July 2015: Lend Me Your Ear

Friends, Romans, Countrymen – Lend me your ears…

For many, the Fourth of July isn’t complete without a grand fireworks display. What began as a festive and patriotic way to dispose of surplus artillery ammunition is now an American tradition like baseball and apple pie. With these colorful explosions comes a lot of sound energy.  At Metropolitan Acoustics, we deal with sound energy every day. Read on to learn more about how you can deal with it, too.

Loud sounds have been a natural phenomenon since the dawn of time. Fortunately, Mother Nature thought of that and devised the acoustic reflex. When sound pressure levels cross a certain threshold, the muscles attached to the ossicles (the hearing bones) stiffen, allowing less stimulus through to the cochlea (the auditory portion of the inner ear). This works great for sounds that build relatively slowly or that repeat within 2-3 seconds; otherwise, the inherent latency of the reflex prevents it from protecting your ears.  In the case of a sudden, high amplitude impulse, the acoustic reflex simply has no time to react.

Impulsive sound is of very short duration and contains a broad range of frequencies. While this energy can be harnessed and put to good use (say, for measuring the reverberation time of a concert hall), repeated exposure to high amplitude impulses can be damaging to hearing. A common source of impulse sound energy is gunfire, which is the leading cause of recreational hearing loss with sound power levels in excess of 150 dB. Exposure to sounds at lower levels but longer duration can be damaging as well. This exposure can come from loud music, either through earbuds or at concerts, or working on a construction site. Regardless of the source, the long-term effects are the same: noise-induced hearing loss.

To prevent hearing loss due to workplace conditions, the Occupational Safety & Health Administration (OSHA) has established guidelines for the maximum sound exposure level and duration: 90 dBA for 8 hours. Every increase of 5 dBA reduces the permissible exposure time by one-half. This implies that one shouldn’t spend more than a half-hour in environments at levels continuously over 110 dBA…a level that could be reached at loud summer events.

Enter hearing protection.  Existing in innumerable forms, from the ubiquitous memory foam plugs to over-ear muffs to custom-fitted silicone inserts, these all serve as physical barriers to prevent excessive sound energy from reaching our hearing organs. When choosing ear protection, consider the nature of the sound source. If you are looking to block out intrusive noise while you sleep, consider a lightweight foam plug that provides 6-12 dB attenuation. Headed to a concert? Consider wearing a musicians’ plug that is designed to attenuate evenly across the audible spectrum and without distorting the music. If you use power tools or work in a loud environment, try over-ear muffs that attenuate sound up to 25 dB.

If it’s too loud, we suggest that you listen to us and protect your ears; you only get one pair! Next time you head to a good summer concert, grab some earplugs and rock out.

Philadelphia Inquirer: 7-14-15
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