Speaker
Description
At low temperatures, air-source heat pumps (ASHPs) regularly perform a so-called defrosting cycle where the compressor of the ASHP is switched off, the circuit reversed, and the compressor switched on to defrost the heat exchanger. After a brief period the circuit is reversed back and the ASHP returns to regular operation. During this cycle the characteristics of the noise produced by the ASHP may change a number of times, e.g., when the compressor is switched off or on. The main interest of our work was to investigate how these different types of noise disturb people. For this, a recording of an ASHP in a climate chamber during a defrosting cycle were used. Six partial and shortened defrosting cycles with relatively stationary sections in between were concatenated to a single continuous recording of 27 minutes duration. Three overall sound pressure level settings ranging over 20 dB were used to scale the various sections. 20 listeners performed a low-level recreational task, i.e. coloring a Mandala, while listening to the continuous ASHP recording and to report whenever they felt disturbed by the noise. Pooled overall responses normalized by the duration of the state or event showed increased disturbance during phases in which the sound changes, in particular the restart of regular operation and the first valve switch. The relation between normalized responses and (psycho)acoustic quantities such as loudness varied also with the ASHP state suggesting that defrosting cycles may need to be considered separately when assessing the annoyance of ASHPs.