Earlier experimental studies showed that during exposure to energy equivalent continuous noise, a reduction in exposure time along with the associated increase in the noise level leads to a significant reduction in “physiological costs” in terms of TTS₂, t(0 dB), and IRTTS values. For exposure to impulse noise, however, a significant increase of physiological responses occurred when the impulse duration was shortened and the number of impulses was increased accordingly. However, the kind of consequences of a reduction of the time interval between noise impulses to approximately 1 s (so that impulse salvos would result) are not clear since an effect of the stapedius reflex can be expected. Another unanswered question concerns the particular impact of a reduction in the impulse duration on the experienced strain when the number of impulses remains constant.

In order to examine these issues, 10 test subjects (Ss) participated in 3 Test Series (TS) which were carried out in a “cross-over” test design. The 8 male and 2 female Ss ranged in age from 20 to 48 years (28.7 ± 9.5 years). The hearing of all 10 individuals satisfied quality criteria according to DIN ISO 4869 at all frequencies. In TS I, the Ss were exposed to the reference exposure of 94 dB(A) for 1 h which is equivalent to a rating level L_Ard of 85 dB(A) for 8 h. In TS II, the exposure consisted of energy equivalent 9,000 short-term impulses with a duration of 5 ms and an exposure level of 113 dB(A). In contrast to an earlier study, the time interval between the impulses was reduced from 3 s to 1 s. In TS III, the Ss were exposed to 9,000 impulses with a duration of only 2.5 ms. Using the same exposure level and time interval between impulses as in TS II, the noise dose is reduced by half in terms of energy (L_Ard value of 82 dB(A)).

The results of this study show that the reduction in the time interval between the impulses from 3 s to 1 s leads to significantly reduced threshold shifts. These reductions cannot be explained by inter-individual differences since different Ss were used relative to previous studies. Thus, these differences must be caused by the stapedius reflex. While no impact of the stapedius reflex on the threshold shift could be shown with 3-s time intervals between noise impulses, some of the positive, level-reducing effect of this reflex seems to be present with 1-s time intervals between the impulses. While the halving of the impulse duration led to reduced “physiological costs,” the reduction was not statistically significant. This suggests that the parameter “number of impulses” has a stronger impact on the hearing's physiological reactions than the impulse duration.