Hearing Loss—Decibels or Percent?
© December 2000 by Neil Bauman, Ph.D. (Revised February 2003)
Question: From time to time, I see people writing, "I have 78% hearing loss in my right ear and 95% in the left."
What does this percent mean? I thought sound was measured in decibels (dB), not percent? If this is the case what percent is 115 dB?—R. D.
Answer: Excellent questions.
You have good reason to be confused because you cannot equate decibels to percentages no matter what anyone tells you.
Decibels vs. Percent
Sound intensities are indeed measured in decibels (dB). There are two reasons
why you can never equate
decibels to percentages. First, the decibel scale is open-ended like that of the Richter scale used for measuring
earthquake intensities. To calculate a percent you need to know the maximum value possible. In both of these scales
there is no limiting maximum value. Therefore, you cannot calculate a percentage. Any attempt to do so is just a bunch of meaningless gibberish!
Second, the decibel scale is logarithmic, while the percent scale is linear.
Numbers that appear to be similar have vastly differing meanings. They are as different as trying to compare apples
When people (ignorantly) talk about having a 50 percent hearing loss they
likely mean that they have a 50 dB loss. Where did the idea come from that we can measure hearing loss in percentages?
Here is how Brad Ingrao, an outstanding audiologist, explained it.
To measure sound intensity (the way audiologists measure it) you need to
do a mathematical calculation that is so strange that 20 + 20 = 26 dB (SPL).
"To make a scale that makes sense to most people (including us knucklehead
audiologists), a different equation is used to convert sound intensity using the Sound Pressure Level (SPL) scale
to the Hearing Level (HL) scale that goes from 0 dB HL (normal threshold) to 120 dB HL (pain).
If we forget about hearing losses greater than 100 dB (like most people tend
to do), we get 0 dB to 100 dB as the usable (dynamic) range of hearing for the average 'normal' ear.
Since doctors and audiologists tend to under-estimate their patient's ability
to understand such things (or they don't understand it themselves), the erroneous concept of dB = % evolved."
There you have it folks. It seems health care professionals think we are
too stupid to understand much, so they let us believe error rather than teach us the truth.
We can put a stop to this nonsense right now. Let's understand how this decibel
scale works and why using a percentage value to describe our hearing losses is so very wrong.
First we need to understand that a decibel is not a given intensity (loudness)
of sound, but rather, it is a ratio
of how many times louder (or softer) a sound is than a given reference sound level.
This means that 0 dB is not the absence of sound, but is an arbitrary zero.
We define it as the faintest sound that a young sensitive human ear can hear. Furthermore, because the decibel
scale is logarithmic, every 10 dB increase in sound intensity is actually a ten-fold increase. Therefore, a sound intensity of 20 dB is not twice as loud as a sound intensity
of 10 dB, but is 10 times as loud, and a sound intensity of 30 dB is 100 times as loud as a sound intensity of
10 dB. Similarly, a sound intensity of 50 dB would be 100,000 times as loud (10 x 10 x 10 x 10 x 10). This is how
the decibel scale works. It is totally unlike the linear percent scale.
Now lets see the fallacy of trying to compare this "funny" decibel
scale to the percent scale. To illustrate this, let's assume (remember this assumption we're making here is totally
wrong) that 0 dB is equal to 0 percent hearing loss and that 100 dB equals a 100 percent loss. This would then
mean that 50 percent would equal a 50 dB hearing loss, right? Wrong! Not by a long shot! A 50 percent hearing loss would equal, believe it
or not, only a 3 dB loss! Looking at it the other way, a 50 decibel loss is not just half as loud, like it would be in a percentage scale, but would
only be one thousandth of one percent as loud!
Here is another example. I have a 70 dB loss. This is not equal to a 70 percent
loss by any means. In actual fact it means that the softest sound I can hear needs to be 10,000,000 times louder than the softest sound
a person with normal hearing can hear. One out of ten million is definitely not a 70 per cent loss but would be
a loss of 99.9999999%! Quite a difference, isn't it? Now you can see why we must never use percentages when talking
about our hearing losses. They just do not equate. They are absolutely meaningless!
Percent Used to Describe Discrimination
Although we cannot use percentages to describe our hearing losses, we correctly
use percentages to describe our ability to discriminate sounds. To determine our ability to discriminate between words, our audiologist sets
the volume at our most comfortable listening level. She then has us listen to a list of words and we repeat back
what we think we heard. The number we get right, converted to a percentage, becomes our discrimination score. Therefore,
if I understood 80 out of 100 words in my right ear, my discrimination is 80% for that ear. I may have an entirely
different result for my other ear. Consequently, we can correctly describe our ability to understand what we hear as a percentage. A person
could correctly say that his discrimination is 78% in his right ear and 95% in his left ear. But this has nothing
to do with the severity
of our hearing losses as such.
Percentage and Hearing Disability
If your hearing loss resulted from an accident on the job, there is a formula
that is used to calculate the percent disability pension for which you may be eligible. Don't get mixed up. This
is not your hearing
loss expressed as a percentage. Rather, this formula calculates how much your degree of hearing loss supposedly
impacts your ability to remain employed at full wages.
For example, plunking your hearing loss levels into the formula may yield
a result of 75%. This means that with your particular hearing loss, you may be
entitled to a 75% disability pension. Again, this is not your
average hearing loss expressed as a percentage. If you are interested in how
they calculate a percentage disability for any given hearing loss, see my article you
can read it at "How Much Are You Worth as a Hard of Hearing Person?".
Classifying Our Hearing Losses
Hearing health care professionals classify hearing into several categories
such as normal, slight, mild, moderate, moderately severe, severe, profound and deaf. Not all of them use all of
these categories, nor do they all use the same hearing loss ranges in each one. In the past, most used this simple
Simple Hearing Classification Hearing Threshold
||down to 20 dB
|Mild hearing loss
||21 to 40 dB
|Moderate hearing loss
||41 to 60 dB
|Severe hearing loss
||61 to 90 dB
|Profound hearing loss
||greater than 90 dB
Today, research has shown that even hearing losses of only a few decibels
can cause significant hearing problems. As a result, many hearing health care professionals have fine-tuned this
scale to better reflect this reality. (Note that these ranges are arbitrary and may vary slightly among authorities.)
Today's Hearing Classification Hearing Threshold
||-10 to 15 dB
|Slight hearing loss
||16 to 25 dB
|Mild hearing loss
||26 to 40 dB
|Moderate hearing loss
||41 to 55 dB
|Moderately severe loss
||56 to 70 dB
|Severe hearing loss
||71 to 90 dB
|Profound hearing loss
||91 to 120 dB
||greater than 120 dB
Describing Our Hearing Losses
Unless you have a "flat" curve on your audiogram, how can you accurately
describe your hearing loss? Your hearing loss could be different at every frequency so one word could be meaningless.
The best way is to be specific. If I have the typical "ski slope"
hearing loss, I could describe it as, "I have a 30 dB loss at 500 Hz, dropping to 100 dB at 4,000 Hz."
A more general way, but still accurate, would be to describe it as, "I have a mild loss in the low frequencies,
dropping to profound in the higher frequencies.
The next best way to describe our hearing losses is to average the 4 frequencies
that carry most of the speech information to arrive at a single figure. Use the
following four frequencies—500 Hz, 1,000 Hz, 2,000 Hz and 3,000 Hz—and average
the hearing loss at these frequencies to come up with one figure. However this
method falls down if we only have a bit of hearing left in the very low frequencies.
Incidentally, it is not right to take the average of our best and worst figures.
That could give a very wrong impression of our hearing losses.
If you want a very simple way to describe your hearing loss, the most accurate
(and simple) is to say you have either a mild, moderate, severe, or profound hearing loss. Your audiologist can
tell you which category your hearing is generally in. (Remember, you could be mild in the low frequencies and profound
in the highs—but to oversimplify, you can reasonably accurately reflect your practical hearing loss by using one of these categories.) It is much more meaningful, and far
more accurate than trying to use a meaningless percentage. Let's get back to using these standard audiological terms and stamp out this absurd percent business.