Sound: Pure tone audiometry in diagnosing hearing loss

The most prevalent type of hearing loss, sensorineural, is attributed to damage to the cochlea or the auditory nerve and cannot consequently be corrected through surgical or medicinal means. Causes for this permanent and irreversible hearing loss include physical trauma, viral infection, exposure to loud noises, and aging. On the other hand, conductive hearing loss occurs when there is impairment in the conduction of sound through the middle into the inner ear due to fluid blockage or wax buildup. The distinction can be made by using a tuning fork and placing it behind the ear on the bone to determine whether sound can be heard via bone conduction.

In clinical settings, pure tone audiometry (PTA) is one of the diagnostic tests used to assess the degree and extent of hearing loss by identifying the hearing threshold levels at various frequencies. In a controlled environment, the patient is allowed to increase the decibel level until the provided calibrated tones are heard through the headphones with each ear tested separately.

Figure 1. Audiogram of the left ear of 4 different patients

The audiogram displaying the results is normalized to the hearing curve so that the horizontal line at 0 dB represents normal hearing. Sound intensity does not accurately reflect the change in the ear’s sensitivity with different frequencies and sound levels. Loudness is the strength of the ear’s perception of the sound and is expressed in units of phon. The equal-loudness contours in the following chart reveal the ear’s discrimination versus its sensitivity towards certain frequencies.

Figure 2. Equal-Loudness Contours for the Human Ear