Noise Control Engineering

Learning Outcomes

Noise control requirements: Motivation for noise control, EC directives on machinery noise (2006/42/EC) and outdoor equipment (2000/14/EC), specification of noise control targets. Units of noise measurement: Overview of decibels for sound pressure, intensity and power levels; combining sound pressures (incoherent and coherent); basic frequency analysis including one-third octave bands; A-weighting and other measures of sound. Characterization of noise sources: Physical nature of noise sources, idealizations; acoustical efficiency; frequency spectrum; parametric dependencies including operational speed; directivity; estimation of source sound power (including engines, fans etc). Summary of sound power measurement methods (ISO 3745, ISO 3744, ISO 3741, ISO 9614). Sound propagation outdoors and indoors: Point source and line source; geometric spreading; ground effects; meteorological effects; noise barriers; sound in rooms, reverberant field. Principles of passive noise control: Effect of multiple sources and multiple paths; noise path models; control at source; airborne transmission; structure-borne transmission. Sound radiation from vibrating structures (engineering approach) Definition of radiation ratio; radiation from monopole and dipole sources; radiation from bending waves in plates; corner modes, edge modes, coincidence; means of reducing radiation ratio. Transmission of airborne sound through partitions Transmission loss of a single partition, mathematical derivation for normal incidence; coincidence and the transmission loss for particular angles of incidence and for a diffuse field (qualitative); double partitions (qualitative); measurement methods for sound reduction index; machinery enclosures using Sabine formula. Sound absorbent materials and applications: Surface impedance and its relation to absorption coefficient; qualitative treatment of dissipation mechanisms; practical forms of sound absorber; measurement techniques for absorption (ISO10534, ISO354). Vibration control: Force and velocity excitation, blocked force and free velocity; vibration isolation - low and high frequency models; damping treatments; effects of damping; structural modification; vibration absorbers and neutralisers. Silencer design: Acoustic impedance; insertion loss; reactive silencers: side branches, expansion chambers; flow-generated noise; lined ducts, splitter attenuators; pressure drop; break-out noise.

This is a one-semester course, normally three lectures per week. Detailed lecture notes are provided and one-to-one assistance and verbal feedback is facilitated through tutorial classes relating to the assignment. Various practical demonstrations are included within the lectures. Blackboard is used to allow the lectures and additional material to be disseminated. Students are encouraged to read supporting texts and a booklist is provided.