Sound reduction techniques of the axial 1st mode transmission of an aperture for ventilation purpose in a plane rigid wall were studied by conducting boundary element numerical simulations. First, the effectiveness of the boundary element approach, and the dependence of the insertion loss of a wall with an aperture on the incidence angle were investigated. Then, the effectiveness of the sound reduction techniques, such as mounting acoustical liners to the aperture perimeter, attachment of a disk shape hoods, and insertion of a Helmholtz resonator were examined.会議名称:The 30th International Congress and Exposition on Noise Control Engineering主催学会:NA

Okawa Shinpei

Sekine Hidehisa

Terao Michihito

大川 真平

寺尾 道仁

関根 秀久

Sound reduction techniques of the axial 1st mode transmission of an aperture for ventilation purpose in a plane rigid wall were studied by conducting boundary element numerical simulations. First, the effectiveness of the boundary element approach, and the dependence of the insertion loss of a wall with an aperture on the incidence angle were investigated. Then, the effectiveness of the sound reduction techniques, such as mounting acoustical liners to the aperture perimeter, attachment of a disk shape hoods, and insertion of a Helmholtz resonator were examined.Article会議名称:The 30th International Congress and Exposition on Noise Control Engineering主催学会:NAGconference pape

寺尾, 道仁

Terao, Michihito

関根, 秀久

Sekine, Hidehisa

大川, 真平

Okawa, Shinpei

KANAGAWA University Repository

inter.noi/e 1001The 2001 International Congress and Exhibitionon Noise Control EngineeringThe Hague, The Netherlands, 2001 August 27-30Effect of acoustic liners on sound transmissionof wall apertures for ventilationMichihito Terao, Hidehisa Sekine, Shinpei OhkawaKanagawa university; Rokkaku-bashi 3-27-1, Kanagawa-ku, Yokohama, 221-8686, JapanAbstractSound reduction techniques of the axial 1st mode transmission of an aperture for ventilationpurpose in a plane rigid wall were studied by conducting boundary element numerical simulations.First, the effectiveness of the boundary element approach, and the dependence of the insertion lossof a wall with an aperture on the incidence angle were investigated. Then, the effectiveness of thesound reduction techniques, such as mounting acoustical liners to the aperture perimeter,attachment of a disk shape hoods, and insertion of a Helmholtz resonator were examined.1. IntroductionTo improve the sound insulation of a rigid plane wall with an aperture for ventilation as shown inFigure 1, the suppression of the axial 1st mode resonance of the aperture is most important. Forthis purpose, sound reduction techniques such as installment of a sound absorbent liner,attachment of a hood, and insertion of a Helmholtz resonator will be examined numerically.2. Descriptions for sound insulation performanceof a wall apertureG. P. Wilson and W. W. Soroka [1] defined the sound transmission loss of a wall aperture asR =IOloglo ~ /~ (1)where J: denotes the acoustic power of the transmitted wave of the aperture, ~ is the acousticpower of the incident wave passing through the area SF cosB (where SF is the face area of theaperture and (J is the angle of incidence).On the other hand, we can define a representation in terms of insertion loss of a rigid wall (i.e., awall with infinitely large sound transmission loss) with an aperture asDIL =Lp,i - Lw,t (2)where Lp,i denotes the sound pressure level of the incident wave at the aperture location when thewall is not present. Lw,t refers to the sound power level of the transmitted wave. Since we canwrite as Lw,t =::: Lp ,O.4m where Lp ,o.4m denotes the sound pressure level at a position of O.4m apartfrom the center of the aperture end section of the transmitted wave side, D1L is equivalent to thesound pressure level difference Lp,i - L p,o.4m. This implies that D1L is equivalent to the differencebetween the sound pressure levels with and without the wall.Incidentally Ohkawa, Tachibana and Koyasu introduced the normalized sound transmission loss[2], which is defined as the decibel expression of the ratio between the transmitted sound powerand the incident sound power per unit wall area. Since the sound pressure level Lp,i is equivalentto the sound intensity level or the sound power level passing through unit area normal to thedirection of the incident wave propagation, D1L is equivalent to the normalized soundtransmission loss when the incidence angle is normal to the wall. From Equations (1) and (2) wehave a relationshipsR - D1L = 1010glo (SF cosO)and D1L will be primarily used in the succeeding sections.(3)3. Numerical simulationTo obtain the sound pressure and particle velocity distribution around an aperture of interest, aboundary element method of constant element is employed. The largest element size taken is25mm. To provide a plane wave incidence around the aperture, the distance between the apertureand a point source (the amplitude of Ss =100/ 4Jl") taken was 1km. For the sound absorbent liner,the locally reacting model is employed for simplicity. A fiberglass board of the density of32kg/m3 and the flow resistivity of 8500Pa· s/m2 are assumed for the sound absorbing liner. Itsacoustical property was estimated by using the empirical expression given by M. E. Delany and E.N. Bazley [3]. The insertion loss of a rigid wall (300 mm thick) with an aperture (the diameter of100 mm) of a rigid perimeter was investigated. Figure 2 compares the analytical formula fornormal incidence, the experiment (by the two reverberant chambers method) of G. P. Wilson and W.W. Soroka [1] and the present numerical simulation for normal incidence, in terms of the insertionloss. They agree excellent in the frequencies below 1000Hz, where the largest element size of thepresent numerical simulation is 1/12 of the wavelength. This implies the effectiveness of thenumerical simulation. It is important to suppress the axial 1st mode sound transmission.Point sourceLW,tHalf spaceFig.! Sound transmission through a wall aperture- Present numerical......... Analytical } .o Experiment WI1son et al.oo--~2----&..-4.1.....-~6~.....8....1~O~OO~--200--0Frequency f [Hz]Fig.2 Insertion loss of a wall with an aperture4. Acoustic lining on aperture perimeterTo reduce the transmission at the resonant frequency of the axial 1st mode, It, the effect ofmounting a sound absorbent liner to the aperture perimeter was investigated for a simple wallaperture case. Figure 3 shows the effect of a sound absorbent liner on the insertion loss.Incidentally, the dependence of the insertion loss DIL on the incident angle (} is practicallysmall enough to ignore. The sound absorbent liner shifts the resonant frequency towardremarkably smaller, as the liner becomes thicker. However the liner improves only 5dB, at most,the insertion loss D1L around the resonant frequency It.5. Resonant frequency of axial modeFor an aperture as shown in Figure 4, nth (n being an integer) mode resonant frequency takesplace when the phase difference of the traveling wave between the effective open ends of incidentand transmitted wave sides coincides with n1C, i.e.,J" =n / 2(IM i / c + lw / Cw+ IM t / c) (4), ,where c denotes the sound speed in the free field, Cw denotes the phase velocity in the airwayof the aperture, IM,i and IM,t are the end correction length of the incident and transmitted waveside, respectively. When the perimeter is unlined and rigid, Cw =c, and IMi =IMt ::::: 0.38D aroundf::::: 500Hz. For these values, Equation (4) gives It::::: 450 which agrees very well withIt ::::: 460Hz for d =Omm of Figure 3.On the other hand, when the aperture perimeter is lined, the values of cw(;t c), IM,i and [M,t areunknown. To determine Cw ' attaching pipes (with the same diameter as the aperture) both side ofthe aperture, a 2-port equation set was composed for the characteristic impedance and thepropagation constant of the aperture [4]. To determine either [M,i or [M,t , removing the pipecorresponding [Mi or IMt ,the impedance of another pipe was used. For these, the sound fields, ,of the pipes were observed numerically. In this way cw ::::: 189m/s, [M,i =[M,t ::::: 0.89D aroundf ::::: 250Hz were obtained when d =50mm. For these values Equation (4) gives It::::: 232Hzwhich agrees excellently with It::::: 240Hz for d = 50mm of Figure 3.30o--2----4-----1~6--8--1--00-0--20-00Frequency f [Hz]Fig.3 Insertion of acoustic linersi~. c--·1--1Fig.4 Mass end corrections IM,i and [M,t ofa open-ended duct, and phase speed in aperture, Cw6. Attachment of a hoodAs an alternative sound suppression technique, a disk shape hood was attached to the sound sourceside of the aperture as shown in Figure 5. When the hood is rigid, at the resonant frequency It,the sound insulation becomes poorer as the hood size increases. When the hood is acousticallylined, although the improvement in the insertion loss is extensive for higher frequencies, thataround the resonant frequency It is only a few dB.7. Insertion of a Helmholtz resonatorFigure 6 shows the effect of the insertion of the Helmholtz resonator (the cavity volume of 0.55£,the neck length of 5mm, the neck cross-sectional area of 3.5cm2 ) side-branched at the midway ofthe aperture. The insertion of the Helmholtz resonator is effective and promising for the purpose.50 .ll~lw=300~[Dimmension in mm](C) A lined hood attached to an aperture"Rigid hood~~I1:s Lined hoodl:~II:!(d) A lined hood(Dh=200mm)o---""!!-2----1.--+4 ----I~6.......&...""!!-8~1~0~00~--2000--llFrequency f [Hz]cS 20O---~_...a.-~----I---I.--I......L...I.......I--__~2 8 1000 2000Frequency f [Hz](b) Rigid hoods30Fig.5 Attachment of hoodso-~2----~4---a~6----8...1:1~00~0----20"""00Frequency f [Hz]Fig.6 Insertion of a Helmholtz resonatorConclusionsSound reduction techniques of the axial 1st modetransmission of a wall aperture for ventilation purposewere investigated by conducting a boundary elementnumerical simulation. The important findings are asfollows: The dependence of the insertion loss on theincident angle is small. A sound absorbent liner mountedto the aperture perimeter decreases considerably theresonant frequency, as the liner becomes thicker. Theattachment of a hood shifts the resonant frequencydownward and decreases the sound insulation, as thehood size increases. To suppress the 1st mode soundtransmission of an aperture, the insertion of a Helmholtzresonator is promising while the sound absorbent linersand the hoods are not effective enough.,......, 30~~1111"""""~")'i300~ 300~l_ " ~8rag ll~~~:~\~~:\1\~~~~;~~\~1~:::1Insertion of resonator - rOriginalReferences[1] G. P. Wilson and W. W. Soroka, "Approximation to the diffraction of sound by a circularaperture in a rigid wall of finite thickness" JASA, 37,1965, nr2, pp286-297.[2] H. Ohkawa, H. Tachibana, M. Koyasu, "Sound transmission characteristics of buildingelements with small openings for ventilation purposes", inter-noise 84, 1984, pp603-606.[3] M. E. Delany and E. N. Bazley, "Acoustical properties of fibrous absorbent materials",Applied Acoustics, vol. 3,1970, pp. 105-116.[4] M. Terao and H. Sekine, "In-duct pressure measurements to determine sound generation,characteristic reflection and transmission factors on an air moving device in air flow", inter-noise89, 1989,ppI43-146.

Effect of acoustic liners on sound transmission of wall apertures for ventilation

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Effect of acoustic liners on sound transmission of wall apertures for ventilation

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