog electric signal; the first sensor and the first sensor Two analog-digital converters for converting the analog electric signals from the sensors into corresponding digital electric signals, and only the signals in the frequency domain to be silenced are passed from the digital electric signals from the analog-digital converters. A frequency cutoff filter by digital signal processing, the first sensor output from the frequency cutoff filter, and (2) A calculation unit that creates a muffling digital electrical signal from the information of the sensor, and a frequency cut-off filter by digital signal processing that passes only the signal in the frequency range to be muffled from the muffling digital electrical signal created by this calculation unit A digital-analog converter for converting the noise-suppressing digital electric signal that has passed through the frequency cutoff filter into an analog electric signal, and the analog-electric signal converted by the digital-analog converter for converting into an acoustic signal for noise reduction. An active noise control system, comprising: a speaker that emits sound waves into the duct.