JPH06282278A - Standing-wave corresponding type active noise eliminator - Google Patents

Abstract

PURPOSE:To provide an active noise eliminator by which a standing-wave noise can be eliminated effectively at low cost in the active noise eliminator to eliminate the standing-wave noise generated in a conduit. CONSTITUTION:In an active noise eliminator to eliminate a noise propagating through a conduit by generating a sound wave to offset it from a sounding means 6, plural sound receiving means 1 receive the noise, and a dummy means 5 samples a signal from either of the plural sound receiving means 1, and inputs it to a constitutive element having the prescribed number of stages, and dummies behavior of the noise, and outputs a signal such as offsetting the noise to the sounding means 6. A detecting means 2 inputs signals from the plural sound receiving means 1, and detects that a standing-wave noise is generated in the conduit 8, and a control means 3 controls so that the dummy means 5 carries out dummy operation by changing a sampling period according to a repetition period of a detected standing-wave and also by changing the number of stages of the constitutive element to be used among the constitutive element having the prescribed number of stages.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a standing wave type active noise canceller. In particular, it relates to a standing wave type active noise canceller that cancels the noise of standing waves generated in a conduit.

[0002] Recently, environmental problems are increasingly raising social problems. Noise also causes social problems in various fields, such as damaging the living environment and working environment and physiologically adversely affecting the human body. Further, in recent years, as a means for preventing noise, not only is it erased by absorbing it, but it is also possible to eliminate noise by generating sound waves of the same amplitude as the noise waveform but in the positive and negative opposite directions, and canceling the noise. , Active noise cancellers are attracting attention.

FIG. 4 is an explanatory view of a cooling and silencing control system for a computer system, and particularly shows a silencing control system for a large-sized and high-speed computer system for cooling by blowing cooling air. Cooling air is sent from the cooling device to the floor under free access, and the cooling control system draws the cooling air into the duct by the fan and exhausts it through the duct. In this way, the heat generated from a heat source such as a printed board constituting the computer and guided to the duct is exhausted through the duct, and the computer is cooled. In addition, in a small computer, instead of cooling air, room temperature air is cooled by passing it through a heat source such as a printed board. In either case, the active noise canceller (ANC
C) drives a sounding device such as a speaker based on noise from a fan (called a fan sound) received by a sensor microphone to generate a sound wave whose waveform is the same as that of the fan sound but opposite in direction. The fan sound and the sound generated from the speaker (called speaker sound) are combined and offset to actively cancel the fan sound.

As described above, the active noise canceling apparatus can be applied to all devices and equipments that generate noises such as home electric appliances and computer systems, and can effectively and inexpensively cancel the noises generated from them. Is desired.

[0005]

2. Description of the Related Art FIG. 5 is a block diagram of a conventional silencing control system, showing an example applied to the system of FIG.

Fan for cooling printed boards (noise source)
A fan noise signal received by a microphone (hereinafter referred to as a sensor microphone) installed in the vicinity of is converted from an analog sound signal into a digital signal by an analog / digital converter (referred to as an A / D converter). An adaptive FIR (finite impulse respo) that represents the transfer function that simulates the physical propagation path of sound through a duct
nse) Input to the filter. The output of the FIR filter is converted from a digital signal to an analog signal by a digital / analog converter (referred to as a D / A converter), and a speaker is driven by the signal, and the noise and waveform generated by the fan have the same amplitude and positive / negative inversion. Directional sound waves are generated, the speaker sound is combined with the fan sound, and the sound is canceled to cancel the fan sound.

As a result of combining the speaker sound and the fan sound,
The residual noise that remains when the noise cannot be completely eliminated, that is,
The sound generated based on the error (called residual error) in the simulation result of the fan sound by the FIR filter is received by the error microphone, and its analog sound signal is A
It is converted into a digital error signal by the / D converter. Based on this error signal, the filter coefficient (or tap coefficient) of the FIR filter is changed so that the residual error, that is, the residual noise is brought close to zero, and the noise generated by the fan is controlled to be completely eliminated.

The above processing is normally executed within the sampling cycle t _{s of} the A / D converter connected to the sensor microphone,
The fan sound is erased by repeating the processing for each cycle t _s .

FIG. 6 is a block diagram of the FIR filter.
A FIR filter is a circuit (called a component or tap) consisting of N sets (called N taps) of multipliers and delay elements.
, And one adder, and the discrete input and output signal sequences on the time axis are {x _i }, {y _i }, respectively, the output y _{n at} time n is calculated by the convolution operation shown in the figure. Given. h _i is a filter coefficient, which is automatically updated based on the error signal so as to minimize the error in y _n .

The number of taps N is determined based on the frequency component of the sound wave of noise and the length L (m) of the duct. That is,
The processing by the FIR filter is usually performed by sampling a component having the highest frequency of a noise signal (hereinafter referred to as a fundamental wave) a required number of times (for example, 8 times) during one cycle [the frequency is a sampling frequency f _s (time). / sec), row by type and represented] at the sampling period t _s (seconds), and shifts to a subsequent stage for each cycle t _s by the delay elements, by performing a convolution operation for each period t _s and outputs a y _n Be seen.
Therefore, the FIR filter is

[0011]

## EQU1 ## A tap number N that satisfies t _s × N = L / sound velocity (m / sec) is required.

However, a sound wave whose amplitude distribution is spatially determined (a standing wave) is generated by superposition of two sound waves of noise propagating from the noise source to the outlet of the duct and speaker sound propagating in the opposite direction. ) May occur. When a standing wave having a period longer than the time for which a sound wave propagates in the duct is generated, the number of taps is insufficient to accommodate the signal for one period in the FIR filter.

In the conventional method for coping with the standing wave, the influence of the standing wave is not taken into consideration, or the duct length is extended so that
A method of providing a necessary number of taps by covering the time for a cycle and adding the number of taps for the shortage was used.

[0014]

As described above, according to the conventional method, the influence of the standing wave is not taken into consideration, or the duct is extended to cover the time corresponding to one cycle of the standing wave, and the shortage of taps is required. Since the required number of taps was provided by adding the number, the former case had a problem that the noise due to standing waves could not be sufficiently eliminated. Further, in the latter case, there is a problem that the noise canceling system becomes large and expensive as the duct is extended and the number of taps is increased.

An object of the present invention is to provide a standing wave type active noise canceller capable of canceling the noise of the standing wave generated in the conduit effectively and at low cost.

[0016]

FIG. 1 shows a block diagram of the principle of the present invention. In the figure, 7 is a noise source, 8 is a conduit, and 6
Is a sounding means for canceling the noise generated by the noise source 7 and propagating through the conduit 8 and canceling the noise. 1 is a plurality of sound receiving means for receiving the noise. Is to sample the output signal from any one of the plurality of sound receiving means 1 and input it to the components of a predetermined number of stages, simulate the behavior of the noise, and output to the sound generating means 6 a signal that cancels the noise. The pseudo means for outputting 2, the detection means for inputting the signals output from the plurality of sound receiving means 1 and detecting that the noise of the standing wave is generated in the conduit 8, and 3 are the detecting means. According to the repeating period of the standing wave detected by 2,
The simulating means 5 is a control means for changing the sampling cycle and changing the number of constituent elements to be used among the constituent elements of the predetermined number of steps so as to perform the pseudo operation.

[0017]

According to the present invention, the noise is generated from the noise source 7, and the conduit 8
In the active noise canceling device for canceling the noise propagating through the inside by generating a sound wave from the sounding means 6 to cancel it, a plurality of sound receiving means 1 receives the noise and the pseudo means 5 Is an output signal from any one of the plurality of sound receiving means 1 and inputs it to the components of a predetermined number of stages, and simulates the behavior of the noise and outputs a signal for canceling the noise to the sound producing means 6. To do. The detection means 2 inputs the signals output from the plurality of sound reception means 1 and detects that noise of a standing wave is generated in the conduit 8, and the control means 3 is detected by the detection means 2. The pseudo means 5 changes the sampling period in accordance with the repeating period of the standing wave, and controls the pseudo operation by changing the number of stages of the components to be used among the components of the predetermined number of stages. Therefore, it becomes possible to deal with a standing wave having a long repetition period.

[0018]

FIG. 2 is a block diagram of muffling control showing an embodiment of the present invention. Throughout the drawings, the same reference numerals indicate the same or similar components.

According to the present invention, when a standing wave having a repetitive cycle longer than the time for which noise propagates in the duct 8a is generated, in accordance with the above-mentioned mathematical expression 1, the standing wave is matched with the standing wave (required for one cycle). The number of taps required is reduced below a given number N of taps of the FIR filter 5a by increasing the sampling period (so that sampling is performed a number of times).

The sensor microphones 1a and 1b are microphones arranged adjacent to each other in the duct 8a, and receive noise passing through the duct at their respective positions. The standing wave detection unit 2a samples the noise signals received by the sensor microphones 1a and 1b in time series to detect the standing wave from the sample data by the method shown in FIG.
Estimate the repetition period of the waveform. Also, one of the noise signals from the sensor microphones 1a and 1b is selectively switched and output. In FIG. 3, the points at which the sensor microphones 1a and 1b are installed are d ₁ and d ₂ , respectively, and the distance between them is d. The lowest natural resonance frequency f ₀ is uniquely determined by the length of the duct, and the relative sound pressure levels L ₁ and L _{2 at} points d ₁ and d ₂ can be predicted based on this frequency f ₀ . However, when a standing wave is generated, the sound pressure level L _{S at} point d ₂ does not become the predicted value L ₂ but becomes L ₂ ± ΔL. Therefore, the standing wave detector 2a
Detects the frequency of the standing wave by this difference ± ΔL,
The sampling period is changed based on the frequency.

The sampling controller 3a includes a standing wave detector 2a.
Based on the period of the standing wave detected by, the magnification r of the period of the standing wave with respect to the period t _s of the fundamental wave (the highest frequency among the frequency components of the fan sound) is obtained.

When a standing wave is generated, an analog / digital converter (referred to as an ADC) c1 controls sampling of a noise signal (from the sensor microphone 1a or 1b) selected through the standing wave detection unit 2a. Based on the magnification signal r from the section 3a, sampling is performed at a cycle of t _s × r, the sample value is converted from an analog signal into digital sample data, and F
Output to the IR filter 5a.

The FIR filter 5a is a digital filter having a predetermined number of taps N determined by the frequency component of the fan sound (in particular, the frequency of the fundamental wave) and the length L of the duct 8a, and samples each time sampling is performed by the ADCc1. The behavior of noise propagating in the duct 8a is simulated by inputting data and shifting each tap to the rear stage. When a standing wave is generated, in order to synchronize the standing wave sound propagating in the duct 8a and reaching the outlet with the output of the simulation result by the FIR filter 5a, t _{s is changed} to t _Assuming that the number of effective taps is N, which satisfies Equation 1, when replaced by _s × r,
The convolution operation shown in is performed and y _n is output.

A digital / analog converter (referred to as DAC) c3 converts the digital signal y _n of the convolution operation result from the FIR filter 5a from a digital signal to an analog signal, and drives the speaker SP by the signal, so that the inside of the duct 8a. The standing wave sound and the generated sound wave generate a sound wave with the same amplitude and opposite directions, and the speaker sound is combined with the fan sound to cancel the noise due to the standing wave.

As a result of synthesizing the speaker sound and the standing wave sound,
The residual noise remaining when the noise due to the standing wave cannot be completely eliminated, that is, the residual noise generated based on the standing wave sound simulation error (residual error) by the adaptive FIR filter 5a is received by the error microphone SM. ,analog/
A digital converter (referred to as ADC) c2 converts from analog to digital error signal.

The FIR filter 5a changes the filter coefficient (or tap coefficient) based on this error signal to bring the residual error, that is, the residual noise close to zero, and the noise due to the standing wave generated in the duct 8a. Control to erase.

Therefore, when the noise of a standing wave having a long repetition period is generated in the duct 8a, the magnification r of the period of the standing wave with respect to the period of the fundamental wave is obtained, and the sampling period is set to r times, and the FIR filter is used. The number of taps included in 5a, r
It is configured so that the number of taps does not become insufficient by using the number of taps that is one-third.

[0028]

As described above, according to the present invention, the behavior of noise propagating in the duct is simulated by the digital filter, and the active noise canceller cancels the noise based on the simulated signal. It is detected that noise of standing wave is generated in the duct by receiving the noise, and the cycle of sampling the noise from the microphone is changed according to the repetition cycle of the noise. Change the number of taps. With this configuration, a standing wave with a long repetition period can be easily dealt with by lengthening the sampling period and reducing the number of taps used, and extending the duct as in the conventional method. However, there is no need to increase the number of taps. Therefore, according to the present invention, there is an effect that the noise of the standing wave generated in the duct can be effectively and economically eliminated.

[Brief description of drawings]

FIG. 1 is a block diagram of the principle of the present invention.

FIG. 2 is a block diagram of muffling control showing an embodiment of the present invention.

FIG. 3 is an explanatory diagram of a standing wave detection method.

FIG. 4 is an explanatory diagram of a computer organization cooling and silencing control system.

FIG. 5 is a block diagram of a silence control system showing a conventional example.

FIG. 6 is a block diagram of a FIR filter

[Explanation of symbols]

 1 plural sound receiving means 2 detection means 3 control means 5 pseudo means 6 sounding means 7 noise source 8 conduits 1a, 1b sensor microphone 2a standing wave detection section 3a sampling control section 5a FIR filter 8a duct 9a heat source c1, c2 analog / Digital converter (ADC) c3 Digital / analog converter (DAC) SP speaker

Claims (2)

[Claims] 1. Active noise cancellation for canceling noise generated from a noise source (7) and propagating through a conduit (8) by generating sound waves from the sounding means (6) to cancel it. A plurality of sound receiving means for receiving the noise
(1) and the output signal from any of the plurality of sound receiving means (1) is sampled and input to the constituent elements of a predetermined number of stages,
Pseudo means (5) for simulating the behavior of the noise and outputting a signal for canceling the noise to the sounding means (6)
And a detection means (2) for inputting signals output from the plurality of sound receiving means (1) to detect the occurrence of standing wave noise in the conduit (8), and the detection means (2) The pseudo means (5) changes the sampling period according to the repetition period of the standing wave detected by the means (2), and changes the number of stages of the constituent elements used among the predetermined number of constituent elements. A standing wave type active noise canceller, which is provided with a control means (3) for controlling so as to perform a pseudo operation. 2. The pseudo means (5) inputs an analog / digital converter that performs a sampling operation at a predetermined cycle t _s and sample data from the analog / digital converter to a component having a predetermined number N of stages. The control means (3) obtains a magnification r of the period of the standing wave with respect to the period of the fundamental wave of the noise generated from the noise source (7), and performs the analog / digital conversion. The sampling unit performs sampling operation at a cycle in which the cycle t _s is multiplied by the scaling factor r, and the digital filter changes the number of stages N by the scaling factor r.
2. The standing-wave-corresponding active noise canceller according to claim 1, wherein control is performed so as to perform a pseudo operation using the number of stages of the components divided by.