Understanding Heat Pump Noise: Challenges and Solutions

Heat pumps have gained significant popularity as energy-efficient and eco-friendly solutions for heating and cooling homes. However, concerns about the noise generated by these systems are common among homeowners. As acoustical engineers, we see many misinterpretations related to heat pump noise. In this blog post, we will hopefully manage to resolve a few. We will delve deeper into the issues surrounding heat pump noise and explore the various factors that need to be considered when characterizing and addressing this problem.

The sound power almighty?

Traditionally, the sound power level has been the go-to metric for rating heat pump noise, much like any other machinery noise. It represents the total energy output of a sound source, considering all frequencies and directions of sound propagation. Sound power provides a valuable single-number baseline for comparing noise levels between different heat pumps. Sound power is a boiled-down metric that describes the noise emitting an object under test; however, it fails to provide any information related to the factors that influence human perception of that same noise. Arguably the most significant limitation of heat pump sound power level metric is its disregard for the impact of heat pump placement near boundaries such as walls, bushes, or fences. These physical structures can reflect, absorb, or diffract sound waves, altering the perceived loudness and quality of the noise at the listening position. Additionally, sound power level treats all frequencies equally, neglecting the varying ability of different frequencies to penetrate windows. This selective transmission renders sound power level ineffective in accurately assessing noise levels indoors and outdoors.

The same heat pump that reached 46 dB in the lab can be perceived as loud or quiet, depending on where it’s placed, what it’s surrounded with, and where we are listening.

There is a lot more to noise than just it’s loudness

The annoyance caused by a noise cannot be solely attributed to its loudness. Take the example of a mosquito buzz – it may have low sound power, yet it is considered one of the most annoying sounds. This annoyance is primarily due to the high  tonality of the sound, which measures the prominence of tones in relation to the overall spectrum (see bottom of the page for further explanation). By addressing tonality and sound quality, we can significantly decrease the perceived “annoyingness” of a noise without significantly affecting its overall energy (and conversely, the total sound power).

Innovative noise reduction solutions like Hummble focus on targeting specific lower frequency tones within a sound spectrum to reduce annoyance.

The way noise is regulated

Noise regulations typically rely on sound pressure level as a reference metric. These regulations often define permissible average noise levels during specific times of day. However, noise regulations can vary between countries and even within regions, making compliance challenging for manufacturers and installers. Furthermore, translating sound power level into sound pressure level at a specific location is a complex task. The measurement conditions in a laboratory setting may not accurately represent the noise behaviour of heat pumps once they are installed in the field. This discrepancy poses challenges for manufacturers in meeting noise requirements based solely on sound power level measurements in a laboratory setup.

Manufacturers may focus on optimizing noise levels under those defined conditions, creating a “lower number on a sticker” competition. This approach may overlook other conditions where the noise could be significantly louder, potentially compromising the system’s overall performance.

Conclusion

Characterizing heat pump noise involves acknowledging and overcoming various challenges. While sound power level offers a baseline for comparison, it fails to account for factors like placement and all frequency-dependent phenomena (i.e. transmission of sound through windows). Evaluating the tonality and sound quality provides a much more comprehensive understanding of the noise, by adding an additional layer of human perception on top of just the sound power. Compliance with noise regulations relies on sound pressure level, posing numerous challenges for manufacturers. We remain optimistic that as the industry progresses, efforts to harmonize noise-related matters will contribute to improved heat pump noise characterization and enhanced solutions. In conclusion, it is worth highlighting the remarkable advancements being made by the HPT TCP member consortia, particularly through Annex51 and the recently established Annex63. These international groups of researchers are playing a pivotal role in the field of heat pump noise, working diligently towards the harmonization and unification of noise-related issues in the heat pump industry. Their efforts signify significant progress and hold promise for a more standardized and improved approach to addressing noise concerns in heat pump systems.    

Tonality

• Tonality is a metric that identifies a strength of tones compared to the rest of the spectrum. It shows how much the tones stick out, compared to the broadband noise. Tones are the spikes we see on a frequency spectrum. Although the amount of energy that is encapsulated in those tones is very small, it is about how much they stick out that makes them significant. Here is an example that demonstrates gradual increase in tonality.