Low frequency noise in ventilation systems

Brandstätt, Peter (2002) Low frequency noise in ventilation systems. University of Southampton, Doctoral Thesis.

Abstract

The main acoustical problems of Heating, Ventilation and Air-Conditioning systems (HVAC) are found more and more in the low frequency range as silencers and other acoustic methods are generally relatively ineffective in this range. In addition, at low frequencies coupled resonances in the duct system may occur which are caused by, for example branches, connected rooms with their modal sound field or other terminations. These resonances can generate high noise levels at low frequencies leading to a situation where desirable levels cannot be met. In rooms with high acoustical requirements, such as for music production and reproduction, the excitation of single modes can lead to a distortion of the sound impression and timbre as well as to a reduce speech intelligibility or a noticeable disturbance even if, for example the required legal levels are met.

The goal of the work presented in this thesis is to provide a calculation scheme for complete HVAC-systems in the low frequency range starting at the fan and ending at the sound pressure level in rooms or at outlets facing and possibly disturbing the neighbourhood. The whole system is modelled using well established theory which makes it possible to determine resonances in the system and the effects on sound pressure levels in rooms. Measurements were performed and approximations derived for the determination of the acoustic properties of single HVAC components together with an investigation of the modal sound field in laboratory and office rooms. Calculation procedures for attenuation, sound pressure level, impedance and reflection coefficient were prepared employing 1-dimensional waveguide theory in matrix notation for the ducted system parts enabling a complete simulation and acoustic analysis of HVAC-systems. In particular, the detection of resonances, which are caused by the system at low frequencies, and the possibility to relate them to specific system components offers extended analysis capabilities.

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