Pure-tone audiometry

I am going to explain the basic concepts of an audiogram so that beginners can interpret it correctly. For this, I will use an audiogram kindly provided by a user from the APAT forum.

There are different types of hearing tests. I will explain the pure-tone audiometry, which is the most common, as well as the test used to determine the presence of hyperacusis.

Pure-tone audiometry is a test that determines the degree of hearing loss, both quantitatively (hearing thresholds for each frequency) and qualitatively (transmission/perception). To determine qualitative hearing loss, both bone conduction (vibrator on the mastoid) and air conduction (headphones) are tested.

Example of pure-tone audiometry with UCL measurement

Pure-tone audiometry

Example of a pure-tone audiogram with UCL measurement

The audiogram consists of two graphs, one for each ear. On the vertical axis is the sound intensity needed to hear each frequency, expressed in dB HL, and on the horizontal axis is a range of frequencies from 125 Hz to 8000 Hz, in octave steps, meaning each indicated frequency is double the previous one.

dB HL units are designed to facilitate audiogram interpretation. dB means decibels, the logarithmic unit used to express sound intensity. HL stands for Hearing Level. If the audiogram used dB SPL (Sound Pressure Level), which expresses the actual sound pressure required to hear a sound, the graph would be curved and harder to interpret due to the ear’s different sensitivity to each frequency. Human ears hear mid-range frequencies better (like the human voice) and require higher intensity for low and high frequencies. For practicality, audiograms use dB HL, giving a straight line for someone with perfect hearing.

Thus, 0 dB HL does not mean silence but represents the intensity at which there is no hearing loss for a given frequency. This value is statistically derived from tests on young people with normal hearing. This is why the vertical axis may show negative dB HL values. Some people, especially those with hyperacusis, may show negative dB HL, meaning they hear better than average. This makes sense because hyperacusis is an abnormal amplification of sound.

Hertz (Hz) measure the frequency of a sound wave—cycles per second. The human audible range is from 20 Hz to 20,000 Hz. Low frequencies are 20–250 Hz, mid frequencies 250–2000 Hz, and high frequencies 2000–20,000 Hz.

Bone conduction is represented by “>” and “<” symbols for the left and right ears, respectively. Air conduction uses “X” and “O” for left and right ears, respectively. Typically, the two pathways overlap, but bone conduction is shown above air conduction. A symbol resembling a lowercase “m” indicates the UCL (uncomfortable loudness level).

Normal dB HL values range from 0 to 30 dB, indicating normal hearing (normoacusis).

If both pathways overlap above 30 dB, it indicates sensorineural hearing loss—hearing loss caused by damage to neurons in the inner ear, often affecting high frequencies.

If both pathways are separated by more than 20 dB, it indicates conductive hearing loss, usually affecting low frequencies.

Finally, UCLs indicate the dB at which a person feels discomfort from pure tones. Lower values indicate more severe hyperacusis. In a healthy person, these values are around 100 dB.

• UCLs around 80–85 dB indicate mild hyperacusis.
• 65–80/85 dB indicates moderate hyperacusis.
• 50–65 dB indicates severe hyperacusis.
• Below 50 dB indicates very severe, critical hyperacusis.

Analyzing the example audiogram: both ears show almost complete overlap of bone and air conduction, with no separation exceeding 20 dB, so there is no conductive hearing loss.

For air conduction: the left ear is between 0–30 dB across frequencies, reaching perfect hearing at 8000 Hz. The right ear shows a significant drop from 4000 Hz, likely due to acoustic trauma, indicating difficulty hearing above this frequency (though frequencies above 8000 Hz were not tested, so the full extent is unknown). Otherwise, hearing is normal, indicating normoacusis.

UCL values are around 65 dB, reaching 60 dB at some frequencies in the left ear. Overall hearing is normal. Considering these results and ruling out other possibilities, this person has hyperacusis between moderate and severe.