In a hearing test, a hearing curve is drawn for each ear in a graph called an audiogram. The hearing curve for the right ear (red) is plotted on the left-hand side of the audiogram, with the curve for the left ear (blue) on the right-hand side. The reason for this ‘mirror inversion’ is because this is how the tester sees the patient sitting in front of them. Anyone looking at the resulting audiogram will have this perspective, too. The frequency (pitch) is measured in Hertz (Hz) and shown from low (left) to high (right) across the top on the horizontal axis. The intensity in decibels (dB) required for an individual to perceive a tone at a given frequency is plotted from soft to loud down the vertical axis, and gives an indication of the individual’s degree of hearing loss. The hearing level is standardised in such a way that a straight line at 0 dB represents normal hearing, although deviations of up to 20 dB are also normal. Many years ago, hearing curves were determined for a large number of individuals with normal hearing, resulting in the measuring unit dB HL (‘Hearing Level’). The greater the hearing level, the more severe is the individual’s hearing loss in relation to normal hearing.
This is the hearing curve of an individual with normal hearing.
This is the hearing curve of a patient suffering from typical high-frequency sensorineural hearing loss. Sensorineural hearing loss is located in the inner ear and often affects the high frequencies, because these frequencies are detected at the base of the cochlea and are thus most susceptible to problems and functional deterioration. The air conduction threshold is plotted onto the graph using circles (representing the right ear) and crosses (representing the left ear). The circles or crosses are connected by drawing a continuous line. The bone conduction threshold is plotted on the graph using arrowheads, which are connected by a dotted line.
In the example above, air and bone conduction thresholds are identical, indicating that the middle ear is fully functional and the origin of the hearing loss must be located in the inner ear or further up. The patient in our example is also suffering from tinnitus in his left ear, which he perceives as a pure tone (beeping sound) with a frequency of 2,340 Hz and an intensity of 50 dB. The tinnitus is represented by a small black horizontal line marked at the relevant place in the audiogram. If the tinnitus is not a pure tone but a noise, it is represented by a black zigzag line. The patient with the above audiogram is a good example of middle ear hearing loss. The bone conduction threshold is normal on both sides, which means that the inner ear is fully functional and the hearing loss has its origin in the middle ear.