Real World Gear: What We Know AND What We Don’t
An expert view on IEM and hearing
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Both in print and in casual conversation among musicians and sound professionals, several claims are regularly made regarding the potential benefits in-ear personal monitoring systems (IEM) provide in terms of hearing conservation, and often, this is contrasted to the approach of using conventional wedge monitors. While studies by both manufacturers as well as third parties have tried to identify and/or quantify the benefits, to date no studies have been published, often because proprietary information about a device is necessarily withheld by the group performing the study. |
So... What do we really know about what happens when we couple an IEM
system to a healthy hearing mechanism? Let’s review what we currently
know about the hearing mechanism (better known as the human ear), IEM,
and the interaction between the two, then compare this information to
questions that have not yet been addressed but need to be answered before
making an educated decision about any benefits and limitations of IEM.
Professional musicians and sound people face a common dilemma. The accuracy
and emotional nature of performance is (to some degree) dep-endent upon
the acoustic feedback they receive. How-ever, overexposure to these (too
often) loud sounds can put the end-user at risk of losing the ability
to hear accurately.
Regardless of musical genre, we know that loudness levels at concerts
regularly exceed safe levels of exposure, at least as defined by several
government agencies like OSHA and NIOSH. When levels are excessive, functional
and physiological hearing problems are almost certain to follow.
THE BIG THREE
In the broadest sense, there are three types of hearing loss people can
inherit or acquire over their lifetime.
Conductive loss refers to damage in the ear before the cochlea. Examples
of conductive losses are an object in the ear canal (such as too much
earwax or a cyst), a perforation or plaque growth on or around the eardrum,
a growth onto, or disarticulation of the middle ear “hearing bones” and
etc. Typically, this type of loss can be fixed with either pharmaceutical
or surgical therapy.
Central auditory processing disorder indicates damage or faulty processing
in the neural pathways and/or hearing centers of the brain.
And sensorineural loss refers to damage to (or after) the cochlea. This
may or not include neural damage from the auditory nerve to the early
auditory processing centers in the brain. A sensorineural loss is permanent,
often progressive (it becomes worse over time), and cannot be completely
or perfectly restored to normal hearing by medical or surgical intervention.
Damage from overexposure to loud sound can affect the hearing mechanism
(the cochlea in particular) as well as the body. Our ears were designed
to hear over a 100 dB range of acoustic sounds, but not to tolerate sounds
over 130 dB SPL or greater.
The human cochlea is a real-time frequency analyzer, capable of bioelectric
transduction of signals ranging from about 20 Hz to 20 kHz, as well as
recognizing streams of acoustic information, while at the same time being
able to discriminate as little as
1 Hz differences in the mid-frequency bands. The healthy hearing mechanism
can focus on a sound source in order to extract information in noise,
and can localize these sound sources in space.
The typical “noise induced” hearing loss (NIHL) results in a sensorineural
loss with a characteristic drop in hearing sensitivity at or near 4 kHz.
Ultimately (and sometimes prior to seeing a reduction in thresholds at
4 kHz), there is an accompanying high frequency hearing loss above 8 kHz.
Loss of high-frequency hearing impedes our ability to localize sounds
(the high-frequency cues we use for this task are no longer available
to the listener with a high-frequency hearing loss) and also effectively
causes a subtle “smearing” effect across different frequency regions of
the cochlea.
In addition, loss of hearing due to overexposure to loud sounds may affect
the way a sound is referenced or perceived (loudness) in relationship
to its actual sound pressure level. Historically, audiologists call this
“loudness recruitment”, but recently, work by Dr. Mary Florentine at Northeastern
University in Boston has demonstrated that what is really occurring is
a loss of sensitivity to soft sounds, rather than an abnormal growth of
loudness. Loudness in fact, is not “growing abnormally”, rather, NIHL
results in a loss of the lower portion of the listeners dynamic range.
It should also be noted (Mueller and Hall, 1998) that hearing loss from
overexposure to loud sounds could also result in non-auditory problems.
Examples include: illness, neuroticism, colitis, headache, endocrine disorders,
fatigue, hypertension, biochemical disorders, insomnia, cardiac disease,
ulcers, and irritability.
IEM ENTERS THE PICTURE
There are two broad classes of IEM earpieces: custom-made, one-piece;
and modular, where the output transducer is coupled to the earpiece. Custom-made
units are created from ear-mold impressions of the user’s ear, and typically,
have greater bandwidth, which is perceived as superior in terms of overall
sound quality.
Modular earpieces, on the other hand, can have a custom-made ear-mold
or can use foam or silicone to create “one-size-fits-all” earpieces. Their
bandwidth is typically more limited and may provide a poorer signal to
noise ratio across the bandwidth.
Since the development of IEM, many manufacturers have recommended it as
a method of improving fidelity for performers while decreasing overall
stage volumes. While it’s true that IEM offers the advantages of being
tiny, light weight and providing significant improvement over stage wedges
in terms of overall fidelity (due to the proximity of the amplifier to
the eardrum), it has yet to be demonstrated whether IEM can be successfully
used as a method of hearing conservation.
In hearing literature, as well as in some consumer-based audio magazines,
many authors regularly cite the superior acoustic isolation provided by
custom-made earpieces versus modular models. However, no actual data has
been published to date to substantiate most if any of these claims.
At the same time, IEM does provide several obvious benefits including
increased mobility on stage, individualized mixes with individual volume
controls on the performers belt-pack, and differing amounts of acoustic
isolation from amplifiers.
Of course, acoustic isolation can also present a problem for the performer.
Most commercially available IEM systems don’t allow easy communication
between performers, since they don’t include microphones or transmitters.
The earpiece needs to be taken out and re-inserted in order to talk onstage.
This can be an inconvenience.
Another complaint often mentioned is that the acoustic isolation gives
the listener the sensation of isolation from the audience. This is usually
remedied by mixing an ambient mic into each performer’s mix.
LIABILITY ISSUES
While most professional touring sound companies do not report statistics
regarding hearing loss of employees, given the increasing incidence of
litigation from audience members at concerts where sound levels have been
extremely high, it may become important for such companies to pay increased
attention to the requirements of other industries where reporting audiometric
data is commonplace.
According to a recent report in the November/December 2003 Access Audiology
newsletter, published by the American Speech Language Hearing Association,
effective January 2004, OSHA will put into effect a new rule applicable
to employers who record and report hearing loss of their employees. New
requirements include use of a “two-step criterion for determining when
to report recordability of hearing losses”. Work-related hearing losses
must be reported to OSHA when:
1) There has been a “standard threshold shift”, defined as an average shift in hearing threshold of 10 dB or greater at 2 kHz, 3 kHz, and 4 kHz, relative to the audiometric baseline (formerly 25 dB);
2) The average hearing level in the same ear is 25 dB or greater at 2 kHz, 3 kHz and 4 kHz.
OSHA hopes that implementation of these new rules will cause employers
to become more aware of noise exposure as an occupational hazard, and
therefore more motivated to provide improved hearing conservation and
noise control programs. Audio professionals might consider incorporating
these guidelines into their own private record keeping of employee audiometric
data.
OSHA will also begin recording and tracking hearing loss on the “OSHA
300 Log of Workplace Injuries and Illnesses”. This means that accurate
statistics regarding hearing loss will now be monitored by a central agency,
and employers will be able to quantify work-related hearing loss in addition
to other injuries acquired on the job.
According to author Claire Bernstein: “For audiologists, this represents
a dramatic advance and is expected to have a major impact on tracking
the real incidence of work-related hearing loss. Driven by financial considerations,
most employers rely on hearing protection in lieu of noise control. With
these statistics OSHA will now have more thorough tracking information
so it can better direct research, training, and enforcement activities.”
Since listening is our life’s work, and exposure to loud sound is part
of the job, it’s important to take preventative steps, rather than attempt
to eradicate loud sound altogether, in an effort to care for our ears.
There are several easy things you can do to improve your hearing health.
• Monitor both the intensity level and duration of exposure. If the duration of exposure will be long, try to limit the intensity level. Conversely, louder levels may be acceptable, but for a shorter duration of exposure.
• Pay attention to your physical relationship to other sound sources. Standing directly in the path of the sound source can cause greater levels of exposure, compared to enclosing or baffling a sound source or standing at an angle from the source.
• After exposure, give your ears a rest. Take at least an hour to sit somewhere quiet. If your session runs long, take frequent breaks.
• Be aware of exposure to non-musical, “recreational” noises like loud television or driving with the windows open at high speeds. If possible, use hearing protective devices when environmental noises seem excessive.
TAKING CARE
If a musician chooses to purchase IEM, an experienced audiologist should
“shoot” the earmolds for the end-user. Responsible audiologists should
make themselves available to attend either a rehearsal and/or a soundcheck
where they can take “probe microphone measurements”. By placing a small
microphone in the ear canal, an audiologist can take a measure of how
loud (in dB SPL) the user “wears” their IEM. This information makes it
possible for the audiologist to counsel the user on a preferred range
of loudness to monitor at during performances, which should help minimize
the risk of hearing loss.
In addition to careful fitting and the measuring of loudness levels, music
and audio professional should have their hearing professionally tested
at least once a year. If possible, it would be ideal to find an audiologist
who can perform “high-frequency audiometry”.
By industry standards, audiologists typically test up to only 8 kHz, but
some audiologists have the specialized equipment necessary to test up
to 18 kHz or 20 kHz. Although there are no established “normal” criterion
for high frequency audiometry, serial test results my be compared over
time, which may be a useful clinical indicator of early hearing loss in
the long run.
Regular recordings of “otoacoustic emissions” may also provide evidence
of sub-clinical hearing loss (damage to the cochlea that is seen before
it effects you to the point where there is an audiometric decrease in
hearing thresholds).
When used cautiously, IEM may offer a dramatic range of improvements for
the professional audio engineer or touring musician. Appropriate use may
also serve to protect your most valuable and irreplaceable instrument;
your ears.
Hearing Note 1
Using an IEM system does not guarantee hearing protection! Some IEM rigs are capable of producing levels in excess of 130 dB SPL. Prolonged exposure to these kinds of levels will likely cause hearing loss. It is up to each user to be responsible for protecting his/her hearing.Hearing Note 2
Tinnitus (head noise) is a common occurrence and occurs in many forms. It can be intermittent or constant, mild or severe, and vary from a low roar to a high-pitched type of sound. It also may be subjective (audible only to the patient) or objective (audible to others). Further, it may or may not be associated with hearing impairment.
Tinnitus must always be thought of as a symptom and not a disease. Because the function of the auditory (hearing) nerve is to carry sound, when it is irritated from any cause, the brain can interpret the impulse as noise. (Courtesy of House Ear Institute)Limiting Your Exposure
The National Institute for Occupational Safety and Health (NIOSH) guidelines are stated in terms of the maximum time that you can safely be exposed to different time-weighted averages (TWAs) of sound pressure on a daily basis over a 40-year period.
TWA dB Levels, Maximum Exposure Time
85 dB 8 hours
88 dB 4 hours
91 dB 2 hours
94 dB 1 hour
97 dB 30 minutes
100 dB 15 minutes
103 dB 7-1/2 minutes
106 dB 3-3/4 minutes
Exposure to 115+ dB or greater may pose a serious health risk.
Rachel Cruz holds a Bachelors degree in Music Production and Engineering from Berklee College of Music and a Masters degree in Audiology and Hearing Sciences from Northwestern University. She is a research associate at the House Ear Institute in Los Angeles, in the Department of Auditory Implants and Perception, and works as a private contractor providing musicians’ hearing services (fitting IEM) with Sensaphonics Hearing Conservation. She can be contacted at rcruz@hei.org. For the full bibliography used in research of this article, see ProSoundWeb
January 2004 Live Sound International