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.
Heat pump noise diagnosis
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:-
- fan: tones at 70Hz + harmonic - cannot be reduced using silencers or barriers
- mains hum: 100Hz tone
- compressor: several tones
- air noise: broadband sound
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.
Noise-reducing modifications
These involved the following:-
- fan tones: aerodynamic modifications
- mains hum + compressor: high-performance damping of key components + local tuned acoustic absorbent
- broadband air noise: once the low-frequency components had been cut, a simple small carefully designed screen was very effective
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.
View a more detailed guide to heat pump noise reduction covering both air-source and ground-source noise control.
