The standard audiogram is a graph that plots hearing sensitivity.
The top of the graph from left to right is sound frequency measured in Hertz (Hz) from low to high, typically 250-8000 Hz. The side of the graph is the volume from soft to loud measured in decibels hearing level (dB HL). Decibels in hearing level is a relative scale of hearing developed in the 1920s which was considered to be normal hearing. When hearing is tested, the lowest level detected at each pure-tone frequency (pitch) is plotted on the graph by air conduction, via headphones or earphones, and bone conduction to determine the presence and nature of hearing loss. The result is a pure tone threshold test.
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Speech does not occur at a stagnant pitch. It is constantly changing from low to high pitch and soft to loud. Each sound in speech has its own pitch range which can be plotted on the audiogram as shown above. Collectively, the bulk of speech shaded on the audiogram is called the speech banana. The vowels (26 different vowel sounds) and some of the consonants fall in the low pitch range and are louder (farther down on the graph). Other consonants fall in the middle and high pitch range and are relatively softer (higher up on the graph). The vowels and lower-pitched consonants give us our volume for speech while middle and high-pitched consonants give us our clarity for speech. When an individual’s threshold test is plotted on a speech sound audiogram, it is evident which sounds in speech are easily heard, barely heard or not heard at all. This explains why often speech is “heard” but not “understood.”