Educational Articles

Noise and the Audio Engineer
by
Michael J. Nilsson, Ph.D.
Experimental Psychologist, House Ear Institute

NOTE:  The following article was prepared by Dr. Nilsson especially for this website.  You will find more information regarding hearing loss at the House Ear Institute website.

Traditional concerns about noise induced hearing loss have focused on industrial settings with continuous, high levels of noise.  Much progress has been made in reducing noise levels and noise exposure, but additional high volume settings deserve attention when attempting to prevent noise induced hearing loss.  Audio engineers and musicians are frequently exposed to sound levels that exceed recommended levels, but little information is available discussing the dangers associated with these professions.  Below is some pertinent information regarding noise exposure concerns for audio engineers and performers. 
 
Are amplitude modulated signals as damaging as industrial noise? 
Industrial noise is commonly steady-state with little variation in level and duration.  Few recording environments experience this type of exposure.  Signals modulated in amplitude are much more common, with frequent changes in spectrum and level.  Some attempts have been made to quantify the danger of these non-industrial environments. 

Previous work has focused on music exposure in night clubs and discos, with measurements made of patrons and employees.  One study (Gunderson et al., 1997) found indications of Noise Induced Hearing Loss (NIHL) in patrons as well as employees of discos and concerts which were associated with dangerous exposure levels, signs of NIHL, and complaints of difficulty hearing.  Peak levels easily exceeded safe levels, but were dependent upon the music venue, the distance to the stage, the reverberation qualities of the room.  Noise levels exceed recommended levels even without live performers on the stage.  Darcy (1977) found high sound pressure levels, but acceptable exposure because of intermittent attendance (high levels are not maintained for long periods of time in the nightclubs where the measurements were made).  One interesting conclusion is the change in exposure patterns with age.  The time spent in the environments described is age dependent, and long term exposures are difficult to follow.  It is safe to assume that exposure levels can easily reach dangerous levels, and the length of exposure is critical.  Long term exposure to high levels of music (or other amplitude modulated signals) can lead to permanent noise induced hearing loss. 

Lindgren & Axelsson (1983) exposed 10 volunteers to music and noise and compared temporary thresholds shifts for two stimuli (white noise and music) across five exposures.  Across all exposures, there was no significant difference in threshold shifts associated with the music or noise.  When considering the five sessions independently, the threshold shifts began smaller for the music, and ended up larger, averaging out to approximately the same shift.  The authors attribute this to an enjoyment factor that decreased as the music got boring. 

How easily can damaging levels be reached under headphones? 
Damaging levels can easily be reached under headphones, and the prevalence of NIHL from headphone use is highly correlated with length of exposure.  A precise measurement of typical volume settings in personal cassette systems showed that the volume was usually set at between 7 and 10 dB below the maximum values allowed by the technical characteristics of the stereo-headset combination, i.e. between 104 and 126 dB A.  Ising et al. (1994) found exposure levels of 95 or higher dB in 11% of young students (age 10-19 years).  Buffe et al (1986) found hearing loss in young males who listened to music under headphones at moderate levels for more than seven hours per week.  Lee et al. (1985) found temporary threshold shifts of up to 30 dB after 3 hours exposure to headphones at the listeners usual maximum level.  Axelsson and Lindgren (1977) found increased high frequency hearing loss in musicians and audio engineers who listened using headphones compared to those who did not.  This is attributed to the high levels possible at the ear drum when using headphones. 

Do levels change with length of exposure? 
Subjective judgments about loudness are often based on relative changes in level, and can change with exposure.  For example, when asked to judge the loudness of sound, ratings will change when the testing is done after resting in a quiet environment, or after exposure to a loud environment. 

Hetu (1994) reports that temporary thresholds shifts associated with exposure to loud music for 1 hour are also associated with altered perception of loudness, impaired pitch perception, reduced frequency selectivity, and temporal resolution, as well as reduced spatial resolution at high sound levels.  In other words, auditory acuity is impaired. 

What exposure times have been shown to lead to threshold shifts? 
Meyer-Bisch in a survey of French youths found increased auditory suffering in groups using personal cassette players > 7 hours per week, versus groups with 2-7 hours, and groups with no use.  Measurements made with a symphony orchestra found maximum exposure times of 10 hours per week in exposed positions (such as in front of trumpets), and 25 hours in normal positions.  Exposures of longer duration were associated with permanent threshold shifts. 

How reliable are personal evaluations of level? 
Miyake and Kumashiro (1986) measured comfortable levels for three kinds of music listened to under headphones in six noisy conditions.  Comfort levels were found that produced temporary thresholds shifts after thirty minutes of exposure.  This shows that comfort levels are dependent upon external factors, and should not be trusted to determine safe levels. 

Sound level judgments have also been shown to be related to age (Coren, 1994).  This is partly explained by changes in hearing sensitivity.  Fucci et al (1993, 1994) have shown that estimates of loudness are related to music preference, with individuals preferring rock music assigning the same level judgment to a higher level, i.e., a higher signal level is required before the music is considered too loud. 

What other interactions are critical? 
High level noise exposure has been shown to be related to serum lipid/lipoprotein levels, which is related to cardiovascular disease (Melamed et al, 1997).  Solas et al (1995) relate noise exposure to increased blood pressure, especially in young listeners (under age 45).  Cary et al. (1997) reviewed many toxic substances, that when combined with noisy situations, increase the chances of noise induced hearing loss.  Most are industrial solvents, which are ototoxic on their own, but carbon monoxide (as found in cigarette smoke and exhaust fumes) has been shown to increase the probability of hearing loss from exposure to noise. 

Conclusions 
What can we conclude from this information?  Loud music can be just as dangerous as industrial noise, but because of typical listening patterns, and differences in the duration of these intense levels, noise-induced hearing loss from music is less frequently reported, and is thought of as more cumulative.  Long exposure times, or long term exposure over many years can lead to hearing loss, especially if combined with ototoxic chemicals.  Any factor that can lead to increased levels (such as background noise or use of headphones) should be controlled, and sound levels should be measured to ensure safe exposures. 

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