Timothy C. Hain, MD Page last modified: April 5, 2014 Return to testing index
Audiometry is the term used to describe formal measurement of hearing. The measurement is usually performed using an "audiometer" by an "audiologist", although recently as audiologists are in short supply, more testing is being done by technicians.
Audiometry is a subjective test -- it is not an objective measurement. It depends on people pushing a button or raising their hand, when they hear a tone. Cooperation is needed.
This page is meant to give a "quick" overview of how audiometry is done and how it is is interpreted.
|Audiogram||Aurical Audiometer such as used in Chicago Dizziness and Hearing|
In pure tone audiometry, hearing is measured at frequencies varying from low pitches (250 Hz) to high pitches (8000 Hz). This is just a part of the entire human auditory range, which extends between 20 and 20,000 hz. Neverthless, most audiometers are designed so that they cannot go as low or high as most good stero systems. The audiometer that we use in my practice is shown above.
The core method of pure tone audiometry is to present a series of tones in one ear, close to threshold (the loudness that the person can just barely detect), and keep dropping the intensity in 10 db steps until the person stops responding - -raising their hand or pushing a button. Then the person testing the hearing goes back up in 5 db steps until the person starts responding again. This is conventionally done at 6 octaves - -250, 500, 1000, 2000, 4000, and 8000. That's most of it right there !
Coding for displaying the output.
The threshold for each frequency, for each ear is plotted on a graph (see above). The right ear is usually plotted in RED, and as an 'x', and the left in Blue as a circle. There are special symbols for the conditions underwhich the hearing test is done (see below).
|Key used to interpret audiogram|
There are two main types of ear phones that are used for audiometry -- insert ("ER-3"), and ordinary headphones ("TDK"). Other kinds can be used as long as someone calibrates them.
The insert earphones have the advantage that they block out environmental noise, and they also have less of a tendency to be heard by the other ear. Their main disadvantage is that they are a little tricker to use (one has to insert them properly), and also that they give wrong readings in persons who have perforations of their ear drums.
Example normal audiogram (more examples are at the end)
An example of an audiogram in a person with normal hearing is shown above in figure 1. The hearing level (HL) is quantified relative to "normal" hearing in decibels (dB), with higher numbers of dB indicating worse hearing. The dB score is not really percent loss, but neverthless 100 dB hearing loss is nearly equivalent to complete deafness for that particular frequency. A score of 0 is normal. It is possible to have scores less than 0, which indicate better than average hearing. Pure-tone average (PTA) is the average of pure tone hearing thresholds at 500, 1000, and 2000 Hz.
Figure 4:Audiogram for a Patient With an Acoustic Neuroma
Hearing loss is often described in words as follows:
- Normal hearing
- < 25 db HL (adults)
- < 15 db HL (children)
- Mild hearing loss = 25-40 db HL
- Moderate hearing loss = 41-65 dB Hl
- Severe hearing loss = 66-90 db HL
- Profound hearing loss = 90+db HL
Hearing testing is not a difficult process at all, and many "industrial" computer audiometers can easily do this process themselves. They don't always do a good job -- as is shown on this page concerning hearing test fraud.
There are a few places where human input is needed because there are sometimes few decisions to make during hearing testing. These decisions generally are only necessary when hearing is significantly abnormal.
If hearing is abnormal consider doing:
Of course, there are many far more complex decisions and thought processes that can potentially present in persons with hearing disorders. However, the three decisions above are the most important ones.
When there is a hearing loss, the next step is to try and determine whether the loss is caused by a sensory problem (sensorineural hearing loss) or a mechanical problem (conductive hearing loss). This distinction is made by using a bone vibrator, which bypasses the mechanical parts of the middle ear. If hearing is better using bone than air, this suggests a conductive hearing loss.
Bone conduction testing is performed in a similar way as is air conduction, but the sound is transmitted to the ear through a "bone oscillator" rather than through an earphone.
For the most part, there is rarely a reason to do bone conduction if the air conduction audiogram is normal. Thus it is somewhat discretionary. An exeception to this general rule is superior canal dehiscence. When an otologist and audiologist or technicianwork together as a team, the otologist should tell the audiologist or technician whether or not bone is needed even if hearing is normal.
Bone conduction testing in persons with hearing loss should be done with masking (see below) to prevent sound from the stimulated side from going over to the good side.
Masking means that one puts in some "noise" in the opposite ear while testing an ear. The reason to do this is to prevent sound from the side being tested from going over to the good side.
Speech audiometry for person with normal hearing
There are a number of special subtests that are optionally included in the audiometry procedure. Speech audiometry involves presenting a list of words to see if patients can discriminate between words. By comparing speech comprehension with anticipated speech comprehension, inferences can be made about central processing and central hearing deficits. Speech should be part of the initial "full" audiogram.
One of the most basic measurements as the speech reception threshold (SRT). This test determines the lowest intensity level (in db HL) at which the patient can correctly identify 50% of common two-syllable words such as: baseball, airplane, mushroom. These two-syllable words are called "spondees", from the Greek word "spondeios" -- meter used at a libation. The word lists are best presented from a CD -- several should be purchased. Of course your CD player's volume should be calibrated so that the volume of words presented is identical to the volume from your audiometer.
|SRT (dB)||Degree of Disability|
|-10 to 15||None|
The SRT should be in close agreement with pure-tone threshold results. As a rule of thumb, the pure tone average or PTA (see above) should match the SRT, within 5 dB, and the speech detection threshold (SDT), within 6-8 dB. A significant difference between the two thresholds would raise questions about the validity of the pure tone thresholds, or an exaggerated hearing loss. The SRT is usually better than the PTA in malingerers. There are many other methods that can be used to detect exaggerated hearing loss.
Word recognition tests (also known as speech discrimination tests) assess the person's ability to understand speech when presented at a loudness that is well above their threshold. This test is administered by using single syllable, single words. The result is presented as a percentage score.
There should be a correlation between the type and degree of hearing loss and the word recognition score (WRS), but this depends on the cause of the hearing loss. For example, a person with a moderate conductive loss might score 88% on a word recognition test, but a person with a similar moderate retrocochlear hearing loss, might only score 28%.
Roll-over refers to distortion in words that occurs at high volumes. With rollover, WRS gets worse with louder presentations. This is usually due to a lesion in the 8th nerve.
The WRS can be helpful in predicting the usefulness of a hearing aid. An increase in the WRS with amplification, suggests that a hearing aid might be useful. WRS has some limitations - - most of us don't communicate with monosyllabic speech, so a good score on the WRS may not necessarily correlated with good functional performance. Scores are also weighted towards perception of high frequency consonants. Poor performance overestimates everyday communication impairment for patients with high frequency hearing loss. WRS also underestimates hearing problems in noise.
Complex speech tests
Complex speech tests are mainly used in evaluations of central auditory processing (CAP). Persons with CAP may have normal pure tone thresholds, and perhaps even normal word recognition ability, but are unable to process complex speech signals. One commonly used test presents two different words to each ear simultaneously (a dichotic task). Persons with normal CAP can repeat both words easily, while someone with a temporal lobe problem might be unable to repeat the word presented to the ear contralateral to the lesion. This results resembles the results of simultaneous visual or sensory stimuli in persons with parietal lobe disturbances.
Practically, hearing measurements are meaningless unless your stimuli are calibrated. Every single part of the system that you use should be calibrated - -the electrical device that produces the sound, and the headphones or speakers that deliver the sound.
Practically, electrical devices (such as digital audiometers or CD players) will never drift in frequency or volume. Once their intensity is checked, formal electrical calibrations are more likely to cause trouble (i.e. noise in the calibration process) than be helpful.
On the other hand, mechanical devices (such as headphones, and insert headphones in particular) nearly always break down over time. They need to be checked everyday with a "sound check", and formally every 3 months. This can be very expensive if one asks one's hearing equipment vendor to do this. So there is a conflict between the financial needs of one's hearing equipment vendor and patient care.
Figure :Audiogram for a Patient With an Acoustic Neuroma
This figure illustrates an audiogram of a person with a tumor called an acoustic neuroma. Hearing is worse for the left ear (squares) than the right ear (circles) although both ears are at least partially outside the normal range. This is a sensorineural (neural more precisely) hearing loss.
|Mild Age-related hearing loss|
This figure illustrates a person whit a mild age-related sensorineural hearing loss. This is not a severe hearing loss and ordinarily very little benefit would be obtained from a hearing aid.
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