I often hear people say that they were told that they had lost a certain percentage of their hearing or that they were, for example, 60% deaf in their left ear. Most of the time, these patients have been tested elsewhere and the “loss percentage” discussion was a practical commercial pressure tactic.
I want to explain why using a percentage to describe the degree of hearing loss is almost always incorrect, and a better way to describe the degree of hearing loss.
The percentage of hearing loss is a concept used to describe the level of incapacity. It’s an estimate. The percentage is based on hearing thresholds, which cannot fully capture a person’s ability to understand speech and communicate. Threshold is the lowest sound where a patient is able to hear a tone or a word. Therefore, in practice, certain considerations go into determining and classifying hearing impairment.
Hearing care professionals record hearing test results on an audiogram, which is often simplified to cover the zero to one hundred decibel range. Each value on an audiogram is a hearing threshold and is a different concept of the level of disability. To see the dangers of confusing these concepts, one must understand that the decibel scale is logarithmic.
For example, a sound at 36 decibels has twice the sound pressure of a sound at 30 decibels. Logarithmic means that data must be added or multiplied by a number to get a result. It’s like adding the tax percentage to your receipt. The final answer is not final until a factor is added.
The percentage scale, however, is linear, which is a statistical term to describe a linear relationship between two variables. These are great mathematical concepts. The man who apprenticed me in the hearing aid industry said comparing logarithmic to linear is like trying to measure the volume of gas in a barbecue gas cylinder by placing it on a scale weighted. The reading on the scale in pounds or kilograms tells me nothing about the volume and pressure of gas in the cylinder.
In 1979, the American Academy of Otolaryngology published a formula for calculating the percentage of hearing loss, which remains in use in many states of the United States to this day. According to Incus.com (a scientific audio technology company in Hong Kong):
“The percentage hearing loss formula has several shortcomings:
— This is a single statistic that aims to summarize an individual’s overall hearing health situation. A lot of valuable information is lost when calculating this value.
— Since Percentage Hearing Loss is an average value, it does not tell us if the hearing loss is flat (with similar thresholds across all frequencies), sloped, bumpy, or differently shaped.
— The formula does not account for any hearing loss equal to or less than 25 dB, when in reality it is possible to see greatly reduced speech understanding even at relatively low thresholds.
— The inputs to the formula are pure tone thresholds at four specific frequencies over a range of 500 Hz to 3000 Hz.
— Pure tone thresholds do not necessarily translate to our ability to understand speech or distinguish between different sounds.
During a hearing test, I measure hearing thresholds, which should not be confused with a statistic used to quantify the level of disability caused by hearing loss. The severity of hearing loss can be described using the categories of mild, moderate, severe or profound hearing loss or normal hearing.
A patient can clearly see how far their scores go in the graph. This, coupled with their experience of poor hearing, gives them enough objective information to know if they need hearing aids. A percentage of the loss score is possible, but it is complicated and time consuming to determine and has the built in gaps described above.
Other than an otolaryngologist with decades of experience, no one can look at your audiogram results and pronounce an immediate accurate percentage of loss. Those who try to convince you they can are truly the least professional in the hearing aid industry.
If you have symptoms of hearing loss, call a hearing care professional today.
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Jeff Bayliff owns Hear the Birds in Lock Haven and can be reached by calling 570-660-6914.