Figure 1. Frequency spectrum of the wide band noise (WBN) used for the exposure. The curve drawn with a thick line displays the frequency distribution of the WBN at level of 90 dBlin SPLrms measured with free field condenser microphone (B&K 4133). The curve drawn with a fine line display the frequency distribution of the WBN measured at the ear drum with an ER-7C tube microphone of an anaesthetised rat, laying in the bottom of the cage. The overall noise level was 9 dB higher when measured in the ear canal, and in the 6-12 kHz range even up to 15 dB higher than the measurements made with the free field microphone.