Innovative heat pump noise control techniques
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...
Inadequate heat pump installation noise specifications...
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...
...and the noise impact consequences
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.
What are the heat pump noise features?
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:-
- mid-high frequency broadband noise control: usually from air-source fans. Whilst enclosures, silencers and barriers can reduce this noise component, they can also reduce heat pump efficiency (geometry dependent)
- heat pump hum: low-frequency tonal noise travels, passing through windows with little attenuation, causing noise complaints. Silencers and barriers are not effective in attenuating this type of noise. Tonal hum can be caused by fans (out-of-balance or blade pass-related frequencies), pump-related frequencies or electromagnetic excitation (multiples of mains hum)
- heat pump vibration: installing heat pumps on buildings causes vibration transmission into the structure that can then radiated as low-frequency structure-borne noise. This noise can be heard inside or outside the building or both. In the latter case, for example, a flat roof can act as a large loudspeaker. As above, traditional attenuators and acoustic enclosures etc are not effective noise-reduction options
Heat pump noise control examples
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Heat pump noise diagnosis - the key to effective mitigation
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
How to reduce air-source heat pump noise - controlling fan noise
How to reduce the fan noise at source
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.
How to reduce heat pump fan noise using silencers
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.
Heat pump and compressor noise control (hum or whine)
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.
Reducing pump/compressor noise at source
The most effective heat pump or compressor options to reduce the noise at source involve:-
- vibration damping
- vibration isolation
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.
Attenuating heat pump noise using acoustic barriers or enclosures
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.
- Note: we have developed an innovative barrier system that provides high attenuation without affecting efficiency. Just contact us (below) with noise and geometric details and we can assess if this system would be applicable to your project.
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.
Heat pump noise reduction made simple: the process
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.
- Noise problem analysis: use a smartphone spectrum analysis app (or specialist instrumentation) to check tonal content - or email smartphone recordings to us for free analysis
- Source diagnosis: match the tones to heat pump elements as above and assess the key frequency range over which attenuation must be achieved
- Select the noise source control measures: choose the optimum noise reduction options as described above for the dominant source(s)
- Noise mitigation check: reanalyse both the tonal frequency components and the overall noise level
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.
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.
Why was this air source heat pump so noisy?
A newly installed air-source heat pump domestic central heating system immediately generated noise complaints abut low-frequency sound from a neighbour. The owner contacted us and subsequently provided the requested smartphone video clips (as per the recommended diagnostic proceedure) of the noise:
- at the neighbour,
- close to the units
- inside the adjacent garage
This allowed us to rule out structure-borne noise (a common noise issue for A/C and heat pump installations) and to diagnose that the cause of the noise problem was primarily sound reflections that created an amplified 93Hz pumping frequency due to standing waves.
How was the heat pump noise reduced?
This precise diagnosis coupled with the photos provided, allowed us to devise a simple solution involving low-cost components available in DIY stores (plywood and loft insulation). Once installed, these noise control measures broke up the standing waves and reduced the tone at the neighbour by about 30dB (as shown in this noise signature), eliminating the problem.
This was a fast, low-cost project completed remotely (without any site visits) via the supplied smartphone data.
We have provided a more detailed guide to heat pump noise reduction covering both air-source and ground-source noise control.
