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) dependent upon the acoustic feedback they receive. However, 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.
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 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.