Hearing Testing: The Audiogram and The Audio Booth


This is an audiology booth.  It is not a torture chamber.  Nor is it a “Time-Out” booth for your loud, unruly kids.  Yet, the booth is designed to prevent sound from entering (or leaving); its metal walls are 4 inches thick and the window is ¼ inch of sound-proof glass.  So it does dampen one’s screams and protests.  An outsider can view the occupant in quiet comfort, sipping on coffee (or beer) as if watching an entertaining video on mute.  But that’s not its purpose.  Nor is it a bomb shelter; but admittedly I’d be the first to dive into it should we be invaded by a hostile country, like Canada.

To obtain a good assessment of hearing, you need three things: (1) a highly trained audiologist (ideally one with doctorate degree); (2) good audiology equipment, including a sound-proof audio booth.  As you can see from the photo, we need an entire  room just to house the booth.  Oh yeah, you also need:  (3) a cooperative patient.

As you guessed it, we do a lot of hearing testing in an ENT practice, since “Ear” is one of the major categories of “ENT” and obviously the ear includes hearing, and is not just an accessory appendage to hang fancy jewelry or to be pulled upon when you’re misbehaving.

Many times, patients are referred to us from other doctors to assess the severity of a hearing loss, determine a cause and render treatment.  Often, we have people who failed a hearing test at school or work and need a more thorough assessment. 

We also get folks coming in with their spouses, the latter of whom tell us, “either he can’t hear me or he’s ignoring me!” to which the poor guy shrugs his shoulders saying, “she doesn’t speak clearly,” or “she speaks too soft!  Especially when I’m watching football or pro wrestling!”  Or the complaint, “He has that T.V. turned up way too loud!”  Sometimes it’s a matter of misunderstanding or not hearing human speech in its entirety: “I can hear them talking a mile a minute, but can’t understand a thing they’re saying!” which falls into “the-problem-is-everyone-else-and-not-me” category.

We get to see all these folks.  Sometimes they come alone.  Sometimes with a spouse or family member to explain things the patient wouldn’t readily admit. 

An audiogram is a test of hearing.  This requires testing hearing in a variety of ways.  First there is pure tone audiometry.  Headphones are placed over the ears and sound is delivered by one of two ways: (1) air-conduction, where sound is transmitted through the headphones directly into the ears, the normal way sound is travels into your ears, and (2) bone-conduction (BC) where sound is vibrated through the skull to directly stimulate the cochlea, thereby bypassing the tympanic membrane TM and ossicles (tiny bones of hearing).  You can check out Hearing and The Tuning Fork for a better understanding of this concept.  In both methods, the volume of sound the person can detect is measured at different frequencies (or pitches). 

Sound travels in waves of pressure.  For an explanation of the science of sound, check out Hearing and the Physics of Sound Mechanics for an entertaining explanation.  We’ll go over some of this briefly.

The sound wave goes through a cycle, where it starts at a baseline then travels upwards to a maximal peak (see Figure 1).  The baseline in this case is normal atmospheric pressure.  Then it travels downward, past the baseline and reaches a minimal trough.  It then travels upwards to the baseline.  The interval from baseline to peak to trough and back to baseline is one wave cycle.

Figure 1. Sound wave

A sound wave has two important features: amplitude and wavelength.   Amplitude is the height of the wave or intensity of sound energy; the taller the amplitude, the greater the intensity and the louder the sound.  Wavelength is the total distance (in fractions of a second) that one complete wave cycle travels.  An easier way to measure this is the number of waves that travel per second (the number of sound waves that pass a fixed point in one second).  The unit of measurement in this manner is Hertz (Hz), which is the same as cycles-per-second (cps).  So, 1,000 Hz is the same as 1,000 cps, or 1,000 waves travelling per second. 

With pure-tone audiometry, sound at a particular frequency (Hz) and particular intensity (dB) is transmitted to the patient via the headphones.  He and she then must signal (with a hand-held device) when they can barely hear the sound.  The audiologist runs through each major frequency with varying degrees of intensity and records the results, which are displayed in graphical form as shown in Figure 2. 

By the way, the audiologist knows if you’re faking (called functional hearing loss, a form of malingering).  So don’t pull any tricks on her, since she’ll call you out, report you to your spouse or boss (sometimes they’re one in the same), or lock you in the audio booth forever, or until lunchtime.

The audiogram in Figure 2 shows the results from a particular patient.  Frequency is on the horizontal axis and intensity (or hearing level) is measured on the vertical access.  The circles (O) represent the right ear and the crosses (X) the left, when tested by air conduction, unmasked.  When masked (explained below) the symbols for right ear and for the left are used.   The symbols < for right and > for left are used for unmasked bone conduction tests.   For masked bone-conduction tests, the brackets [ for right and ] for left are used.

Figure 2.  An audiogram

Frequency is measured in Hertz (Hz) or the number of sound waves that travel per second.  The tested frequency increasing as one goes from left to right on the graph.  Hearing level is measure in Decibels (dB) where loudness is increased as one goes from top to bottom on that graph.  Of note, that vertical axis is logarithmic; meaning every 10dB increase is actually a ten-fold increase in sound energy delivered through the headphones (i.e., 30dB is 100-fold greater than 10dB).

From the sample audio in Figure 2, you can see the pure-tone testing for the right ear for both air-conduction and bone-conduction are normal other than bordering on mild loss at 4,000 and 8,000 Hz. (marked by ’s and <’s respectively).  The left ear shows a profound hearing loss.  The square symbol means masking was used for air conduction.  With masking, sound is presented to the rt ear while the left is tested, since very loud sounds can cross over to the better ear on the other side (right ear in this case) and give a false result for the left, meaning the hearing would seem better than it really is if masking were not used.  For the bone-conduction test for the left ear, the little arrow on the bottom of the ] symbol shows that bone-conduction testing was done at maximal limits (one cannot vibrate bone louder than this level).  This essentially means hearing loss in this case is predominately air-conduction. 

So this patient has a profound sensorineural hearing loss, meaning the loss is from the cochlea (inner ear) or the auditory nerve that travels from the cochlea to the brain.  Conductive hearing loss refers to problems with sound transmission from the ear canal, tympanic membrane (TM) or ossicle.  See Hearing Loss: Explanation of the Different Types for more information.

The way I describe the pure-tone testing on the audiogram to patients is this: as one goes from left to right across the graph, the frequency is higher, or goes from bass to treble; and as one goes from top to bottom, the sound presented gets louder.

I won’t go into the details of the other parts of the audiogram, namely speech testing, where actual words are presented to the patient at different sound levels, other than to say Speech Discrimination (SD) scores should be in the 90-100% levels.  Poorer SDs result in poorer understanding of speech despite higher intensities (ie, turning up the volume still results in poor comprehension, even when someone is yelling at you at the top of their lungs.  An example of wasted breath).

There are many other aspects of hearing testing that would take an entire book to explain, which is the reason for the years and years of education and training to become an audiologist.  We’ll touch on some of these in later posts.  Hopefully this provides an understanding on what is done in that booth and rest assured you’ll come out of it alive and untraumatized.

©Randall S. Fong, M.D.


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