Fergus R. Fricke

Determination of blocking locations and cross‐sectional area in a duct by eigenfrequency shifts

A theoretical determination of the location and size of blockages in the one‐dimensional duct has been carried out. It is found that the blockage cross‐section area function can be calculated from the measured eigenfrequencies obtained using two boundary conditions, i.e., closed–closed (or open–open) and closed–open ends. When the blockages are small, the area function of the blocked duct can be expressed as a one‐dimensional spatial Fourier transform, whose spatial frequencies are related to the eigenfrequency shifts caused by the blockages in the duct. The method developed can be applied to the detection of the multiple blockages. The experimental results show that the accuracy of the calculated blockage area function is dependent on the number of the eigenfrequencies used. The agreements between the calculated blockage area and the actual blockage are excellent when the half‐wavelength of the eigenfrequency used is greater than the length of the blockage.

Theory and applications of quarter-wave resonators: A prelude to their use for attenuating noise entering buildings through ventilation openings

Abstract This paper concerns the use of quarter-wave resonators to attenuate noise. Qualitative and quantitative derivations of the relationship between cavity length and wavelength of sound to be attenuated are presented. The mechanism by which resonators can be used to attenuate noise is developed in terms of the optimum scattering and absorption conditions at resonance. The aim is not to present detailed mathematical expressions for the design of quarter-wave resonators, rather to illustrate the design principles involved for practical applications and to demonstrate that the advantage of such resonators is their simplicity of design. Recent applications of quarter-wave resonators are described including their use to attenuate fan noise and in conjunction with conventional barriers to reduce transportation noise. A new application is proposed involving the use of such resonators outside building ventilation openings to attenuate noise entering buildings. Unlike other applications discussed, resonators of different lengths are used to achieve attenuation over a wide frequency range rather than at an isolated, discrete frequency.

Sound propagation through vegetation

Abstract The propagation of sound through a large number of scatterers (i.e., trees) is treated in a similar way to a classical diffusion problem. A general differential equation governing the sound intensity is derived which is valid under certain conditions, notably that the depth of the belt of vegetation is large, and absorption small. The predictions of this theory are compared with results derived from a small scale model study, and with some field measurements. They are also compared with published field data. The implications of some of the conclusions reached for the practical achievement of effective sound attenuation are pointed out. In general, it would appear that significant noise reductions may be achieved for a predominantly high frequency source if the existing ground cover is acoustically hard, or if there is no “ground effect” attenuation between source and receiver for some other reason. In other cases, the noise reduction will be much lower and may be negative.

The interference index and its prediction using a neural network analysis of wind-tunnel data

Abstract This paper reports on recent progress in the authors’ ongoing efforts to quantify the effects of shielding and interference between pairs of buildings located in proximity in a variety of geometric configurations and boundary-layer wind flows. Recent developments in numerical analytical techniques and expert systems have made neural network analysis available as a potentially useful tool in the investigation of this problem. Analysis using neural networks allows the quantification of variables over a continuous range of values, whereas results have previously been limited to the analysis of specific configurations which have been wind-tunnel tested or to the identification of qualitative trends. In this study, we have applied neural network methodology to wind-tunnel data obtained from a variety of sources which describe shielding and interference behavior between two buildings. The results are presented here in terms of a newly defined “Interference Index”. Once a neural network has been properly configured and trained, it can easily generate results for building configurations that have not been tested experimentally, based on the patterns it has derived from the available wind-tunnel data.

Sound attenuation in forests

Abstract Many measurements of sound attenuation rates in forests have been made but there is little in common in the measuring procedures used or the results obtained. Consequently there is a considerable divergence of opinion on the effectiveness of vegetation as a noise control measure. In this paper the factors controlling the transmission of sound through vegetation are examined and the attenuation rates achieved in pine plantations are presented.

Pressure fluctuations in separated flows

Abstract This paper presents measurements of wall pressure spectra and r.m.s. levels in subsonic separated flows and it is shown that there is a relation between the r.m.s. pressures in various separated flow configurations. Using pressure correlation measurements, turbulence properties and a similarity analysis, a flow model is postulated which gives good estimates of the fluctuating wall pressures.

Sound propagation in a street

Abstract The propagation of sound in a street has previously been treated almost exclusively by the use of ray-tracing techniques. As a result, the effects of scattering from objects and protrusions in the street have not been taken adequately into account. In this paper, the sound field is analysed in terms of its propagating modes. The use of this technique allows the effect of scattering to be calculated, assuming a rather simplified model of a scattering surface. Although a number of important assumptions are made, the predicted attenuations at points down a street are in good agreement with results obtained from model studies. These attenuation curves are also similar in form to ihose obtained in full-scale tests.

The prediction of reverberation time using neural network analysis

Abstract An alternative method of predicting the reverberation time for enclosures, using artificial neural networks, has been investigated. The study begins with the hypothesis that, at the conceptual design stage, reverberation time predictions are too difficult to derive using existing computer models, and too inaccurate when using other methods and that a more explicit and quicker method of predicting reverberation time is required. Assessments are made of the predictive powers of trained neural networks by comparing the predicted reverberation times obtained using neural networks with those using Sabines and Eyrings ‘classical equations’ and the ray tracing model ODEON 2.6D. The results indicate that there is a good basis for using trained neural networks to predict the reverberation time for enclosures but that 15 input variables are required to achieve accuracy of ±10%. In addition, the results show that neural network analysis can identify those variables which have the greatest effect on the predicted reverberation times.

Duration of Perceived and Performed Sounds

This paper reports the results of a study on the perception and reproduction of sound durations. First, the duration discrimination abilities of musician and non-musician subjects were measured. An experiment was then carried out to find the effect of frequency difference on duration discrimination in pairs of pure tones (musical note durations). Finally, three experiments were undertaken to investigate any effect of frequency difference on the performed duration of tones. The major findings were: (i) that musicians show lower discrimination thresholds than non-musicians for short stimuli durations (25-100 msec), (ii) that higher frequencies are perceived as being longer in duration than lower frequencies, and (iii) that when the performed sound should be double or longer than a presented tone, subjects typically perform the sound longer than is appropriate.

An evaluation of the importance of surface diffusivity in concert halls

Abstract The nature of the relationship between the diffusion of sound and the acoustical quality of halls is investigated. The acoustical quality of 53 concert halls was determined using a questionnaire survey of visiting musicians. The ‘surface diffusivity’ of the interiors of the same halls was estimated subjectively. Through the several statistical analyses it was found that the correlation between the surface diffusivity and the acoustic quality of the halls was surprisingly high (r2 > 0.6).