 This limit is a day–night 24-hr average noise level (LDN), with a 10-dBA penalty applied to nighttime levels between 2200 and 0700 hours to account for sleep disruption and no penalty applied to daytime levels.The 10-dB(A) penalty makes the permitted U.S. nighttime noise level equal to 45 dB(A), which is more than is accepted in some European countries but less than the noise produced by some heat pumps.Another feature of air source heat pumps (ASHPs) external heat exchangers 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.