This page illustrates the relation between the frequency of an audio signal and its power perceived by the human ear. The frequency interval of audio waves is defined between 20 Hz and 20 KHz. Frequencies below 20 Hz are called infrasonic sounds and above 20 kHz are called ultrasounds. Sounds between 50 and 300 Hz are perceived to be ‘strong’, they are called the bass range. Frequencies between 300 and 3300 Hz are in the ‘mid-range’ and all audio frequencies above 3.3 kHz are called ’trebble’ and they are perceived to be weak. Human voice is situated between 200 and 4 kHz. The 4 kHz value plays an important role in different communications systems. For example the highest audio frequency transmitted in radio AM is 5 kHz. In telephony systems, VoIP or cellular the highest transmitted frequency is cong; à 3500 Hz.