JPH0272395A - Active noise control method - Google Patents

Abstract

PURPOSE:To reduce a sound pressure ratio and to apply the active noise control method even to a large noise of low frequency at low cost by connecting a branch pipe to a noise propagation conduit and generating a control sound through the branch pipe. CONSTITUTION:The branch pipe 4 is fitted halfway to the duct 3 which connects a noise source 1 such as an air blower and an air compressor to an air intake (or air outlet) 2 and the control sound is generated by the speaker 6 of a control sound generating device 5 installed at a terminal of the branch pipe. In this state, the nondirectional length from, for example, the noise source 1 to the fitting position of the branch pipe 4 is set to 0, namely, the fitting position of the branch pipe 4 is at a distance which is an integral multiple of the half of the wavelength of the sound from the noise source 1 and the length of the branch pipe 4 is 6/12 time or an odd multiple of the quarter of the wavelength of the sound to reduce the sound pressure ratio greatly. Consequently, the low- frequency noise of low-frequency air vibration, etc., can be canceled with a small sound and the active noise control method is applicable.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to reducing the sound pressure ratio of radiated sound for noise cancellation to noise in an active noise control method.

(Prior art and its problems) Noise generated by large blowers, air compressors, etc. used in thermal power plants and other plants may have a negative impact on the environment. For this reason, the following methods have been implemented and proposed in the past.

One of them is a method using a silencer. This is due to the equipment that generates most of the noise, such as large blowers, and the air intake (
Since the sound propagates through the pipes (ducts) that connect the pipes (or the air discharge ports) and is radiated from the openings, this method involves installing an expansion type or sound absorbing type muffler in the middle of the pipe, and is currently widely used. It is something that exists. However, in order to reduce low-frequency sounds using this method, the muffler must be made larger, which increases the installation space and cost of countermeasures.

Another method is to cancel the noise by radiating a sound with the opposite phase to the noise, which is called the active noise control method.This method is based on the fact that the longer the wavelength of the sound, the easier it is to control the phase. In principle, this is a method that can be used as an effective mitigation measure for low-frequency noise such as low-frequency air vibration.

However, as is well known, the speakers used to generate the opposite phase sound in this method have a characteristic that the lower the frequency, the more difficult it is to generate a loud sound. Therefore, it is difficult to apply this method to equipment that often generates loud and low-frequency noise, such as the large-sized blowers and air compressors mentioned above. Furthermore, even if it were applied, control sound generating equipment such as speakers would be extremely large and expensive. Therefore, the cost required for mitigation is high, making it difficult to implement in practice, and this is a cause of hindering the practical application of active noise control methods.

(Objective of the Invention) An object of the present invention is to provide a method that can reduce noise from pipes, particularly low-frequency noise, at low cost using an active noise control method.

(Means of the Invention for Solving the Problems) A device that enables the application of active noise control methods in low frequency noise is a blower that cancels out loud noises with small sounds from a loudspeaker. It is necessary to be able to reduce the ratio between the loudness of the sound generated by the equipment and the loudness of the control sound required to cancel it (hereinafter referred to as the sound pressure ratio).

As a result of theoretical and experimental research, it was confirmed that the sound pressure ratio can be significantly reduced by connecting a branch pipe to the noise propagation pipe and generating control sound through this, and the present invention has been applied to the present invention. We have demonstrated that active noise control methods can be applied at low cost even to large low-frequency noises, which have been considered difficult, and that excellent noise reduction effects can be obtained.

That is, as shown in Figure 1, a branch pipe (4) is installed in the middle of a pipe line (3) connecting a noise source (1) such as a blower or an air compressor to an air intake port (or air discharge port) (2). A control sound is generated from the speaker (6) of the control sound generator (5) installed at the end of the installation, and in this state, the installation position of the branch pipe (4) from the noise source (1) in the pipe (3) is When we theoretically determined the relationship between the length of the pipe, the length of the branch pipe, and the sound pressure ratio, we obtained the following results.

Figure 2 shows the noise fi (
1) is a diagram showing how the sound pressure ratio differs depending on the dimensionless length from the branch pipe (4) to the attachment position.

Furthermore, Figure 3 shows how the sound pressure ratio changes depending on the dimensionless length of the branch pipe (4), using the dimensionless length from the noise source (1) to the installation position of the branch pipe (4) as a parameter. be.

As is clear from these figures, the dimensionless length from noise #(1) to the installation position of the branch pipe (4) is O1, that is, the installation position of the branch pipe (4) is the wavelength of the sound from the noise source (1). The length of the branch pipe (4) is 6712 times the wavelength of the sound, that is, the length of the branch pipe (4) is an odd multiple of the wavelength of the sound A. The pressure ratio can be significantly reduced. Therefore, large low-frequency noises such as low-frequency air vibrations can be canceled out with small sounds, making it possible to apply active noise control methods that have been difficult to apply in practice in the past.

Next, experimental results of the present invention will be shown.

FIG. 4 is an example of the results of an experimental investigation of the relationship between the dimensionless length of a branch pipe and the sound pressure ratio, using the dimensionless length (ml) from the noise source to the branch pipe installation position as a parameter. Although this result does not determine the minimum value of the sound pressure ratio, it is quite possible to lower the sound pressure ratio to 0.1 or less since the results are almost the same as the theoretical value in Figure 3. .

Further, FIG. 5 is an experimental example in which the amount of noise attenuation by the active method was investigated, and the value is the value obtained by subtracting the noise level when the control sound is generated from the noise level when the control sound is not generated. On average, attenuation of a little less than 50 dB is observed. The amount of attenuation when the dimensionless length of the branch pipe is 6712 is smaller than the others, but in this case, the branch pipe itself acts as a muffler, so the noise level when no control sound is generated is already higher than that in other cases. This is because the attenuation is more significant and the waveform is disordered, and the amount of attenuation combined for both is actually larger in this case.

(Effects of the Invention) As is clear from the above explanation, according to the present invention, the control sound may be small, so the active noise control method can be applied to large low-frequency noises where it is difficult to generate large control sounds. Furthermore, since the capacity of the control sound generating device can be reduced, the cost for noise countermeasures can be reduced. Therefore, it makes a significant contribution as a measure to reduce noise generated from pipe systems such as pipes connected to blowers and the like.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the principle of the present invention, Figures 2 and 3 are
The figure shows the installation position of the branch pipe to the pipeline, the length of the branch pipe,
The side view of the relationship between the sound pressure ratio of the control sound necessary for cancellation, and FIGS. 4 and 5 are diagrams showing experimental results. (1)...Noise sources such as blowers, (2)...Open ends such as air intake ports and discharge ports, (3)...Pipe lines (ducts), (4)...Branch pipes, (5)...Control sound generator, (6)...Speaker. Patent applicant: Central Research Institute of Electric Power Industry

Claims (1)

[Claims] (1) In an active noise control method in which noise transmitted through a conduit connected to a noise source is canceled by emitting sound with an opposite phase to the noise, a branch pipe is provided in the conduit, and the noise transmitted through the conduit is In addition to radiating the opposite phase sound, the length of the branch pipe and the length of the pipe from the noise source to the branch pipe are selected to reduce the sound pressure ratio of the sound required for cancellation to the noise. active noise control method.