• Title, Summary, Keyword: Acoustic Impedance

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Note on the Two-Microphone Methods for the Measurement of Acoustic Impedance (음향 임피던스 측정을 위한 이중 마이크로폰 기법에 대한 고찰)

  • SEO, SEONGHYEON
    • Transactions of the Korean hydrogen and new energy society
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    • v.29 no.2
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    • pp.163-169
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    • 2018
  • The present article discusses about the measurement techniques of acoustic impedance that becomes one of the important acoustic characteristics of various boundaries found inside of propulsion systems. Acoustic characteristics including acoustic impedance and reflection coefficient can be often assessed and estimated by use of the two-microphone method. Theoretical expressions of acoustic impedance and reflection coefficient measured in an impedance tube are presented for both cases with mean flow and without flow, and the practical application of the method through calibration is also provided. The acoustic impedance and the reflection coefficient are related with axial locations of microphones, thermodynamic characteristics of gas inside, and the transfer function between the pressure wave measurements at multiple locations.

An Investigation on the Acoustic Impedances and Estimation Models of Multiple Layer Perforated Plate Systems (다중 다공판 시스템의 음향임피던스와 계산모델에 관한 고찰)

  • 이동훈;허성춘;허성욱;김민배
    • Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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    • pp.1238-1243
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    • 2002
  • In this study, the validity of the acoustic impedance model and the estimation model by electro-acoustic analogy suggested by Maa for predicting the absorptive performance of multiple layer perforated plate systems is investigated. From the comparison between the experiment and calculation for the absorption performance of double layer perforated plate system, the calculated results of using Rao and Munjal's impedance model and transfer matrix method are closer to the experimental values than those of using Maa's impedance model and electro-acoustic analogy. Therefore, in order to apply the acoustic impedance model and the estimation model by electro-acoustic analogy suggested by Maa to the multiple layer perforated plate systems, it is necessary that the suggested acoustic impedance and estimation models should be re-examined.

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A Study on the Acoustic Damping Characteristics of Acoustic Cavities in a Liquid Rocket Engine (로켓엔진에서 음향공의 음향학적 감쇠능력에 대한 고찰)

  • Kim Hong Jip;Kim Seong-Ku;Choi Hwan Seok
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • v.y2005m4
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    • pp.291-298
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    • 2005
  • Linear acoustic analysis has been performed to elucidate damping characteristics of acoustic cavities. Results have shown that resonant frequencies of acoustic cavity obtained by classical theoretic approach and present linear analysis are somewhat different from each other. This difference is due to the limitation of classical theory. To quantify the damping characteristics, acoustic impedance has been introduced and resultant absorption and conductance have been evaluated. Satisfactory agreement has been achieved with previous experiment. Finally the design procedure for optimal tuning of acoustic cavity has been established

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An Empirical Acoustic Impedance Model for the Design of Acoustic Resonator with Extended Neck at a High Pressure Environment (높은 음압에서의 내부 확장관형 음향 공명기의 설계를 위한 실험적 음향 임피던스 모델)

  • Park, Soon-Hong;Seo, Sang-Hyun
    • Transactions of the Korean Society for Noise and Vibration Engineering
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    • v.22 no.12
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    • pp.1199-1205
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    • 2012
  • An empirical acoustic impedance model of acoustic resonators with extended neck at a high sound pressure environment is proposed. The acoustic resonator with extended neck into its cavity is appropriate for the launcher fairing application because the length of neck does not increase the total height of the resonator. This enables one to design slim and light acoustic resonators for launch vehicles. The suggested acoustic impedance model considers the incident pressure and geometric variables(the neck length, the perforation ratio and the hole diameter) in terms of non-dimensional variables. Several acoustic resonators with extended neck are manufactured and their wall impedances are measured according to the pre-defined incident pressure levels. Effects of non-dimensional variables on the non-linear acoustic impedance are investigated so that a simple non-linear impedance model for the launcher fairing application can be proposed. It is demonstrated that the estimated acoustic resistance and acoustic length correction show reasonable agreement with the measured ones within the range of design parameters for launcher fairings.

Modifications of Numerical Impedance Boundary Conditions Considering Incident Acoustic Pressure (음향 입사파를 고려한 수치적 임피던스 경계조건의 보정)

  • Kim, Min-Woo;Park, Yong-Hwan;Kim, Sung-Tae;Lee, Soo-Gab
    • Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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    • pp.344-348
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    • 2007
  • The acoustic liner has been used for the suppression of noise. The impedance characteristics of the acoustic liner are increased by the incident pressure. For the estimation of the acoustic liner on the incident acoustic pressure effect, the modified impedance model is suggested on the basis of the GE impedance prediction model. The modified impedance model is originated from the 3 parameter impedance model, and extended to the incident pressure parameter. The modified model is applied on the simple duct analysis with variant source pressure. Through the computation, it is observed that the fore directivity patterns of the duct are varied by the incident SPL level.

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Effect of Moisture Content on Acoustic Absorption and Impedance of Wood (목재(木材)의 흡음(吸音)과 음향(音響)임피이던스에 미치는 함수율(含水率)의 영향(影響))

  • Hong, Byung-Wha
    • Journal of the Korean Wood Science and Technology
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    • v.22 no.2
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    • pp.25-29
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    • 1994
  • The effect of moisture content on acoustic absorption coefficient and impedance of the woods of Pinus densiflora, Chamaecyparis pisifela, Cryptomeria japonica, Abies koreana, Melia azedarch var. japonica, Shorea bracteolata and Shorea acuminata was investigated by standing wave method. The results obtained are summarized as follows: The acoustic absorption coefficient increased with increasing moisture content, but acoustic impedance decreased with increasing moisture content.

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A Study on the Acoustic Performance Design Technique of Underwater Acoustic Material (수중 음향재료의 음향성능 설계기법 연구)

  • Seo, Youngsoo;Ham, Ilbae;Jeon, Jaejin
    • Transactions of the Korean Society for Noise and Vibration Engineering
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    • v.23 no.10
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    • pp.920-927
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    • 2013
  • The requirement of acoustic performance about underwater acoustic material which is used in underwater environment more increases. Underwater acoustic material was made by viscoelastic material such as a rubber and a polyurethane etc. In order to increase an acoustic performance, several kinds of inclusions were added to viscoelastic material. In this paper, acoustic modelling and analysis techniques were introduced and the acoustic characteristics of underwater acoustic material were studied. Echo reduction and transmission loss were calculated with volume fraction of inclusion in the material. Also the characteristic impedance and the input impedance of underwater acoustic material were obtained and effects on the echo reduction and transmission loss of material were discussed.

Acoustic Source Power Control and Global Noise Reduction by Selection of Distribution and Impedance of Absorptive Materials in Acoustically Small Enclosures (흡음재의 배치와 임피던스 선정을 통한 음원 방사파워 제어와 전역 소음 감소)

  • 김양한;조성호
    • Transactions of the Korean Society for Noise and Vibration Engineering
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    • v.14 no.8
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    • pp.668-674
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    • 2004
  • The possibility of global noise reduction by the sound power control through selection of distribution and impedance of absorptive materials is discussed. It is necessary to investigate the relation between the global sound energy in the field and the total sound power radiated by sources. In the previous work,$^{(1.2)}$ the authors presented a useful design method to change boundary condition that can be useful to reduce noise in acoustically small enclosures. Changing boundary condition Is related to not only enclosure’s geometrical shape but also acoustical treatment on walls for example, attaching of impedance patches (ex: absorptive material). In many practical situations, we often meet situation to change acoustical treatment on walls. The possibility of total acoustic potential energy(globa1 noise) reduction by acoustic source power control is examined in an acoustically small cavity Using acoustic energy balance equation, the relation between global noise control performance and absorptive material’s arrangement/impedance is deduced. Numerical simulation is performed to interpret its physical meaning in terms of absorbent’s distribution and impedance.

A Study on the Acoustic Damping Characteristics of Acoustic Cavities in a Liquid Rocket Combustor (로켓연소실에서 음향공의 음향학적 감쇠에 대한 정량적 고찰)

  • Kim, Hong-Jip;Kim, Seong-Gu;Choe, Hwan-Seok
    • Aerospace Engineering and Technology
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    • v.5 no.2
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    • pp.195-204
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    • 2006
  • A linear acoustic analysis has been performed to elucidate damping characteristics of acoustic cavities in a liquid rocket combustor. Results have shown that resonant frequencies of acoustic cavity obtained by classical theoretic approach and by the present linear analysis are somewhat different with each other. This difference is attributed to the limitation of the simplified classical theory. To quantify the damping characteristics, acoustic impedance has been introduced and resultant absorption coefficient and conductance have been evaluated. Satisfactory agreement has been achieved with previous experiment. Finally the design procedure for an optimal tuning of acoustic cavity has been established.

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Radiation Power Control by Means of Absorptive Material Arrangement in an Enclosure (흡음재 배치를 통한 닫힌 공간에서의 소음원 방사 파워 제어)

  • 조성호;김양한
    • Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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    • pp.688-691
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    • 2004
  • We have studied the possibility of global noise reduction by the sound power control through selection of distribution and impedance of absorptive materials. It is necessary to investigate the relation between the global sound energy in the field and the total sound power radiated by sources. In the previous work (1,2), the authors presented a useful design method to change boundary condition that can be useful to reduce noise in acoustically small enclosures. The possibility of total acoustic potential energy reduction by acoustic source power control is examined in an acoustically small cavity. Using acoustic energy balance equation, the relation between global noise control performance and absorptive material's arrangement/impedance is deduced. Numerical simulation is performed to interpret its physical meaning in terms of absorbent's distribution and impedance.

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