Auditory Fatigue following Tone - Burst Trains at 2

Three hundred and two sailors were exposed by phones in groups of 11-20 men to 2.2 kilocycle/sec (kc) pulses (5 msec rise-fall time) of 37-250 msec pulse duration at 105-120 dB sound pressure level, duty cycle (ontime/total time) from 0.1—100%, and for 5-120 min/session. Permutations were selected to throw direct light on the four stimulus variables and on their interactions as well. Pulse-tone audiometry by magnetic tape was accomplished 7 times in 6 min at 2.5, 3, 4, 6, and 8 kc, before and immediately after exposure. The concept of a criterion temporary threshold shift (TTS) was developed in which one Kryt represents the area of ITS plotted on an audiometric chart from 2-8 kc. Any TTS can then be expressed in terms of the Kryt. Thus, for each of our subjects, for each of seven audiograms taken approximately once per minute, we have seven successive Kryt values. When plotted over recovery time, these values can be compared to the area enclosed on the graph representing one Kryt lasting for two min; this summation datum is termed the Nox. Mean Nox values for each experimental condition were determined. Within our data, (1) each 5 dB SPL adds 0.5 Nox; (2) pulse duration as such is negligible; (3) each log session duration adds 0.65 Nox; and (4) the Nox is a linear function of duty cycle. Subjective and, to some extent, objective observations lead us to set a damage risk criterion of two Nox for pulsed stimulation at 2.2 kc. Graphs are provided by which any combination of stimulus parameters which would yield two or more Nox may be predicted. AUDITORY FATIGUE FOLLOWING TONE-BURST TRAINS AT 2.2 KC Very many studies on auditory fatigue have explored certain parameters of the fatiguing stimulus. This has been accomplished to some extent for pure tones and for noises of various bandwidth, both continuous and intermittent. Unfortunately, the shorter tonebursts, and the smaller duty cycles (ontime/total time) have not been well sampled. This laboratory covered these conditions (Harris, 1959) for a stimulus of 3.5 kilocycles (kc), but confined attention to the temporary threshold shift (TTS) (always in decibels (dB)) at 4 kc. Some preliminary observations led us to the unhappy conclusion that predictions of TTS from a 2.2 kc stimulus could not be made as precise as desirable from the earlier data at 3.5 kc. It was therefore decided to complete observations at 2.2 kc, using a variety of parametric combinations, checking for the effect of each parameter separately and in interaction. The parameters of interest include individual tone-burst duration, sound pressure level (SPL; in this experiment SPL is always computed in dB from .0002 microbar), duty cycle (on time/on and off time), and overall exposure duration. a. Tone-Burst Duration. Ward, etal (1958) derived an equation for recovery of TTS: t TTSt = TTS2mln (1 — 0.37 log10 2) where t = any interval after stimulus exposure cessation; but they pointed out that for burst durations shorter than 250 milliseconds (msec), or longer than 1 min, the equation might not—probably would not—hold. No equation predictive of TTS exists for toneburst durations of less than 250 msec. b. SPL. Experiments too numerous to list show TTS to be a linear function of SPL. This is expresssed in the equation of Ward, et al (1958) for white noise stimulation: TTSamin at 4 kc = 1.06 duty cycle (SPL-85) logio Stimulus Duration/1.7, provided TTS2min is less than 50, and stimulus duration is at least five minutes. Harris (1967) has shown the generality of this equation form for continuous pure tone stimulation, the constants differing with frequency. c. Duty Cycle. The equation of Ward, et al, above, shows TTS to be a linear function of duty cycle. This was in general corroborated for tone-bursts at 3.5 kc (Harris, 1959), but considerable uncertainty exists for the very small duty cycles. The data of Selters and Ward (1962), for example, were confined to the 50 percent duty cycle and to noiseburst durations of 30 seconds and longer. For high-intensity clicks, the duty cycle is a function of click rate; Ward (1962) showed that interclick intervals of 1-9 sec yield the same TTS after 60 clicks each, but that if as much as 30 sec elapses between clicks, TTS is less, especially at 4 kc as compared with other frequencies, and especially for the earlier recovery process Rl (see Hirsh and Bilger, 1955). Evidently the smaller duty cycles must be studied with some care. Differential effects of frequency, of recovery processes, and of middle ear muscle activity may all exist. d. Overall Exposure Duration. Almost complete unanimity exists that TTS is proportional to log exposure duration; but with high-intensity clicks at 25/min, Ward, et al (1961) found it proportional to linear time. Evidently this matter must be looked into in any particular set of stimulus parameters. As a consequence of these considerations, it was deemed advisable to create stimulus conditions incorporating a broad range through each of the four parameters listed here: tone-burst durations 37-250 msec; SPL 105-120; duty cycles 0.1—50% and 100%; session duration 5-120 min. Our practical purpose was to explore the borders of permanent damage for brief, high-intensity tonebursts at 2.2 kc, and to write damage risk criteria (DRC) for such acoustic conditions. A word must here be said as to the index of TTS. Many writers have noted by audiometry that 4 kc is earliest and hardest hit by acoustic insult; and in our earlier paper using 3.5 kc tone-bursts we confined ourselves to studying TTS at 4 kc. However, with stimulation at 2.2 kc it seemed likely that frequencies lower than 4 kc would also be affected, perhaps even more profoundly affected. Furthermore, as is well known, the frequency of maximum TTS may change with stimulus SPL; and Van Dishoeck (1948) showed that after 100 dB SPL at 1 kc, the frequency of maximum TTS changed from 2.8 kc after one min stimulation to 1.4 kc after five minutes. We therefore felt it necessary, to present a true picture of TTS and its recovery, to sample several frequencies from 2.5—8 kc, and several times each within the first few minutes of recovery. Thus our index of TTS must condense information much in excess of the usual TTS2 min at 4 kc. METHOD Subjects. In this laboratory, young healthy males 18-25 yrs old are available in quantity for a few hours each, but cannot be detailed for many successive days. This fact dictated our experimental design. Subjects were selected in groups of 8-20, but usually 12, from a 50man squad, as having no audiometric loss (ASA, 1951) greater than 10 dB at any frequency through 8 kc. There were 26 such groups. Although at the upper limits of fatigability, significant subjective discomfort was produced, in general all subjects were cooperative; none asked to be excused. Apparatus and Procedure. a. Workspace and phones. A double-wall soundproof room lined with acoustical tile was used, seating 20 men. Each arm chair was provided with two Permoflux PDR-8 phones in MX cushions. The 40 phones had been selected from among a larger number, with a National Bureau of Standards 9-A coupler, Western Electric 640AA microphone and SPL meter, as not deviating from the mean phone by more than 3 dB at each of the frequencies 2, 3, 4, 6, and 8 kc. All left ear phones were wired in a parallel circuit, and all right phones in another. b. Stimulus Creating TTS. The output of a General Radio Model 1304 oscillator, set to 2.2 kc with a Hewlett Packard Model 500A frequency meter, was led to a Grason-Stadler switch, timer and attenuation pad, then to an Altec Model 1569A amplifier, and finally switched to one of the phone circuits. The phone exhibiting median output at 2.2 kc was fitted to the 9A coupler, and the voltage to that phone yielding 105-120 SPL were noted for continuous tones. The GrasonStadler electronic switch was adjusted to a rise-fall time of 5 msec, and set at either 37, 70, or 250 msec duration (measured at the frequencies representing half power), or set to the continuous mode when called for by the design. These conditions were checked with a Tektronix oscilloscope. For group stimulation, then, a 2.2-kc tone either interrupted or continuous, was presented to each subject's L or R ear, at some duty cycle at some SPL, for 5-120 min. These conditions, and the number of ears involved, are in Table 1. For duty cycles, 4 SPL's, 3 tone-burst durations, and 3 session durations, there are 324 permutations, an impossible and unnecessary experimental design. Table I shows that each stimulus parameter was sampled in a fashion which allowed it to vary while other parameters were changed in any of several ways. Thus, interactions could be spotted early in the experiment, and a sort of sequenGROUPS AND NUMBER OF CARS GIVEN EACH EXPERIMENTAL CONDITION 37-MSEC TONE-BURST 70-MSEC TONE-BURST 250-HSEC TONE-BURST SOUND PRESSURE MINUTES DURATION LEVEL 5 25 MINUTES DURATION MINUTES DURATION 5 25 120 5 25