The audiogram records the softest level of sound a person detects at each frequency, according to specific test criteria. These detectable levels of sound are referred to as the person’s hearing thresholds. During a hearing test, the audiologist will usually attempt to obtain thresholds for a number of different frequencies in each ear, often using different methods (air conduction and bone conduction) to deliver the sound to the ear.
Look at the blank audiogram graph below. Along the top of the graph the numbers range from 125 to 8000. These numbers refer to frequencies, or different pitches of sounds.
The frequency of a sound refers to the frequency of vibration of the sound source. The faster the rate of vibration, the higher the frequency of the sound. Sound frequency is usually measured in Hertz (Hz). A 250 Hertz (250Hz) tone sounds like a deep, low-pitched horn. A small, tinkling bell has a high-pitch sound, probably measuring around 3,000 to 4,000 Hz.
Normal, healthy, young human ears can hear frequencies as low as 20Hz and as high as 20,000Hz. However, audiologists test hearing in the range 250Hz to 8000Hz because most of the sounds of speech occur in this frequency range.
The intensity (loudness) of a sound is measured in decibels (dB). The decibel scale is shown down the side of the audiogram. Audiologists need to use a number of different decibel scales, depending on how the sound is measured. For hearing tests, sound is usually measured in decibels of Hearing Level (dB HL). This decibel scale reflects the sensitivity of the normally-hearing human ear.
- The softest sounds that young people with normal hearing can detect in ideal listening conditions, is between zero and 20 (0 – 20) dB HL, across the frequency range.
- Conversational speech measured from one metre away is around 50 dB HL, though some of the speech sounds will be around 35 to 40 dB HL.
- Conversational speech measured from four metres away will be quieter, about 35 dB HL, with the softer speech sounds only measuring around 20 dB HL.
- Really loud rock concerts can measure 110 to 120 dB HL. Many people find sounds over 100 dB HL unpleasantly loud.
What is masking?
During a hearing test, sound being delivered to one ear can sometimes be heard by the opposite ear. This is usually only an issue if there is a difference in hearing levels between the two ears. Masking involves playing a special noise into one ear while testing the other ear. Audiologists use masking to find out which ear (cochlea) is hearing the test sound.
Sound heard in a room reaches both ears at very similar levels. When very young children are tested with sounds played out of a loudspeaker, the sound is assumed to be heard by the “better ear”, regardless of which ear is nearest the loudspeaker.
Sound delivered by headphones or insert earphones is less likely to be cross-heard. Masking is usually only needed if there is a big difference in hearing levels between the ears.
Sound delivered by a bone conductor is easily cross-heard by the opposite ear. Masking is often needed for bone conduction testing.