Basics: What is Sound?
Humans hear sound by processing vibrating air waves, which are created by the source of the sound. Our eardrum translates these waves to pressure differences through use of fluid in the ear. The inner ear interprets these pressures, and turns them to a nerve impulse, which is sent to the brain via auditory nerve. Once the brain receives these signals, it interprets them in our experience as sound.
Microphones and Speakers
A microphone is used to capture the sound waves, and turn them to an electrical signal via an electromagnet. The electrical signal is passed through a wire to the speaker, which uses an electromagnet to vibrate the speaker diaphragm to create sound waves. There is no measuring of sound waves with a microphone. The physical sound waves vibrate the diaphragm, and the signal the electromagnet creates depends entirely on the intensity and frequency of the wave.
Digital audio is a measured, numerical representation of sound. A computer will receive the electrical signal a microphone creates using a sound card. This signal is measured at a rate known as the sampling frequency. The measured electrical levels are recorded at values that are limited on a range known as bit depth.