Optimisation and implementation of a hybrid passive silencer for improved sound quality

The noise from domestic products can cause significant annoyance to the user. As this is a subjective experience, the quality of the sound is as important as the overall level. A listening test of the noise from the Dyson Air Multiplier fans showed the loudness and tonality to be the key factors influencing the subjective quality. In order to improve the sound quality of the product, a hybrid silencer has been implemented consisting of porous media combined with a Helmholtz resonator which target the loudness and tonality respectively. The silencer was placed at the inlet of the product with a single resonator tuned to the blade passing frequency of the compressor at the maximum speed setting. There was a limitation on the volume available for the silencer as a larger silencer would have disturbed the flow path into the compressor which can cause an increase in noise. The silencer volume had to be divided between the resonator and the porous media. Various configurations were tested experimentally for insertion loss and the resulting product noise. From these results, a preference index based on the weighted sum of the psychoacoustic loudness and tonality can be calculated. The optimal division of the available volume between porous media and resonator can be determined by using the preference index as a cost function. The work demonstrates that using sound quality as a performance metric can have a significant impact on the design and optimisation of silencers.