Introduction
Hearing loss is a gradual process that may not be noticed for years. When it does happen, people generally notice that speech is mumbled and unclear. People may report a ringing (or tinnitus) in their ears or head. By that time, the only thing that may help is a hearing aid. While hearing aids have improved dramatically, they are not perfect.
Once you leave work, there are many sources of noise encountered in everyday life: traffic, loud music, MP3 players, lawn mowers, snowmobiles, and motor boats, to name a few. Even a noisy hockey arena can be damaging! Yet, even quiet noise, depending on how long you listen, can damage your hearing. It is quite surprising how quiet an 85 dBA noise actually sounds. A permanent hearing loss can be the result of a single loud blast (acoustic trauma), but more often it is the result of years of exposure to sounds that one would not normally think of as damaging. Unlike industrial noise exposure, there are unfortunately some potential sources of acoustic trauma in the musical venue. These may include feedback squeals during sound checks, inappropriately-set limiters, purcussive blasts from cannons and pieces of wood being smashed together, or being seated in front of a large stack of speakers for an extended performance. While there is scant research in the literature on this subject area, clinically, hearing loss is reported (and confirmed) where the source was a single or relatively short duration blast. Industrial environments are in this sense, a much more controlled environment than many musical venues.
Acoustic Trauma and the Musician
Most of the models of noise-induced hearing loss are adequate for levels up to 115 dBA; however, they tend to break down for more intense impulse stimuli. For a list of common sounds and intensities, download PDF Addendum C. Price & Kalb (1991) and Price (1994) investigated the effects of intense impulse sounds and found that the motion of the basilar membrane during the impulse sound was also important for the prediction of hearing loss (other than intensity and duration). Price (1994) notes that "at lower SPLs losses are in all likelihood largely a function of the metabolic demand on the inner ear (it gets 'tired out') and that above some spectrally dependent critical level, the loss mechanism changes to one of mechanical disruption . . . (the ear gets 'torn up')". He argues that if the basilar membrane is allowed to oscillate past the zero (atmospheric pressure) point, then more damage will be sustained by the hair cells in the Organ of Corti. If impulses possess either completely positive or completely negative pressure waves, the displacement of the middle ear ossicles cannot impart sufficient energy to create a "tearing" action to the inner ear structures.
Although a cap pistol (at 30 cm) and two small wooden blocks (found in some Christmas carrols) have almost identical peak sound pressure levels (at 150-153 dB SPL), because of the shape of the pressure wave, the small wooden blocks would cause a 25 dB permanent hearing loss but the cap pistol would only cause a 10 dB permanent hearing loss.
A Couple of Words Regarding Audiometry
Acoustic trauma typically shows up at, or near, the spectral peak frequency of the offending stimulus. A feedback squeal at 3000 Hz will generate a sensorineural hearing loss at about 3000 Hz. In contrast to acoustic trauma, hearing loss from long-term noise or music exposure is typically in the 3000 to 6000 Hz region, and although there is a small dependence on spectral shape, this notched loss tends to be a hallmark of extended noise or music exposure. What are the causes of the nonmonotonic nature of noise-induced hearing loss that creates an audiometric notch? Several explanations have been proposed. These include (a) a poor blood supply to the part of the cochlea that corresponds to the 3000 to 6000 Hz region (Crow, Guild, & Polvogot, 1934); (b) a greater susceptibility for damage to the supporting structures of the hair cells in this region (Bohne, 1976); (c) the orientation of the stapes footplate into the inner ear is such that its primary force vector aims toward those hair cells in this region, with the effect of eventual failure because of the constant hydromechanical action (Hilding, 1953; Schuknecht & Tonndorf, 1960); and (d) permanent noise exposure has its greatest effect approximately one-half octave above the peak frequency of the noise spectrum. Since all spectra are enhanced at 3000 Hz by the outer ear canal resonance, the greatest loss will be in the 4000 to 6000 Hz region (Tonndorf, 1976). Because of these phenomena, hearing losses due to noise (including music) exposure are relatively easy to spot.
However, many clinical cases of music or noise exposure do not possess an audiometric notch. For example, Barrs, Althoff, Krueger, & Olsson (1994) found that only 37% of workers suffering from noise exposure possessed an audiometric notch. It is quite possible that in advanced cases of exposure or advanced age where there is a significant age-related hearing loss (presbycusis), the hearing sensitivity at 8000 Hz may have also deteriorated, leaving a flat audiometric configuration. In addition, depending on the noise spectrum, the frequency region of greatest damage may be above the audiometric test frequencies. For example, using data derived from violin players, the frequency of greatest damage can be at 8000 Hz, and unless a higher frequency pure tone were to be assessed (e.g. 10,000 Hz), a notch would not be apparent.
Alberti (1982) argued that noise induced hearing loss should be symmetrical- roughly equal amounts of loss in both ears with similar configurations. However, asymmetrical hearing losses are commonly found amoung those in the performing arts. Typical occupational environments for industrial workers are highly reverberant, with most of the sound pressure in the lower frequencies. Potentially damaging sounds that may emanate from one side, because of the reverberant conditions, may be almost as intense at the other side. In addition, because most of the sound is lower frequency, the head does not appreciably attenuate the sound from one side to the other. That is, there is no head-shadow effect. Subsequently, even in asymmetrical noise situations, exposure tends to be symmetrical, with resulting symmetrical audiograms. In contrast, those in the performing arts work in relatively non-reverberant conditions where assymetrical musical exposures (e.g., drummer near the right ear) may result in asymmetrical hearing loss as measured on an audiogram. In addition, because of the significant mid- and high-frequency sound pressures (i.e., short wavelengths), the head acts to further attenuate the off-side music exposure such that the other ear is in an acoustic shadow and is subsequently more protected. Having said this, audiometric assymetries can be signs of serious medical problems and patients should thus be referred to the appropriate hearing health care professional for further assessment.
Intermittent Nature of Music
The vast majority of the research in the area of hearing loss has been in the industrial/occupational domain. While it is known that occupational levels in excess of 85 dBA can permanently damage hearing (and data exists that even levels above 80 dBA can be damaging), the levels from exposure to recreational noise such as music are not as well defined. An implicit assumption in all noise exposure research is that intermittent noise with regular quiet periods would be less damaging than steady-state noise. Various regulatory agencies and standards have handled this differently (e.g. Committee on Hearing and Bioacoustics "on-off" fraction rule), but how quiet do the spaces in between the noise (and music) bursts have to be for there to be a reduction in the level of damage? Since the dynamics of music are more variable than typical noise spectra, with music having intense periods followed by periods of relative quiet, this may result in a different exposure for music and noise of "equal intensities". This leads us to our first hearing loss prevention strategy.
"Hum while you work." Humans (and all other mammals) have a small muscle in their middle ears (behind the ear drum) that contracts upon hearing loud sounds. From an evolutionary perspective, we have such a muscle so that our own voice would not be too loud for us. When this muscle (called the stapedius muscle) contracts, it pulls on the small chain of bones in the ear that conducts sounds, making these bones temporarily less efficient as conductors. Sound from the environment therefore cannot get through to our inner ears as readily, thus providing us with significant protection. Borg, Canlon, and Engstrom (1995) argued that the level of one's stapedial reflex, something routinely measured in an audiology assessment, is the primary reason why some people are more susceptible to hearing loss than others, if given the same noise source. If you know that a loud sound is about to occur, such as when you are walking past a construction site, start humming before you encounter the loud sound, and try to sustain your humming until the noise is finished. Rock drummers have used this strategy for years- humming (and grunting) as they smash away at their drum sets. One unfortunate feature of the stapedius muscle is that it loses its efficiency after about 10-15 seconds. In an industrial environment, therefore, after 15 seconds of constant steady-state noise, the stapedius muscle yields no further protection. However, music has loud and soft passages, and it is this intermittent loud/soft alteration of music that allows the stapedius muscle to "reset" and once again provide protection for intense sounds.
Tuned Hearing Protection
Hearing protection has been available for years. It may not be the lawn mower or the chain saw that is the main source of noise exposure in your situation, but using hearing protection for these noisier recreational parts of life will afford you better hearing years later (assuming, of course, that you consider mowing your lawn to be recreational). Earplugs are usually small foam or rubber inserts that can be placed in the outer ear. Earmuffs that fit over the ear and are bulkier than earplugs, can also be quite useful for very noisy situations. Due to the laws of acoustics, hearing protection attenuates the shorter wavelength, higher frequencies more than their longer wavelength, low-frequency neighbors. Subsequently, hearing protection treats different frequencies in different ways. This may be acceptable for many industrial workers, but is disastrous for many musicians. In the past 15 years, there have been a series of earplugs available that are ideal for listening to music. These "flat" or uniform attenuator earplugs lessen the sound or noise energy equally across the spectrum. Music still sounds like music, but without that "dead" feeling. Several manufacturers offer this flat earplug in both custom-molded and pre-molded versions. For musician-specific fact sheets regarding appropriate ear protection and preventative measures, download PDF Addendum A, and for auditory dangers associated with specific instrument locations and the recommended ear attenuators, download PDF Addendum B.
It has been shown that a flat or uniform type of hearing protection can actually reduce the incidence of front-back warning signal confusion in a noisy environment (Chasin and Chong, 1999). For this reason and for purposes of improved communication ability, a flat attenuation hearing protector may be quite useful on the factory floor, as long as the noise level in the factory is not too excessive. (i.e., less than 100 dBA).
Moderation
Another difference between musicians and industrial workers is that most musicians do not work a 40 hour week, with the exception of classical musicians who can be exposed for a significant number of hours each week from their own instruments, their colleagues, or their students. Musicians have the advantage of being able to rest for long periods of time in relative silence (as well as sleep in until noon each day!). This is not the case for industrial workers who like to go to rock concerts. The loud concert could easily add to their total weekly dosage of noise exposure.
Permanent hearing loss starts as a series of temporary hearing losses. When you come out of a rock concert or other loud place, your hearing may be temporarily reduced. One might notice this as a muffled or dead feeling in the ears, which is sometimes accompanied by ringing. This temporary hearing loss resolves after about 16-18 hours. If exposure to the loud noise is repeated often enough, temporary hearing loss can become permanent. The strategy would therefore involve moderation. If you go to a rock concert on Friday night, don't mow your lawn Saturday. Wait until Sunday, or better yet, get someone else to do it!
Addendums
Several addendums have been provided as a part of this article. This includes:
- PDF Addendum A - Contains musician-specific fact sheets regarding appropriate ear protection and preventative measures - click to download.
- PDF Addendum B - A list of auditory dangers associated with specific instrument locations and the recommended ear attenuators - click to download.
- PDF Addendum C - A list of common sounds and intensities - click to download.
Alberti, P. W. (Ed.). (1982). Personal hearing protection in industry. New York: Raven Press.
Barrs, D., Althoff, L., Krueger, W., & Olsson, J. (1994). Work-related, noise-induced hearing loss: Evaluation including evoked potential audiometry. Otolaryngology Head and Neck Surgery, 110(2), 177-184.
Bohne, B. A. (1976). Safe level for noise exposure? Annals of Otology, Rhinology, and Laryngology, 85(1), 711-724.
Borg, E., Canlon, B., & Engstrom, B. (1995). Noise-induced hearing loss: Literature review and experiments in rabbits. Scandinavian Audiology, Suppl. 40.
Chasin, M. & Chong, J. (1999). "Improved Localization Ability in Noise using Musician Earplugs", in Industrial and Occupational Ergonomics: Users' Encyclopedia, International Journal of Industrial Engineering, ISBN# 0-9654506-0-0.
Crow, S., Guild, S., & Polvogot, L. (1934). Observation on pathology of high-tone deafness. Johns Hopkins Medical Journal, 54, 315-318.
Hilding, A. C. (1953). Studies on otic labyrinth: Anatomic explanation for hearing dip at 4096 Hz characteristic of acoustic trauma and presbycusis. Annals of Otology, Rhinology, and Laryngology, 62, 950.
Price, G. R. (1994). Occasional exposure to impulsive sounds: Significant noise exposure? Forum presented at the 19th annual National Hearing Conservation Association (NHCA) Conference, Atlanta, GA.
Price, G. R., & Kalb, J. T. (1991). Insights into hazard from intense impulses from a mathematical model of the ear. Journal of the Acoustical Society of America, 90(1), 219-227.
Schuknecht, H., & Tonndorf, J. (1960). Acoustic trauma of the cochlea from ear surgery. Laryngoscope, 70, 479.
Tonndorf, J. (1976). Relationship between the transmission characteristics of conductive system and noise-induced hearing loss. In D. Henderson, R. P. Hamernik, D. S. Dosanjh, & J. H. Mills (Eds.) Effects of noise on hearing (pp. 159-178). New York: Raven Press.
