Wesley A. Bulla
Belmont University
Terminology
The author acknowledges some readers may not recognize terminology specific to the investigation of noise exposure and OSHA defined criteria. In order to help the reader through unfamiliar territory, a list of definitions is provided at the end of this article.
Introduction
Previous research has established that noise-induced hearing loss (NIHL) is a product of excessive and prolonged exposure to high-intensity sound. Under the direction of the National Institutes of Health (NIH), a consortium from the biomedical, behavioral, and health care professions stated three key conclusions regarding NIHL: (1) sounds of sufficient intensity and duration will damage the hearing mechanism and result in temporary and possibly permanent hearing loss (HL) regardless of the age of the individual; (2) sound levels of less than 75 decibels A-weighted (dB-A) are unlikely to cause permanent HL; and (3) sound levels of 85 dB-A with exposures of eight hours per day will produce HL after prolonged exposure over the years of an individual’s working lifespan. The National Institute for Occupational Safety and Health (NIOSH) estimates that one in four workers exposed to hazardous noise levels will develop HL as a result of their exposure.1 Before 1990, NIHL was generally associated with everyday work or leisure activities such as working with industrial machinery, power tools used in the construction industries and for yard work, or sport shooting and hunting with firearms. Cited as “occupational noise exposure,” in recognition of the effect of activities such as repeated listening to loud music at concerts or nightclubs, the NIH Noise and Hearing-Loss Conference (1990) listed populations exposed to “live or recorded high-volume music” as among the 10 million people in the U.S. at risk for damage to the hearing mechanism.2
While audio engineering is not directly listed as a high-risk occupation, it is commonly presumed that many audio and music engineering professionals, such as those employed in recording studio activities, listen to music for prolonged periods of time often at excessive sound levels. However, previous studies of cumulative exposure to loud music and its effect on HL have focused almost entirely on live-performance musicians, conductors and directors of band and orchestral ensembles, and their audiences. Unfortunately, there is a general lack of data regarding noise-expo-sure among non-performance-oriented music industry professionals, such as recording engineers, technicians, and music producers. A search of the Medline PubMed database lists 186 publications concerned with music-related HL or NIHL in the music professions.3 None of these studies involve exposure levels of audio engineers, music producers, or other non-performance-oriented professionals in the audio and music recording industries. To date, there have been no empirical studies of noise exposure levels among “behind-the-scenes” non-musician participants in audio- and music-recording activities.
The working audio engineer is the focus of the present study. We documented work-related sound exposure levels of ten professional music recording studio engineers during their daily activities and accumulated over 400 hours of data revealing peak, maximum, and average sound pressure level (SPL) exposures.
Background: The Impact of Noise Exposure
It is well documented by hearing scientists that excessive exposure to high SPL noise leads to a shift in the hearing sensitivity threshold. Research has shown that seven hours of exposure to industrial noise levels between 85 dB-A and 90 dB-A causes a condition of reduced hearing sensitivity known as “temporary threshold shift” (TTS). In 1995, Kvaerner, Engdahl, Arnessen, and Mair found significant changes in the hearing sensitivity threshold of workers in an ironworks factory after exposure to the noise of their work environment.4 Additionally, there is evidence that after continual exposure, this temporary shift in sensitivity may, and often does, become permanent through irreparable damage of the sensory hair cells in the cochlear mechanism.5 It has also been established that TTS is an autonomic function and is uncontrollable by the listener. In other words, even when a person is anticipating the noise, TTS will still occur and presumably may result in permanent HL.6
It should also be noted that while scientists generally agree that TTS and HL may be plausibly related and inherently linked, their connection, as well as an individual’s susceptibility to NIHL, is difficult to determine and their precise relationship is still unclear. Unfortunately, individuals who are experiencing TTS, or are in the early stages of mild HL, are often unaware of its presence.7, 8 This lack of self-awareness at the onset of TTS or NIHL, in addition to making assessment difficult, is an indication of the insidious and covert nature of NIHL.
Music vs. Noise
In an effort to understand the effect of exposure to loud music, Axelsson and Lindgren9, 10 examined pure-tone hearing thresholds in a population that included disk jockeys, entertainment artist managers, live-sound engineers, and musicians. They found HL demonstrated in 13 to 30% of their populations. Interestingly, as shown in Figure 1, TTS examinations conducted by Kvaerner’s research team in an ironworks factory revealed hearing sensitivity thresholds reduced in the same frequency bands (4 and 6 kHz) as those observed in the music professionals studied by Axelson and Lindgren.4 This observation provides evidence that prolonged or repeated exposures to loud music in common performance and listening environments may have the same impact as industrial noise in the workplace.
Over the past thirty years, scientists have exhibited varied degrees of concern as to whether exposure to loud music, especially the rock genre, would result in music-induced hearing-loss (MIHL) that has similar characteristics to NIHL. In 1972, Barry and Thomas12 conducted an investigation to determine if noise and music could possibly result in similar levels of TTS, and therefore, similar types of NIHL. After an exposure of 60 minutes to 95 dB (linear) of white noise, rock music, or symphonic music their subjects demonstrated an average TTS for the noise stimulus that was greater than the average for either music stimulus. Shifts in lower frequencies (750 - 2000 Hz) were similar across conditions. However, shifts in higher frequencies (2 - 5 kHz) were 3 to 7 dB greater for the noise stimulus. Additionally, they found large variations across subjects, but consistency within subjects. That is, if a subject had large measures of TTS for one stimulus, s/ he also demonstrated large measures for the other two stimuli. Interestingly, several subjects complained that the symphonic music was too loud even though all three stimuli had equal average intensity levels.
Figure 1. Average pure-tone audiogram of 69 pop musicians, 4 disk-jockeys, 4 managers, and 6 live-sound engineers. A similar pattern was observed in TTS examinations after sustained periods of exposure to industrial noise in an ironworks factory.3 The ordinates represent a decrease in hearing threshold when compared to a normalized hearing sensitivity for a given frequency where 0 dB-HL represents the threshold of an established population of normal listeners. (Adapted from Axelsson and Lindgren11)
While the Barry and Thomas study seemingly provides evidence that music has a lesser effect on TTS than noise, it must be noted that their noise stimulus contained equal energy per band and therefore had greater energy content in the higher frequency bands when compared to their music program. More importantly, their study provides testament to the covert quality of excessive noise exposure, in that even though their noise stimulus contained greater energy in the spectral bands normally associated with auditory fatigue, several subjects went to sleep while listening to the noise stimulus. As noted by Barry and Thomas, this is disturbing because subjects in general found the potentially more dangerous stimuli to be the least bothersome.
Recently, Dibble13 published a concise review of related noise- and music-induced HL literature in an attempt to ascertain the relationship between exposure to loud music and HL. While there is evidence that many music professionals suffer from some form of HL, there is still much debate as to whether extended exposure to loud music and noise result in the same levels and types of damage to the sensory organ. Dibble reported that the weight of evidence suggests that music does not have the same effect as industrial noise. He also reported, as a subjective note, that due to frequency content, he did not experience stress while listening to music at average levels of over 100 dB (no indication of weighting), while he did with industrial noise levels of 92 - 96 dB-A. Due to differences in spectral content, transient variations, or some other acoustic factor(s), when compared directly to some types of noise, it is plausible that our sensory organ may tolerate greater average levels of music before damage occurs. If this is true of the general population of audio professionals, it is also possible that they are more prone to expose themselves to greater levels of music for greater periods of time because they find it less disturbing than noise per se.
Musician Studies
Over the years there have been many studies examining exposure levels and HL among musicians. While not consistent in methods of documentation, most have noted high SPL at both rehearsals and concert performances and many have documented some degree of hearing impairment among musician populations. Consistent with results found with industrial workers, two and one-half hours of unprotected exposure during rehearsals caused TTS of 15 dB, 25 dB, and 20 dB at 2 kHz, 4 kHz, and 6 kHz, respectively, among musicians in a rock and roll band.13 All of the subjects examined by Axelsson and Lindgren demonstrated HL in excess of 30 dB in at least one frequency band in at least one ear.11 Of their ten “worst case” musicians, the mean HL between 3 and 8 kHz was shown to be greater than 20 dB.
The impact of excessive sound level exposures on musicians is not limited to the pop and rock genres. School band directors have exhibited similar levels of TTS and HL. Cutietta, Millin, and Royse found 66% of their population exhibited signs of HL at one or more of the tested frequen-cies.15 Thirty-four percent of those were attributable to NIHL (as opposed to age-induced HL). A more disturbing finding revealed band directors under 30 years of age exhibiting HL normally associated with a population of about 50 years of age.
There is great variety and contradiction among results from similar studies. Behroozi and Luz pointed out that HL was demonstrated in 58% of the musicians in a Danish classical orchestra while it was reported in only 15% among German orchestral musicians.17 A follow-up investigation of subjects by Axelsson found no significant differences in a comparison of individual audiograms taken 16 years apart. Interestingly, even though subjects had originally demonstrated mild HL in their earlier study, it appears that their population had “well-preserved” hearing that was not deteriorating at a significant rate.18
Audio Engineer Studies
Audiometric surveys were conducted at the 51st Audio Engineering Society (AES) Conference and at a meeting of the Los Angeles section of the AES.19, 20 The two surveys involved 480 professionals in the recording industry. The authors of the earlier study found that 10% of the participants demonstrated a moderate to severe HL at 4000 Hz. Results of the later study suggested a small but consistent HL which could not be attributed to age-related factors. As shown in Figure 2, average pure-tone thresholds of 100 recording engineers show hearing sensitivity thresholds reduced at 4, 6 and 8 kHz—the same “notch” found in industrial workers (see also Figure 1).