JP7144853B2 - Manufacturing method of muffler with acoustic phase shifter - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method of manufacturing a silencer equipped with a pure acoustic phase shifter that does not require a power source.

(Conventional interference silencer)
An interference muffler (so-called passive muffler) is a muffler composed of a main pipe and a branch pipe. The principle of these mufflers is that the sound wave that entered the main pipe is branched into a branch pipe, and when it is rejoined with the main pipe, the sound wave branched into the branch pipe is made into a sound wave of opposite phase to the sound wave of the main pipe. , cancels sound waves by using the principle of superposition.

(Problems with conventional interference silencers)
However, with the above-described interference type muffler, significant muffling performance can be obtained only at specific frequencies, but it is often not very effective at other frequencies.

(traditional active noise control)
On the other hand, active noise control aims to reduce noise in a wider frequency range. This method normally uses an electronic control circuit to reverse the phases of sound waves measured by a microphone and output them from a speaker to merge them, thereby enabling noise reduction over a wider band.

(Problem 1 of conventional active noise control)
However, active noise control is (1) costly, (2) requires a power source, and (3) frequently breaks down, 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 a target frequency range, subdivide the range, and detect amplitude and phase for each frequency. A signal for the adaptive sound source is obtained by assuming the position of the adaptive sound source, generating signals of (1) opposite phases and (2) equal amplitudes for each frequency component, and synthesizing them. In addition, a control for correcting errors is also required.

JP-A-02-041953 JP-A-62-110400

Stinson, M. R. 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 .

(Object of the present invention)
SUMMARY OF THE INVENTION It is an object of the present invention to provide a low-cost, low-cost, power-free, less likely to malfunction, and easy-to-maintain noise suppressor equipped with a phase shifter.

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

Among the processes essential for active noise control, the present inventors have applied the process of generating the opposite-phase signal in (1) above to a pure acoustic phase shifter (that is, , an acoustic structure with no moving parts), and completed the present invention.

ここで、ｘは、前記フェイズシフタの長手方向の位置、ｙおよびｚは隙間断面内の座標、ｃは空気中の音速、κは空気の比熱比、σはプラントル数、ρは空気の密度、μは空気の粘度、ωは角周波数であり、
前記伝搬定数γを以下の数式２により実部αと虚部βとに分けたうえで、

以下の数式３により前記音速Ｖの関係式が前記隙間厚さｂの変数のみで表されるようにし、

前記工程Ｓ３では、以下の数式４を用いて、音速Ｖ’の関係式が、前記主管と前記フェイズシフタの管長Ｌ_Ａ，Ｌ_Ｆの変数のみで表されるようにし、

前記工程Ｓ４で決定される前記主管の寸法と前記フェイズシフタの寸法とは、前記隙間厚さｂ、前記管長Ｌ_Ａ，Ｌ_Ｆであること、
を特徴とする消音装置の製造方法。
（態様２）
前記工程Ｓ４では、前記主管の各周波数における位相変化量θ_Ａと、前記フェイズシフタ位相変化量θ_Ｆと、の差（θ_Ａ－θ_Ｆ）を求め、
前記差（θ_Ａ－θ_Ｆ）が１８０°から±１５°以内であることを、前記音速Ｖと前記音速Ｖ’とが一致するかどうかの判断基準とすること
を特徴とする態様１に記載の消音装置の製造方法。
（態様３）
前記消音装置に、マイクと、フィルタと、増幅器と、スピーカと、を用意する工程を、
さらに含み、
前記マイクは、前記フェイズシフタ通過後の音波を測定し、
前記増幅器は、前記フェイズシフタ内で減衰した分だけ前記音波を増幅し、
前記スピーカは、前記増幅器で増幅した前記音波を、前記フェイズシフタと前記主管との合流部に向けて出力することで、前記主管中を伝搬する音波を消音すること
を特徴とする態様１又は２に記載の消音装置の製造方法。 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,
step S1 of using an empty pipe as the main pipe and using an acoustic phase shifter having a hollow cylinder partitioned by a plurality of plate members separated by a gap thickness b as the branch pipe;
a step S2 of deriving a relational expression of the sound velocity V in the gap between the plate materials;
A step S3 of deriving a relational expression of the sound speed V′ having an antiphase with respect to the sound speed of the main pipe;
step S4 of determining the dimensions of the main pipe and the phase shifter so that the sound velocity V and the sound velocity V'match;
and
In the step S2, using the propagation constant γ represented by the following formula 1,

Here, x is the longitudinal position of the phase shifter, y and z are the coordinates in the cross section of the gap, c is the speed of sound in air, κ is the specific heat ratio of air, σ is the Prandtl number, 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 formula 2,

The relational expression of the sound velocity V is represented by only the variable of the gap thickness b by the following formula 3,

In the step S3, using the following formula 4, the relational expression of the speed of sound _V ' is expressed only by the variables of the pipe lengths LA and _LF of the main pipe and the phase shifter,

The dimension of the main pipe and the dimension of the phase shifter determined in step S4 are the gap thickness b and the pipe lengths _LA and _LF ;
A method of manufacturing a silencer, characterized by:
(Aspect 2)
In step S4, the difference (θ _A - θ _F ) between the phase change amount θ _A at each frequency of the main channel and the phase shifter phase change amount θ _F is obtained,
Aspect 1, wherein the difference (θ _A −θ _F ) being within ±15° from 180° is used as a criterion for determining whether or not the speed of sound V and the speed of sound V′ match. method of manufacturing a silencer.
(Aspect 3)
preparing a microphone, a filter, an amplifier, and a speaker in the muffler;
further includes
The microphone measures sound waves after passing through the phase shifter,
The amplifier amplifies the sound wave by the amount attenuated in the phase shifter,
Aspect 1 or 2, wherein the speaker outputs the sound wave amplified by the amplifier toward a confluence portion of the phase shifter and the main pipe, thereby muffling the sound wave propagating in the main pipe. 3. The method for manufacturing the silencer according to 1.

The acoustic phase shifter of the present invention can automatically and passively generate a signal in a wide frequency range that is opposite in phase to the sound waves of the main pipe, using only an acoustic structure that does not require a power source or moving parts. Therefore, the "digital signal processing to generate opposite phases", which is essential for active noise control, is completely unnecessary. In addition, 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 using a linear filter.

According to the present invention, it is possible to provide a muffler that has a very simple structure, is low in cost, does not require a power supply, is trouble-free, and does not require maintenance. In other words, the muffler of the present invention contributes to simplification of noise control, reduction in power consumption, and improvement in reliability, which are difficult to achieve with ordinary active noise control.

Figure 2 shows a flow chart for designing an acoustic phase shifter of the present invention; 1A and 1B are a schematic diagram of a silencer of the present invention, a diagram showing a gap between plate materials (between two planes) in an orthogonal coordinate system, and a diagram showing a sine wave of a sound wave entering a pipe. FIG. 10 is a diagram showing a specific example of comparing the speed of sound V and the speed of sound V′; FIG. 5 is a diagram comparing the phase difference (theoretical value) between the sound wave of the phase shifter and the sound wave of the main pipe when the pipe lengths _LA and _LF are varied with the gap thickness kept constant (b=0.1 mm). 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. FIG. 3 shows a diagram comparing theoretical values and experimental values of the phase shift amount _θF in the phase shifter, and a diagram comparing theoretical values and experimental values of the phase shift amount _θA in the main pipe. FIG. 10 is a diagram showing a result of calculating a phase difference between a phase shifter and a main pipe, and a diagram showing a sine wave at each point (incident portion, merging portion) of the silencer; FIG. FIG. 10 is a diagram showing the result of calculating the phase difference between the phase shifter and the main pipe (reposted) and the estimated value of the transmission loss due to the superimposition of the sound waves (the transmission loss of the sound wave at the junction of the main pipes).

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 flow chart for designing the acoustic phase shifter of the present invention. The present invention also assumes the adoption of an interference muffling structure having a main pipe and branch pipes, as in the conventional side branch muffler. (step S1).

(Structure of Acoustic Phase Shifter)
In this embodiment, as shown in FIG. 1, an empty pipe is used as the main pipe 2, and an acoustic phase shifter 1 (hereinafter simply referred to as "phase shifter") is used as the branch pipe (step S1). As shown in FIG. 5(b), this is an analog control device in which plate materials (thin plates) are stacked with a gap in a hollow cylindrical body having a rectangular cross section. In this acoustic phase shifter 1, unlike the case of a branch tube made of a hollow tube, the sound velocity of the sound wave incident on the gap between the plate materials changes depending on the frequency due to the action of the boundary layer. The inventors have found that there is a possibility of changing also.

(Difference between acoustic phase shifter and normal branch pipe)
If a normal hollow pipe (not shown) is used for the branch pipe, the sound velocity of the incident sound wave is constant, and even if this is rejoined with the main pipe 2, Only with this method, significant noise reduction performance can be obtained.

(muffler with acoustic phase shifter)
FIG. 2(a) shows an outline of a silencer 10 including the phase shifter 1. As shown in FIG. In this silencer 10, a sound wave generated from a noise source 3 passes through a hollow pipe that is a main pipe 2 and a phase shifter 1 that is a branch pipe. When a 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 attenuated amount. After that, by outputting this amplified sound wave from the speaker 8 to the confluence portion 4 in the main pipe 2, the sound waves of the opposite phase and the same amplitude are merged toward the sound wave propagating in the main pipe 2, and the sound waves overlap. Sound waves are canceled and silenced by the principle of combination.

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 restore the flat frequency characteristic. For this reason, the filter 6 performs processing to give the sound wave a frequency characteristic opposite to the frequency characteristic described above. As the filter 6, for example, a linear filter that is technically easy can be used.

(Parameters of silencer to be determined)
However, the main pipe 2 and the phase shifter 1 must have a geometrical relationship that satisfactorily exhibits the above-described noise reduction mechanism, and it is necessary to determine the dimensions of each element. In this embodiment, the sonic velocity V in the gap between the plate materials (between two planes) and the sonic velocity V′ having an opposite phase to the sonic velocity of the main pipe 2 are calculated (steps S2 and S3), and these V and By comparing V′, the pipe length L _F and gap b of the phase shifter 1 and the pipe length _LA of the main pipe 2, which are key factors for the silencing effect, are determined.

(Derivation of sound velocity V in gap between plate materials (between two planes))
There are various reports on the derivation of the propagation constant and the characteristic impedance considering the viscosity of the air inside the tube. In this example, Stinson's method (see Non-Patent Document 1) was applied to attempt to calculate the sound velocity V in the gap between plate materials (between two planes). Specifically, a rectangular coordinate system is taken as shown in FIG. The propagation constant γ in frequency and the sound velocity V in the gap between the plate materials were derived (step S2).

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

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 (equation (2)).

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

From the above equations (1) and (2), the only variable in equation (3) is the gap thickness b (ie, V=V(b)).

(Derivation of sonic velocity V′ that is in opposite phase to the sonic velocity of the main pipe)
The sine wave _yI at the incident portion _XI and the sine wave _yC at the end portion _XC of the sound waves entering the pipe are expressed by the following equations (4) and (5), respectively. (See also FIG. 2(c)).

At this time, the phase change amount .theta. at each frequency in the tube is obtained from the following equation (6). Here, f is the frequency, L is the tube length, and ν is the speed of sound in the tube.

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, _LA is the length of the main pipe, _LF is the length of the phase shifter 1, c is the speed of sound in air (343.7 m/s), and V' is the speed of sound on the phase shifter 1 side.

In order to join opposite-phase sound waves 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 must be 180°. (π radians). That is, it is necessary to satisfy the following formula (9).

That is, the condition of the sound velocity V' on the phase shifter 1 side for the opposite phase at each frequency is expressed by the following equation (10) using equations (7), (8), and (9). can be asked for.

Variables in expression (10) are the pipe lengths L _A and L _F of the main pipe 2 and the phase shifter 1 . That is, the sound velocity V' on the side of the phase shifter 1 for the opposite phase at each frequency can be obtained by determining the respective tube lengths L _A and L _F (V'=V'(L _A , _LF )).

(Determining the dimensions of the silencer by comparing the sound velocities 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 side of the phase shifter 1 so that the phase is opposite to the sound velocity V in the gap between the plate materials. Specifically, these are compared, and the gap thickness b and the pipe lengths L _A and L _F are obtained so that the condition of the speed of sound V and the condition of the speed of sound V′ are matched (step S4).

(Step S5: Judgment method effective for comparison of sound velocities V and V')
As shown in FIG. 1, in the above 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. , and whether or not this difference (θ _A −θ _F ) is within ±15° from 180° is preferably used as a criterion for judging whether or not the speed of sound V and the speed of sound V′ match (step S5).

(Step S6: Measures against Attenuation of Sound Waves in Phase Shifter)
It should be noted that the determination of the desired dimensions and the like of the phase shifter 1 is completed when the above step S4 (preferably step S5) is finished. However, since sound waves are attenuated in the phase shifter 1 when actually used as a muffler, it is preferable to add the following step S6. That is, as shown in FIG. 1, the microphone 5, the filter 6, the amplifier 7, and the speaker 8 are prepared for the muffler 10 (step S6).

Here, the microphone 5 measures the sound wave after passing through the phase shifter 1 . The filter 6 performs processing so that the phase shifter 1 does not amplify the sound wave in the frequency range where 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 muffle the sound wave propagating in the main pipe 2 by outputting the sound wave amplified by the amplifier 7 toward the junction 4 of the phase shifter 1 and the main pipe 2 .

(Comparison of sound velocities V and V' at a certain gap thickness b)
A specific example of step S4 is shown below. In FIG. 3A, the solid line shows the sound velocity V when the thickness b of the gap of the phase shifter 1 is 0.1 mm. FIG. 3(a) also plots three types of curves of sound velocity _V ' obtained by appropriately inputting the values of LA and _LF .

From FIG. 3(a), when the thickness b of the gap is 0.1 mm, the pipe length _LA of the main pipe 2 is 290 mm, and the pipe length _LF of the phase shifter 1 is 270 mm (that is, in FIG. 3(a) ), the difference between the two sound velocities V and V' is the smallest.

(Examination of the phase difference between the sound wave of the phase shifter and the sound wave of the main pipe)
In determining the pipe _lengths LA and _LF , the phase difference between the sound wave of the phase shifter 1 and the sound wave of the main pipe 2 was also examined and compared. Specifically, the determined gap thickness b=0.1 mm was substituted into the equation (1), and the sound velocity V was obtained from the equations (2) and (3). This value of V was substituted for the speed of sound V' in Equation (8). After that, the theoretical value of each phase change amount when the pipe lengths L _A and _LF are changed is calculated from the equations (7) and (8), and the theoretical value of the phase difference between the sound waves in the phase shifter 1 and the main pipe 2 is calculated. compared. The results are shown in FIG.

Since the transmission loss can take a value of 10 dB or more when the phase difference is within ±15° from 180°, it is preferable to adopt the tube lengths _LA and _LF under the condition that the phase difference falls within this range. (Step S5). Based on this point as well, when the thickness b of the gap is set to 0.1 mm as described above, it was determined that the pipe length L _F of the phase shifter 1 =270 mm and the pipe length L A of the main pipe L _A =290 mm were most appropriate.

(Comparison of sound velocities V and V' at different gap thickness b)
On the other hand, FIG. 3B shows the sound velocity V when the thickness b of the gap of the phase shifter 1 is set to 0.2 mm by a solid line. Also, in FIG. 3B, curves of three types of sound velocity V' obtained by appropriately inputting the values of L _A and L _F are also drawn.

When the thickness b of the gap is 0.2 mm, the pipe length _LA of the main pipe 2 is 1080 mm, and the pipe length _LF of the phase shifter 1 is 1000 mm (that is, in the case of the dashed 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, when trying to reduce the difference between the sound velocities _V and V', the tube lengths LA and _LF exceed 1.0 m. Further, if the thickness b of the gap between the plate members is further increased, the pipe lengths _LA and _LF are also increased accordingly.

(Examination of the production of silencers)
Next, the phase shifter 1 of the present invention was produced as a trial, and its acoustic performance was evaluated. By comparing the conditions of the sound velocities V and V' in the above step S4, some important dimensions of the muffler 10 (the thickness b of the gap between the plate materials, the pipe lengths L _A and L _F ) are determined. was made. Among these conditions, it was determined that the optimum condition for the case where the thickness b of the gap is 0.1 mm is the one that can be realistically considered for the noise reduction device 10 .

(Prototype phase shifter)
FIG. 5(a) shows an image of the phase shifter 1 actually manufactured as a prototype, FIG. 5(b) shows a schematic diagram of the phase shifter 1, and Table 1 below shows the specifications thereof.

The phase shifter 1 of this embodiment is a jig (thickness b= 0.1 mm) and plate materials (width 24.6 mm, thickness 0.1 mm) were alternately stacked, and after fixing these plate materials, the jig was pulled out. FIG. 5(a) shows that a paper with "A" written on it is placed behind the manufactured phase shifter 1 to show that a gap is formed. For the gap thickness b and the tube length _LF of the phase shifter 1, b=0.1 mm and _LF =270 mm obtained by the design process described above are adopted. Also, the main pipe 2 was made with a pipe length of L _A =290 mm.

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

(Modified example of phase shifter)
In addition, in the phase shifter 1 (1A) of the present embodiment, a hollow cylindrical body having a rectangular cross section is assumed, but it is not necessarily limited to this. For example, the hollow cylinder 1B may have a circular cross-section or a polygonal cross-section other than a quadrangle (for example, a hexagon shown on the left side of FIG. 5(c)). The plate material (an object that partitions 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. For example, as shown on the right side of FIG. 5(c), a structure in which the inside of a hollow cylindrical body having a circular cross section is partitioned by a plurality of concentric circular cylindrical bodies with gradually decreasing diameters separated by a gap thickness b. phase shifter 1C may be employed.

(Equipment for measuring the amount of phase change)
A four-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). A measurement sample (main pipe 2 or phase shifter 1) is installed at a predetermined position, and microphones are attached one by one in front of and behind the measurement sample.

(Method for measuring phase change amount)
A sound wave is generated by a reference signal (linear multisine wave) from a sound source, and the sound waves on the incident side and transmission side of the measurement sample are detected by a microphone. The amount of change was calculated. In this embodiment, as a pre-stage evaluation of the performance evaluation of the silencer 10 of the present invention, the amount of phase change of the phase shifter 1 and the amount of phase change of the main pipe are separately measured, and the results of these measurements are used to evaluate the silencer. We predicted and verified the acoustic performance of

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

(Measurement result of phase change amount of main pipe)
On the other hand, FIG. 7(b) shows a comparison between the theoretical value and the experimental value of the phase change amount θA in the main pipe 2. _As shown in FIG. The experimental values here are generally consistent with the theoretical values throughout. However, at a frequency of 2000 Hz or less, the experimental values are somewhat disturbed.

(Phase difference between sound wave in phase shifter and sound wave in main pipe)
FIG. 8(a) shows the result of calculating the phase difference between the phase shifter 1 and the main pipe 2 using the results of each phase change amount described above. According to the theoretical values indicated by the dashed-dotted line in the figure, the phase difference asymptotically approaches 180° when the frequency exceeds 1500 Hz, and coincides with 180° when the frequency becomes 5000 Hz. It can be seen that the phase difference gradually increases beyond 180° at higher frequencies.

On the other hand, the experimental values deviate greatly from the theoretical values at frequencies of 1600 Hz or less, indicating that the phase cannot be significantly inverted. A phase difference close to 180° can be obtained at frequencies of 1600 Hz to 4500 Hz, but the phase difference deviates from the theoretical value at frequencies of 4500 Hz to 6000 Hz. It is considered that the reason why such a deviation from the theoretical value occurs is that the phase shift on the phase shifter 1 side is not ideal due to errors in the dimensional accuracy of the prototype phase shifter 1 .

(Calculation of transmission loss)
Next, using the phase change amount data, the sound wave is represented by a sine wave, and the transmission loss TL when the sound waves are combined is calculated from the amplitude data. The calculation method is shown below. First, the sine wave _yI at the incident portion XI of the phase shifter ₁ and the main pipe 2 is expressed by the following equation (11). In addition, FIG. 8B is a diagram showing sine waves at each point (incident portion, merging 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 using the amplifier 7 to make it equal to the amplitude of the original sound waves in the main pipe 2. Therefore, when the sound waves are actually merged, A sine wave of the sound wave on the side of the phase shifter 1 is expressed as follows.

From this, the sine wave yC of the composite wave when the sound waves join is expressed by the following _formula 15 according to 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 Equation 16 below.

(Evaluation of transmission loss by 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 using the result of the phase change amount described above. For reference, FIG. 9A shows the result of calculating the phase difference between the phase shifter 1 and the main pipe 2 . FIG. 9(a) shows the results of the phase difference in FIG. 8(a), with a dashed line indicating the phase difference within ±15° and a solid line indicating the phase difference within ±60°. is.

Comparing FIG. 9(a) and FIG. 9(b), in the frequency range where the phase difference between the phase shifter 1 and the main tube 2 is within the range of 180° to ±15°, the transmission loss is stably 10 dB or more. It can be seen that TL can be obtained. Also, in FIG. 9B, there is a frequency range where the transmission loss TL has a negative value. This is because it becomes larger than before synthesis. When actually using the phase shifter 1, it is preferable to apply a filter 6 that does not amplify sound waves in the frequency range where the transmission loss TL becomes a negative value.

According to the present invention, the muffler 10 is provided which has a very simple structure, is low in cost, does not require a power source, and is trouble-free and maintenance-free. Therefore, the present invention has very high industrial applicability and utility value.

1 acoustic phase shifter 2 main pipe 3 noise source 4 junction 5 microphone 6 filter 7 amplifier 8 speaker 10 silencer b thickness of gap between plates L _A , _LF length of main pipe, length of phase shifter

Claims (3)

A method for manufacturing a silencer having a main pipe and a branch pipe,
step S1 of using an empty pipe as the main pipe and using an acoustic phase shifter having a hollow cylinder partitioned by a plurality of plate members separated by a gap thickness b as the branch pipe;
a step S2 of deriving a relational expression of the sound velocity V in the gap between the plate materials;
A step S3 of deriving a relational expression of the sound speed V′ having an antiphase with respect to the sound speed of the main pipe;
step S4 of determining the dimensions of the main pipe and the phase shifter so that the sound velocity V and the sound velocity V'match;
and
In the step S2, using the propagation constant γ represented by the following formula 1,

Here, x is the longitudinal position of the phase shifter, y and z are the coordinates in the cross section of the gap, c is the speed of sound in air, κ is the specific heat ratio of air, σ is the Prandtl number, 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 formula 2,

The relational expression of the sound velocity V is represented by only the variable of the gap thickness b by the following formula 3,

In the step S3, using the following formula 4, the relational expression of the speed of sound _V ' is expressed only by the variables of the pipe lengths LA and _LF of the main pipe and the phase shifter,

The dimension of the main pipe and the dimension of the phase shifter determined in step S4 are the gap thickness b and the pipe lengths _LA and _LF ;
A method of manufacturing a silencer, characterized by: In step S4, the difference (θ _A - θ _F ) between the phase change amount θ _A at each frequency of the main channel and the phase shifter phase change amount θ _F is obtained,
The method according to claim 1, wherein the difference (θ _A - θ _F ) is within ±15° from 180° as a criterion for determining whether the speed of sound V and the speed of sound V' match. A method of manufacturing the described silencer. preparing a microphone, a filter, an amplifier, and a speaker in the muffler;
further includes
The microphone measures sound waves after passing through the phase shifter,
The amplifier amplifies the sound wave by the amount attenuated in the phase shifter,
2. The speaker muffles the sound wave propagating in the main pipe by outputting the sound wave amplified by the amplifier toward the junction of the phase shifter and the main pipe. 3. The method for manufacturing the silencer according to 2.

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