Environmental Noise

Noise from transportation sources is referred to as environmental noise. Exposure can cause annoyance and sleep disturbance; both of which impact on quality of life. In addition, annoyance and sleep disturbance can cause adverse health effects.

Research has been carried out into the effect of urban streets on the noise from low-flying aircraft [1]. The effect of a variety of street configurations on the transmission gain was investigated and analysed in terms of changes in street geometry. The relative Effective Perceived Noise Level (EPNL) was predicted which increases in proportion to the ratio of building height to flying altitude for specular reflections. However, changing the street width has only a minor effect. The effect of mixed specular and diffuse reflections was investigated using a commercial modelling package indicating that although the effect on the transmission gain was small, the scattering of sound energy out of the street resulted in a decrease in the relative EPNL with the ratio of building height to flying altitude resulting in a value of the relative EPNL which was negative for most street configurations. It was concluded that although simple specular models can predict the transmission gain with reasonable accuracy, more complex models involving the effects of diffuse reflections are required for noise units involving integration over time.

A simple model for predicting the sound reflected from building facades has been developed based upon the assumption that the scattering coefficient is small [2]. This model is then used as the basis of an experimental attempt to measure the scattering properties of scale model facades featuring a similar degree of surface irregularity to that found on real buildings. Scale model measurements were used to examine the effect of a non-uniform distribution of facade scattering. The measured value of the scattering coefficient is found to be small and not very sensitive to the degree of surface irregularity. A progression of energy from a specular reflection field to a diffuse reflection field for successive orders of reflections is observed. It is suggested that the dominant mechanism of sound propagation for higher order reflections is via random scattering and that the development of propagation models based upon purely random scattering are valid.

Investigations have been carried out into sound fields that exist near building facades with sources near the ground [3]. The sound field in front of, and close to a building facade is relevant to the measurement and prediction of environmental noise (e.g. road traffic, railway traffic and industrial noise) and facade sound insulation. For simplicity it is often assumed that the facade can be treated as a semi-infinite reflector, however in the low-frequency range this is no longer appropriate as the wavelengths are similar or larger than the facade dimensions. Scale model measurements and predictions using integral equation methods have been used to investigate the effect of diffraction on the sound field in front of finite size reflectors. For building facades, the results indicate that diffraction effects are only likely to be significant in the low-frequency range (50Hz to 200Hz) when facade dimensions are less than 5m assuming a point source close to the ground and microphones at a height of 1.2m or 1.5m, at a distance between 1m and 2m in front of the facade. 

Effects of building façade and balcony design on the reduction of exterior noise have been investigated by measuring the noise from traffic at an apartment complex located by a road side as well as the sound field characteristics of an area surrounded by four apartment buildings in Korea [4]. The efficiency of different balcony forms for reducing exterior noise was determined using a 1:50 scale model and a single spark source. It was found that parapets were more effective in reducing exterior noise than lintels. Based on the measurements of the parapet used for this study and the absorptive materials in the scale model, a maximum noise reduction of 23dB was obtained. Computer simulations were conducted to predict the noise reduction level of lintels and parapets. The results were compared to the scale model test and indicate that this method of exterior noise reduction can be useful in high-rise buildings where tall barriers cannot be built.

Due to the importance of measuring maximum sound pressure levels in environmental noise, research has been carried out to investigate the effects of signal processing with different sound level meters [5]. This has investigated Fast and Slow time-weighting detectors when combined with an A-weighting filter. Measurements were used to quantify the variation in measured maximum levels using tone bursts, half-sine pulses, ramped noise and recorded transients. Tone bursts indicate that Slow time-weighting is inappropriate for maximum level measurements due to the large bias error. In general there is more variation between sound level meters when considering Fast time-weighted maximum levels in octave bands or one-third octave bands than with A-weighted levels. However, to reduce the variation between measurements with different sound level meters, it is proposed that limits could be prescribed on the phase response for CPB filters and A-weighting filters.

A parametric study [6] was performed in order to investigate the influence of height-to-width ratio (H/W) on sound fields in urban streets in terms of sound pressure level (SPL), reverberation time (T30), and early decay time (EDT). A computer simulation technique based on a hybrid method combining ray tracing and image source modeling was adopted, and an omnidirectional sound source was used. It was found that the variation of SPL was affected by the H/W ratio only for narrow urban streets, and the T30 and EDT increased with an increase in the H/W ratio for both narrow and wide streets. It was also observed that the T30 decreased with increasing scattering coefficients, and the EDT significantly increased as scattering coefficients increased for H/W of 3 and 6.

Selected publications

[1] Ismail MR and Oldham DJ (2002) The effect of the urban street canyon on the noise from low flying aircraft. Building Acoustics vol 9 pp 233-251.

[2] Ismail MR and Oldham DJ (2005) A scale model investigation of sound reflection from building facades. Applied Acoustics vol 66 issue 2 pp 123-147.

[3] Hopkins C and Lam Y (2009) Sound fields near building facades - comparison of finite and semi-infinite reflectors on a rigid ground plane. Applied Acoustics vol 70 issue 2 pp 300-308.

[4] Lee PJ, Kim YH, Jeon JY, Song KD (2007) Effect of apartment building façade and balcony design on the reduction of exterior noise. Building and Environment vol 42 pp 3517-3528.

[5] Robinson M and Hopkins C (2014) Effects of signal processing on the measurement of maximum sound pressure levels. Applied Acoustics vol 77 issue C pp 11-19.

[6] Lee PJ, Kang J (2015) Effect of height-to-width ratio on the sound propagation in urban streets, Acta Acustica united with Acustica vol 101 pp 73-87.