Mark R. Avis

Low frequency sound propagation in activated carbon

Activated carbon can adsorb and desorb gas molecules onto and off its surface. Research has examined whether this sorption affects low frequency sound waves, with pressures typical of audible sound, interacting with granular activated carbon. Impedance tube measurements were undertaken examining the resonant frequencies of Helmholtz resonators with different backing materials. It was found that the addition of activated carbon increased the compliance of the backing volume. The effect was observed up to the highest frequency measured (500 Hz), but was most significant at lower frequencies (at higher frequencies another phenomenon can explain the behavior). An apparatus was constructed to measure the effective porosity of the activated carbon as well as the number of moles adsorbed at sound pressures between 104 and 118 dB and low frequencies between 20 and 55 Hz. Whilst the results were consistent with adsorption affecting sound propagation, other phenomena cannot be ruled out. Measurements of sorption isotherms showed that additional energy losses can be caused by water vapor condensing onto and then evaporating from the surface of the material. However, the excess absorption measured for low frequency sound waves is primarily caused by decreases in surface reactance rather than changes in surface resistance.

Prediction of the Random-Incidence Scattering Coefficient Using a FDTD Scheme

This paper is focused on the evaluation of the scattering coefficient of sound diffusers which are based on the incoherency of diffusely reflected sound. A new approach for predicting the scattering coefficient is proposed; the method is based on a finite difference time domain (FDTD) scheme. Two established scattering coefficient measurement methods, proposed by Mommertz and Vorlander [1], are simulated; these correspond to measurements in a reverberant chamber, and free field. The results are also compared to those obtained in a previous paper [2], wherein it was demonstrated that FDTD schemes can be used to predict polar responses. These free field polar responses are used to find the correlation scattering coefficient, which in turn is used to validate the free field case. In modelling the reverberant chamber method, 2D simulations have been used to reduce computation time; hence it is necessary to derive a diffuse field formulation for a 2D reverberation chamber, which is presented. In this case, 1:5 scale experimental data is used for validation.

Adsorption in activated carbon and its effects on the low frequency performance of hearing defenders

Activated carbon displays interesting behaviours at low frequencies due to its large internal surface area and complex network of pores of various sizes and shapes. The material can produce larger than expected absorption and change the compliance of acoustic enclosures. This paper investigates the performance of hearing defenders which utilize activated carbon as the lining material of the cup. Compared to a standard foam liner, the introduction of activated carbon increases the insertion loss by up to 15 to 20 dB at frequencies between 31.5 and 250 Hz. The enhanced insertion loss is due to the increase in stiffness and apparent volume of the cup cavity as the activated carbon is introduced. This is probably due to the substantial change in local density as air molecules adsorb onto, and desorb from, the activated carbon pores during sound propagation. There is a change in entropy and energy loss from the sound wave during the adsorption/ desorption process due to the existence of a hysteresis loop. This...

On the analysis of the time spreading of sound diffusers

Since the invention of sound diffusers three decades ago a substantial effort has been made to predict the acoustic behaviour of these structures. BEM methods are well established for this purpose after a systematic comparison between simulations and experimental data. Volumetric methods such as finite element methods (FEM) or the finite difference time domain method (FDTD) are not often used, due to their large computational cost. However, near to far field transformations (NFFT) can overcome that problem. Recently some of the authors have shown that the FDTD method is a useful technique to analyse the time domain signature of sound diffusers. In this paper a careful analysis of the performance of diffusers in the time domain (“time spreading”) are reported, opening a new field of research.

Enhancing bass absorbers using activated carbon

Bass absorbers, either Helmholtz or membrane devices, are commonly used in small rooms to reduce the effects of standing wave modes. Previously, it has been shown that activated carbon can be used to change the compliance of loudspeaker enclosures. It has also been shown that activated carbon displays excess absorption at low frequencies. It has been suggested that the change in compliance and absorption is due to adsorption and desorption of air molecules on the surface of the carbon, caused by the compressions and rarefactions of sound waves. This paper investigates the extent of these effects to and their significance for resonant absorbers. The results show that activated carbon can lower the resonant frequency of a Helmholtz absorber, and also increases the quality factor of the resonance. The paper will discuss what the results mean in terms of the physical interaction between activated carbon and sound, as well as the practical application of the material to bass absorbers.

Nonlinear Finite Element Analysis of Moving Coil Loudspeaker Suspension

The surround and the spider of the loudspeaker suspension are modelled in ANSYS to carry out finite element analysis. The displacement dependent nonlinearities arising from the suspension are studied and the material and geometric effects leading to the nonlinearities are parameterised. The ANSYS models are simulated to be excited by a sinusoidal load and the results are evaluated by comparison with the results obtained by a physical model. The paper illustrates how practical models can be analysed using cost effective finite element models and also the extension of the models to experiment on various parameters, like changing the geometry for optimisation, by computer simulation.Copyright