Kimihiro Sakagami

Sound absorption of a cavity-backed membrane: A step towards design method for membrane-type absorbers

Abstract This paper deals with a theoretical study of the sound absorption characteristics of a membrane-type sound absorber. A closed-form analytical solution for the sound absorption coefficient of an infinite membrane with an air-back cavity is produced by making a minor modification to the solution for a cavitybacked infinite elastic plate which was derived in the previous paper. To analyse the mechanism of absorption, the solution is rearranged in a form which points out the contribution of each element of a membrane-type sound absorber. The contribution of the cavity is found to be dominant, and that of the absorptivity of the source side surface of the membrane is rather small. In the case of an elastic plate, the contribution of the loss factor is found negligible. The effects of the parameters of the sound absorption system are discussed in the light of calculated results. Furthermore, the method to predict the peak-frequency and the peak value of the oblique-incident absorption coefficient of the membrane-type sound absorber is presented. The method satisfactorily explains the relationship between the absorption characteristics and the parameters. It is also applicable to panel absorbers. It can be useful for the design of the purpose-made membrane-type or panel absorbers, and for the sound field prediction within boundaries made from this type of absorber.

Acoustic properties of permeable membranes

Membranes used for building materials have a certain degree of acoustic permeability, which has been disregarded in general membrane‐vibration theory, and may cause serious effects especially on the acoustic properties. In this study, a theory for sound absorption of, and sound transmission through, a single permeable membrane is developed. Subsequently, sound absorption of structures composed of air layers, absorptive layers, and the facings of permeable membranes is investigated theoretically, and discussed in comparison with the experimental data measured by using the reverberation‐room method. The results are in fairly good agreement; thus the present theory should give an effective tool for prediction of the acoustic properties of this type of membrane structure.

The role of reflections from behind the listener in spatial impression

Abstract This paper describes the results of two subjective experiments to clarify the role of reflections arriving from behind the listener in the perception of spatial impression. The experiments investigate the effects of reflections from behind the listener on both listener envelopment (LEV) and auditory source width (ASW) and which is more effective for LEV, the early or late reflections. The results of experiments clearly show that: (1) The listener can perceive LEV and ASW as two distinct senses of a sound image. (2) The role of reflections arriving from behind the listener is to increase LEV in spatial impression. Namely LEV increases as the relative reflection energy of sound arriving from behind the listener increases. (3) The early reflections also contributes to the perception of LEV, while (4) the late reflections are more effective for LEV than the early ones. However, it cannot be definitely concluded whether C 80 affects LEV or not.

Detailed analysis of the acoustic properties of a permeable membrane

Abstract A detailed analysis is carried out to clarify the mechanisms governing the acoustic performance of a permeable membrane and the effects of various material parameters. For this purpose, the theoretical solutions for the reflected and transmitted sound fields by an infinite permeable membrane which have been derived in a previous paper [1] are approximated to obtain simple expressions for normal incidence absorption and transmission coefficients. An electrical circuit analogy is employed for a detailed analysis in which the particle velocity is separated into two components, i.e. the mass and the permeability, which is helpful in understanding their contributions to the acoustic properties of the permeable membrane. These considerations are aimed at demonstrating the effects of the parameters on the acoustic properties as well as explaining the following particular phenomena which are observed in the acoustic properties of a permeable membrane: the decrease of the sound energy absorbed in the structure at low frequencies and the increase of transmission loss at low frequencies due to the permeability. The optimal value of flow resistance for the most effective absorption is also obtained from those solutions.

A Basic Study on Acoustic Properties of Double-leaf Membranes

Abstract Low absorptivity is a problem associated with many membrane-structure buildings and can result in a long reverberation time which degrades the acoustic environment. One possible solution to this problem may be the use of membranes to form the ceiling. The present study examines the acoustic properties of double-leaf membranes, which have recently become widely used in the construction of membrane-structure buildings. Parametric studies with theoretically calculated results show the effects of various parameters on absorptivity and transmissibility. Experimental results confirm the theory. The possibility of using double-leaf membranes to construct the absorption structures is then discussed.

A method to calculate the acoustic response of a thin, baffled, simply supported poroelastic plate

The Helmholtz integral equation formulation is used to produce the solution for the acoustic field reflected from a finite, thin, poroelastic plate in a rigid baffle with simply supported edges. The acoustic properties of the porous material are predicted using the effective fluid assumption. The solutions for the displacement of the plate and for the loading acoustic pressures are given in the form of the sine transform. The sine transform coefficients are obtained from the solution of a system of linear equations resulting from three integral Helmholtz formulations which relate the displacement of the plate and the acoustic pressures on the front and on the back of the plate. The effect of an air gap behind the plate in the front of a rigid wall is also considered. A parametric study is performed to predict the effect of variations in the parameters of the poroelastic plate. It is shown that thin, light, poroelastic plates can provide high values of the acoustic absorption even for low frequency sound. ...

Sound radiation from a double-leaf elastic plate with a point force excitation: effect of an interior panel on the structure-borne sound radiation

Abstract The sound radiation from a double-leaf elastic plate subjected to a point force excitation is investigated theoretically, to gain a fundamental insight into the sound radiation from an interior panel of a double-leaf structure in buildings. The effects of the interior panel on the sound radiation, which show a negative effect at low frequencies due to the mass–air–mass resonance, are discussed in detail. The theory is validated experimentally. As a measure of the efficiency of the interior leaf in reducing noise radiation, the radiation reduction is defined in this study, and it is found useful for predicting the sound radiation due to the structure-borne sound in building elements. Parametric studies through theoretical results are made to clarify the effects of the parameters of the sound radiation system, and to gain a fundamental insight into the control of structure-borne noise radiation. It is shown that it is difficult to reduce the radiated sound power by an interior panel alone, even if its mass is increased.

SOUND RADIATION FROM AN UNBAFFLED ELASTIC PLATE STRIP OF INFINITE LENGTH

Abstract The noise which is caused by vibration of steel plate girders of a motorway is studied. This type of noise is often referred to as elevated structure noise, and has its main component within the frequency range 500 Hz–1 kHz. Since a steel plate girder can be considered as a rectangular plate with an extreme aspect ratio, the sound field radiated by a baffled elastic plate strip of infinite length, which is the simplest model of a steel plate girder, was theoretically analysed in the preceding paper (Sakagami, K., Michishita, K., Morimoto, M., and Kitamura, Y. Sound radiation from a baffled elastic plate strip of infinite length with various concentrated exitation forces. Appl Acoust 55: 181–202). However, a real motorway construction is more appropriately modelled by an unbaffled strip. Thus the unbaffled condition has been analysed numerically in this paper. The method of equivalent sources is used. The main features of the radiated sound field are discussed in the light of numerical examples, and compared with those of a baffled strip. Two vibration patterns, i.e. an idealised piston vibration and flexural vibration excited by a moment are considered. Furthermore, in order to determine the far-field range of an infinitely long plate, the variation of the radiated sound field with the distance is discussed.

Acoustic properties of double-leaf membranes with a permeable leaf on sound incidence side

Abstract Double-leaf membranes with a permeable leaf on the sound incidence side are usually used as roofs in actual membrane-structure buildings. They are modelled as of infinite extent, and their acoustic properties are theoretically analysed. The theory is experimentally validated. The permeability can significantly improve the sound absorption performance at high frequencies, and makes the characteristics similar to those of cavity-backed porous absorbents. At low frequencies the permeability also affects the sound absorption and insulation performance. At middle frequencies, the characteristics changed according to the permeability: a mass–spring peak appears if it is very high, and vanishes if lower. These effects are strongly dependent upon the mass of leaves, flow resistivity, cavity depth, etc. A detailed discussion is also given to shed some light on the mechanism of these effects.

Effect of an air-back cavity on the sound field reflected by a vibrating plate

Abstract The effect of an air-back cavity on the spherical wave reflected by the stage floor is analyzed theoretically. An air-cavity backed infinite elastic plate is driven to vibration by a point force. A closed form expression for the far-field solution is derived by coupling the Helmholtz integrals for sound field and the equation of the plate motion. Numerical examples demonstrate four main features of the sound field. Two of them are absent for the plate without the air-cavity: one is a panel absorption dip owing to the plate and air-cavity resonance system. The other is small peaks and dips due to the acoustic resonances of the air-cavity.