Chris Bibby

Acoustical and Airflow Performance of Interior Natural Ventilation Openings and Silencers - Optimisation and Application

Abstract There is a need for a greater understanding of the acoustical and airflow performance of interior openings, and of silencers implemented to improve their acoustical performance, in naturally ventilated buildings. This paper discusses preliminary, fundamental aspects of a study done to provide engineers and architects with optimal design techniques. It discusses the characterization of ventilator performance, including the definition of the open area ratio - a combined acoustical and airflow performance-optimisation metric. This is calculated from conventional standardized metrics, takes a value of unity for a simple opening, and increases with increasing overall ventilator performance. Theory describing the open area ratio for ventilator characterization, and its limitations, are reviewed. Finally, to illustrate its benefits and capabilities, the proposed method is applied to the analysis of existing ventilator experimental results.

Optimization of silencers for interior natural ventilation openings.

Naturally ventilated buildings are being constructed around the world due to advantages they offer in reduced HVAC operating costs, increased indoor air quality, and government subsidies for “green” construction and certification. Unfortunately, natural ventilation is often associated with a decrease in occupant acoustical satisfaction. One reason for this is the requirement to create acceptable air exchange rates in naturally ventilated buildings that have very limited pressure differentials; airflow paths must offer very low resistance. This leads designers to create paths by connecting adjacent spaces with apertures in the partitions or to use short transfer ducts rather than conventional duct systems. These apertures can be detrimental to the noise isolation provided by the partition. Products exist to attenuate noise through these apertures, but little exists in the literature about their performance. In this work, current methods and new methods of passively silencing interior natural ventilation op...

Applicability of flow-rate-independent discharge coefficients in purpose-provided, interior natural-ventilation openings

ABSTRACT Natural ventilation involves low pressures, necessitating large interior ventilation openings (ventilators) with low airflow resistance in interior partitions, allowing required ventilation airflow. These ventilators are detrimental to the noise isolation between spaces. Design methods often use flow-rate-independent discharge coefficients, which rely on a high-Reynolds-number (Re) assumption. This paper evaluates this assumption for purpose-provided ventilators, based on theory, field measurement data and prediction. If a high-Re discharge coefficient is used to predict airflow at low Re, the flow rate will be over-predicted and the system under-designed. Reynolds numbers in some existing interior ventilators are low enough that flow rates are inaccurately described by a high-Re discharge coefficient. If the ventilator is highly restrictive to flow or if the hydraulic diameter of the flow path is very small, low-Re behaviour may be critical to system design.

Measurement and improvement of the diffuse‐reflection coefficients of profiled‐wood surfaces.

An apparatus and procedure were developed for measuring the diffuse‐reflection coefficients of surfaces in an anechoic chamber according to the ISO‐17497 method. This involves determining the proportion of incoherent energy in the impulse response measured in the presence of the surface. Surface absorption was measured by the reverberation‐room method. The sound‐absorption and sound‐diffusion properties of two existing profiled‐wood architectural panels, one with sinusoidal corrugations, the other comprising rectangular sections of different heights, were characterized. Investigations into how to improve the absorption and diffusion characteristics were made, and prototype surfaces tested and evaluated.