JPS6397817A - Muffler - Google Patents

Abstract

PURPOSE:To provide a high insertion loss at a given frequency, by a method wherein the diameter of a cavity is increased three or more times as large as that of an inlet pipe, the length of the cavity is set to a value being 0.35 times as long as the diameter of the cavity. CONSTITUTION:A muffler is constituted such that an inlet pipe 1, formed in a circle in cross section, a cylindrical cavity 2, and an outlet pipe 3, formed in a circle in cross section, are coaxially interconnected, in order named. In this case, a diameter phi2r2 of a cavity 2 is increased three or more times as large as a diameter phi2r1 of a inlet pipe 1, a length 12 of the cavity 2 is set to a value being 0.35 or less times as long as the diameter phi2r2 of the cavity 2, and a diameter phi2r3 of an outlet pipe 3 is set to a value being 1/3 times as large as the diameter phi2r2 of the cavity 2. In this constitution, a sound wave generated from a sound source 4 is branched into a sound wave reflected in the direction of the inlet pipe 1 in a position II, a sound wave passing in the direction of the outlet pipe 3, and a sound wave incoming to the interior of the cavity 2. A sound wave propagated to the outlet pipe 3 is completely absorbed by a non-reflection end 5.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a muffler for attenuating noise from intake and exhaust pipes and ventilation ducts of internal combustion engines, compressors, etc., for example.

[Conventional technology]

FIG. 9 is a front view showing a conventional muffler described in, for example, the Journal of the Acoustical Society of Japan, Vol. 26, No. 8. Figure (here, (1
) is an inlet pipe with a circular cross section, (2) is a cylindrical cavity, (3
) are the outlet tubes with circular cross section, and these inlet tubes (18 cylindrical cavity (2) and outlet w (3) are in turn connected to the coaxial IC
It is connected.

The silencing performance of a silencer is not determined by a single unit, but is influenced by the acoustic systems before and after it, so the operation will be explained using an acoustic system as shown in FIG. Figure 10: (4)
is the sound source, and (5) is the non-reflection end.

Location of sound source (4)! The sound waves generated at 1 travel in the tube as plane waves and are reflected at position 1 and at position I. Therefore, a positive direction wave and a negative direction wave exist at the same time between position fal and position l, but since the sound wave that has passed through the position type is completely absorbed at the non-reflection end, the positive direction wave is more positive than the 1 position type. Only waves can exist.

At this time, if the length of the cavity (2) is sufficiently long compared to the diameter of the cavity, the sound wave will become a plane wave that travels in the positive and negative directions even if ζ is inside the cavity.At this time, the sound pressure at position 1.ff PIg P4m The density of the medium is e, the sound velocity is C1 the cross-sectional area of the inlet pipe (1) is Sl & the length is 1. The cross-sectional area of the cavity (2) is S, m the length is g2. When the cross-sectional area is equal to that of the inlet pipe, the length is e3, and the angular frequency of the sound wave is ω, the insertion loss IL is approximated by the following formula.

(dB) [1] Since the second term and @8 term on the right side of equation (1) are negative, the maximum value of insertion loss I Lmax at this time is as follows: ILmax = 20 Jogl l (dB)
(2)S.

However, the effective high-frequency limit frequency F of equations (1) and (2)
c is the lowest resonant frequency in the radial direction, and when the cavity diameter is D, Fc = 1.22 (Hz)
(3). At frequencies above Fc, the assumption of a plane wave no longer holds true, so equation (1) no longer holds true.

The transmission loss IL of the conventional muffler was actually determined through experiments and is shown in FIG. 11 together with the result of equation (1). However, the values of each constant are as shown below.

Ra-274,2cm Jzsw12.0cm 13
m19.6cm5@-18,4cm' S1=249
Cm"D -17.8cm at this time.

In Fig. 11, the peaks of small period are the length of the inlet pipe 11.
This is due to the influence of , and is expressed by the second term on the right side of equation (1). If the period of this mountain is fl, -e1■π
(6) a! becomes. The large period peak is due to the length a2 of the cavity, and is expressed by the eighth term on the right side of equation (1).

If the period of this mountain is f2, it is 162■π (9).
It can be seen that the experimental results agree well with the calculated results below Hz.

General one-stage silencers include those with various cross-sectional shapes, those with the center axes of the inlet pipe (1) and outlet pipe (3) shifted,
Various shapes of cavities are used, such as those with a stubby shape, but in general, as mentioned above, the cavity length is sufficiently long, and compared to an axially symmetric cavity, resonance due to a plane wave occurs more clearly. It has been confirmed that it is difficult to obtain high insertion loss when using

[Problem that the invention seeks to solve]

Since the conventional muffler is configured as described above, the sound wave inside the cavity becomes a plane wave traveling in the X-axis direction, and the transmission loss IL at this time is expressed as in equation (1). Therefore, there is a problem in that the insertion loss IL cannot exceed ILmax shown in equation (2).

The present invention was made to solve the above-mentioned problems, and an object of the present invention is to obtain a muffler that can obtain a high insertion loss at a predetermined frequency.

[Means for solving problems]

The silencer according to the present invention is one in which an inlet pipe with a circular cross section, a cylindrical cavity, and an outlet pipe with a circular cross section are sequentially connected coaxially to each other, and J: the cavity has a diameter equal to the diameter of the inlet pipe. The length is 8 times or more and the length is 0.85 times or less than the diameter of the mud cavity, and the diameter of the outlet pipe is 1/8 or less of the diameter of the cavity.

[Effect]

In the silencer according to the present invention, since the cavity length is short relative to the cavity diameter, the sound field within the cavity is not composed of a plane wave traveling in the axial direction, but is composed of an axially symmetrical cylindrical wave traveling in the radial direction. Therefore, the entire cavity acts acoustically like a side branch. Therefore, the graph of insertion loss IL with respect to frequency has a peak at a specific frequency, and a very high insertion loss can be obtained at a specific frequency.

[Example] An example of the present invention will be described below with reference to the drawings. 1st
The figure is a front view showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an installation example of the silencer shown in FIG. 1. The numerical values in the figure are as follows.

glm 274.2cm h-2, Ocm 6m1
9.6cmSum 18.4cm” 5zm 24
9cm”r, 1111 2.42cm rz=
8.9 cm, that is, the diameter 2r of the cavity (2) is 17.9 cm.
8cm, diameter of inlet pipe (1) and outlet pipe (3) 2r
1 is 4.84 cm, the diameter of the cavity (2) is the inlet pipe (1)
and is set at least eight times the diameter of the outlet pipe (3). Further, the length e2 of the cavity (2) is 2.0 cm, which is 0.85 times or less of the diameter 2r217.8 Cm'' of the cavity (2).

In the silencer configured in this way, the sound waves generated at the position of the sound source (4) travel in the pipe as plane waves and become 1
Sound waves reflected toward the inlet pipe (b) at position l.

The sound waves are branched into sound waves passing toward the exit pipe (3) and sound waves entering the cavity (2). The sound waves incident into the cavity (2) travel in the radial direction as cylindrical waves due to the short length of the cavity, and are reflected by the outer wall of the cavity and returned to the inside.

The inlet pipe (1) interferes with the sound waves in the outlet pipe (3). Since the sound waves that have reached the exit pipe (3) are completely absorbed at the non-reflection end (5), only positive waves can exist in the positive direction from the 1st position l.

At this time, the cavity (2) functions as a side branch whose boundary is the cylindrical surface (r+wr, ) of the extension of the wall surfaces of the inlet pipe and the outlet pipe within the cavity. If the entrance acoustic impedance of the cavity seen from this boundary is Zc, then Zc is Be5se
l Function Jy(z) h Neumann function Ny
(z) is expressed as follows. However, j is an imaginary unit (j''=-1).

and pc pc Zos11□-time S1 πrI', so the entrance acoustic impedance Zc of the cavity becomes α◆. Since the frequency F is, substitute the values of each constant into the α4 formula and get Zc
/ If you draw a graph of Zo versus frequency F, it will look like Figure 8. In FIG. 3, the frequency at which Zc/Zo 0 is the resonant frequency, and a high insertion loss can be obtained near the resonant frequency.

Let the sound pressures at digits III and ff in Fig. 2 be Pl and P.

Insertion loss IL is expressed by the following formula.

)! +sln”/4)” (dB)
When M, IL becomes infinite at the resonant frequency.

Further, the effective maximum limit frequency Fc of the 00 type is the minimum resonant frequency in the axial direction.

Fc--8600Hz'17)gz. At frequencies higher than Fc, the assumption of a cylindrical wave no longer holds true, so the open equation no longer holds true.

The insertion loss IL of the silencer in this example was actually determined through experiments and is shown in FIG. 4 together with the results of the α0 equation.

In FIG. 4, the peaks with a small period have a period of 62.7 due to the influence of the length e of the inlet pipe, as in the case of the conventional embodiment.
) 1z mountain.

From Figure 4, the experimental results agree well with the calculated results in almost the entire frequency range, and the volume of the cavity is 1 compared to the conventional example.
Despite being as small as /6, the resonance frequency is F-1080.
Hz, it can be seen that a very large insertion loss can be obtained near 8850 Hz.

In addition, in the above example, the cavity length e: was set to 2Cm, but
Experiments were conducted to determine the transmission loss in the same manner as in the examples described above, using a cavity in which the cavity diameter was not changed and the cavity length e2 was 4 cm by 6 cm. Figure 5 shows the results superimposed on the calculation results assuming a cylindrical wave.

It is shown in FIG. In this case, the limit frequency Fc is 4
800Hz and 8860Hz.

From these results, the cavity length I12 is 0.3 of the cavity diameter.
5 times or less, that is, J: 12≦6co1 in the above example
If this is the case, below the critical frequency, the frequency characteristics of the insertion loss will be close to those assumed by a cylindrical wave, and a high insertion loss can be obtained at a specific frequency.In this way.

Increasing the cavity length increases the overall insertion loss. Moreover, the resonance frequency shifts to the high frequency side.

In a similar cavity, the cavity length a2 is 8 cm, locm
An experiment was conducted to determine the transmission loss of a comparative example when The results are shown in Figure 8 for the seventh factor, superimposed on the calculation results for the plane wave plate constant. From these figures, if the cavity length e2 is larger than 0.86 times the cavity diameter, that is, 8228 cm in the above comparative example, the frequency characteristics of the insertion loss will be close to those of a plane wave plate constant, and the insertion loss at a specific frequency will be No peak loss appears.

Also, the diameter of the cavity (2), the inlet pipe (1) and the outlet pipe (
As a result of extensive experimental studies regarding the diameter of the cavity (2)
) is 8 times the diameter of the inlet pipe (1) and outlet pipe (3).
It was confirmed that a good silencing effect can be obtained when the amount is more than twice that.

Further, in the above embodiment, no sound absorbing material was used.

A sound absorbing material may be provided within the cavity (2).

〔Effect of the invention〕

As described above, according to the present invention, an inlet pipe having a circular cross section, a cylindrical cavity, and an outlet pipe having a circular cross section are sequentially connected coaxially, and the cavity has a diameter of h is at least 8 times the diameter of the cavity, and its length is 0.35 times or less the diameter of the cavity, and h the outlet pipe has a diameter of 1/1 of the diameter of the cavity.
Since it is 8 or less, the insertion loss is improved, and a muffler with a more excellent muffling effect can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a front view showing a silencer according to an embodiment of the present invention, FIG. 2 is an explanatory diagram showing the installed state of the silencer of FIG. 1, and FIG. FIG. 4 is a characteristic diagram showing the results of calculation of impedance; FIG. 4 is a characteristic diagram showing experimental results and calculation results of insertion loss IL in an embodiment of the present invention shown in FIG. 1; FIGS. Other Examples and Comparative Examples of the Invention Insertion Loss I in Hot Heat
A characteristic diagram showing the experimental results and calculation results of L, Fig. 9 is a front view showing a conventional silencer, Fig. 10 is an explanatory diagram showing the installed state of the silencer in Fig. 9, and Fig. 11 is Fig. FIG. 3 is a characteristic diagram showing experimental results and calculation results of transmission loss IL in the conventional silencer shown in FIG. In the figure, (1) is the inlet pipe, (2) is the cylindrical cavity, (
3) is the outlet pipe, (4) is the sound source, and (5) is the non-reflection end. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (2)

[Claims] (1) An inlet pipe with a circular cross section, a cylindrical cavity, and an outlet pipe with a circular cross section are sequentially connected coaxially, and the cavity has a diameter of three times or more the diameter of the inlet pipe and a length of the same diameter. A silencer characterized in that the diameter of the outlet pipe is 0.35 times or less than the diameter of the cavity, and the diameter of the outlet pipe is 1/3 or less of the diameter of the cavity. (2) The muffler according to claim 1, characterized in that a sound absorbing material is provided in the cavity.