JP2020134548A - Manufacturing method of silencer with acoustic phase shifter - Google Patents

Abstract

To provide an inexpensive silencer with a phase shifter, which does not require a power source, has less failures and whose maintenance is easy.SOLUTION: A manufacturing method of a silencer having a main pipe and a branch pipe comprises: a process S1 for using, as the branch pipe, an acoustic phase shifter having a hollow cylinder partitioned by a plurality of plates separated from one another by a gap thickness b; a process S2 for deriving a relation expression of sound speed V in a gap between the plates; a process S3 for deriving a relation expression of sound speed V' becoming an opposite phase to the sound speed of the main pipe; and a process S4 for determining a size of the main pipe and a size of the phase shifter so that the sound speed V is matched with the sound speed V'. In the process S4, it is desirable that the gap thickness b and pipe lengths Land Lof the main pipe and the phase shifter are determined, for example.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for manufacturing a sound deadening device provided with a pure acoustic phase shifter that does not require a power source.

(Conventional interference type silencer)
An interference type silencer (so-called passive type silencer) is a silencer composed of a main pipe and a branch pipe. The principle of these silencers is that when a sound wave incident on a main pipe is branched into a branch pipe and rejoined to the main pipe, the sound wave branched into the branch pipe is made a sound wave having a phase opposite to that of the main pipe. , The sound is muted by canceling the sound waves using the principle of superposition.

(Problems with conventional interference type silencers)
However, with the above-mentioned interference type silencer, meaningful sound deadening performance can be obtained only at a specific frequency, but it is often not so effective at other frequencies.

(Conventional active noise control)
On the other hand, active noise control aims to mute the sound in a wider frequency range. In this method, sound waves measured by a microphone using an electronic control circuit are usually out-phased and output from a speaker to be merged, thereby enabling sound deadening in a wider band.

(Problem 1 of conventional active noise control)
However, active noise control will continue to be the mainstream of mufflers in the future because (1) it is costly, (2) a power supply is essential, and (3) there are many equipment failures and maintenance is difficult. Very Hard to think.

(Problem 2 of conventional active noise control)
Another problem with active noise control is that it requires a huge amount of calculation during control. It is necessary to determine the target frequency range, divide the band into small pieces, and detect the amplitude and phase for each frequency. The signal for the adaptive sound source is obtained by assuming the position of the adaptive sound source, generating signals (1) having opposite phases and (2) having the same amplitude for each frequency component, and synthesizing them. Further control is needed to correct the error.

Japanese Unexamined Patent Publication No. 02-041953 Japanese Unexamined Patent Publication No. 62-110400

Stinson, MR and Champou, Y., Propagation of sound and the assignment of shape factors in model porous materials having simple pore geometries, Journal of the Acoustical Society of America. Vol 91, No. 2 (1992), pp. 685-695 ..

(Purpose of the present invention)
The present invention has been made in view of such circumstances, and an object of the present invention is to provide a sound deadening device provided with a phase shifter, which is low in cost, does not require a power supply, has few failures, and is easy to maintain.

(Acoustic phase shifter)
Here, the phase shifter in the present invention is a pure acoustic phase shifter, unlike the conventional analog or digital phase shifter (for example, Patent Documents 1 and 2).

Among the processes essential for active noise control, the present inventor performs the above-mentioned (1) anti-phase signal generation process with a pure acoustic phase shifter (that is, a pure acoustic phase shifter that does not require a power supply and does not use a calculation or a control circuit. , An acoustic structure without moving parts), and have completed the present invention.

That is, the present invention has, for example, the following configurations and features.
(Aspect 1)
A method for manufacturing a silencer having a main pipe and a branch pipe.
A step S1 in which an empty pipe is used for the main pipe and an acoustic phase shifter having a hollow cylinder partitioned by a plurality of plate materials separated by a gap thickness b is used for the branch pipe.
Step S2 for deriving the relational expression of the sound velocity V in the gap between the plate materials, and
Step S3 for deriving the relational expression of the sound velocity V'that has the opposite phase to the sound velocity of the main pipe,
Step S4 in which the dimensions of the main pipe and the dimensions of the phase shifter are determined so that the sound velocity V and the sound velocity V'match.
Including and
In the step S2, the propagation constant γ represented by the following mathematical formula 1 is used.
Here, x is the position in the longitudinal direction of the phase shifter, y and z are the coordinates in the gap cross section, c is the speed of sound in air, κ is the specific heat ratio of air, σ is the Prandtl number, and ρ is the density of air. μ is the viscosity of air, ω is the angular frequency,
After dividing the propagation constant γ into a real part α and an imaginary part β by the following mathematical formula 2,
According to the following mathematical formula 3, the relational expression of the sound velocity V is expressed only by the variable of the gap thickness b.
In the step S3, using Equation 4 below, as relation of acoustic velocity V 'is the pipe length L _A of the phase shifter and the main _pipe, it represented only by variables L _F,
The dimensions of the main dimensions and the phase shifter to be determined at the step S4, the gap thickness b, the pipe length L _A, it is L _F,
A method of manufacturing a silencer, which comprises.
(Aspect 2)
In the step S4, the difference (θ _A − θ _F ) between the phase change amount θ _A and the phase shifter phase change amount θ _F at each frequency of the main pipe is obtained.
The first aspect described in the first aspect, wherein the difference (θ _A − θ _F ) is within ± 15 ° from 180 ° as a criterion for determining whether or not the sound velocity V and the sound velocity V'match. How to manufacture a silencer.
(Aspect 3)
A step of preparing a microphone, a filter, an amplifier, and a speaker in the silencer.
Including more
The microphone measures the sound wave after passing through the phase shifter.
The amplifier amplifies the sound wave by the amount attenuated in the phase shifter.
Aspect 1 or 2 characterized in that the speaker outputs the sound wave amplified by the amplifier toward the confluence portion of the phase shifter and the main pipe to mute the sound wave propagating in the main pipe. The method for manufacturing a silencer according to.

The acoustic phase shifter of the present invention can automatically and passively generate a signal having a phase opposite to that of the main sound wave in a wide frequency range by using only an acoustic structure that does not require a power supply or a moving part at all. Therefore, the "digital signal processing that generates the opposite phase", which is indispensable for active noise control, becomes completely unnecessary. Further, since the acoustic phase shifter of the present invention does not require phase control, the adaptive signal can be generated only by adjusting the frequency characteristic of the gain with a linear filter.

According to the present invention, there is provided a muffling device having a very simple structure, low cost, no power supply, no failure, and no maintenance. That is, the muffling device of the present invention contributes to simplification of noise control, reduction of power consumption, and improvement of reliability, which are difficult to achieve with ordinary active noise control.

The flowchart for designing the acoustic phase shifter of this invention is shown. It is a schematic view of the muffling device of this invention, the figure which showed the gap between plate materials (between two planes) in a Cartesian coordinate system, and the figure which showed the sound wave incident in a pipe by a sine wave. It is a figure which showed the specific example which compared the sound velocity V and the sound velocity V'. Constant gap thickness (b = 0.1 mm) on to the pipe length L _A, upon changing the L _F, is a diagram comparing a phase difference between the sound waves and main phases shifter (theoretical value). An image of an actually prototyped phase shifter and a schematic diagram of the phase shifter are shown. A schematic diagram and a photograph of an acoustic performance (phase change amount) measuring device are shown. A diagram comparing the theoretical value of the phase change amount θ _F in the phase shifter with the experimental value, and a diagram comparing the theoretical value of the phase change amount θ _A in the main pipe with the experimental value are shown. It is a figure which showed the result of having calculated the phase difference of a phase shifter and a main pipe, and the figure which showed the sine wave at each point (incident part, confluence part) of a silencer. It is the figure which showed the result of calculating the phase difference of a phase shifter and a main pipe (repost), and the estimated value of the transmission loss (the transmission loss of a sound wave at the confluence part of a main pipe) by superposition of sound waves.

Hereinafter, the present invention will be described based on the following specific embodiments with reference to the accompanying drawings, but the present invention is not limited to these embodiments.

(Outline of design method of acoustic phase shifter)
FIG. 1 shows a flowchart for designing the acoustic phase shifter of the present invention. The present invention also presupposes the adoption of an interference type silencer structure including a main pipe and a branch pipe as in the conventional side branch type silencer, but in the present invention, an acoustic phase shifter described later is used for this branch pipe. Note what to do (step S1).

(Structure of acoustic phase shifter)
In this embodiment, as shown in FIG. 1, an empty pipe is used for the main pipe 2, and an acoustic phase shifter 1 (hereinafter, also simply referred to as “phase shifter”) is used for the branch pipe (step S1). As shown in FIG. 5B, this is an analog control device in which plate materials (thin plates) are stacked with a gap inside a hollow cylinder having a rectangular cross section. In this acoustic phase shifter 1, unlike the case of a branch pipe made of a hollow pipe, the sound velocity of a sound wave incident on a gap between plate materials changes depending on the frequency due to the action of the boundary layer, and the phase is utilized by utilizing this. The inventor has found that it may also be possible to change.

(Difference between acoustic phase shifter and normal branch pipe)
If a normal hollow tube (not shown) is used for the branch tube, the sound velocity of the sound wave incident on it is constant, and even if it is rejoined to the main tube 2, it will be in a specific frequency range. Only meaningful muffling performance can be obtained.

(Silencer with acoustic phase shifter)
FIG. 2A shows an outline of the muffling device 10 including the phase shifter 1. In the sound deadening device 10, sound waves generated from the noise source 3 pass through the hollow pipe which is the main pipe 2 and the phase shifter 1 which is a branch pipe. When the sound wave passes through the phase shifter 1, a sound wave having a phase opposite to that of the sound wave passing through the main pipe 2 is obtained. Since the sound wave in the phase shifter 1 is attenuated, the sound wave after passing is measured by the microphone 5, and after passing through the filter 6 (equalizer), the sound wave is amplified by the amplifier 7 by the amount of attenuation. After that, by outputting this amplified sound wave from the speaker 8 to the confluence portion 4 in the main pipe 2, sound waves having the opposite phase and the same amplitude merge toward the sound wave propagating in the main pipe 2, and the sound waves are superposed. Sound waves are canceled and muted by the principle of matching.

Since the sound wave that has passed through the phase shifter 1 has a gain frequency characteristic, it is necessary to cancel the frequency characteristic and return it to a flat frequency characteristic. Therefore, the filter 6 performs a process of giving the sound wave a frequency characteristic opposite to the above frequency characteristic. As the filter 6, for example, a technically easy linear filter can be used.

(Parameter of silencer to be determined)
However, it is necessary that the main pipe 2 and the phase shifter 1 have a geometric relationship in which the above-mentioned sound deadening mechanism is exhibited well, and it is necessary to determine the dimensions of each element. In this embodiment, the sound velocity V in the gap between the plate materials (between two planes) and the sound velocity V'that is in the opposite phase to the sound velocity of the main pipe 2 are calculated (steps S2 and S3), respectively, and these Vs are used. by comparing the V ', a key factor for silencing effect, determines the pipe length L _F and the gap b of the phase shifter 1, a main tube 2 and a tube length L _a.

(Drivation of sound velocity V in the gap between plate materials (between two planes))
There are various reports on the derivation of propagation constants and characteristic impedances in consideration of the viscosity of air in the pipe. In this example, the method of Stinson (see Non-Patent Document 1) was applied to attempt to calculate the sound velocity V in the gap between the plate materials (between two planes). Specifically, the Cartesian coordinate system is taken as shown in FIG. 2 (b), and each is performed by three-dimensional analysis using the Navier-Stokes equation, the continuity equation, the gas state equation, the energy equation, and the dissipation function representing heat transfer. It was decided to derive the propagation constant γ at the frequency and the sound velocity V in the gap between the plate materials (step S2).

(Propagation constant γ)
The propagation constant γ in the two planes considering the attenuation is expressed by the following mathematical formula (1). Here, x is the position in the longitudinal direction of the Cartesian coordinate system, y and z are the coordinates in the gap cross section, c is the speed of sound in air (343.7 m / s), κ is the specific heat ratio of air, and σ is the Prandtl number. ρ is the density of air, μ is the viscosity of air, ω is the angular frequency, and b is the thickness of the gap.

Further, if the real part of the propagation constant γ is the attenuation constant α and the imaginary part is the phase constant β, the equation (1) can be replaced with the following form (formula (2)).

The sound velocity V at the thickness b of the gap between the two planes at each frequency can be obtained by using the imaginary part (phase constant) β of the propagation constant as in the following equation (3).

From the above equations (1) and (2), since only the gap thickness b is a variable in the equation (3), the sound velocity V in the gap between the two planes at each frequency f is the gap thickness b. Can be obtained by determining (ie, V = V (b)).

(Derivation of sound velocity V'that is in phase opposite to the sound velocity of the main pipe)
A sine wave y _I at the entrance portion X _I of the acoustic waves incident on the tube, the sine wave y _C at the end portion X _C, respectively, are expressed out with the following equation (4), and Equation (5) (See also FIG. 2 (c)).

The phase change amount θ at each frequency in the pipe at this time can be obtained from the following equation (6). Here, f is the frequency, L is the pipe length, and ν is the speed of sound in the pipe.

By using this formula (6), the phase change amounts θ _A and θ _F at each frequency of the main pipe 2 (hollow pipe) and the branch pipe 1 (phase shifter) can be obtained as follows. Here, _{L A} is the pipe length of the main pipe, _{L F} is the pipe length of the phase shifter 1, c is the speed of sound in air (343.7m / s), V 'is the velocity of sound phase shifter 1 side.

In order to merge the sound waves of opposite phases from the phase shifter 1 at the confluence portion 4 of the main pipe 2, the difference between the phase change amounts θ _A and θ _F at each frequency of the main pipe 2 and the phase shifter 1 is 180 °. Must be (π radians). That is, it is necessary to satisfy the following mathematical formula (9).

That is, the condition of the speed of sound V'on the phase shifter 1 side for having the opposite phase at each frequency is as shown in the following mathematical formula (10) using the mathematical formulas (7), (8), and (9). Can be asked for.

Become a variable in the equation (10), the pipe length _L A of the main pipe 2 and the phase shifter 1, a _{L F.} That is, the acoustic velocity V of phase shifter 1 side to the opposite phase at each frequency 'can be determined by determining the respective pipe length L _A, the _{L F (V' = V '} (L A, L _F)).

(Determination of the dimensions of the silencer by comparing the speeds of sound V and V')
By executing the steps S2 and S3 described above, it is possible to obtain the condition of the sound velocity V'on the phase shifter 1 side so that the phase is opposite to the sound velocity V in the gap between the plate materials. Specifically, by comparing these, the thickness of the gap, such as the condition of the above conditions sound velocity V and the sound velocity V 'coincides b and the pipe length L _A, obtains the L _F (step S4).

(Step S5: Effective judgment method for comparing sound velocity V, V')
As shown in FIG. 1, in the above-mentioned step S4, the difference (θ _A − θ _F ) between the phase change amount θ _A of the main pipe 2 at each frequency and the phase change amount θ _F of the phase shifter 1 is obtained. It is preferable that whether or not this difference (θ _A − θ _F ) is within ± 15 ° from 180 ° is used as a criterion for determining whether or not the sound velocity V and the sound velocity V ′ match (step S5).

(Step S6: Measures against attenuation of sound waves in the phase shifter)
When the above-mentioned step S4 (preferably step S5) is completed, the determination of the desired dimensions and the like of the phase shifter 1 is completed. However, when it is actually used as a muffling device, sound waves are attenuated in the phase shifter 1, so it is preferable to add the following step S6 as well. That is, as shown in FIG. 1, a microphone 5, a filter 6, an amplifier 7, and a speaker 8 are prepared in the muffling device 10 (step S6).

Here, the microphone 5 measures the sound wave after passing through the phase shifter 1. In the filter 6, processing is performed so that the phase shifter 1 does not amplify the sound wave in the frequency range in which the transmission loss becomes a negative value. The amplifier 7 amplifies the sound wave by the amount attenuated in the phase shifter 1. The speaker 8 can mute the sound waves propagating in the main pipe 2 by outputting the sound waves amplified by the amplifier 7 toward the confluence portion 4 of the phase shifter 1 and the main pipe 2.

(Comparison of sound velocity V, V'at a certain gap thickness b)
A specific example of step S4 is shown below. In FIG. 3A, the sound velocity V when the gap thickness b of the phase shifter 1 is 0.1 mm is shown by a solid line. Further, in FIG. 3 (a) also draws curves of L _A and L _F 3 types of sound speed V of the values obtained by inputting appropriate for '.

From this FIG. 3 (a), when a 0.1mm thick b of the gap, the pipe length _{L A} of the main pipe 2 290 mm, when the tube length _{L F} of the phase shifter 1 and 270 mm (i.e., FIGS. 3 (a) in (In the case of the broken line of), it was found that the difference between the two sound velocities V and V'was the smallest.

(Examination of the phase difference between the sound wave of the phase shifter and the sound wave of the main pipe)
Each pipe length L _A, Upon determining the L _F, also examined the phase difference of the sound wave and the main pipe 2 of the wave phase shifter 1, it was compared. Specifically, the determined gap thickness b = 0.1 mm was substituted into the mathematical formula (1), and the sound velocity V was obtained from the mathematical formulas (2) and (3). The value of this V was substituted into the sound velocity V'in the equation (8). Then, Equation (7) and the tube length L _A from Equation _(8), to calculate the theoretical values for each phase change amount when changing the L _F, the theoretical value of the phase difference of the sound wave in phase shifter 1 and the main pipe 2 Was compared. The result is shown in FIG.

Transmission loss, since the phase difference can take more than 10dB value when within ± 15 ° from 180 °, it is preferable to employ a tube length L _A, L _F conditions retardation within the range falls (Step S5). This point is also in light, as described above, when a 0.1mm thick b of the gap is determined that the pipe length _L F = 270 mm of the phase shifter 1, the pipe length _L A = 290 mm of the main pipe is most appropriate.

(Comparison of sound velocity V, V'at another gap thickness b)
On the other hand, FIG. 3B shows the sound velocity V when the gap thickness b of the phase shifter 1 is 0.2 mm with a solid line. Further, also FIG. 3 (b), also drawing curves of L _A and L _F 3 types of sound speed V of the values obtained by inputting appropriate for '.

When set to 0.2mm thickness b of the gap, 1080 mm tube length _{L A} of the main pipe 2, the pipe length _{L F} of the phase shifter 1 When 1000 mm (i.e., if the one-dot chain line in FIG. 3 (b)), It was found that the difference between the two sound velocities V and V'was the smallest. However, in this case, the sound velocity V, the difference the less try to the respective pipe length _L A of the V ', _{L F} exceeds the 1.0 m. Moreover, when further increasing the thickness b of the gap between the plate material also increases the pipe length L _A and L _F accordingly.

(Examination of silencer production)
Next, the phase shifter 1 of the present invention was prototyped and its acoustic performance was evaluated. The above steps S4, by comparing the condition of the sound velocity V, V ', to some determined critical dimensions of the silencer 10 (the thickness b of the gap between the plate material, the pipe length L _A, L _F) to Was done. Of these, it was determined that the sound deadening device 10 can be realistically examined under the above-mentioned optimum conditions when the gap thickness b is 0.1 mm.

(Prototype of phase shifter)
FIG. 5 (a) shows an image of the actually prototype phase shifter 1, FIG. 5 (b) shows a schematic view of the phase shifter 1, and the specifications thereof are shown in Table 1 below.

The phase shifter 1 of this embodiment is a jig (thickness b =) for creating a gap in a rectangular tube (outer diameter is 25.6 mm in length × 25.6 mm in width, inner diameter is 24.6 mm in length × 24.6 mm in width). It was manufactured by alternately stacking 0.1 mm) and plate materials (width 24.6 mm, thickness 0.1 mm), fixing these plate materials, and then removing the jig. FIG. 5A shows that a paper with “A” is placed behind the manufactured phase shifter 1 and a gap is formed. Gap thickness b and the pipe length _{L F} of the phase shifter 1 employs a b = 0.1 mm and _L F = 270 mm obtained by the design process described above. Also, was created as a pipe length _L A = 290mm also the main pipe 2.

Further, when performing the acoustic performance test described later, in order to stably install the phase shifter 1 in the measuring device, the phase shifter 1 is surrounded by an aluminum alloy plate (not shown) having a thickness of 10 mm. Fixed and held.

(Modified example of phase shifter)
In the phase shifter 1 (1A) of this embodiment, a hollow cylinder having a rectangular cross section is assumed, but the present invention is not necessarily limited to this. For example, it may be a hollow cylinder 1B having a polygonal cross section other than a circular cross section or a quadrangle (for example, a hexagon shown on the left side of FIG. 5C). The plate material (object partitioning the gap thickness b) of the present invention is not limited to the flat plate of the above embodiment, and may be a plate material having a curved surface, a sheet-like thin plate, or a film-like film body. For example, as shown on the right side of FIG. 5C, a structure in which the inside of a hollow cylinder having a circular cross section is partitioned by a plurality of concentric cylinders whose diameters gradually decrease so as to be separated by a gap thickness b. Phase shifter 1C may be adopted.

(Measuring device for the amount of phase change)
A 4-microphone impedance tube and an FFT analyzer were used to measure the amount of phase change. A schematic diagram of the measuring device is shown in FIG. 6 (a), and a photograph of the measuring device is shown in FIG. 6 (b). The measurement sample (main pipe 2 or phase shifter 1) is installed at a predetermined position, and one microphone is attached to the front and back of the measurement sample.

(Measurement method of phase change amount)
A sound wave based on a reference signal (linear multi-sine wave) is generated from the sound source, the sound wave on the incident side and the transmission side of the measurement sample is detected by the microphone, and the phase is analyzed by the FFT analyzer via the signal amplifier for the microphone. The amount of change was calculated. In this embodiment, as a preliminary evaluation of the performance evaluation of the muffling device 10 of the present invention, the phase change amount of the phase shifter 1 and the phase change amount of the main pipe are separately measured, and the muffling device is used by using these measurement results. Predicted and verified the acoustic performance of.

(Measurement result of phase change amount of phase shifter)
FIG. 7A shows a comparison between the theoretical value and the experimental value of the phase change amount θ _F in the phase shifter 1. Here, when the theoretical value and the experimental value are compared, the experimental value has a smaller phase change than the theoretical value at a frequency of 1600 Hz or less. Further, at frequencies of 4500 Hz to 6000 Hz, the experimental values are larger than the theoretical values.

(Measurement result of phase change of main pipe)
On the other hand, FIG. 7B shows a comparison between the theoretical value and the experimental value of the phase change amount θ _A in the main pipe 2. The experimental values here are generally in agreement with the theoretical values. However, at a frequency of 2000 Hz or less, some disturbance is seen in the experimental values.

(Phase difference between the sound wave in the phase shifter and the sound wave in the main pipe)
FIG. 8A shows the result of calculating the phase difference between the phase shifter 1 and the main pipe 2 by using the result of each phase change amount described above. According to the theoretical value shown by the alternate long and short dash line in the figure, the phase difference gradually approaches 180 ° when the frequency is higher than 1500 Hz, and coincides with 180 ° when the frequency is 5000 Hz. It can be seen that at frequencies higher than that, the phase difference gradually increases from 180 °.

On the other hand, in the experimental values, it can be seen that the frequency deviates greatly from the theoretical value at a frequency of 1600 Hz or less, and the phase cannot be inverted so much. At frequencies of 1600 Hz to 4500 Hz, a phase difference close to 180 ° can be obtained, but at frequencies of 4500 Hz to 6000 Hz, the phase difference deviates from the theoretical value. It is considered that the reason why such a deviation from the theoretical value occurs is that the phase change on the phase shifter 1 side due to an error in the dimensional accuracy of the prototype phase shifter 1 is not ideal.

(Calculation of transmission loss)
Next, using the data of the amount of phase change, the sound wave was represented by a sine wave, and the transmission loss TL when the sound waves were merged was calculated from the amplitude data. The calculation method is shown below. First, a sine wave y _I in the entrance part X _I of the phase shifter 1 and the main pipe 2 is expressed by the following equation 11. Note that FIG. 8B is a diagram showing sine waves at each point (incident portion, confluence portion) of the silencer 10.

At this time, the sine waves y _A and y _F at the end portions X _CA and X _CF of the main pipe 2 and the phase shifter 1 can be expressed by the following equations 12 and 13, respectively.

Further, when the sound waves on the phase shifter 1 side are merged, the amount attenuated in the phase shifter 1 is amplified by the amplifier 7 to be equal to the amplitude of the sound waves of the original main pipe 2, so that the sound waves are actually merged. The sine wave of the sound wave on the phase shifter 1 side is expressed as follows.

From this, the sine wave y _C of the synthesized wave when the sound waves merge is expressed by the following equation 15 by the principle of superposition of sound waves.

From the amplitude C of the composite wave obtained here and the amplitude A of the original sound wave, the transmission loss can be calculated using the following mathematical formula 16.

(Evaluation of transmission loss due to superposition of sound waves)
FIG. 9B shows the result of calculating the transmission loss TL when the sound wave of the phase shifter 1 is merged with the sound wave of the main pipe 2 by using the result of the above-mentioned phase change amount. For reference, the result of calculating the phase difference between the phase shifter 1 and the main pipe 2 is shown again in FIG. 9A. FIG. 9A shows the result of the phase difference of FIG. 8A with the addition of a broken line indicating the range of the phase difference ± 15 ° and a solid line indicating the range of the phase difference ± 60 °. Is.

Comparing FIG. 9A and FIG. 9B, in the frequency range where the phase difference between the phase shifter 1 and the main pipe 2 is within the range of 180 ° to ± 15 °, the transmission loss is stable at 10 dB or more. It turns out that TL can be obtained. Further, in FIG. 9B, there is a frequency range in which the transmission loss TL has a negative value. In this frequency range, when the phase difference is 180 ° to ± 60 ° or more, the sound wave is amplified and the amplitude of the combined wave becomes large. This is because it will be larger than before synthesis. When actually using the phase shifter 1, it is preferable to apply a filter 6 so as not to amplify the sound wave in the frequency range where the transmission loss TL becomes a negative value.

According to the present invention, there is provided a muffling device 10 having a very simple structure, low cost, no power supply, no failure, and no maintenance. Therefore, the present invention has very high industrial applicability and utility value.

1 acoustic phase shifter second main pipe 3 the noise source 4 merging portion 5 microphone 6 filter 7 amplifier 8 speaker 10 silencers b plate between the gap thickness L _A, pipe length _{L F} main, the pipe length of the phase shifter

Claims (3)

A method for manufacturing a silencer having a main pipe and a branch pipe.
A step S1 in which an empty pipe is used for the main pipe and an acoustic phase shifter having a hollow cylinder partitioned by a plurality of plate materials separated by a gap thickness b is used for the branch pipe.
Step S2 for deriving the relational expression of the sound velocity V in the gap between the plate materials, and
Step S3 for deriving the relational expression of the sound velocity V'that has the opposite phase to the sound velocity of the main pipe,
Step S4, in which the dimensions of the main pipe and the dimensions of the phase shifter are determined so that the sound velocity V and the sound velocity V'match.
Including and
In the step S2, the propagation constant γ represented by the following mathematical formula 1 is used.
Here, x is the position in the longitudinal direction of the phase shifter, y and z are the coordinates in the gap cross section, c is the speed of sound in air, κ is the specific heat ratio of air, σ is the Prandtl number, and ρ is the density of air. μ is the viscosity of air, ω is the angular frequency,
After dividing the propagation constant γ into a real part α and an imaginary part β by the following mathematical formula 2,
According to the following mathematical formula 3, the relational expression of the sound velocity V is expressed only by the variable of the gap thickness b.
In the step S3, using Equation 4 below, as relation of acoustic velocity V 'is the pipe length L _A of the phase shifter and the main _pipe, it represented only by variables L _F,
The dimensions of the main dimensions and the phase shifter to be determined at the step S4, the gap thickness b, the pipe length L _A, it is L _F,
A method of manufacturing a silencer, which comprises. In the step S4, the difference (θ _A − θ _F ) between the phase change amount θ _A and the phase shifter phase change amount θ _F at each frequency of the main pipe is obtained.
The first aspect of the present invention is that the difference (θ _A − θ _F ) is within ± 15 ° from 180 ° as a criterion for determining whether or not the sound velocity V and the sound velocity V ′ match. The method of manufacturing the silencer described. The process of preparing a microphone, a filter, an amplifier, and a speaker in the silencer.
Including more
The microphone measures the sound wave after passing through the phase shifter.
The amplifier amplifies the sound wave by the amount attenuated in the phase shifter.
The speaker is characterized in that the sound wave amplified by the amplifier is output toward a confluence portion between the phase shifter and the main pipe to mute the sound wave propagating in the main pipe. 2. The method for manufacturing a silencer according to 2.