When two sounds are heard at the same time, the strength of one sound may be good enough to cause the other to be silent. This change in the threshold is caused by a second sound that coexists with the first sound is called masking. There is no one who has not experienced masking in noisy situations. The noise that causes the interference is called the masker. Masking plays an important role in some parts of clinical audiology.
After a hearing threshold is received, a symbol is placed under the test frequency at a number that matches up with the hearing-level dial setting the represents the threshold. The usual symbols for air conduction are a ? representing the right ear and an X representing the left ear. After all the results are plotted on the audiogram, the symbols are usually connected with a solid red line for the right ear and a solid blue line for the left ear. When masking is used non-test ear, the symbol ? is used for the right ear and the symbol ? is used for the left ear.
When no response (??) is received while the highest frequency is being tested, the appropriate symbol is placed at the point on the intersection of the greatest possible testable level for the test frequency. A lot of people, usually children, have hearing that is more sensitive than the lower limit of strengths shown on the standard audiograms. Our book suggests that an arrow pointing up be placed next to the symbol when threshold readings of -10 dB HL are found.