Last month I completed a project on audiograms for a class. Realizing many people with hearing loss don’t understand what all the symbols mean, I thought I’d condense it into a blog. I am not an audiologist, nor an “expert” on audiograms. I’m just a person with hearing loss, so if something doesn’t make sense, I suggest you ask your audiologist. This is just a very short, simplified overview. I’ve also included links at the bottom for further reading.
In order to understand an audiogram, first you need to understand sound. There have been entire books written about sound alone, but I’m going for simplicity here.
Why does a frog sound different from a bird? Sounds vibrate at different speeds. Low sounds vibrate more slowly than high sounds. Here’s an illustration of low frequency sound (on the top) vs high frequency (on bottom).
The frequency at which sounds vibrate is measured in hertz. The audiogram only tests pure pitch. In real life most sounds aren’t purely one pitch.
The pitches tested on an audiogram may range from 125 hertz to 8000 hertz. The vertical lines of the audiogram indicate pitch. 125 hertz is a low-pitched sound, while 8000 hertz is high-pitched.
The piano above is a great way to represent low-pitched vs high-pitched sounds. Of course many sounds are lower than the lowest piano key. Likewise many other sounds are higher-pitched than the highest piano key. Additionally, there are pitches between each key, and so on. Audiologists do not measure every single pitch known to man, they only measure the pitches at certain intervals to give them an idea of how hearing loss might be affecting you.
You already know some sounds are loud and some soft. A loud bus can sound soft in the distance. It becomes louder as it gets closer. It is the same sound near or far, only the intensity has changed. The loudness of a sound (intensity) is measured in decibels (dbs). Below, the horizontal lines of the audiogram represent the loudness of sounds (decibel intensity).
This makes perfect sense if you think about it. When you turn the volume up on your cd player, the pitch of the music you’re listening to stays the same. You’ll recognize a piece of music whether the volume is high or low, unless it’s too low to hear. Intensity levels measured on audiograms range from about 5 dbs (very soft) up to 110 or 120dbs (very loud).
Putting it all together, your audiogram may look like this if you have good hearing:
Everything above the blue line (the hearing threshold) is what you don’t hear. Everything below the line is what you hear. This audiogram above represents good hearing because the hearing threshold falls at or above 10dbs in all the pitches tested.
Now let’s look at the “speech banana.”
I like this picture with illustrations of where sounds fall on the audiogram. The speech banana is important to audiologists, since understanding speech is what sends most people to the doctor. This banana illustrates the range of human voice and where some phonemes of the English language fall. It isn’t perfectly accurate of course because all voices are different. However, you can see how the speech sounds fall in different areas of the audiogram. ‘M’ and ‘u’ are usually nasally lower pitched sounds, while ‘f’ and ‘s’ are high-pitched sounds.
Some people place the number of English phonemes at around 40, however others say it’s impossible to know how many phonemes there are because of all the different English dialects. Additionally, some people speak a blend of English dialects. Your hearing threshold and exposure to various dialects can impact understanding when you have hearing loss. Also, when you lost your hearing– if you lost hearing pre-lingually as opposed to post-lingually– makes a difference.
Further, individuals all differ in speech comprehension. Two people with the exact same hearing thresholds may not hear exactly the same way. This can be hard for some to understand, but think about runners. All feet are different. Some people have flat feet, some don’t, some people are bow-legged, some may have scar tissue from old injuries, some legs are short and others long. Many factors determine how fast you run. It’s the same with hearing. Many factors determine how well you understand the sounds you hear besides measurements plotted on your audiogram. There are differences in how all your parts work together, as well as voices you’re used to and dialects, and many other variables.
When people lose their hearing they don’t necessarily lose all their pitches equally. In fact, a common type of hearing loss is the “ski-slope” where low tones remain intact, while high tones drop. There’s also a reverse ski-slope, “cookie bite” and reverse cookie bite.
Why does this happen? If you look at the picture of the cochlea below, you will notice different areas of the cochlea are responsible for detecting different frequencies.
Often the nerves in only one area of the cochlea are damaged, while other nerves remain healthy. A similar analogy would be experiencing numbness in just one part of your leg after having a bad accident.
During sickness, parts of the cochlea may become damaged. Those parts may not react to sound unless amplified. When a hearing loss becomes severe to profound, amplification may no longer do the job adequately. A person can begin to experience pain with loud amplification while simultaneously unable to hear– especially when other parts of the cochlea still work just fine. The cochlea has become dysfunctional.
Below is an example of the common ski-slope hearing loss. The slope can be steeper or less steep. Notice the severe hearing loss at 6000 hertz, but “normal” hearing at 250 hertz. Without hearing aids, this person will hear ‘m’ and ‘u’, but not ‘f’ or ‘s’. Listening to speech becomes a constant game of fill-in-the-blanks.
What do the x’s and o’s mean??
The circles indicate measurements for the right ear.
The x’s indicate measurements for the left ear.
What are the brackets? While the “air conduction” test determines hearing threshold (the blue line below), audiologists also perform a bone conduction test, with a vibrator placed near the cochlea. This can help the audiologist determine whether there’s a problem with the cochlea or some other part of the ear. The brackets indicate the results of the bone conduction. They usually look like this > but sometimes they look like this ].
Below there’s a disparity between the bone conduction and air conduction results. The cochlea seems to be working fine, as shown by the brackets. In this case, doctors determined it was due to infection of the middle ear.
For Further Reading: