While the media continue to spotlight the risk of noise-induced hearing loss (NIHL) from personal music players (PMPs), a number of myths have surfaced in the public arena. It's important for professionals to stay informed and provide contemporary perspectives and accurate information to those for whom we care.
MYTH 1: Personal music players are a primary reason for NIHL in children.
PMPs are the most commonly reported "noise" source for children 6–19 years old who have their hearing tested as part of a research project at the Dangerous Decibels exhibit at the Oregon Museum of Science and Industry (OMSI). Hearing tests and noise history data is available online at Dangerous Decibels; data accessed on April 15, 2009, reveals that 65% (n=2,266) of youth use stereo headphones and 20% report using firearms. Sixteen percent (n=25,651) of youth in this age bracket have thresholds worse than 20 dB HL at 4 kHz.
Although PMP use may be implicated in NIHL, it is unlikely that PMPs are the primary reason for the occurrence of notched high-frequency hearing loss configurations at young ages.
NIHL develops gradually over long periods of exposure to a variety of noise sources. NIHL will take years to develop in the small percentage (~5%–10%) of individuals who listen at hazardous sound levels for extended periods of time. Immediate acoustic trauma from fireworks and/or firearms is a much more likely culprit for the notched high-frequency hearing loss evident in young children (Gupta &Vishwakarma, 1989).
MYTH 2: PMP manufacturers have eliminated the risk of NIHL by providing a means of locking the PMP's volume control setting.
Efforts to reduce hearing loss risk by limiting volume level fail to take into account that NIHL risk is due to the combination of time and sound level, not just the listening level. If the level were to be limited to the maximum level known to not increase risk for NIHL, then no headphone would transduce sound above 78 dBA (Melnik, 1991). In many circumstances a person might want or need to turn the sound up higher than 78 dBA, so this solution would not survive in the marketplace. Legislation passed in France in 2005 limited PMP sound output to a maximum 100 dBA and voltage output to to a maximum 150 mV (Legifrance, 2005). However, 100 dBA is not "safe" by any stretch of the imagination; listening for longer than 15 minutes at this "limited" level exceeds the National Institute for Occupational Safety and Health (NIOSH) recommended exposure level (NIOSH, 1998). Some headphone manufacturers claim to limit output to 85 dBA, but this approach neglects to consider the risks of listening duration and inappropriately polices chosen listening level when listening duration is less than several hours per day. Software that can be set by parents to provide a maximum output on the PMP has merit because it can help parents feel confident that at least their child isn't listening at maximum volume. Without expert guidance to assign an appropriate maximum limit, however, parents operate with a false sense of security and potentially put their children at risk.
MYTH 3: Insert earphones are worse than other styles of earphones for your ears.
The media picked up on a detail from a study by Fligor and Cox (2004) that sound levels produced by earbud headphones were several decibels higher than over-the-ear headphones at the same volume control. The intent was not to malign earbuds, but rather to demonstrate the caveats of trying to apply general "rules of thumb" about maximum recommended listening duration and listening level. Data presented at the NIHL in Children at Work and Play meeting in October 2006 and forthcoming in a peer-reviewed journal document that regardless of the output capacity of an earphone (earbud vs. over-the-ear), people use headphones at the same listening level and that this chosen listening level is, in part, determined by the background noise in the listening environment. Some in-the-canal earphones that isolate the ear from background noise are actually used at lower listening levels than over-the-ear earphones that have lower maximum output because the users don't have to ramp up the volume to hear over the ambient sound.
MYTH 4: The music is too loud if you can hear it from your child's headphones.
This myth suggests that being able to overhear someone else's music when the earphones are in the user's ears is a good screening measure for detecting when sound levels exceed some predetermined maximum safe listening level. Conversely, this myth then also suggests that not being able to hear the music means the levels are safe. Despite the earlier arguments that focusing efforts only on the listening level is ill-advised, Jamie Weiner and this article's first author conducted a recent AuD thesis project under the direction of Brian Kreisman at Towson University that debunked this myth.
Considering "If I can hear it, that means it's too loud!" as a screening measure for NIHL risk, Weiner conducted a study to see if overhearing someone's music—or not overhearing it—was closely related to levels above or below 85 dBA. Background noise levels were varied from a quiet sound booth up to 75 dBA of background noise, and PMP users were instructed to turn the music to the level where they liked it. Even in a listening environment with high background noise (where this "screening measure" performed best), the positive predictive value was only 0.42 (i.e., the screening measure was correct only 42% of the time). The study will be published in the May 2009 issue of Audiology Today.
MYTH 5: 85 dBA time-weighted average is a safe noise exposure reference for children when applied to PMPs.
An 85 dBA time-weighted average (TWA) may not be low enough. There are no longitudinal studies that have examined the long-term relationship between PMP listening level, listening duration, and music spectral content in children. In fact, we do not know if children's auditory systems are more or less susceptible to noise damage and if there are developmentally sensitive periods for increased risk of NIHL. The 85 dBA time-weighted average (TWA) exposure criteria arose from occupational policy and guidelines (OSHA, 1983; NIOSH, 1998) and were designed to "risk" a percentage of the adult workforce. The use of an 85 dBA TWA criteria may assume a risk we are unwilling to take in children.
MYTH 6: Sound levels measured at the eardrum can be directly compared to damage risk criteria.
The short answer is "no, definitely not." The ear canal serves to amplify sound at certain frequencies (i.e., ear canal resonance, real-ear unaided gain, etc.) so levels recorded at the eardrum are several decibels higher than outside the ear. How many decibels higher the sound is at the eardrum depends on the frequency content of the signal, so it is difficult to assign a single "correction factor" that applies in all circumstances. Epidemiologic studies conducted in the 1960s and 1970s that form the basis for our understanding of hearing loss as a result of chronic noise exposure were conducted using instrumentation with microphones measuring sound in the free-field or diffuse-field, rather than at the eardrum. There is at least one study (Shotland, 1996) that conducted noise dosimetry at the eardrum and suggested that some of the wide variability seen in individual susceptibility in NIHL is due to differences in the acoustics of ear canals between people. Regardless, sound levels measured in the ear canal will overestimate the risk, resulting in overprotection. Some might say, "that's a good thing!" Except consider that the more permissive a rule, the more easily a person might adhere to it.
MYTH 7: PMPs should never be played at hazardous sound levels.
Have you ever turned up that one favorite song? Go ahead, admit it...we have all done it at some point in time and for most of us there was no immediate hearing damage. The potential noise risk from PMPs comes primarily from repeated exposures to high-intensity sound over extended periods of time. A high-level exposure to one song may not be hazardous and we should be realistic and practical when discussing the risk.
MYTH 8: Noise cancellation earphones provide safe listening because they cancel the hazardous noise.
As described in #3, some earphones block outside noise, allowing the user to moderate listening levels when listening in a noisy environment. However, simply having a tool that allows a person to moderate listening levels does not mean they know how to use it. As well, roughly 5% to 10% of users listen at high levels regardless of background noise, so noise canceling or sound isolating earphones will not reduce their risk.
MYTH 9: A recommended maximum volume control setting and listening duration for adults is appropriate for children and babies.
Listening level and loudness judgment measurements have not been made on very young children and infants and conducting such research presents challenges. Due to the smaller size of a child's ear canal, sound levels will be higher than they would be in an adult's ear at the same volume control setting. So, can we just ask a child if it is "too loud"? At what age can a child make appropriate loudness judgments? For that matter, can adults judge whether or not sound levels are sufficiently high to put them at risk for NIHL? Literature to date suggests otherwise. Macpherson, Elfenbein, Schum, and Bentler (1991) suggest that the threshold of discomfort can be adequately assessed in children whose cognitive age is at or above 5 years. In addition, other childhood health conditions may influence chosen listening level. For instance, a child with otitis media might be expected to turn up the volume on the player. Autism also may have an influence on recommended volume control settings. Intolerance to loud sounds and an increased perception of loudness has been reported in children with autism (Khalfa et al., 2004).
MYTH 10: Today's PMP technology puts listeners at greater risk.
PMPs produce free-field maximum sound levels that are no higher—and in some cases they are lower—than those produced by most portable CD players and portable cassette players. The maximum output of portable cassette players has been reported to range from 98–108 dBA (Turunen-Rise, Flottorp, & Tvete, 1991). The maximum output from CD players has been reported to range from 91–121 dBA (Fligor & Cox, 2004). Perhaps the concern that current PMP technology is more "dangerous" is that their storage and power supply capacity far exceeds earlier technology, allowing a person to listen longer. The data are sparse regarding use duration of earlier technology and it is unclear whether people do in fact listen longer now than they did with the older technology.