Heat pump noise problems are common. They are often caused by a low-frequency tonal hum that is difficult and costly to attenuate using conventional noise control techniques. We have developed more effective alternatives based on elegant engineering source control measures.
Air-source and ground-source (geothermal) heat pumps are a common cause of tonal noise complaints, even when the typical costly noise control measures of barriers, acoustic enclosures and silencers have been installed. These are not only often ineffective at the problem low-frequencies, but they also tend to reduce system efficiency.
Far from reducing efficiency, these innovative, low-cost engineering noise control alternatives can increase heat pump performance...
Unfortunately, most heat pump installation noise specifications are based only on the overall broadband dB(A) (e.g. 42dB LAeq, 5mins in the UK) and do not include a penalty for the tonal content that is so common - and that is such a common cause of complaints. This is a problem that can be avoided as described below...
Heat pump noise reduction often fails as it must be based on evaluating the "character" of the noise (the typical heat pump hum) and not just the overall broadband dB(A) noise level. A simple diagnostic procedure must be followed to determine the optimum noise control techniques.
There are 3 distinct heat pump noise components, the relative contributions of which determine the optimum noise control approach. This critical diagnostic step is very often omitted - which leads to poor outcomes. These components are:-
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The principle requirement for the diagnostic process is to carry out a narrow-band frequency analysis. If this has not been done, then any recommended noise control measures have been based on guesswork. This is despite the fact that more than adequate tonal analysis is available via free smartphone apps, the vast majority of noise reports omit this element and are therefore inadequate (Environment Agency and Environmental Health surveys reveal that 95% of such noise reports are inadequate).
Sound file: typical heat pump low-frequency hum noise
This is a typical heat pump noise signature illustrating the low-frequency (LF) hum associated with these units (in this case at 112Hz and harmonics). The lowest frequency hum components are usually from the fans (at the blade pass frequency, number of blades x speed) caused by disturbed airflow through the impellers and often the cause of complaints. A very common, costly mistake is wasting time and money installing silencers and noise barriers that, due to the laws of physics, cannot attenuate the hum significantly. The typical "noise expert" approach involves measuring the overall dB(A) without diagnosing the fact that low-frequency tonal noise is the issue, not the overall dB(A). LF heat pump hum does not contribute significantly to the overall "A" weighted noise level. Not taking this simple fact into account is a common cause of very costly noise control mistakes.
Considering that diagnosis can be so simple using either a free smartphone spectrum analysis app or more sophisticated instrumentation, it is a depressing fact that it is so rarely done, leading to costly assumptions that silencers or barriers will be effective.
The only practical solution to cut low-frequency tonal heat pump fan noise (silencers are not effective) is to fit low-cost aerodynamic fan modifications. These are innovative retrofit flow modifying devices that minimise the hum-causing pressure fluctuations at source by >90%. This low-cost solution can also improve fan efficiency - by up to 23% on one data centre fan application - potentially making noise control projects self-financing.
If the fan produces substantial low-frequency tones (hum), then conventional sound attenuators will not be effective! The reason is down to simple physics. In order to provide significant attenuation at any frequency, the silencer must be of the order of at least one wavelength long (or have a high backpressure). At the 50Hz - 100Hz typical frequency range tones typically produced by air source heat pump fans, the wavelength of sound is around 3m to 7m, so very large mufflers would be needed...
However, if the fan is not tonal and is only generating broadband noise, then silencers can be an effective way to reduce the overall dB(A) noise level.
Tonal noise from compressors and pumps is caused by structure-borne vibration that it then radiated as sound. These sources have characteristic patterns of tones (called "toothcombs") at harmonics of rotation speed over a wide frequency range with particular peaks at the pumping frequency as shown here. Anyone can easily diagnose this characteristic noise issue using one of the many free spectrum analyser apps on a smartphone.
The most effective heat pump or compressor options to reduce the noise at source involve:-
Unlike enclosures or barriers, these low-cost engineering modifications have no effect on system efficiency.
Vibration damping: pump and compressor pipework vibration is transmitted into structural elements that then radiate the sound like loudspeakers. The simple solution is to use high-efficiency damping on the radiating surfaces to cut the vibration, either by replacing key panels or by retrofitting the damping in situ to the vibrating elements. https://sounddampedsteel.com/laminated-metal-how-does-it-work/.
Vibration isolation: it is difficult to use sound for detailed diagnosis as the source is tonal and generates standing waves. Consequently, vibration measurements are preferred to determine the transmission paths. However, with experience, it is often possible to determine these by inspection. Is the pump efficiently isolated with no vibration short-circuits and not connected to thin panels? What method has been used to connect high-vibration pipes to the frame?
If these components are the dominant sources, then these low-cost engineering source control options can reduce heat pump noise by up to c 15dB.
Assuming that the problem pump or compressor tones are not at the lowest frequencies, then noise barriers can be effective either in addition to or (if only modest attenuation is required), instead of noise control at source. In order to be effective, conventional barriers should be placed as close to the heat pump as practical - which tends to cut system efficiency.
In principle, the same factors apply to acoustic enclosures. Whilst they can provide high levels of higher frequency noise reduction, it can be a very costly balancing act to achieve the required attenuation without seriously compromising heat pump efficiency.
We use this simple process to determine the optimum noise control measures for each application. Unless this procedure is followed, any noise control measures will be based on guesswork - which has too often led to costly mistakes. You can apply this approach to any air-source or ground-source (geothermal) heat pump.
Alternatively, make use of our complete turnkey service, typically without a site visit (anywhere in the world) via remote control of noise. The initial steps (1, 2 and 3) are a free service that allows us to generate an accurate diagnosis and the costed optimum noise control options. This can then be followed by the provision of detailed recommendations that can be implemented using local contractors, minimising costs.