The detectability of clicks perceived at onset or offset of a brief sinusoidal signal is measured as a function of the Sensation Level (SL) of the tone ranging from 30 to 70 dB SL. The tone frequencies are 400 and 2000 Hz, while the two types of amplitude envelopes (linear and exponential) are employed. The results show that the critical rise or decay time (tc_1, tc_2) required to achieve clickless signals depend on the level, the onset of offset slope and the signal frequency, while does not depend on the initial phase of the tone. As the SL of the tone is decreased, it is necessary to decrease the onset or offset slope in order to achieve clickless signals. The critical decay time is about 1/3 times as small as the critical rise time. This means that the transient clicks are less audible at offset than at onset of the tone. It is shown that the average data can be well fitted with the equation (I/I_0)・tc^a_jj=constant, where I and I_0 are the tone intensity and the threshold intensity respectively, and a _j is a consonant depending on the signal frequency, while tc_j is the critical rise (j=1) or the decay (j=2) time. A running average model is shown to be effective to account for the above results.
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