Relation of Hearing Loss to Noise Spectrum

Because of the many uncontrollable factors, field studies to investigate the relation of hearing loss to noise spectrum are not very suitable. Assuming a close relation between temporary and permanent hearing loss to exist, experiments are done to find the “relative traumatizing power” of different frequencies. Octave bands of noise are used as a stimulus to investigate the threshold shift at one frequency (one-half octave above the center of the noise band), as well as the integrated threshold shift over all frequencies. From these data isotraumatic lines are derived, representing as a function of frequency the sound-pressure levels which give equal threshold shifts. These lines have a minimum at about 3600 cps with steep slopes below and above this frequency. A review of recent literature shows that generally flatter curves are found. This difference is discussed, but no reasonable explanation can be given. Considerations based on field studies give the impression that a steep isotraumatic line is more probable than a flat one. It is proposed that the isotraumatic line found by the authors, shifted to an appropriate level corresponding with a minimum of 75 dB, may have some value as a criterion line to conclude whether a noise spectrum will induce hearing losses or not.