The influences of environmental conditions on source localisation using a single vertical array and their exploitation through ground effect inversion

The performance of microphone arrays outdoors is influenced by the environmental conditions. Numerical simulations indicate that, while horizontal arrays are hardly affected, direction-of-arrival (DOA) estimation with vertical arrays becomes biased in presence of ground reflections and sound speed gradients. Turbulence leads to a huge variability in the estimates by reducing the ground effect. Ground effect can be exploited by combining classical source localization with an appropriate propagation model (ground effect inversion). Not only does this allow the source elevation and range to be determined with a single vertical array but also it allows separation of sources which can no longer be distinguished by far field localization methods. Furthermore, simulations provide detail of the achievable spatial resolution depending on frequency range, array size and localization algorithm and show a clear advantage of broadband processing. Outdoor measurements with one or two sources confirm the results of the numerical simulations

Virtual Pitch and Pitch Shifts in Church Bells

It is well established that musical sounds comprising multiple partials with frequencies approximately in the ratio of small integers give rise to a strong sensation of pitch even if the lowest or fundamental partial is missing—the so-called virtual pitch effect. Experiments on thirty test subjects demonstrate that this virtual pitch is shifted significantly by changes in the spacing of the constituent partials. The experiments measured pitch by comparison of sounds of similar timbre and were automated so that they could be performed remotely across the Internet. Analysis of the test sounds used shows that the pitch shifts are not predicted by Terhardt’s classic model of virtual pitch. The test sounds used were modelled on the sounds of church bells, but a further experiment on seventeen test subjects showed that changes in partial amplitude only had a minor effect on the pitch shifts observed, and that a pitch shift was still observed when two of the lowest frequency partials were removed, so that the effects reported are of general interest

Acoustic propagation over periodic and quasi-periodic rough surfaces

Transport noise is an ever present concern in urban areas affecting the quality of life for millions of people. The traditional noise barrier is not always a convenient method of noise control and can divide communities. Deliberate introduction of small scale (0.3 m high or less) periodic roughness on otherwise acoustically-hard ground has been investigated as a way of reducing noise near to a surface transport corridor. The roughness alters the effective surface impedance of the ground and thereby creates a 'soft' ground effect. Moreover the effectiveness if the rough surface is not reduced significantly if there are pathways through it. However the rough ground also creates surface waves that must be absorbed for the noise reduction to be effective. An alternative way of reducing surface waves may be to alter the periodicity. The effects of altering the periodicity of circular rods placed on a hard surface in the laboratory have been investigated. Predictions of multiple scattering theory and a boundary element code have been compared with the experimental data

Sound propagation through forests and tree belts

The potential use of forests or narrow belts of trees alongside surface transport corridors to reduce noise is often dismissed. This may be a consequence of conflicting experimental evidence and incomplete understanding of the various attenuation mechanisms involved. Important mechanisms include (a) destructive interference between sound travelling directly between source and receiver and reflected from the ‘acoustically-soft’ ground formed by decaying leaf litter, (b) the influence on this interference of loss of coherence due to the reverberant scattering by trunks and branches and (c) visco-thermal scattering by foliage. First the paper lists experimental evidence of significant attenuation due to forests and tree belts. Subsequently models for predicting the various contributions to overall attenuation are outlined. Predictions of the ‘soft’ ground effect are made using physically admissible ground impedance models. Incoherence due to trunk and branch scattering is modelled as enhanced turbulence. An empirical formula involving leaf area density and mean leaf size is used to predict foliage attenuation. Predictions that sum these contributions are compared with data. Regular or near-regular tree planting can cause ‘sonic crystal’ effects but the relatively sparse distributions of scatterers in realistic tree planting schemes means that the first band gap due to the periodic structure is weak. On the other hand the first pass band may be reduced and the second band gap can be enhanced by perturbing the tree locations with respect to periodic spacing. Finally results of numerical simulations showing the potential for traffic noise reduction by narrow tree belts are outlined

Meteorological effects on the noise shielding by low parallel wall structures

Numerical calculations, scale model experiments and real-life implementations have shown that the insertion of a closely spaced array of low parallel walls beside a road is potentially a valuable road traffic noise abatement technique. However, all previous studies have assumed a non-refracting and non-turbulent atmosphere. This study carries out a numerical assessment of the extent to which the noise reduction is preserved in the presence of wind gradients and turbulence. Several full-wave calculation techniques have been used to model the noise reduction provided by parallel walls subject to moderate and strong winds, and in a turbulent atmosphere. While meteorological effects do not deteriorate the insertion loss of the parallel wall array in the low frequency range, higher sound frequencies are strongly negatively affected. These numerical results are compared to the noise shielding of traditional highway noise walls with different heights including refraction

Ground characterization for JAPE

Above-ground propagation modelling at the JAPE (Joint Acoustic Propagation Experiment) site requires a reasonably accurate model for the acoustical properties of the ground. Various models for the JAPE site are offered based on theoretical fits to short range data and to longer range data obtained with random noise and pure tones respectively from a loudspeaker under approximately quiescent isothermal conditions

Partial frequencies and Chladni’s law in church bells

The rim partials of a church bell (those with an antinode at the soundbow) generate the strike pitch or perceived note of the bell. The spacing in frequency of the higher rim partials has an important effect on the tonal quality of the bell. Investigations into the partial frequencies of 2752 bells, both bronze and steel, of a wide variety of dates, founders and sizes, show a simple and unexpected relationship between the frequencies of the rim partials. This relationship explains why attempts to tune the higher rim partials independently have failed. A modified version of Chladni’s law provides insight into the musical relationship of the partials, and predicts the partials of individual bells well, but fails to give a simple model of the spacing between the partials seen in bells with different profiles

Correlating differences in the playing properties of five student model clarinets with physical differences between them

This paper reports work that is part of a larger project concerned with correlating differences in the perceived playing characteristics of musical wind instruments with physical differences between them. Here we focus on five different student model clarinets. Some of the practical difficulties of (i) directly measuring the bore profiles of the clarinets and (ii) measuring their input impedances are discussed. Results are presented which show significant differences in bore profile between the five instruments, leading to clear differences in the frequencies and magnitudes of their resonance peaks. In addition, some initial thoughts regarding playing tests designed to establish clarinetists’ perceptions of the instruments are considered

An investigation of the sound field above a surface with periodic roughness

When audio-frequency sound is incident near grazing on acoustically-hard surfaces with periodic sub-wavelength roughness air-borne acoustic surface waves are generated which could be used to amplify acoustic signals and, therefore, improve detection ranges of, perimeter security systems. Experimental and numerical studies using the Boundary Element Method (BEM) of the sound field generated over periodically-spaced rectangular strips also show several enhancements as a result of complex interactions between the sound field and the rough surface. Surface waves result in excess attenuation spectra with anomalous maxima greater than the 6.02 dB that would be expected from constructive interference above a smooth acoustically rigid surface. The enhancements are found to depend on the roughness spacing and can be attributed to effects due to the finite width and periodicity of the array, quarter-wavelength resonances in the gaps between elements and Bragg diffraction. Pressure maps of the total sound field over rough surfaces show the details of the sound field at the frequencies of interest. As well as being useful for amplifying frequencies arriving at a sensor array, detailed study of the enhancements provides understanding of the evolution of the sound field over rough surfaces

Deduction of static surface roughness from complex excess attenuation

Data for complex excess attenuation have been used to determine the effective surface admittance and hence characteristic roughness size of a surface comprising a random distribution of semicylindrical rods on an acoustically hard plane. The inversion for roughness size is based on a simplified boss model. The technique is shown to be effective to within 4%, up to a threshold roughness packing density of 32%, above which the interaction between scattering elements appears to exceed that allowed by the model