Simple and induced loudness adaptation.

Simple loudness adaptation is the decrease in loudness that takes place when a continuous sound is presented alone for a period of time. Simple adaptation normally occurs only when a sound is soft to begin with, no more than 30 dB above threshold; except for some persons with a retrocochlear lesion, sounds above 30 dB SL do not diminish in loudness over time. However, adaptation can be induced in at least two ways: (1) A steady sound to one ear, presented together with an intermittent sound to the contralateral ear, decreases in loudness by 50-60% within 3 min. (2) An otherwise steady sound that is intermittently increased in level by at least 5 dB becomes softer during its weaker periods. When, for example, a 40-dB tone is increased every 20 s to 60 dB for 15 s, its loudness decreases by about 50% within 3 min. We report measurements of both simple and induced adaptation on 10 persons listening to a 1 000-Hz tone via earphones or from a loudspeaker. The results provide an overview of both types of adaptation. They also permitted a correlational analysis that reveals some of the similarities and differences between the two kinds of adaptation.