The effective protection provided by arousal of the acoustic reflex against temporary threshold shift (TTS) at 4 kc from exposure to impulse noise was studied by comparing the rate of growth of TTS produced by 2‐min exposures to clicks of successively higher peak levels under two conditions: first, when the clicks were heard alone, and second, when each click was preceded at various intervals from 25 to 150 msec by a 1000‐cps 100‐dB pure tone presented to the contralateral ear. The results imply that the effective attenuation amounts to 1 dB at 25 msec, 5 dB at 62 msec, and 13 dB at 100 msec. Individual differences were large; some of the most slowly responding ears did not show significant protection until the delay reached 150 msec. The practical conclusion reached is that a reflex‐arousal stimulus to be used to protect gun crews should precede firing by 150 msec or more. In regard to theory, the results support the hypothesis that the value of the acoustic impedance of an ear at any instant is an accurate indicator of the protection provided by the middle ear muscles. However, there is also evidence that part of the protection may stem from a change, with reflex arousal, in the peak‐limiting characteristics of the middle ear.