Measuring oblique incidence sound absorption using a local plane wave assumption

Abstract

SummaryIn this paper a method for the measurement of the oblique incidence sound absorption coefficient is presented. It is based on a local field assumption, in which the acoustic field is locally approximated by one incident-and one specularly reflected plane wave. The amplitudes of these waves can be determined with an unidirectional sound intensity probe. The local active-and incident acoustic intensity are straightforwardly obtained. The area-averaged sound absorption coefficient is calculated after spatial integration of these quantities over the surface area of interest. Alternatively, one may use a three-dimensional intensity probe. In that case, the determination of the amplitudes of the plane waves can be formulated as a least-squares problem. Measurements performed for a sound absorbing foam demonstrate that accurate results can be obtained, even under non-ideal acoustic conditions. Measurements carried out for a periodic absorber show that the method is accurate below the cuton frequency of scattering as long as the amplitude of the evanescent surface waves is significantly smaller than that of the specularly reflected wave. PACS no. 43.20El, 43.55Ev, 43.55Dt

Measuring Sound Absorption: Considerations on the Measurement of the Active Acoustic Power

“…This novelmeasurement method employs aL ocal Plane Wave Assumption, and is therefore called the LPW-method. It yields the exact sound absorption coefficient for plane wavest hat are normally incident upon alocally-reacting surface, butalso yields accurate results for near-normal incidence, even if additional reflections are present [20,21,22,23]. By using the LPW-method, at asmall distance from the surface of interest, one can straightforwardly determine the local, ©S.Hirzel Verlag · EAA Figure 1.…”

Prediction of Sound Absorption of Stacked Granular Materials for Normal and Oblique Incident Sound Waves

“…Here, we implemented the latter: the angle of the incident and reflected wave are estimated beforehand, based on the measurement setup. The LSPWm ethod is described in more detail in [18]. The active,complexpressure pq is measured by microphone q for microphone position q = 1, 2,... N.Ateach microphone the pressure can be described by pq ω, r q = A(ω)e ik I (ω)ξ I (rq)…”

“…The microphones are spaced in acubic array,asshown in Figure 11. Formore information about the design, characteristics and implementation of this 8p-probe, the reader is referred to [18]. Furthermore, in [18,20] the measurement technique is validated for several sound hard and sound absorbing materials for normal and oblique incidence.…”

“…The developer of the 8p-probe validated this measurement technique using alinear x-y guiding system to measure al arge set of grid points, equally divided along the surface and the edges of a( locally behaving)m aterial [18,20]. However, this is av ery time consuming method and is difficult to apply outside of laboratory circumstances.…”

“…In general, for al arger angle of incidence the frequencyo ft he absorption peak increases. This can be explained when decomposing the wave number k in k x = k sin θ j cos ϕ j , (16) k y = k sin θ j sin ϕ j , (17) k z = k cos θ j , (18) with k 2 = k 2 x + k 2 y + k 2 z .For θ = 0 • and ϕ = 0 • ,the wave number in z-direction corresponds with the first resonance frequencyo ft he quarter wavelength tube: k 1 = k z .A si s illustrated in Figure 18, the corresponding resonance wave number in z-direction does not depend on the angle of incidence. Therefore, when the angle of incidence increases, the wave number k 1 and thus the resonance frequency f 1 will increase as well.…”