Hearing:

Hearing tests, known as “audiograms,” measure a person’s ability to hear sounds based on the frequency (pitch) and decibel level (loudness) of sounds.

While the tendency is to think that the only factor in hearing loss is loudness, there are actually two factors involved: loudness and clarity.  Loss generally occurs first in the high pitch, quiet range.  A mild loss can cause one to miss 25-40 percent of speech, depending on the noise level of the surroundings and distance from the speaker.  When there is background noise it becomes difficult to hear well; the speech may be audible but not be understandable.  Hearing loss may have an even greater affect on children than adults.  The degree of hearing loss does not necessarily reflect the degree of impairment.