Understanding an Audiogram

An audiogram (“audio” for sound and “gram” for measurement) is a graphic representation of the status of a person’s hearing acuity. Acuity is the capacity of the ear to hear fine detail, measured by determining the finest detail that can just be detected.

An audiogram-form can be a simple black and white grid that easily fits on an 8-1/2 X 11 page. The vertical (y- axis) represents volume in decibels and the horizontal (x-axis) represents the frequencies (pitch) of speech measured in hertz (Hz). An audiogram form grid is meaningless until it is plotted with data-points during a subject’s hearing evaluation. (See “Exhibit A” for basic orientation).

basic orientation of hearing
Exhibit A

The audiogram is produced by a hearing clinician with the assistance of a calibrated machine called an audiometer. This device makes very quiet tones called “pure tones” at different frequencies. All speech frequencies are simulated by these pure tones. A hearing test will check acuity in the frequencies of 250-8000 Hz because this range represents most of the speech spectrum.

The clinician operating the audiometer turns a dial increasing the loudness, one “click” at a time, as each tone is presented as the subject listens. A data-point is marked on the grid at the exact volume level the subject is first able to hear each tone. These data-points are called thresholds. A horizontal line is drawn connecting all the data-points.

A clinician may use the terms Normal, Mild, Moderate, Severe or Profound to describe a subject’s hearing status. To arrive at a conclusion, four threshold data points from the subject’s audiogram will be averaged – referred to as the Pure Tone Average (PTA). The frequencies averaged are typically 500, 1000, 2000 and 4000 because these frequencies represent the bulk of all speech sounds. If a subject’s PTA is less than 25, his/her overall hearing is considered within normal limits. On the other hand, if the subject’s PTA is 95, his/her hearing is considered in the Profound range. Pure Tone Average is also used by the clinician when comparing one ear to the other in determining symmetry of the subject’s hearing loss.

Audiometer-produced audiograms are universal around-the-world, meaning if a subject has her hearing tested in a London clinic and a New York clinic, her audiograms would graphically look the same and indicate the same hearing status. The units of measure used in hearing testing are also universal around the world; “decibels” are the standardized unit for measuring sound loudness, and “frequency” is the recognized unit for measuring the number of times a tone vibration repeats itself in a second. Frequency is presented as a number-unit followed by the abbreviation (Hz).

The “low frequency” vowel sounds (such as ahhh) are easier to hear at low (quiet) volume while “high frequency” consonant sounds (such as “s” “t” “f” or “k”) are harder to hear at the same low volume. This explains why a subject may only hear the “i” sound clearly in words like “time” and “dime” or the “a” sound in “bathroom” and “vacuum.
An audiogram will independently reveal the acuity of a subject’s left and right ears in all speech frequencies. A red plotted line always represents the right ear while a blue line always represents the left ear. The most important frequencies for speech fall into the 250-6000 Hz range. Thresholds between -10 decibels and +20 decibels HL are universally considered in the normal hearing range. Thresholds above 20 decibels HL are considered diagnostic for mild, moderate, severe or profound hearing loss. (see Exhibit B)

Exhibit B

On an audiogram, the order of speech frequencies is shown from low to high, like a piano’s keyboard. Low frequencies are on the left side (125 or 250Hz), and gradually climb to higher frequencies on the right side (8000Hz). Note: A piano range is from 28 Hz to 4,000 Hz. For example, low frequency speech sounds, such as “Ahhh,” will always be found on the left half of the audiogram, and high frequency speech sounds, such as “Sssss,” will always be plotted on the right side of the audiogram. (See Exhibit C)

Exhibit C

In Summary: An audiogram is a report of a subject’s hearing acuity indicating how much volume is needed in decibels for the subject to detect any given frequency of speech. The results of an audiogram may be presented using the terms Normal Hearing, Mild Loss, Moderate Loss, Severe Loss or Profound Loss. The universal values of each of these descriptions is defined by averaging four specific threshold data-points from the subject’s audiogram – referred to as the Pure Tone Average (PTA).