The manufacturer of air-source heat pumps approached us to use our diagnostic skills and technology help them to reduce the noise from their units without recourse to expensive, efficiency-sapping enclosures and silencers. The objective was to design-in low-cost engineering noise control features to make the units inherently quieter - without compromising efficiency.
The diagnostic analysis showed that there were 4 major noise sources, all of which would need to be reduced in order to achieve the desired noise reduction. The sources identified were:-
Previous resources had been focused on reducing the overall dB(A). However, the low-frequency tones were the dominant noise complaint issue despite only contributing <0.5dB to the overall dB(A). These would not have been reduced by the conventional methods under consideration.
These involved the following:-
The combined effect was to reduce the 70Hz fan tone by 97% (15dB) and the overall noise by 90% (10dB(A)) without affecting the efficiency of the heat pump as demonstrated by the above analysis and the sound file.