Acoustical design of water features and their use for road traffic noise masking

Abstract

This thesis examines the physical and perceptual properties of water sounds generated by small to medium sized water features, and their use for road traffic noise masking. A wide range of design factors have been tested in the laboratory for waterfalls, cascades, fountains and jets which can typically be found in open spaces such as gardens and parks. A number of field tests were also carried out to illustrate the variability of water sounds. The results obtained indicated that estimations can be made on how design factors affect sound pressure levels, frequency content and psychoacoustic properties. Key design factor findings include the higher sounds pressure levels obtained when distributing the same amount of water over several streams rather than over one uniform stream (+2-3 dB), the increase in the overall sound pressure level at high flow rates with increasing waterfall’s width (+2-3 dB), and the significant increases in sound pressure level with increasing height of falling water (+5-10 dB). Impact materials greatly affected acoustical and psychoacoustical properties, results showing however that changes in sound pressure level and spectra become less and less significant with increasing height and flow rate. Overall, water produced more mid and low frequencies (+5-10 dB compared to hard materials in the range 250 Hz – 2 kHz), whilst hard materials tended to increase the high frequency content of approximately 5 dB. Comparisons with road traffic noise showed that there is a mismatch between the frequency responses of traffic noise and water sounds, with the exception of waterfalls with large flow rates which can generate low frequency levels comparable to traffic noise. Auditory tests were carried out to assess water sound preferences in the presence of road traffic noise. These were undertaken in the context of peacefulness and relaxations within gardens or balconies where motorway noise was audible. Results showed that water sounds should be similar or not less than 3 dB below the road traffic noise level, and that stream sounds tend to be preferred to fountain sounds, which are in turn preferred to waterfall sounds. Analysis made on groups of sounds also indicated that low sharpness and large temporal variations were preferred on average, although no acoustical or psychoacoustical parameter correlated well with the individual sound preferences.