KR100749817B1 - Acoustic filter and ite type hearing protectors applied by it - Google Patents

Abstract

The present invention relates to an auditory protective device that can prevent noise-induced hearing loss, and more particularly, to prevent hearing loss caused by noise and at the same time, a sound shielding sound filter that can solve a conversational disorder that may be caused by using an auditory protective device, and the application thereof. In-The-Ear (ITE) Hearing protector, which is an important part of hearing protector using the principle of acoustic filter that changes sound transmission characteristics according to the change of the cross-sectional area of pipe through which wave is transmitted. By setting the voice band as the transmission band, it is possible to prevent the hearing loss due to noise and to solve the speech impairment that may occur by using hearing protection.

Soundproof Hearing Loss, Hearing Protection, Acoustic Filter, Bind Type, Noise Shield, ITE

Description

Acoustic filter for noise shielding and attribution hearing protection applying it {ACOUSTIC FILTER AND ITE TYPE HEARING PROTECTORS APPLIED BY IT}

1 is a view showing the existing ear-type hearing protectors.

FIG. 2 is a diagram illustrating human audible range and speech and listening range. FIG.

3A and 3B illustrate cross-sectional views and an analogous model of an acoustic filter in which a sound pipe is widened, respectively.

4A and 4B are cross-sectional views of an acoustic filter having a narrow sound tube and an electrical similarity model, respectively.

5 is a diagram illustrating an electrical similarity model of the outer ear.

FIG. 6 illustrates an electrical system incorporating an acoustic filter and an electrical analog model of the outer ear.

FIG. 7 shows simulation results of the electrical system shown in FIG. 6.

FIG. 8 is an experimental configuration diagram for measuring sound transmission characteristics of a sound filter having a narrow sound tube including characteristics of the outer ear. FIG.

9A and 9B are graphs showing the results of the characteristic experiment according to FIG. 8.

10 is a diagram showing the structure of an acoustic filter for shielding noise according to the present invention.

11A and 11B are structural and wearing virtual views of the hearing protector according to the present invention.

12A and 12B are cross-sectional views of an acoustic filter structure including a resonator and a worn state.

13A and 13B are cross-sectional views of the acoustic filter structure and the worn state including the tube.

14A and 14B are cross-sectional views of an acoustic filter structure and wearing state including a sound tube having a gradually decreasing cross-sectional area.

15A and 15B are cross-sectional views of an acoustic filter structure and wearing state including a funnel shaped sound tube.

FIG. 16 is a view showing a structure in which a predetermined film is inserted into a sound shielding sound filter according to the present invention.

<Description of the symbols for the main parts of the drawings>

10: sound filter 12: first sound tube

14 second sound tube 16 resonator

18 tube 17 third sound tube

19: funnel-shaped sound tube

20: Ear Mold

The present invention relates to an auditory protective device that can prevent noise-induced hearing loss, and more particularly, to prevent hearing loss caused by noise and at the same time, a sound shielding sound filter that can solve a conversational disorder that may be caused by using an auditory protective device, and the application thereof. In-The-Ear (ITE) hearing protection.

In the modern society, noise-induced hearing loss is a serious occupational disease worldwide, and more than 30 million people are exposed to noise in the United States. Noise-induced hearing loss accounts for more than half of all occupational diseases in Korea. Noise-induced hearing loss, which is caused by hearing loss in stages when exposed to strong noise in prolonged periods of time, may vary depending on the characteristics of the noise. Once it occurs, there is no effective treatment method and recovery is impossible. Can be.

In view of the prevention of hearing loss, various hearing protection devices are used, and they are classified into earmuffs in the form of headphone covering both the ear type and the ear wheel, which are similar to the general earphones, among which the convenience, beauty effect, portability of the wearer, etc. In terms of attribution, preference is given.

FIG. 1 is a view showing existing ear-type hearing protectors. Although conventional hearing protectors provide a great effect in terms of hearing protection, users who are exposed to an actual noise environment are not frequently worn. In particular, when wearing hearing protection in a noisy environment, the difficulty of communicating smoothly and the feeling of closure when wearing is the biggest reason to avoid wearing hearing protection.

In order to prevent hearing loss, it is essential for the user to continuously use hearing protection in a noisy environment. However, as described above, the inconvenience of the existing hearing protector causes the user to wear the hearing protector to reduce the time of hearing loss. Therefore, in order for hearing protection to be effective in preventing hearing loss, it is necessary to improve the problem of use with noise shielding function so that users can always wear hearing protection in a noise environment.

The present invention has been made to solve the above problems, an object of the present invention is to solve the disturbances that may appear by using the hearing protection at the same time to prevent hearing loss due to noise to continue to use the hearing protection.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings.

The sound filter for noise shielding according to the present invention is an important part of the hearing protection device using the principle of the acoustic filter which changes the sound wave transmission characteristics according to the change of the cross-sectional area of the pipe through which the sound wave is transmitted. By setting it up, you can prevent hearing loss caused by noise and at the same time solve the conversational disturbances that may occur by using hearing protection.

In determining the pass band of the acoustic filter, an important audible range is 20 Hz to 20 kHz, as shown in FIG. 2, and is lowered to about 16 kHz with respect to aging since the 20s, and the conversational range involved in communication is It is known as 100Hz to 8kHz. However, in recognizing the size of speech (ie, the energy of sound), the frequency range between 125 Hz and 1 kHz contributes about 95% and the frequency range between 1 Hz and 8 Hz contributes about 5%. In understanding the meaning of speech, the frequency range below 500 kHz contributes only about 5% and the frequency range between 500Hz and 2kHz contributes about 70%. The remaining 25% is related to the frequency range between 2 kHz and 8 kHz. From the characteristics of the conversational sound, it can be seen that the transmission band to be considered in designing a small acoustic filter that can solve the problem of dialogue is less than 2 kHz.

I. Design of Small Acoustic Filter

The basis of an acoustic filter is that the sound energy passing through the sound tube is attenuated by a sudden change in the shape or cross-sectional area of the sound tube. The system operates as a low pass filter, a high pass filter, and a band pass filter according to the input impedance of the part where the sudden change in the shape of the sound tube starts, and it has the characteristics of the low pass filter when the part of which the acoustic cross section is reduced is included. Known.

There are two types of low pass filters to be applied to the small acoustic filter, which can reduce or increase the cross-sectional area drastically.

1. Type of compact acoustic filter

1.1. Type 1

인 음관에 길이

, 단면적

로 넓어지는 부분을 삽입하여 저역통과 특성을 갖는 음향 필터를 설계할 수 있다.

을 만족하는 저주파 영역에서 단면적이 넓어지는 것은 음향 컴플라이언스(compliance)를 포함하는 것과 같이 보인다(

).Cross section

Length to the sound tube

, Cross section

By inserting the part widened into, it is possible to design an acoustic filter with low pass characteristics.

The broader cross-sectional area in the low frequency region that satisfies is seen to include acoustic compliance.

).

3A and 3B illustrate cross-sectional views and an analogous model of an acoustic filter in which a sound pipe is widened, respectively.

는 연속이므로,Volume velocity at the boundary where the cross-sectional area changes rapidly in the structure of the acoustic filter of FIG. 3A

Is continuous,

(1)

(One)

to be. In other words,

(2)

(2)

Therefore, the ratio of the characteristic impedance of the medium can be represented by the area ratio of each part as shown in Equation (3) below.

(3)

(3)

)는 식(4)에 의해 정의된다.Assuming that a layer of constant thickness is between two identical media, the acoustic energy transfer coefficient (

Is defined by equation (4).

(4)

(4)

)을 구할 수 있다. Therefore, from equation (4), the transfer characteristic of the acoustic filter (

) Can be obtained.

(5)

(5)

이라면

,

, 그리고

이므로, 전달특성(

)은if,

Ramen

,

, And

, The transfer characteristic (

)silver

(6)

(6)

Becomes

1.2. Type 2

Another type of acoustic filter having low pass characteristics is a narrow cross-sectional area. This is the same as the acoustic inertance connected in series to the sound tube as shown in FIG. 4A. Therefore, the transfer coefficient of this type of acoustic filter is the same as the result in equation (5).

2. Electrical Modeling of Acoustic Filters

2.1. Outer ear model

FIG. 5 is a diagram illustrating an electrical superior model of an outer ear, in which the outer ear is divided into an external ear, a concha, and an auditory canal, and each is configured as an electroacoustic model.

를 가지고 있는 구의 형태로 가정하였을 때,

,

등은 소리의 회절을 나타내며 다음과 같이 표현된다.When external sound is emitted to the ear, the obstacle structure caused by the head or the remaining chest

Assuming a form of a sphere with

,

The back represents the diffraction of the sound and is expressed as follows.

(7)

(7)

(8)

(8)

는 공기밀도를

는 소리의 속도를 나타내며, 병렬로 연결된

,

은 음향방사 임피던스를 나타낸다.here,

Air density

Denotes the speed of sound and is connected in parallel

,

Denotes the acoustic radiation impedance.

(9)

(9)

(10)

10

의 반경을 가지는 음향공진기와 같으며,

의 길이를 가지고 있다. 회로망 성분

와

는 음향질량과 탄성성분을 나타낸다.

의 경우는 에너지손실에 관련된 값을 점성마찰, 열전도, 막의 진동 등을 나타내는 값이다. 외이의 기하학적 구조는 불규칙적인 음향 도파관의 모양을 나타내지만, 주파수 범위 8,000Hz까지는 반경

, 길이

을 가지는 곧은 관으로 근사할 수 있다.The ear canal can be represented as a cylinder,

Is the same as an acoustic resonator with a radius of

Has the length of. Network components

Wow

Represents acoustic mass and elastic component.

In the case of, the values related to energy loss are values indicating viscous friction, thermal conductivity, film vibration, and the like. The geometry of the outer ear shows the shape of an irregular acoustic waveguide, but the radius is up to a frequency range of 8,000 Hz.

, Length

Can be approximated as a straight tube with

(11)

(11)

(12)

(12)

(13)

(13)

(14)

(14)

(15)

(15)

(16)

(16)

가 각 분절의 길이일 때,

은 각 단위 길이당 청각로의 효과적인 감쇠상수이다.

Is the length of each segment,

Is the effective damping constant of the auditory path for each unit length.

은 이 선에서 특성화된 임피던스이며, 이도(acoustic meatus)의 끝에 나타난 전압

은 고막에 들어가는 음압과 동일하다.

Is the characteristic impedance at this line, and the voltage at the end of the acoustic meatus

Is equal to the sound pressure entering the eardrum.

2.2. Combination of acoustic filter and electrical model of the outer ear

FIG. 6 is a diagram illustrating an electrical system in which an acoustic filter and an electrical superior model of the outer ear are combined, and the path through the filter to the eardrum is considered as a system by using the acoustic filter as an electrical model.

Since sound is transmitted through the acoustic filter inserted into the outer ear, the model of the auricle portion was removed from the outer ear model shown in FIG. 5 and replaced with the electrical model of the acoustic filter.

3. Experimental Results

3.1. Electrical modeling simulation of sound shield and outer ear for noise shielding

Using the structural change of the acoustic filter into an electrical system, the transmission characteristics reaching the eardrum were simulated using the OrCad 9.2 version through electrical analog modeling of the acoustic filter and the outer ear.

The outer ear model was used in 1994 by C. Gigure and C. W. Philip, and the acoustic filter used Type 2 to reduce the area difference between the ear canal and the ear when inserted into the outer ear.

The inductance value was determined according to the equation for the similarity between the acoustic system and the electromagnetic field shown in Table 1 below, and the simulation was performed while changing the inductance value.

FIG. 7 shows simulation results of the electrical system shown in FIG. 6.

Simulation results obtained by varying the inductance values representing the characteristics of the acoustic filter indicate that the type 2 acoustic filter and the characteristics of the outer ear can control the transmission band of sound transmitted to the eardrum to less than 2,000 Hz.

Also. By changing the characteristics of the acoustic filter, it can be seen that the characteristic of the acoustic filter can be changed to the necessary transmission band according to the surrounding environment among the 500 to 2,000 Hz band, which occupies most of the voice power.

3.2. Characteristic Experiment of Designed Small Acoustic Filter

FIG. 8 is an experimental configuration for measuring sound transmission characteristics of a sound filter having a narrowing sound tube including characteristics of the outer ear, and an experiment for measuring and analyzing frequency characteristics of a designed and manufactured small sound filter was performed.

The measurement was performed using Audio Precision's System 2, and the measurement microphone is the Audio Precision's MMK-1 system with a frequency response of 5 to 40,000 Hz. The speaker used in the experiment is a 12-inch low frequency loud speaker (model name: CX-12AW200G) of SammiSpeaker, which has a flat frequency response in the 55 ~ 15,000Hz range. The white signal (DNS #white) provided by the audio precision was sent through the speaker to measure the characteristics of the sound passing through the acoustic filter and the coupler. The chamber was fabricated to block external sound during measurement, and a 2cc coupler was attached between the filter and the microphone. The 2cc coupler follows the ANSI standard, and the 2cc coupler measures the acoustic transmission characteristics including the external ear characteristics.

인 부분의 직경은 6mm로 고정시키고, 단면적의 변화에 따른 특성을 측정하기 위해

인 부분의 직경을 1mm, 1.2mm, 1.5mm 바꾸어서 실험하였다. 또, 단면적이 좁아지는 부분의 길이에 따른 특성을 측정하기 위해

을 2mm, 6mm, 12mm로 바꾸어가며 실험하였다.Since the inlet diameter of the 2cc coupler is 6mm,

The diameter of the phosphorus part is fixed at 6 mm, and in order to measure the characteristics according to the change of the cross

The diameter of the phosphorus part was tested by changing 1 mm, 1.2 mm, and 1.5 mm. Moreover, in order to measure the characteristic according to the length of the part from which a cross-sectional area becomes narrow,

Was changed to 2mm, 6mm, 12mm.

9A and 9B are graphs showing the results of the characteristic experiment according to FIG. 8.

=2mm일 때, m의 변화에 따른 실험결과(

)이며, 도 9b는 m=4일 때,

의 변화에 따른 실험결과이다.

When = 2mm, the experimental results according to the change of m (

And FIG. 9B shows that when m = 4,

The experimental results according to the change of.

이 길어질수록, 전달대역이 좁아진다는 것을 알 수 있다.It can be seen from FIG. 9A that the transmission band is narrower as the change of the cross-sectional area becomes larger at a constant length. Moreover, when the change of cross-sectional area is constant from FIG. 9B, length

It can be seen that the longer this is, the narrower the transmission band.

Designed as shown in the graph, the compact acoustic filter has a passband of less than 2,000Hz, which is important for better speech comprehension. From these results, it can be seen that the change in propagation band, which is shown by changing the value of inductance in the electrical simulation, can be obtained by using the change in the cross-sectional area and the length of the small acoustic filter.

II. Hearing protection

1. Sound filter structure for noise shielding

10 is a view showing a sound filter structure for shielding noise according to the present invention.

을 갖는 제1 음관(12)과, 제1 음관(12)과 직렬연결되고 길이

및 단면적

을 갖는 제2 음관(14)으로 구성된다. 좀 더 상세하게는, 소음 차폐용 음향 필터(즉, 원통 형상의 케이스)(10) 내부에 제1 음관(12)과 제2 음관(14)이 직렬로 설치된다. 단면적

, 단면적

, 및 길이

의 비율을 조절하여 음향 에너지 전달특성을 변화시키는 구성을 가진다. 여기서, 단면적

은 단면적

보다 더 크다. 다시 말해서, 본 발명의 소음 차폐용 음향 필터는 앞서 설명한 바와 같이 저역통과 특성을 갖는 단면적이 좁아지는 타입2의 음향 필터가 사용되고 있음을 알 수 있다.Referring to FIG. 10, the acoustic filter 10 for noise shielding has a cross-sectional area.

A first sound tube 12 having a length and connected in series with the first sound tube 12

And cross section

It consists of a second sound pipe 14 having a. More specifically, the first sound pipe 12 and the second sound pipe 14 are installed in series in the noise shielding acoustic filter (that is, the cylindrical case) 10. Cross section

, Cross section

, And length

It has a configuration that changes the acoustic energy transfer characteristics by adjusting the ratio of. Where cross-sectional area

Silver cross section

Greater than In other words, it can be seen that the acoustic filter for noise shielding of the present invention uses a type 2 acoustic filter having a narrow cross-sectional area having a low pass characteristic as described above.

)은 식(5)에서의 결과와 동일하다.Acoustic energy transfer characteristics of the noise shielding acoustic filter 10 (

) Is the same as the result in equation (5).

)은 최대 6㎜ 범위내에서 원하는 음성 전달대역의 특성에 적합하도록 제1 음관(12)의 직경(

)과 제2 음관(14)의 직경(

)을 조절하여 얻어진다.Diameter of sound filter 10 for noise shielding

Is the diameter of the first sound tube 12 to suit the characteristics of the desired voice transmission band within a range of up to 6 mm.

) And the diameter of the second sound pipe 14

Is obtained by adjusting

)의 합인, 최대 길이(

)는 최대 길이(

)를 정한 후 최대 10㎜ 범위내에서 원하는 음성 전달대역의 특성에 적합하도록 제2 음관(14)의 길이(

)를 조절하여 얻어진다.Further, the length of the first sound pipe 12 and the length of the second sound pipe 14 (

), The maximum length (

) Is the maximum length (

After determining the length of the second sound pipe 14 to suit the characteristics of the desired voice transmission band within a range of up to 10 mm

Is obtained by adjusting

2. Hearing protection

The hearing protector according to the present invention is completed by inserting the noise shielding filter 10 of the above structure into an ear mold 20 modeled after the user's ear as shown in FIGS. 11A and 11B. Here, the pattern of the ear is used to imitate the shape of the ear to the ear canal in the same manner as when manufacturing a hearing aid.

Ear molds modeled after the user's ear can reflect the shape of the user's ear, and as a result, when wearing the hearing protector, the hearing protector follows the shape of the ear, thereby further improving the wearing comfort.

3. Sound Filters of Various Structures

The ear mold may be inserted in the ear filter suitable for the size of the ear type hearing protection, and may be inserted in various structures to shield the noise and secure the conversation area according to the noise environment of the user. The user can solve the communication difficulties by the sound transmission characteristics of the acoustic filter.

12A and 12B are cross-sectional views of an acoustic filter structure including a resonator and a worn state.

As shown in FIG. 12A, the resonator 16 is connected in parallel to one side of the second sound pipe 14 and has a narrow and long neck. That is, as referred to in FIG. 12A, the cross-sectional area of the resonator 16 is smaller than the cross-sectional area of the second sound pipe 14. The sound transmission characteristic correction in this acoustic filter structure is made by changing the volume V of the resonator 16 using the screw 16a which is externally provided.

13A and 13B are cross-sectional views of the acoustic filter structure and the worn state including the tube.

를 갖는다. 이러한 음향 필터 구조에서의 음향전달 특성 보정은 튜브(18)의 단면적

를 조정함으로써 가능하다.Referring to FIG. 13A, the tube 18 communicates with the first sound tube 12 and extends outward from the first sound tube 12 by a predetermined length and has a cross-sectional area.

Has Sound transfer characteristic correction in this acoustic filter structure is the cross-sectional area of the tube (18)

By adjusting

When worn, the tube 18 exits the hearing protector and faces behind the ear as shown in FIG. 13B.

14A and 14B are cross-sectional views of an acoustic filter structure and wearing state including a sound tube having a gradually decreasing cross-sectional area.

로부터 단면적

로 단면적이 서서히 감소한다. 즉, 이러한 음향 필터 구조에서의 음향전달 특성 보정은 단면적이 급격하게 변하는 것을 서서히 변하도록 함으로써 이루어진다.As shown in FIG. 14A, a sound tube having a gradually decreasing cross section, that is, a third sound tube 17 is connected in series between the first sound tube 12 and the second sound tube 14, and has a cross-sectional area.

Cross-sectional area

The cross-sectional area gradually decreases. In other words, the acoustic transfer characteristic correction in such an acoustic filter structure is made by gradually changing the cross-sectional area rapidly changing.

15A and 15B are cross-sectional views of an acoustic filter structure and wearing state including a funnel shaped sound tube.

Referring to FIG. 15A, the funnel-shaped sound pipe 19 is connected in series to the inlet of the first sound pipe 12, and by collecting the external sound efficiently by the funnel shape, the voice band required for the conversation is transferred to the transmission band. The function of the sound filter to be set can be maximized.

4. Another embodiment

FIG. 16 is a view showing a structure in which a predetermined film is inserted into a sound shielding sound filter according to the present invention.

Specifically, FIG. 16 illustrates an embodiment in which the membrane 30 having various characteristics is inserted into the acoustic filter structure for noise shielding according to FIG. 10, and the membrane 30a is inserted into the inlet of the first sound pipe 12. The membrane 30b is inserted into the connection of the first sound tube 12 and the second sound tube 14, and the membrane 30c is inserted into the outlet of the second sound tube 14.

The membrane 30 may be used in various types according to acoustic filtering and size adjustment, and may be applied by adjusting the thickness and structure in consideration of the acoustic and vibration characteristics of the membrane. Thus, the membrane need not be limited to a particular kind.

According to the present invention as described above, by using the ear-type ear mold modeled after the shape of the ear suitable for the characteristics of the user's ear it is possible to solve the problem of long-term wearing of the hearing protection by improving the discomfort of wearing. In addition, by inserting various types of sound filters, it is possible to solve the communication difficulties that may occur when using hearing protection by using the sound transmission characteristics of the sound filter. In addition, by inserting a membrane into the sound pipe of the acoustic filter and by changing the position of the membrane to obtain a variety of sound transmission characteristics, it is possible to introduce a filter suitable for the noise condition exposed to the hearing protector. As a result, it is expected to increase the frequency of wearing hearing protection to have a high effect on the prevention of hearing loss.

Claims (10)

A cylindrical case; A cylindrical first sound pipe installed inside the cylindrical case and having a first cross-sectional area and a first length; And A cylindrical second sound pipe installed inside the cylindrical case and having a second cross-sectional area smaller than the first cross-sectional area and a second length; An inlet of the second sound tube is connected to an outlet of the first sound tube, the diameter of the inlet of the first sound tube is larger than the diameter of the outlet of the first sound tube, The maximum value of the diameter of the sound shielding acoustic filter is set to 6 mm so that the frequency of the sound that has passed sequentially through the first and second sound pipes is less than 2 kHz. The maximum value of the sum of the first and second lengths is set to 10 mm, And when the ratio of the first cross-sectional area to the second cross-sectional area is changed, or when the first and second lengths are changed, the acoustic transmission characteristics of the sound passing through the first and second sound pipes in sequence are changed. Sound filter for noise shielding. delete delete Earmolds shaped after the ear; And An acoustic filter for shielding noise inserted into the ear mold, The sound shielding sound filter, A first sound pipe having a first cross-sectional area and a first length; And A second sound pipe having a second cross-sectional area smaller than the first cross-sectional area, and a second length; The first and second cross-sectional areas and the first and second lengths of the first and second sound pipes are adjusted such that the frequency of sound sequentially passing through the first and second sound pipes is less than 2 kHz, When the ratio of the first cross-sectional area to the second cross-sectional area is changed, or when the first and second lengths are changed, the acoustic transfer characteristic of the sound sequentially passing through the first and second sound pipes is changed. Bind Hearing Protection. The method of claim 4, wherein The first and second sound pipes have a cylindrical shape, one end of the second sound pipe is connected to one end of the first sound pipe, and the first and second sound pipes are installed inside the cylindrical acoustic noise shielding filter. , The maximum value of the diameter of the noise shielding acoustic filter is 6 mm, the maximum value of the sum of the first and second lengths of the first and second sound pipes is 10 mm. The method of claim 4, wherein A resonator connected in parallel to one side of the second sound pipe and having a cross-sectional area smaller than the second cross-sectional area of the second sound pipe, And when the screw fastened to one side of the resonator is tightened or loosened, the volume of the resonator is changed so that the acoustic transmission characteristic of the noise shielding acoustic filter is adjusted. The method of claim 4, wherein And a tube communicating with the first sound pipe at one side of the first sound pipe and extending outwardly by a predetermined length from the first sound pipe, When the cross-sectional area of the tube is adjusted, the acoustic hearing protector, characterized in that the sound transmission characteristics of the sound shielding sound filter is adjusted. The method of claim 4, wherein The first and second sound pipes have a cylindrical shape, one end of the second sound pipe is connected to one end of the first sound pipe, and the first and second sound pipes are installed inside the cylindrical acoustic noise shielding filter. , And a funnel-shaped sound pipe which is connected in series to another inlet of the first sound pipe and protrudes to the outside of the sound shielding sound filter. The method according to any one of claims 4 to 8, And a predetermined membrane is inserted into an inlet of the first sound pipe, a connection between the first sound pipe and the second sound pipe, and an outlet of the second sound pipe, respectively. The method of claim 4, wherein And a third sound pipe having a cross-sectional area gradually decreasing from the first cross-sectional area to the second cross-sectional area in series between the first sound pipe and the second sound pipe.

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