Hear is one of the human senses by which man collects information from an environment that spreads like sound waves through an elastic medium (most often air). We hear the sound because the ears in the ears convert vibrations into the signals that are then sent to the brain. The hearing system consists of the ear, auditory nerve and parts of the central nervous system involved in the hearing. When the auditory organs are unable to receive sufficient quality stimuli from the immediate environment, we talk about hearing impairment, while the complete inability to receive the stimulus of the name of deafness.

Sound is the human sensation of mechanical titers, respectively. Pressure changes spreading by elastic medium (solid, liquid or gaseous). A man observe sound through the hearing sensors. Each sound is determined by the frequency (hertz - Hz) and the strength (decibel - dB). People can hear the frequency ranges between 20Hz and 20000Hz, but there are actually few people who hear the frequencies above 16000Hz. It is natural that by aging, the upper limit of the frequency of the sound that a person can hear decreases. So it is quite normal for people over the age of 25 not to hear sound frequencies above 15000Hz. In the age of 70, this threshold of silence drops below 8000Hz. In addition to the bandwidth (altitude) the sound is determined by its strength, volume. The lowest level of sound that a person hear on a given frequency is called threshold of hearing. The bandwidth between 250Hz and 8000Hz is the typical frequency range of the most important people to hear speech and sounds from everyday life.

The audiogram is a graphic representation of the sensitivity of each ear to the sound of a given frequency and volume resulting from the measurement of hearing by a device called an audiometer. Typical hearing measurements are performed in the frequency range up to 8000Hz. According to the ASHA (American Speech-Language-Hearing Association), the level of hearing is classified according to the following principles:

• • 0 - 19 dB HL: normal hearing
• • 20 to 39 dB HL: mild hearing impairment - a more difficult understanding of speech in loud environments, silent voice or distant speech is difficult to hear even if the ambient noise level is minimal
• • 40 - 59 dB HL: moderate hearing impairment - hearing and communication problem in the group of people when interwoven with different voices and sounds
• • 60 to 89 dB HL: Severe hearing impairment - Difficulties in talking within a group of people, people can not hear the speech unless it is not very loud but it does not detect any words at all, difficult to recognize sounds from the environment
• • 90 and more dB HL: Deafness - it is possible for a person to hear only very loud noises but he can not hear the speech. By using hearing aids, a person can develop basic voice communication

(HL = hearing threshold)