Sound Absorption at Low Frequencies: Modelling a Test Room

Abstract

A description is given of sound absorption in small-furnished rooms at low frequencies. The objective of the work was to characterise room absorption and to develop a model of surface absorption appropriate for a modal description of contained sound fields at low frequencies. An empty test room was measured and numerically modelled, for use as a reference condition for later measurements with obstacles and absorbers in the room. A finite element model of the empty room demonstrated the need to account for even small irregularities in the room geometry. It is also confirmed that the vibration behaviour of the room walls contributes to the whole room absorption mechanism at very low frequencies. A parametric survey, involving model updating, yielded a frequency invariant value of absorption coefficient of 0.02, which produced the best agreement between prediction and measurement. The resultant predicted frequency response functions agreed with measurement within ± 5 dB. It is shown that it is possible to obtain agreement between measurement and prediction, after adjustments of wall acoustic impedance and loudspeaker frequency response.

Sound Absorption at Low Frequencies: Room Contents as Obstacles

“…assumed appropriate. From equation (1) it can be seen that the expression for the scattering cross-section of an obstacle with simple shape and uniform surface sound absorption coefficient is similar to expressions suggested by other researchers [9,10] namely the scattering cross-section of an object is simply equivalent to its total surface area divided by four. At low frequencies, the inclusion of furniture in a room may generate additional eigenmodes, mode shifts and selective damping of modes [11].…”

“…In this paper, a preliminary study of the influence of furniture on low frequency room responses was performed by introducing a "reference obstacle" at three positions within a numerically modelled enclosure. The results for the empty test room, presented in a previous paper [1] were used as a reference. In each case eigenfrequency shifts and selective modal damping were observed throughout the frequency range of interest, and the numerical results were able to take these effects into account, showing overall good agreement with measurements.…”

“…As described in a previous paper, a FE model of the empty reference room was successfully developed [1]. The model now was modified in order to include the reference obstacle.…”

“…In an earlier paper [1] measured and predicted frequency response functions were presented for empty rooms with rectangular and slightly non-rectangular shapes. Although most real rooms have (at least approximately) rectangular shapes, invariably they will be filled with various contents.…”

Sound Absorption at Low Frequencies: Room Contents as Absorbers

“…In previous papers an investigation was described of the influence of room surfaces and hard contents on the acoustic properties of enclosures at low frequencies [1,2]. However, hard-surfaced objects do not exclusively compose real furniture inside rooms.…”