APPENDIX S Production of Startle Noise

Equipment

The startle stimulus used in this study was a white noise generated by a 5837 integrated circuit. The signal from this chip was switched electronically and fed to two stereo high fidelity amplifiers that drove the headphones and speakers. Choice of amplifiers was based on the ability to amplify signal pulses quickly (slew rate) and the amount of power available for amplifying transients (dynamic headroom). The headphones were chosen because they could deliver high sound pressure levels, fit comfortably, and allowed the subject to hear the experimenter. Four high fidelity speaker units and two PA speakers were stacked directly behind the subject's seat, about six inches from her head. The sound field from this array was wide, rather than from a single point, minimizing the effects of room boundaries and head position on intensity.

Speakers were used in addition to the headphones because during pretesting, it was clear that they markedly increased the capacity of the noise to startle. This increase was out of proportion to the increase in actual sound pressure level which was marginal (see below). Subjectively, this effect was compelling, and subjects clearly appeared to have greater responses to the combination of speakers and headphones than to headphones alone. The sound seemed to have a more palpable form and was more noxious. Perhaps this effect was due to sound waves hitting the face or body rather than being confined to the place where the headphones rested. Also, the speakers provided heavier, felt low frequencies that may have resonated with body organs.

Measurement of Sound Pressure Level

Measurement of sound pressure levels (SPL) produced by the audio equipment used in this study was made to provide guidance for replication by other researchers. SPL is, of course, only one factor that contributes to the perceived loudness of a sound, and loudness is only one factor contributing to he capacity of a sound to startle. SPL was measured using a Bruel and Kaejr Precision Sound Level Meter Type 2203 and its associated 4134 microphone element. The "A" weighting, fast response scale was used. To measure the output from the headphones or headphone/speaker combination, a non-standard headphone coupler was used. This coupler was a plexiglass cylinder 1 1/2 inches in inner diameter and 7/8 inches deep fitting around the B & K's microphone which was 3/4 inches from the diaphram of the microphone. The microphone was placed in the approximate center of an imaginary subject's head sitting in the chair, 11 inches from the speaker's front panels. The microphone diaphram was perpendicular to the wavefront of the speakers's sound, but parallel to the headphone's sound. The headphones were placed on top of the coupler.

The background SPL before the subject received the startling noise was 49 dB. The continuous output of the headphones alone was 116 dB, and of the speakers alone (without coupler) was 119 dB. Together, headphones and speakers produced a continuous output of 121 dB. Balance of the speaker stack was assessed by moving the microphone (without coupler) to he centerline of each speaker stack which was 7 inches from the center of the two speaker stacks. Channel balance was within 1 dB. Measurements of headphone balance showed equal output from each cup. Combined SPL from the speaker/headphone combination was also balanced within 1 dB. The variability in SPL of 80 msec pulses was no greater than 1 dB as read from the B & K's "fast" measuring scale. Although the headphone coupler used to make the measurements above was not standard, an audio consultant measured headphone output alone using a standard B & K coupler to assure its balance and output. Comparison of the measurements above with these measurements indicated that the nonstandard measurements were as much as 4 dB lower than the standard measurements.