JPH0772872A - Active silence method - Google Patents

Abstract

(57) [Summary] [Structure] An active sound deadening method in which a sound wave in a one-dimensional sound field is detected by a sound wave detector, and a sound wave that cancels the sound wave is sent from a sound wave generator to mute sound. A plurality of at least one of the sound wave generator and the sound wave generator is used, and the sound wave detectors and the sound wave generators are separated by a distance corresponding to (2n-1) / 4 (n is a natural number) of the wavelength of the sound wave of the highest frequency to be silenced The method is characterized in that they are arranged apart from each other in the traveling direction of sound waves. [Effect] Even when one sound wave detector or sound wave generator hits a node of a one-dimensional plane traveling wave of a frequency to be silenced, at least one other sound wave detector or sound wave generator becomes a non-knot position. , The sound reduction effect does not decrease at a specific frequency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active noise reduction method.

[0002]

2. Description of the Related Art An explosion sound of an engine, a rotation sound of a blower fan, etc. are periodic sounds having a repetitive waveform synchronized with rotation. In order to reduce the noise caused by such periodic sounds, an active noise reduction technique has been attracting attention to reduce the noise by utilizing the interference of sound waves. This is described in, for example, Japanese Patent Application Laid-Open No. 2-94900, and is created by sending a noise signal detected by a monitor microphone to a signal processing device and calculating a signal for canceling a sound wave by the signal processing device. Based on this signal, a sound wave is emitted from the sound deadening speaker and interferes with the noise to reduce the noise. This method is particularly effective when a sound wave from a sound source travels in a tubular body such as a duct and is regarded as a one-dimensional plane traveling wave in fact. Japanese Unexamined Patent Publication No. 4-306477 discloses an example in which this active noise reduction technology is applied to a cooling device.

[0003]

In the conventional active muffler, one detection microphone and one additional sound source speaker (silence speaker) are usually used. However, when the sound wave is considered to be a one-dimensional plane traveling wave, if the detection microphone or the additional sound source speaker hits the node of the vibration of the traveling wave of a certain frequency, it becomes unobservable or uncontrollable. The sound reduction effect is reduced for that frequency. FIG. 6 is a diagram showing this state, in which the microphone 3A and the additional sound source speaker 4A are located at the nodes of the plane traveling wave traveling in the duct.
Shows an example where is located.

In such a case, no sound wave is detected at the position of the microphone 3A. Further, at the position of the additional sound source speaker, it is impossible to cancel the plane traveling wave even if the sound wave is emitted. This problem can be avoided by placing a microphone and an additional sound source speaker at the exit of the duct,
In an actual plant, such arrangement may not be possible. Further, at the exit of the duct, the sound wave cannot be regarded as a one-dimensional plane traveling wave. Therefore, when the microphone and the additional sound source speaker are arranged at the exit of the duct, the sound deadening effect may have directivity. .

The present invention has been made to solve the above problems, and an object of the present invention is to obtain a high-performance active muffling method in which the noise reduction effect at a specific frequency does not deteriorate. .

[0006]

The above object is to provide an active muffling method in which a sound wave in a one-dimensional sound field is detected by a sound wave detector, and a sound wave that cancels the sound wave is transmitted from a sound wave generator to mute the sound wave. Therefore, at least one of the sound wave detector and the sound wave generator is used, and the sound wave detectors and the sound wave generators are combined at the wavelength (2n-
1) / 4 (n is a natural number), which is achieved by a method of arranging the distance in the traveling direction of the sound wave.

[0007]

By arranging a plurality of sound wave detectors and sound wave generators as described above, even when one sound wave detector or sound wave generator hits a node position of a one-dimensional plane traveling wave of a frequency to be silenced, The other at least one acoustic wave detector or acoustic wave generator is in a non-nodal position. FIG. 1 is a diagram showing the principle of the present invention.

As shown in FIG. 1, the first microphone (sound wave detector) 3A and the second microphone 3B, the first additional sound source speaker (sound wave generator) 4A and the second additional sound source speaker 4B are ΔS. (= 1/4 λ: λ is the wavelength at the highest frequency that you want to mute).
The sound wave detected by the first microphone 3A is input to a calculation device (not shown), and a sound wave necessary to cancel it is calculated and output from the first speaker 4A.

Similarly, the sound wave detected by the second microphone 3B is input to an arithmetic unit (not shown), and the sound wave necessary to cancel it is calculated and output from the second speaker 4B. The first microphone 3A and the first additional sound source speaker 3B are in the unobservable and uncontrollable state because they are located at the node of the one-dimensional plane traveling wave, but the second microphone 3A and the second additional sound source speaker Since 3B is 1 / 4λ away from this position, it is located at the antinode of the one-dimensional plane traveling wave, and these can be used for silencing.

Although two microphones and two loudspeakers are used in FIG. 1, only the microphones are used if it is certain that the mounting positions of the loudspeakers do not become the nodes of the plane traveling wave. It is also possible to adopt a method of using a plurality of. In this case, the sound waves necessary to cancel the sound waves detected by the respective microphones may be calculated by the calculation device, and the speaker may be driven by the combined output.

On the other hand, when it is certain that the position where the microphone is attached does not become a node of the plane traveling wave, it is possible to use a plurality of speakers only. In this case, the sound waves to be sent out by the respective speakers in order to cancel the sound waves detected by the microphone may be obtained, and the respective speakers may be driven accordingly.

[0012]

EXAMPLE Next, as an example of the present invention, an example in which the present invention is used for silencing exhaust noise of a gas engine will be described. In this embodiment, two sound wave detectors are used and one sound wave generator is used. The exhaust sound of a gas engine has a high sound pressure level in a low frequency range of about 200 Hz or less, which has an engine speed as a fundamental frequency and is mainly an integral multiple and a 0.5th order component. In others, the sound pressure level is gradually reduced and the sound pressure level is expanded to around 500 Hz.

The structure of the active silencer will be described with reference to FIGS. 2 and 3. When the active silencer is installed in the middle of the duct 2, the additional sound source speaker 4 and the microphone 3 must be protected from the heat of the exhaust gas 8 and the contained components. The temperature of the exhaust gas 8 of the gas engine 1 is about 300 ° C. or higher along the road of the exhaust duct 2. In addition, the components of the exhaust gas 8 include water vapor. In order to withstand such an environment and have a compact structure that can be accommodated in the dead space of the pipe, the structure shown in FIG. 3 is used here.

That is, a heat-resistant film 7 is provided to prevent high-temperature exhaust gas containing water vapor from directly flowing into the muffler side, and the additional sound source speaker 4 and the additional sound source speaker 4 are provided through the cooling unit 6 in which many thin pipes are embedded. The structure is such that the microphone 3 is installed. The silencer uses two 40 cm cone speakers 5 and a condenser microphone, and does not use a special speaker or microphone but uses a commercially available inexpensive one. In this way, we realized a silencer with a transparent structure that cuts off the heat by only natural cooling, but acoustically. Since the engine exhaust sound is a periodic sound, the muffling algorithm used the synchronous waveform synthesis method. The signal processing is performed using the equation (1) and the equation (2).
The logical formula shown in the formula is used.

[0015]

[Equation 1]

In order to realize the expressions (1) and (2), the active silencer shown in FIG. 4 was manufactured. That is, A / D
The converter 11 externally samples the sound pressure signal from the microphone 3 based on the pulse 13 synchronized with the engine speed, and transfers it to the computer 10 by direct memory access. The computer 10 divides the transferred data into four blocks and performs a Fourier transform for each block.

Next, with respect to all the frequency components to be silenced, the vector operation shown in the equation (1) is performed to obtain the optimum value, and then the inverse Fourier transform is performed. The result is D
Direct memory access transfer is again performed to the / A converter 12, and the synthesized sound is emitted from the additional sound source speaker 4. At this time, the input from the A / D converter 11 and the calculation of the next output signal by the computer 10 are simultaneously executed. By repeating this series of cycles, data input / output is continuously processed.

The noise reduction effect of the active silencer will be described. The following experiment was conducted to investigate the noise reduction effect of the active silencer alone. The test object is output 500kW, fundamental frequency 1
It is a 6.7 Hz gas engine. The active silencer is turned on and off when the engine is operating, and the control point of the microphone 3A in FIG. 2 and the point 1 m from the outlet of the duct 2 (see FIG.
The sound reduction effect of C) was measured. As a result, 80Hz, 1
The sound reduction effect was not obtained in the vicinity of 80 Hz. Therefore, the microphone 3B was separated by 50 cm and incorporated into the A / D converter 11. This is the control point microphone 3
This is because the input sound pressure level becomes small because point A is close to the sound pressure nodes at both frequencies, and sufficient control could not be performed. Therefore, the microphone 3A shown in FIG.
When control was performed at two points of 3B, the sound was reduced at all frequency peaks, and an effect of about 30 dB at maximum could be obtained.

In order to examine the directivity of the exhaust gas 8 from the outlet of the duct 2, the sound reduction effect at the measurement points D (45 ° direction) and E (90 ° direction) in FIG. 2 was measured. As a matter of course, since the sound is silenced inside the duct 2, the same sound reduction effect is obtained in any direction, and the directivity for the sound reduction is not shown.

In order to confirm the durability of the active silencer, a long-term test was conducted on the equipment currently in operation. FIG. 5 shows the time variation of the temperature inside the active silencer. The temperature is saturated in about 1 hour, and when the temperature of the exhaust gas 8 is 350 ° C,
Immediately before the additional sound source speaker 4 and the microphones 3A and 3B, the temperature environment was kept at 40 to 50 ° C. at the most, and the temperature environment was sufficient to withstand. Also, regarding water vapor, there is no water that passes through the heat resistant film 7, and there is no particular problem. The system ran smoothly for 1000 hours.

[0021]

As described above, in the present invention, a plurality of at least one of the sound wave detector and the sound wave generator are used, and the sound wave detectors and the sound wave generators are provided with the highest frequency sound wave to be silenced. One-dimensional plane of the frequency at which one sound wave detector or sound wave generator wants to muff because it is arranged in the traveling direction of the sound wave by a distance corresponding to (2n-1) / 4 (n is a natural number) of the wavelength. Even when hitting the node position of the traveling wave, the other at least one sound wave detector or sound wave generator is located at a position other than the node, and the sound reduction effect does not deteriorate at the specific frequency.

[Brief description of drawings]

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

FIG. 2 is a diagram in which a microphone and an additional sound source speaker are arranged in an embodiment of the present invention.

FIG. 3 is a detailed diagram when a microphone and an additional sound source speaker are arranged in a duct according to an embodiment of the present invention.

FIG. 4 is a block diagram of an active silencer.

FIG. 5 is a diagram showing a temporal change in temperature inside the active silencer.

FIG. 6 is a layout diagram of a microphone and an additional sound source speaker in a conventional active silencer.

[Explanation of symbols]

 1 Gas engine 2 Duct 3 3A, 3B Microphone 4 4A, 4B Additional sound source speaker 5 Speaker alone

Claims (1)

[Claims] 1. An active muffling method, wherein a sound wave in a one-dimensional sound field is detected by a sound wave detector, and a sound wave that cancels the sound wave is sent from the sound wave generator to muffle the sound wave. Using at least one of the sound wave detectors, the sound wave generators and the sound wave generators are moved in the sound wave traveling direction by a distance corresponding to (2n-1) / 4 (n is a natural number) of the wavelength of the sound wave with the highest frequency to be silenced. An active noise reduction method characterized by being placed apart.