Another characteristic of ASHP exterior heat ex-changer is their need to stop the fan from time to time for a period of several minutes in order to get rid of frost that accumulates in the outdoor unit in the heating mode. After that, the heat pump begins to work again. This part of the work cycle brings about two sudden changes of the noise made by the fan. The acoustic effect of such disruption on neighbors is especially powerful in quiet environments where background nighttime noise may be as low as 0 to 10dBA. This is included in legislation in France. According to the French concept of noise nuisance, “noise emergence” is the difference between ambient noise including the disturbing noise, and ambient noise without the disturbing noise. The emergence should be below 3 dB in the evening (between 10.00 pm and 7.00 am), so the pump in France, once stopped after dark, theoretically should not be allowed to engage again until morning. Additionally, the level of ambient noise including the disturbing noise must be below 30 dB in France, a condition which definitely can not be met without an effective sound barrier by most ASHPs which were commercially available by the end of 2015.