Sound Industry and Hearing Loss - Part 2

Here's Part One

HEARING 101

The healthy ear is a sophisticated system of acoustical, mechanical, chemical, hydraulic and electrical engineering. It involves the passage of a signal through the ear canal, where sound energy is given a boost between 1.5 and 2.7 kHz due to the natural resonance of the canal. It is then mechanically transduced through the action of an efficient impedance transformer - the middle ear. Most of the sound entering the middle ear is passed on into the fluid-filled space of the cochlea, or the inner ear. The cochlea, the sensory mechanism for hearing, is essentially a real-time frequency analyzer. Intensity and timing or phasic (directional) differences of a sound source also begin their processing here.

Not only does the hearing system receive sound information from the environment; it also sends it to the brain, where highly complex bioelectric processes allow the listener to associate sounds with meaning, emotions and other complex cognitive processes. Interestingly, after sound information reaches the brain, it travels back to the ear. This efferent auditory system, while not well understood, is thought to be responsible for the high degree of frequency specificity with which we can hear.

If there is any damage to sensory cells in the inner ear, there will be a loss of frequency selectivity in the cochlea. Damage in the cochlea means that the ear's dynamic range is also reduced. After hearing loss has occurred, the brain becomes starved for the auditory information it was accustomed to receiving. Once the sensory cells in the cochlea are damaged, the loss of those cells begins a chain reaction of cell death. Supporting cells in the cochlea are destroyed, as are the neural connections from the cochlea to the brain. This loss can occur over the course of several years. Therefore, even audio professionals' ears may not immediately notice the perceptual changes taking place. Almost always, hearing loss from overexposure to loud sounds will first result in a loss of the ability to hear high frequencies and, later, middle and low frequencies.

EFFECTS OF HEARING LOSS Not always discussed, but of equal importance to many professional listeners, are the functional effects of hearing loss as well as preventative measures one can take to maintain hearing health. Most people begin to realize a change in their hearing ability during day- to-day activities. They may struggle to understand a conversation in a noisy environment, or even in a quiet room with more subtle background noise. In more extreme cases, people may notice timbral changes in music. They may find themselves increasing the volume on the stereo or television. The problem is that some people suffering from hearing loss need more than an increase in volume. They need more clarity of sound.

Audiologists can fit a person with a hearing aid to amplify sound, but not to clarify or distinguish sound. In order to improve hearing- aid performance under noisy conditions, scientists at the House Ear Institute have focused their research on binaural hearing. Sigfrid Soli, Ph.D., and Michael Nilsson, Ph.D., have found that the binaural system uses the time and level differences of sound as it arrives at the left and right ears to improve hearing in noise. They applied this knowledge to develop the normalized Hearing in Noise Test (HINT) in order to measure the functional ability to hear in noisy conditions.

"The results of our tests showed that some people, who generally demonstrated normal hearing thresholds in a clinical test, may show a handicap in the binaural/directional HINT - especially if they have a mild to moderate high-frequency hearing loss," said Dr. Soli. These results demonstrate how the HINT can be used to screen sound professionals and others in hearing-critical jobs. Sound professionals need to detect, recognize and distinguish sounds that occur in the presence of background noise.

By testing hundreds of people, House Ear Institute scientists found that normal pure tone sensitivity did not necessarily indicate normal functional ability. That is, clinical hearing test results may differ from those that test for ability to function in daily life. Consequently, hearing is being evaluated in new ways, especially for hearing-sensitive occupations. This may lead ear specialists to redefine the measures associated with hearing loss for disability purposes. This new direction of thought is important because hearing specialists will be able to maximize the benefits for users of prosthetic devices and apply this knowledge to hearing aid research.

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Rachel Cruz is an audio engineer and licensed audiologist currently doing research on music perception. Dilys Jones is House Ear Institute's marketing/communications director. She develops and administers the Sound Partners program. Contact them at soundpartners@hei.org, and find more information at www.hei.org.

Advances in Audio Testing Two organizations recently made donations to the HEI's Sound Partners program, enabling audiologists to offer hearing tests to audio professionals in Los Angeles. Sennheiser Electronic Corporation donated two pairs of closed dynamic HAD 200 Audiometric headphones. The Western Lion's Ear Foundation donated funds for the purchase of a 2-channel audiometer that measures hearing thresholds up to 16 kHz. The House Ear Clinic will use this equipment to conduct EHF tests as part of a pilot study and gather data from the clinic's test results to establish clinical norms and improve testing methods.

Some audiologists are now performing a diagnostic test called otoacoustic emissions. A healthy cochlea not only receives energy, but also emits a small amount of energy. As sensory cells in the cochlea are damaged, the cochlea will emit less energy at various frequencies. This objective test reveals information about cochlear health. Typically, a loss of sensory cells in the cochlea will be seen as a loss of OAEs prior to a subjectively perceived hearing loss in pure tone thresholds. Often, this is a warning sign to reassess daily sound exposure.

Managing Your Hearing Health The initial point of entry into a hearing health program is usually through an audiologist, who will test the health of the entire hearing system. You should find an audiologist able to perform extended high-frequency (EHF) testing or high-frequency audiometry, pure-tone threshold testing from 12 to 16 kHz or higher.