noise induced hearing loss

Another repost from my old website. Hope you find it interesting!

Structure of a human ear
Structure of a human ear

Last Thursday I sat at the back of a noisy bus on my way to the station. In my ears were ear buds playing music at my usual fairly moderate level. You can imagine my astonishment then when I realised that I could hear somebody else’s music playing. I took a look around the bus and realised that across the aisle on the other side of another person was a girl listening to thumping techno at volumes that to her must’ve sounded roughly like a 737 landing on her face.

Unlike the irritating clowns who whinge in mX (the free afternoon newspaper distributed on public transport) about iPod listeners’ music distracting them from their furious navel-gazing on their journey home from their dreary jobs, I have no particular problem with being able to hear someone else’s headphones. Since I respect my ears I wouldn’t listen so loud but whatever floats your boat I suppose. Rant aside, it did get me thinking about noise induced hearing loss. So I did a bit of reading, learnt some stuff and hope to enlighten you a little.

The image of the human ear to the right gives a basic overview of what a human ear looks like. Sound waves travel down the ear canal and cause the eardrum to resonate. This vibration is transmitted through the ossicles and into the fluid filled cochlea. Within the spiral-shaped cochlea are a large number of very fine hairs. Their arrangement is somewhat analogous to the keys on a piano where hairs toward one end of the cochlea detect high frequency sound and those toward the other end detect low-frequency sound.

Noise-induced hearing loss is a result of excessive sound intensity being passed through the auditory system and causing damage. This produces a variety of abnormalities in the cochlear hair bundles including “…metabolic exhaustion of the hair cells, structural changes and degeneration of structures within the hair cells, morphological changes of the cilia, ruptures of cell membranes, and complete degeneration and loss of hair cells, neural cells and supporting cells” [3]. The result of this damage is a reduction in sound perception usually beginning in the 4-5kHz range and progressing into lower frequency range as severity increases. [1]

Cross-section of the cochlea
Cross-section of the cochlea

You may claim your music isn’t noise. Well for the purposes of this article we can class it as noise and the National Occupational Health and Safety Commission in Australia has set the national standard for exposure to noise in the occupational environment to be a daily average of 85dB(A). The decibel unit (dB) given here is a logarithmic scale for measuring sound intensity. It can also be used in the context of electrical current. The zero point for sound is defined to be the threshold of human hearing though some people may be able to hear sound at lower levels than this. A perceived doubling in sound levels corresponds to a 3dB increase on the scale. The (A) indicates that the scale has been weighted to correspond more closely to human hearing characteristics. As a reference, normal conversation is about 60dB and a noisy restaurant is about 80dB.

Preliminary testing data on iPods and similar music players sound output levels place the maximum between 100 and 115dB. Sustained exposure to sound of these levels causes hearing damage in time ranging from 2 hours to 15 minutes respectively. “In informal research, Wichita State University audiologist Ray Hull asked students to take off their headphones in the name of science. Taking readings with “a fairly sophisticated” sound level meter, Hull found typical listening levels approaching 120 decibels” [5].

It’s worth noting at this point that there is a level of redundancy in the ear to improve tolerance to auditory damage. The bad news is that generally by the time hearing-loss symptoms prompt medical attention, “the damage as measured by audiometry will be severe and, even with cessation of noise exposure, progressive” [1]. To avoid irreversible hearing damage, “[some] hearing experts recommend that you set the volume while in a quiet environment, turn the volume down if you can’t hear people speaking near you, avoid turning up the volume to block out noisy surroundings, and limit the amount of time that you use earbuds or headphones at high volume” [6].

I love my music, both playing and listening so am concerned enough to have set a maximum volume set on my player that I can converse over and I listen for 2 hours or less per day. If you want to do some more reading of your own, check out some of my references or do some searches for “noise-induced hearing loss”.

[3] Gelfand, S. (2001). Auditory System and Related Disorders. Essentials of Audiology: Second Edition (p. 202). New York: Thieme.
[8] National Standard for Occupational Noise [NOHSC:1007(2000)]

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