Observations of non-zero asymptotes for high annoyance at low sound exposure levels

Social survey data relating the prevalence of a high annoyance to transportation noise typically indicate a decrease in annoyance rate with decreasing exposure. At very low exposure levels, the expectation is that the percentage of highly annoyed respondents will asymptotically approach a value of zero. In many studies this has in fact been the case. In some studies, however, the lower asymptote does not appear to be zero, however, but rather some small, non-zero value. Such non-zero asymptotic values are sometimes nearly constant over an exposure range of as much as 10 decibels. In other words, the observed prevalence of noise-induced annoyance appears to be independent of sound level over an extended range, before beginning to depend on noise dose at higher levels. Several examples of this phenomenon are presented and discussed.