David Oldham

Noise control strategies for naturally ventilated buildings

Natural ventilation openings offer little resistance to noise passage as the introduction of conventional passive noise control treatments will generally incur a large flow constraint. This paper reviews various noise control techniques and assesses the variation of airflow characteristics with sound insulation to road traffic noise of a facade incorporating ventilation openings. It is suggested that reactive and active techniques can possibly supplement conventional passive attenuation especially in the limiting low frequency region while adding minimal flow constraint to the ventilation path. Using such hybrid systems useful broad band sound reduction may be a real possibility for natural ventilation openings.

Measurement of the sound transmission loss of circular and slit-shaped apertures in rigid walls of finite thickness by intensimetry

A series of experiments was conducted to determine the transmission loss of circular and slit-shaped apertures. An effective measurement method, sound intensimetry, was employed in the investigation. The measured transmission loss was in very good agreement with existing approximate theories over their accepted ranges of validity. However, the effect of viscosity in small apertures was found to be significant and to vary systematically with the dimensions of the apertures.

Modelling the acoustical and airflow performance of simple lined ventilation apertures

The pressure differences that can be used to drive a natural ventilation system are very small and thus large apertures are required to allow sufficient air to enter and leave a building to ensure good air quality or thermal comfort. Large apertures are potential acoustic weak points on a façade and may require some form of acoustic treatment such as absorbent linings, in which case the ventilator is similar to a short section of lined duct. In ducts, the performance of absorbent linings increases with the length of lining and the ratio of the length of lined perimeter to the cross sectional area of the duct. Thus, for a duct of a given cross sectional area, a lining is more effective for a duct with a high aspect ratio than for a duct with a square cross section. However, the high aspect ratio cross section will result in greater flow resistance and impede the airflow performance. In this paper numerical methods are employed to investigate the effect of different configurations of a lined aperture on the acoustical and ventilation performance of the aperture in order to establish the optimum configurations.

Computer Modelling of Urban Noise Propagation

The nature of noise propagation through the urban fabric is not well understood and this reflects the difficulty in obtaining good quality data. The most promising approach to developing urban noise prediction models is computer simulation. Reflections from building facades are a mixture of specular and diffuse. However, as the order of reflection increases the relative fraction of energy reflected purely specularly will decrease and the reflection characteristics will resemble those of the pure diffuse case. It is proposed that an appropriate approach to the development of propagation models could be based upon specular reflections for the lower orders of reflections and diffuse reflections for higher orders of reflection. An alternative statement of the above is that the specular approach would be most appropriate for modelling near field propagation whilst the diffuse approach might be more appropriate for modelling mid and far field propagation. Two approaches to the development of diffuse prediction models have been identified. The radiosity method would allow the investigation of the effect of the disposition of individual buildings on noise propagation but would be computationally demanding if applied to large scale studies. Conversely, an approach based upon the classical diffusion equation is potentially capable of being applied to large scale studies but will not yield any local detail. It is suggested that both approaches might be suitable for incorporating into a comprehensive model for far field urban noise propagation.

A computer model for predicting the daylight performance of complex parallel shading systems

Abstract In tropical areas where the exclusion of solar heat from buildings is a paramount consideration, shading systems consisting of adjustable parallel multiple slats have evolved which exclude direct solar radiation whilst permitting some daylight penetration. The modelling of daylight illuminance from these systems is not easy using conventional graphical or computational quantitative techniques. This paper sets out a computer-based model for the analysis of the general case of parallel shading systems. The model is illustrated by reference to the Rowshan shading system used in the Hedjazi architecture of Saudi Arabia.

The Effect of the Urban Street Canyon on the Noise from Low Flying Aircraft

The effect of urban streets on the noise from a low-flying aircraft has been investigated. The study first involved the development of a prediction method based on geometrical acoustics with the assumption of specular reflections. 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 was also predicted. The predictions show that for specular reflections the relative Effective Perceived Noise Level increases in proportion to the ratio of building height to flying altitude, however, changing the street width has only a minor effect. The effect of mixed specular and diffuse reflections was investigated using a commercial acoustical modelling package and it was shown that although the effect on the transmission gain was small, the scattering of sound energy out of the street channel resulted in a decrease in the value of the relative Effective Perceived Noise Level with the ratio of building height to flying altitude resulting in a value of the relative Effective Perceived Noise Level which was negative for most street configurations examined. It can be 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 rapid technique to determine the internal area function of finite-length ducts using maximum length sequence analysis

This paper describes a rapid technique for reconstruction of the internal area function of a duct using blockage-induced eigenvalue shifts determined from eigenfrequencies measured under two sets of duct termination boundary conditions. A single broad band maximum length sequence (MLS) measurement of short duration is utilized to obtain the transfer function of the duct, which in turn can be utilized to determine its eigenvalue shifts and subsequently its internal area function using an inverse perturbation technique. The reconstruction results display the same order of accuracy as those obtained previously using swept sine measurements of extended duration. An expression for the determination of the area function is presented utilizing resonant frequency information alone, thus rendering duct length determination unnecessary. A computational routine further simplifies the process such that the accuracy of the technique could be ascertained for a range of configurations including longer ducts and ducts that initially have nonuniform internal cross section over their length. Development of a relationship between obstacle length and wavelength of the lowest eigenfrequency required for successful reconstruction is also described. This is an important result for longer ducts where measurement of lower eigenfrequencies may present problems using standard measurement equipment.

The Absorption Characteristics of Muslim Worshippers

The Masjid (Mosque) is the most important architectural space in the religious life of the Muslim. However, although considerable effort has been spent on the investigation of the absorption coefficient of an audience in many other situations, the absorption characteristics of people performing prayers according to the Islamic rules have never been measured. As this information is essential for the design of the acoustics of Masjids, this work describes an investigation of the absorption per person, Aobj, for the worshippers performing prayer according to Islamic rules. The absorption characteristics of Muslim worshippers have been investigated in the reverberation chamber of the Acoustic Research Unit, University of Liverpool according to BS- EN-ISO 354:2003. The four positions of the group prayer according to the Islamic rules (standing, bowing, prostration and sitting in rows on carpet) were simulated with thirteen male persons in the reverberation chamber in addition to a fifth position where the audience were sitting down in a random way on carpet to simulate the position of listening to the Friday sermon. Results were calculated for the sound absorption per person and compared with other reported values of audience absorption. Results showed that at the mid and high frequencies the maximum absorption occurs for the standing position followed by the bowing position, the random sitting position, the sitting in rows position and finally the prostrate position which exhibited the minimum values.

An acoustic window for sustainable buildings

Encouraging the use of natural ventilation is an important tendency in the green building movement, but opening windows can often cause noise problems. This research develops a window system which allows natural ventilation while reducing noise transmission. The core idea is to create a ventilation path by staggering two layers of glass and using micro‐perforated absorbers (MPA) along the path created to reduce noise. The MPA are made from transparent materials so that daylighting is relatively unaffected. Starting with a brief introduction of the MPA theory and its application in ducts, the paper presents a series of numerical simulations using finite element method based software FEMLAB, and experiment results measured between a semi‐anechoic chamber and a reverberation chamber. Performance in acoustics, ventilation and daylighting are all taken into account. A basic window configuration is first considered, studying the effectiveness of various window parameters. A number of strategic designs are then ...

The Use of Virtual Reflectors to Improve Speech Intelligibility in Open Stage Auditoria

A potential problem encountered with open stage auditoria is poor speech intelligibility arising from the directional nature of human speech sources. The conventional technique of employing large overhead reflecting surfaces to ensure a large early energy fraction to improve intelligibility is shown to be unsuitable in this situation. The use of Virtual Reflectors consisting of microphones and loudspeakers symmetrically located relative to the stage is proposed as a possible solution to the problems of open stage auditoria. It is suggested that the Speech Transmission Index is a suitable parameter for assessing the performance of Virtual Reflectors when in use and that computer modelling could provide a suitable method for investigating the action of Virtual Reflectors.