Introduction
An operating assumption of diagnostic audiology is that hearing level of a young adult with no known hearing loss or history of noise exposure should be 0dB HL.
Hearing thresholds of 0 – 25 dB HL are generally
considered a loose clinical standard of “normal hearing,” yet hearing loss is consistently measured as elevation
from 0 dB.
Previous Research
Mostafapour et al. (1998) tested young adults ages 18 – 30 for the presence of noise induced hearing loss. They were particularly interested in the role of personal
listening devices and other sources of leisure noise on young peoples’ hearing thresholds.
Results: All hearing thresholds were found to be within normal limits (less than 25 dB), and although notches were present in a percentage of the sample, the notches did not correlate with any single or cumulative noise
Previous Research
The authors of the study concluded that “the majority of young users of personal listening devices are at low risk for substantive NIHL.”
However they warn that NIHL is an additive process, and “even subtle deficits may contribute to unequivocal
Previous Research
Carter at al. (1982) studied a group of 16 – 20 year olds to explore the occurrence of hearing loss caused by
exposure to loud music.
The study found no evidence of widespread hearing loss, but it was noted that “the accumulated noise exposure of some of [the subjects] to noise is such that, if their
An Additive Process
The controversy in the literature raises an important question about the accuracy of our standards for
“normal” hearing in young adults.
Determining a correct baseline is important because the concept of 0 dB HL as a baseline of normal hearing is
critical for determining the degree of hearing injury in cases of occupational noise exposure.
Noise induced hearing loss is an additive process, so even a small trend toward NIHL in the early adult years may
The purpose of this study
The Sample
All subjects were between 20 and 29 years of age and identified themselves as having “normal hearing.”
Exclusion criteria included diagnosed hearing loss, history of chronic middle ear dysfunction, history of occupational noise exposure, occluded ear canals, and abnormal
immitance on the day of testing.
The Sample
21 graduate students volunteered to participate; one
student was excluded due to abnormal tympanograms on the day of testing, leaving 20 people in the sample.
Materials and Methods
Audiological examination was performed by the author, an AuD student in her fourth year of study.
Otoscopy
Bilateral screening tympanometry
Pure tone air conduction audiometry with ER-3 insert ear phones using a GSI 61 audiometer
250, 500, 1000, 2000, 4000 and 8000 Hz in 2 dB steps.
In order to insure accurate thresholds, each subject was carefully instructed about response criteria, and
Results
To explore audiogram configuration, the thresholds of both ears of each participant were averaged to create a single average threshold for each frequency.
Examined under a series of comparisons to determine how close thresholds were to 0 dB HL and if significant differences were present based on age, sex or ear.
Average Thresholds of All
Participants
Average Threshold of All Test Subjects
250 500 1000 2000 4000 8000 -6 -2 2 6 10 14 18 Average Threshold Frequency (Hz) d B H L
Average Thresholds of
Participants by AGE
Average thresholds of subjects divided by age, and SEM. Average Threshold by Age
Ear Effect?
To determine the presence of a threshold difference between the ears, thresholds for all subjects were
Average Thresholds of all
participants by SEX
Average thresholds of sample divided by sex, and SEM. Average Threshold of Males vs. Females
ISO 1999 Annexes A and B
The mean thresholds of males and females were
compared to ISO 1999(1990), a document produced by the International Organization of Standardization to
explore the prevalence of occupational noise induced hearing loss by duration and intensity of noise exposure in the work week.
ISO 1999 Annexes A and B
The ISO 1999 data is not representative of hearing
threshold levels for young adults in their twenties, but to obtain a general comparison, data from the present study was compared to the 30 year age group.
Comparison to ISO 1999
Annexes A and B
Both males and females showed elevated thresholds compared to Annexes A and B data for 30 year old subjects.
This is an interesting finding, as we would expect the
Males compared to ISO 1999
Average thresholds of males in sample compared to ISO Annexes A and B. Average Thresholds of Males in Sample vs ISO 1999
Females compared to ISO 1999
Average Thresholds of Females in Sample vs ISO 1999500 1000 2000 4000 -6 -2 2 6 10 14 18 Annex-A Annex-B Frequency (Hz) Females in Sample d B H L
Discussion
The results obtained in this study show that the average thresholds of a small carefully screened group of graduate students in their twenties are not 0 dB.
Although individual subjects presented with some thresholds equal to or better than 0 dB across the
Discussion
Despite elevated thresholds in the sample, the
audiometric configuration does not generally show the pattern of loss we would expect to see from the
emergence of noise induced hearing loss.
Discussion
Comparison of thresholds by age:
As a whole, the group of subjects in their younger twenties
presented with lower thresholds.
The elevation of high frequency thresholds in the older
Discussion
It should be noted that the samples of each age group were not even. The 21-25 year old group included 8 subjects while the 26 – 29 year old group included 12 subjects.
Low Frequency Loss?
Consistent elevation of low frequency thresholds for males and females of both age groups
There is no audiological reason to suspect low frequency hearing loss, particularly since each subject was screened on the day of testing for middle ear involvement.
Low frequency loss is not consistent with age or noise induced hearing loss.
It is possible that there was some source of low frequency masking present in the booth.
Conclusion
The results of this study indicate that 0 dB HL may be an
optimistic assumption of normal hearing for young adults with no known otologic insult or history of noise exposure.
The configurations of mean thresholds in the older vs younger samples imply the possible beginning of high frequency
hearing loss.
It is impossible to determine the cause of threshold elevation from this study, but it is important to note that even adults under age 30 may have emerging hearing loss and be
References
Carter, N.L., et al. (1982). Amplified music and young people’s hearing. Medical
Journal of Australia, 2(3), 125-128.
Dobie, R. A. (1993). Age-Related Hearing Loss. Medical-Legal Evaluation of Hearing
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Dobie, R.A. (2008). The burdens of age-related and occupational noise-induced hearing loss in the United States. Ear & Hearing, 29, 1-13.
International Organization for Standardization. 1990. Acoustics: Determination of
Occupational Noise Exposure and Estimation of Noise-Induced Hearing Impairment, ISO-1999. Geneva: International Organization for Standardization.
Mostafapour, S.P., et al (1998). Noise-induced hearing loss in young adults: the role of personal listening devices and other sources of leisure noise. American
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