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FIRST YEARS > How to Read an Audiogram: Auditory Thresholds

Terminology and Symbols

The goal of audiometric testing is to produce an audiogram. The audiogram charts hearing ability, specifically, the softest sounds that can be heard in ears at various low-to-high frequencies. These sounds are called thresholds. Technically, a person's hearing threshold is defined as "the softest sounds a person hears at each frequency approximately 50% of the time." Important: Keep in mind that an audiogram is a quantitative measure, not a measure of quality (i.e. clarity of speech and other sounds).

An audiogram is a graph, with frequency, from low to high, across the top or horizontal axis and intensity, from soft to loud, down the vertical axis.

Intensity is measured in decibels (dB HL). The top line, at 0 decibels (dB HL), represents a very soft sound, with each horizontal line below representing successively louder sounds.

Zero (0) dB HL does not mean that there is no sound at all. Rather, it is the softest sound that a person with "normal" hearing ability would be able to detect at least 50% of the time. (Some audiograms, in fact, begin at -10 dB HL or lower.)

Audiologists consider 0 -15 dB HL to be "normal hearing" in children (0 - 25 dB HL in adults).

Frequency is measured in Hertz (Hz). Just like a piano's keyboard, the frequencies are low on the left side (125 or 250Hz), and then gradually climb to higher frequencies on the right side (8000Hz). Note: The piano ranges actually go from 28 Hz to 4,000 Hz.

The most important frequencies for speech fall into the 250-6000 Hz range. The vowel sounds of speech are typically low frequency sounds that make up the loudness of speech. The consonant sounds like "f", "s", and "th" are high frequency sounds.

Frequency, or frequency, defines low vs. high-frequencyed sounds.

Audiometric Testing
In testing, an audiometer delivers "pure tones" at different frequencies (Hz) from low to high at different intensities (dB). Air conduction testing uses earphones or loudspeakers (in a sound-proof room/soundfield). These send sounds to the ear canal and through the middle ear to reach the inner ear. Hence air conduction evaluates the whole ear system -- outer, middle, and inner ear. In bone conduction testing (shown at right) Bone conduction testing

, a bone vibrator is placed behind the ear to deliver the sound vibrations to the cochlea (the hearing organ of the inner ear). This allows the examiner to bypass the entire outside and middle ear areas and test the sensitivity of the inner ear directly.

In "conventional" audiometry, the child (5+ yrs.) presses a button or raises a hand each time he or she hears a sound. For younger children, examiners introduce reward (e.g. visual reinforcement audiometry (VRA): 7-30 mos.) and/or play (e.g. conditioned play audiometry (CPA): 30 mos. - 5 yrs.) "incentives" into testing. As the child responds to the presented tones and thresholds are determined and marked across the audiogram, the graph fills in to present a picture of the child's hearing ability.

In this audiogram, the white represents what the child CANNOT hear and the yellow, what he CAN hear. In the audiogram to the left, the white area represents the sounds that the child cannot hear (softer than his/her thresholds) and the yellow area indicates all of the sounds that the child can hear (louder than his/her thresholds). Recall that audiologists consider 0-15 dB HL to be normal in children. It is obvious that this child has a hearing problem!

There are different styles of audiograms, but most use a standard set of symbols for representing items on the chart.

Always review the key that accompanies each audiogram to verify that it uses symbols you are familiar with.

The terms and symbols below may be used on the audiogram to describe information about hearing.

Cheat Sheet for Reading an Audiogram. A handy reference, designed to be given to parents/caregivers, to make audiogram information less complicated to understand.

• Hertz (Hz): The measurement standard for the frequency or frequency of sound. On an audiogram, these typically range from 250 Hz to 8000 Hz.

• decibels (dB HL): The measurement standard for the amplitude or loudness/intensity of sound. On an audiogram, these typically range from 0 to 110 dB HL.

• red and blue: When both ears are tested, thresholds for the left ear typically appear in blue and for the right ear, in red. Right is Red.

• OandX: In air conduction testing, the symbol 0 stands for the right ear and is usually recorded in red. The symbol X, typically marked in blue, represents the left ear.

• < and >: When a bone-conduction vibrator is used to test for thresholds, a > symbol is used for the left ear and a < symbol is used for the right ear. (Sometimes there will be no color or shapes. So, remember: check the key/code.) An easy way to remember which symbols are right vs. left is to imagine the child is facing you. <O>Their right ear will be on your left, so you use the symbol < and vice versa.

These symbols indicate there was no response to the given stimulus.

air conduction

bone conduction
• [ and ]: Masking symbols: The term masking refers to noise presented to the non-test ear to keep it “busy," preventing it from hearing the sound presented to the test ear. The sound is a noise, which sounds like static, but has tonality. By properly presenting masking noise to the non-test ear, you can ensure that you are testing the ear to which the pure tone is presented.

• Pure Tone
Average (PTA): The average of the three thresholds at 500 Hz, 1000 Hz. and 2000 Hz. For example, when a hearing loss is described as 70 dB HL, that number probably represents the PTA.

• A/C Symbol for aided audiogram, representing the hearing level with amplification with hearing aids (A) or cochlear implants (C or CI). While some audiologists may still conduct functional-gain measurements, recent technological advances, i.e. Real-Ear Measurements (REM) have made these obsolete.
Audiogram A shows a 20 dB LOSS FALLING to 50 dB AT 4000 HZ audiogram with cochlear implant symbol

•L (or C₁)
andR (or C₂) For bilateral cochlear implants, Left or Right, you may see these symbols.

Hearing Loss: A Matter of Degrees

Recall that 0 dB HL is the softest sound that a person with "normal" hearing ability would be able to detect at least 50% of the time. What exactly is "normal?" Hearing loss ranges, defined in dB HLs, have been established to help people identify how much difficulty they should expect from a hearing loss. The ranges that are listed below -- normal, mild, moderate, severe, and profound -- are fairly standard across the United States, although some variations do exist.

Degree of hearing loss based on PTA:

Note: Click on to hear the effect of the hearing loss.

NORMAL 0–15 dB HL (infant)
0-25 dB HL (adult) degrees of hearing loss

MILD 20-40 dB HL
Unable to hear soft sounds. Can hear a normal conversation in a quiet room but has difficulty in a noisy environment. Cannot hear whispered conversation or speech from a distance.

MODERATE 40-70 dB HL
Has difficulty hearing a normal conversation in a quiet room. Must lip-read or use amplification to understand most words.

SEVERE 70-90 dB HL
Cannot hear a conversation unless the speaker speaks loudly near the ear.

PROFOUND 90+ dB HL
Cannot understand speech even if the speaker shouts. Can only hear very loud sounds such as a motorcycle engine, if at all.

For more detail, see Hearing Loss.

Let's look at 4 sample audiograms illustrating air conduction testing:

sample air conduction audiogram
This shows a mild hearing loss in both ears in the high frequencies. On average, however, hearing is normal.
Masked air thresholds: Note that in the lower frequencies, the hearing sensitivity is normal in both ears, but falls sharply into the moderate hearing loss range beginning at 2000 Hz.

sample air conduction audiogram
Here we see striking differences between the ears. The left ear shows normal hearing in the low frequencies sloping to a severe high frequency hearing loss. The right ear shows a moderate to severe hearing loss.
This shows a bilateral mild sloping to profound hearing loss.

Although these audiograms help us to identify the degree of hearing loss with pure tones, they do not indicate the type of hearing loss (because bone-conduction testing is not included) and provide little detail on the person's ability to understand speech. Mapping conversational speech on an audiogram (the "speech banana"), then superimposing the audiograms over these sounds, gives us more more information to interpret the results. When we look at the speech banana, we see that:

vowels fall in the "louder" ranges (They lie lower on the chart.);
consonants are higher-pitched than vowels (They lie more to the right on the chart.); and
consonants are spoken more softly than vowel. (They lie higher on the chart, in the lower decibel ranges.)

Consonants play big role in our ability to understand speech. For example, they indicate possession or whether a word is plural and help us to distinguish one word from another (hot versus hop). How does this apply to our sample audiograms?

Bilateral normal sloping to mild hearing loss. Under perfect listening conditions, this child will miss a few words off and on because of not hearing "s" and "th," for example. Under noisy conditions, he/she will have even more difficulty.	Bilateral normal sloping to moderate hearing loss. Both ears hear the vowels and the lower-frequency consonants, but miss "s", "th" and "t," among others. This child would not be able to distinguish plural from singular forms.
Normal sloping to profound hearing loss in the left ear. Moderate to severe hearing loss in the right ear. This child can hear no conversation in the right ear. The left ear, however, hears all but the soft, upper frequency consonants. Reference: The Speech Banana	Bilateral mild sloping to profound hearing loss. With only a few consonant and vowels sounds audible, this child would miss most of the distinguishing speech features.

Type of Hearing Loss

Hearing Loss
So far we have examined audiogram results with air conduction testing. Recall that the air conduction test evaluates the entire hearing mechanism -- the outer, middle, and inner ear -- and helps pinpoint whether a hearing loss exists in one ear or both ears at frequencies that are critical to normal speech and language development.

If a hearing loss exists, bone conduction helps us to determine whether the problem is in the outer, middle, or inner ear. Recall that the bone conduction test zeroes in on the inner ear, bypassing the middle and outer ear. If the air conduction thresholds show a hearing loss but the bone conduction thresholds are normal, the hearing loss is conductive -- there is a problem involving the conduction of sound through the outer or middle ear. If both the air conduction thresholds and the bone conduction thresholds show the same amount of hearing loss, this indicates the problem is in the inner ear alone, since the results are the same regardless of whether the outer and middle ears are being tested -- the hearing loss is sensorineural. And finally, a mixed hearing loss is when the bone conducted thresholds show a hearing loss and the air conducted thresholds show an even greater hearing loss. In this case, the problem lies in the inner
ear (the sensorineural component) and the outer and/or middle ear as well (the conductive component), hence mixed.

Summarizing the test implications:

• When air conduction tests show a hearing loss, but there is no loss identified with bone conduction tests, there may be a conductive loss.

• When both air and bone conduction results show hearing loss at the same level, the loss is considered sensorineural.

• If different degrees of hearing loss are found via air and bone conduction testing, the loss is mixed.

With this in mind, let's look at some sample audiograms showing bone-conduction testing:

Normal sloping to moderate sensorineural hearing loss in the left ear.	Moderate mixed hearing loss for the left ear.
Moderate conductive hearing loss in the left ear. Normal sloping to moderate sensorineural hearing loss in the right ear.	Mild to moderate conductive hearing loss in the right ear

To summarize, by comparing air conduction with bone conduction thresholds at each frequency, we can determine 1) how well a child hears at low, medium and high frequencies and 2) if a hearing loss is present, whether it is conductive (potentially medically treatable) or sensorineural (usually permanent and not medically treatable) or mixed.

A final note: The ability to process and interpret sound, as well as the ability to speak, depends on the interaction of numerous factors in each individual's background. Two people with similar audiograms may function very differently!

There are excellent interactive tutorials available at AuDStudent.com's "Tutorial on Understanding Audiograms," specifically:

Acknowledgments:

FIRST YEARS wishes to thank Cathy Constantine, audiology doctoral student in the UNC-CH Division of Speech and Hearing Sciences, for her assistance in developing this unit.

Many of the audiogram examples, although modified for use here, were taken from originals from two sources: the Hearing Alliance of America and Raising Deaf Kids, both by permission.

Photograph depicting bone conduction testing from My Baby's Hearing (by permission).

05/05/04

Alexander Graham Bell Association | UNC-CH Division of Speech and Hearing Sciences