by Brian J. Fligor, Children's Hospital and L. Clarke Cox, Boston University Medical Center, Boston, Massachusetts
Using headphones with personal stereosystems (PSS) is a common sight in everyday society. For the past couple decades, the "Walkman" has been demonized by "responsible adults" while fully embraced by the youth. In the 1970s and early 1980s a few articles came out in peer-reviewed journals warning that maximum output levels from PSS could be outrageously high (Wood and Lipscomb, 1972; Katz et al, 1982), and the inevitable over-interpretation by well-meaning mothers and hearing conservationists said headphones are dangerous.
Are PSS headphones dangerous or not? Articles presenting research into this question fall on both sides of the fence, but the sum of the literature suggests PSS can cause hearing damage if used at high volume control settings for long durations over a period of time. But the question begs, what is the cut-off level? How loud is too loud? It's not just how loud (that is, the level), but it's the Time-Weighted Average (TWA is the combination of the level and the duration of exposure) that should be considered. Another way to describe the maximum allowable TWA is "noise dose" where a 100% noise dose reflects the highest permissible exposure. Numerous studies into occupational noise exposure and regulations to protect the hearing of the noise-exposed worker tell us that if a person regularly exceeds the maximum safe exposure (greater than a 100% noise dose), then over time, that person will sustain noise-induced hearing loss. How can this be translated to using headphones? Does it need to be?
To date, audiologists have not had useful guidelines for telling people what listening levels are ok and what levels are not. Such guidelines would help the audiologist counsel the patient how to keep from regularly exceeding a 100% noise dose. True, it's likely that a good number of people self-regulate their listening (listen at levels and for durations that don't exceed a 100% noise dose). But the literature suggests that between 5% and 20% do not (Catalano and Levin, 1985; Rice et al, 1987a, 1987b).
Journal articles have reported sound levels from PSS measured on audiometric calibration equipment, measured in the listener's ear canal, and measured in the ear of KEMAR (an acoustically "correct" manikin). The output signal that has been measured from headphones has been primarily music of one genre or another. The different methods used to make measurements confound making easy comparison from one study to another. The standard for deciding if an exposure (that is, noise dose) is hazardous comes from the damage-risk criteria set forth by various agencies, such as the International Standards Organization (ISO), National Institute for Occupational Safety and Health (NIOSH), and the Occupational Safety and Health Administration (OSHA). These damage-risk criteria are based on population studies of hearing loss in unprotected workers and noise survey data that was collected in the soundfield. The acoustical properties of the ear canal and the effect of coupling a PSS to the ear are fundamentally different than were the conditions in the studies used to develop the damage-risk criteria (Prince et al, 1997). Thus, measures of output from headphones must be adjusted carefully in order for it to be meaningfully compared to damage-risk criteria (that is, determined whether or not an exposure is hazardous).
To overcome these challenges, Fligor and Cox (2004) used KEMAR (with appropriate acoustical conversions) and a reference output signal to measure output levels from headphones. Since the output levels can be dramatically different from one song to another across and within music genres, a reference signal of white noise was used. The output using white noise was compared to a song from each of eight music genres. Some individual songs were overestimated by the white noise reference signal, but for the most part, output levels using white noise matched output levels using music.
Rather than survey the hearing of individuals who use PSS or determine (again) how many were at risk for chronic noise overexposures, we measured output levels from multiple manufacturers and multiple styles of headphones. Based on these measurements, we calculated the time it would take a listener to reach a 100% noise dose for a given PSS, headphone, and volume control setting. Although there were significant differences between PSS manufacturers, with maximum output levels being between 91 and 121 dBA, the smaller the headphone (i.e., in-the-ear earphones vs. over-the-ear earphones), the higher the output levels at a given volume control setting. Across models within one manufacturer, output levels were similar as long as the same headphone was used. Our study also looked at peak output levels from percussion beats in the music. At the highest volume control setting, some peaks in music samples exceeded 130 dB SPL (with the highest level recorded at 139 dB SPL).
In all, we concluded that a person would reach a 100% noise dose within 1 hour of listening with the volume control set to "7" (where "10" is the highest level), if the person were using supra-aural headphones (that is, headphones that rest on top of the ear). For safety recommendations, it was suggested that a PSS user limit the volume control to no higher than "6" and limit daily listening to one hour, if using supra-aural headphones. Using headphones that resulted in boosting the output levels (in-the-ear earphones used in this study) decreased the tolerable listening time at level "6" to one-quarter (that is, 15 minutes of listening) to one-twelfth (only 5 minutes of listening). The actual decrease varied considerably from model to model.
The recommendations and specific guidelines based on our research data reported (Fligor and Cox, 2004) are theoretical and assume that damage-risk from music is the same as damage-risk from occupational noise. This may not be a forgone conclusion. Further studies into the effect of temporally and spectrally varying sounds on susceptibility to noise-induced hearing loss are needed. Enjoy your headphones, but watch your noise dose.
REFERENCES
Catalano, P.J., & Levin, S.M. (1985). Noise-induced hearing loss and portable radios with headphones. International Journal of Pediatric Otorhinolaryngology, 9(1), 59–67.
Fligor, B.J., & Cox, L.C. (2004). Output levels of commercially available portable compact disc players and the potential risk to hearing. Ear and Hearing, 25, 513–527.
Katz, A.E., Gertsman, H.L., Sanderson, R.G., & Buchanan, R. (1982). Stereo earphones and hearing loss. New England Journal of Medicine, 307, 1460–1461.
Prince, M.M., Stayner, L.T., Smith, R.J., & Gilbert, S.J.(1997). A re-examination of risk estimates from the NIOSH Occupational Noise and Hearing Survey (ONHS). Journal of the Acoustical Society of America, 101(2), 950–963.
Rice, C.G., Breslin, M., & Roper, R.G. (1987a). Sound levels from personal cassette players. British Journal of Audiology, 21, 273–278.
Rice, C.G., Rossi, G., & Olina, M. (1987b). Damage risk from personal cassette players. British Journal of Audiology, 21, 279–288.
Wood, W.S., & Lipscomb, D.M. (1972). Maximum available sound pressure levels from stereo components. Journal of the Acoustical Society of America, 52, 484–487.
Originally published in Audiology Today Volume 17, Number 2.