Psychoacoustical Investigation of Sounds from Heat Pumps

Psychoacoustics

Heat pumps become increasingly popular as heating systems for houses. Purchasing a heat pump or replacing the conventional heating system in favor of a heat pump is mainly driven by a growing ecological sensibility, costs for energy and legal requirements.
The air delivering fan is an essential component in an air-to-air heat pump and simultaneously the main acoustic source. Typically, air-to-air heat pumps are placed outside of the house, and hence a source of annoyance for the neighborhood. According to the German regulations the maximum rating level is 55 dB(A) in residential areas by day and 40 dB(A) at night. Although this rating level already contains A-weighting and penalties for tonality and impulsiveness, the degree of annoyance of different heat pumps with equal rating levels is assessed as completely different. Humming for instance is a frequent reason for complains.
The aim of the current study is to evaluate subjective response from jury tests to objective metrics such as loudness (DIN 45631, ISO 532), sharpness (DIN 45692), tonality (DIN 45681, ISO 1996-2) and others.
Different sounds of heat pumps were evaluated by a jury of in total 40 participants. The selected sounds were adjusted to the same A-weighted overall sound pressure level to determine level independent influences as for example tonality and impulsiveness used for the rating level.
The test was based on seven-point adjective scales for sound description and assessment. In addition the test persons were interviewed concerning their sound preferences. Based on the jury test results the sound samples and the adjective scales were grouped. The results show no correlation to the rating level at all, e.g. no correlation to possible tonality or impulsiveness. Furthermore the location of the maximum spectral energy is important for heat pump sounds avoiding the frequency range that generates a sensation of "humming" as well as "whistling" or "hissing". In addition, the time structure of the sound plays an enormous role for the perceived pleasantness or to blank out the sounds which could not be represented by the impulsiveness used in the rating level. Eventually, a preliminary regression model for sound quality is presented. It is proven that - beyond the standard rating level - psychoacoustic parameters are suitable for sound quality assessment of heat pumps.