Speech is sound in motion,
but talking produces acoustic pressure. A telephone reproduces sound by electrical means. However, in wireless technology, a coder inside the mobile telephone converts sound to digital impulses on the transmitting side and on the receiving side it converts these impulses back to analog sounds. A coder or vocoder is a speech analyzer and synthesizer all in one. A vocoder is found in every digital wireless telephone and is part of a larger chip set called a digital signal processor. In this process, sound gets modeled and transmitted on one end of the vocoder and on the receiving end, the speech synthesizer part, interprets the signal and produces a close match of the original.
These sounds can vary because of telephone circuit's resistance, while electrically representing speech with a continuous (analog) electromagnetic wave; however, digital signals remain stable for the length of their travel because digital signals are a mathematical or numerical representation of sound, with each sonic nuance captured as a binary number.
When you hear sound, your ears are responding to tiny, rapid changes in the pressure of the air. These changes are called sound waves. They can have a single frequency and constant amplitude. Hearing is a complex mixture of waves with different frequencies and amplitudes. Sound waves range from pure sine waves to complex combinations of waves. The normal human ear can perceive sound ranging in frequency from 20 to 20,000 Hz.