For 140 male subjects (20 per decade between the ages 20 and 89) and 72 female subjects (20 per decade between 60 and 89, and 12 for the age interval 90-96), the monaural speech-reception threshold (SRT) for sentences was investigated in quiet and at four noise levels (22.2, 37.5, 52.5, and 67.5 dBA noise with long-term average speech spectra). The median SRT as well as the quartiles are given as a function of age. The data are described in terms of a model published earlier [J. Acoust. Soc. Am. 63, 533-549 (1978)]. According to this model every hearing loss for speech (SHL) is interpreted as the sum of a loss class A (attenuation), characterized by a reduction of the levels of both speech signal and noise, and a loss class D (distortion), comparable with a decrease in signal-to-noise ratio. Both SHLA+D (hearing loss in quiet) and SHLD (hearing loss at high noise levels) increase progressively above the age of 50 (reaching typical values of 30 and 6 dB, respectively, at age 85). The spread of SHLD as a function of SHLA+D for the individual ears is so large (sigma = 2.7 dB) that subjects with the same hearing loss for speech in quiet may differ considerably in their ability to understand speech in noise. The data confirm that the hearing handicap of many elderly subjects manifests itself primarily in a noisy environment. Acceptable noise levels in rooms used by the aged must be 5 to 10 dB lower than those for normal-hearing subjects.
Previous experiments on the limit of the ear's frequency-analyzing power, carried out with only two subjects, were repeated for four other subjects. The limit was investigated by measuring the number of harmonics of a complex tone that can be heard separately. It was found that, even under the most favorable conditions, not more than the first five to seven harmonics can be distinguished. This limit agrees with the critical-band concept, that is to say, a partial of a multitone stimulus can be heard only when the adjacent tones are separated by more than the critical bandwidth.
Following a pilot study (Smoorenburg et al., Scand. Audiol., Suppl. 16, 123–133) based on 44 ears (22 subjects) with noise-induced hearing loss (NIHL) we conducted a study based on 400 ears with different degrees of NIHL. The study included speech reception thresholds for sentences presented in quiet and in noise, tone audiograms measured in quiet and against a low-frequency background noise, and self-rating of the hearing handicap according to the Social Hearing Handicap Index. The results show virtually no correlation between the hearing loss for speech presented in quiet and the loss for speech presented in background noise. Speech thresholds found in quiet show the highest correlation with losses in the tone audiogram at 0.5 and 1 kHz, whereas speech thresholds in noise show the highest correlation with the losses at 2–4 kHz. The speech thresholds will be compared with predictions from the audiogram using the Articulation Index procedure and with the audiograms measured against a low-frequency background noise. Only those questions in the Social Hearing Handicap questionnaire that refer directly to practical situations show a correlation with the speech thresholds. The results for speech perception in noise show that the AAOO fence of 25-dB average hearing loss across 1/2, 1, and 2 kHz implies a severe auditory handicap. The fence should be set at a lower level and it should include audiometric frequencies.
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.
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