The EM Poll

Say What?

Jan 11, 2008 4:14 PM, By Joanna Cazden

           

I lay on the hotel bed, flat-out wasted. I'd spent the day walking around the 2001 Winter NAMM convention (the annual trade show for all kinds of musical gear), so my feet hurt. Far worse, I'd spent the evening with Spinal Tap, proudly known as England's loudest rock band.

I'd brought my trusty custom-fitted earplugs to the concert, of course, and I had beamed approvingly at the ticket takers who offered foam plugs from big bowls at the door. Nevertheless, Spinal Tap lived up to its reputation (Nigel's amp was definitely at “11”), and my pocket SPL meter routinely clocked the sound at 118 dB. I was plugged, it was great fun, but it was loud! By the time I crashed, my ears were ringing — roaring like surf on the residue of too much noise.

Most bands don't approach Spinal Tap's dizzying decibels, but these days 100 dB or higher is normal for smaller rooms. That's more than enough to do real damage during the typical three- or four-hour club gig. If you regularly perform at these levels, you're at serious risk for hearing loss. But before you hear the bad news about permanent injury — and the good news about increasingly affordable, high-quality protection — it's important to understand the exquisite yet vulnerable ear.

ANATOMY LESSON

Scientists divide the human auditory system into three areas: the outer, middle, and inner ears (see Fig. 1). These components form a delicate instrument that transforms acoustic energy into electrical impulses that the brain interprets as sound.

FIG. 1: The human auditory system consists of three parts: the outer, middle, and inner ears. Noise-induced hearing loss occurs in the cochlea within the inner ear.

The outer ear gathers and directs sound through the ear canal to the eardrum or tympanic membrane. The eardrum vibrates in response to the acoustic energy and transmits this vibration to a series of three tiny bones, called the ossicular chain, in the middle ear. The last bone in the chain sends the vibration into the fluid-filled inner ear, or cochlea, a structure coiled like a snail shell.

FIG. 2: These are electron micrographs of cochlear hair cells. The ones on the left are healthy, whereas the ones on the right have been damaged by noise exposure.
Click here to enlarge image

Extending the entire length of the cochlea is a small, flat structure called the basilar membrane, covered with more than 30,000 microscopic hair cells (see Fig. 2). Think of a fuzzy carpet on a coiled, spiral-shaped ramp. Each hair cell responds to a particular frequency, depending on its location between the base of the spiral (high frequency) and the end or apex (low frequency). The hair cells are also connected to sensory nerve fibers.

As the incoming vibrations travel through the cochlear fluid, they stimulate different areas of the basilar membrane to vibrate, depending on which frequencies the sound contains. The hair cells in each area generate electrical impulses in the corresponding nerve endings. The fibers of the auditory nerve then transmit these electrical signals to the brain.

In addition to frequency, the intensity or amplitude of the sound is encoded into the electrical impulses. However, when the intensity is too great for too long a period, it can damage the hair cells. Once hair cells collapse or die, contact with the nerve fibers is broken, and the perception of a particular frequency range is reduced or lost entirely.

Noise-induced hearing loss (NIHL) is the most common occupational injury, according to government statistics. In activities such as mining, manufacturing, and woodworking, NIHL is a side effect that may not interfere with the primary task. But for musicians and audio professionals, it distorts and then destroys the most important tool of your trade.

Overexposure to high sound levels can also lead to tinnitus, a debilitating ringing, buzzing, or roaring in the ears. An hour with Spinal Tap after a day at NAMM (with instruments and sound gear blaring from almost every booth) gave me some moderate tinnitus, which fortunately went away by morning. But if I worked in such a loud environment for a long time, I could develop permanent problems.

NIHL is insidious and irreversible, but it is also preventable. Like cigarette smoking or unsafe sex, exposure to high sound levels brings real dangers. So don't sacrifice your future musical enjoyment — and your livelihood — for a quick bone-buzzing blast. Take the long-range view and play it safe. Protection is up to you.

WORK RULES

Damage to the hair cells of the inner ear occurs with long-term exposure to sound-pressure levels (SPL) of 90 dB and above. The Occupational Safety and Health Administration (OSHA) defines long-term as eight hours per day for ten years. However, the higher the sound level, the faster the damage accumulates (see Fig. 3).

FIG. 3: The louder the sound, the less time you can safely be exposed to it without protection.

The OSHA standard cuts the safe exposure time in half for every 5 dB above 90. In other words, 95 dB is considered safe for four hours, 100 dB is safe for two hours, 105 dB is safe for one hour, 110 dB is safe for half an hour, and so on. In other countries, these guidelines are 5 to 10 dB lower, and studies show that at 85 dB long-term exposure will definitely cause hearing loss in a certain percentage of those exposed.

Consider that rock concerts at sustained levels above 100 dB can easily last three hours or more, placing everyone's hearing at risk. Last year, in a series of articles in the Bay Area Reporter on the blaring levels at dance clubs, journalist Ed Walsh documented music as high as 115 dB at the loudest club in San Francisco. That's not quite as bad as Spinal Tap, but it's worse than sandblasting. If this club were a factory, workers would legally be allowed in for only 15 minutes a day. Yet very few patrons or employees were observed wearing hearing protection of any kind.