Psychoacoustics

When we hear something, it first arrives as a mechanical sound wave. But that is not what our brain "hears." The sound wave is transformed into nerve pulses which our brain perceives.

Our perception of sound is somewhat limited. When two sounds are made, which are close to the same frequency, we are unable to detect the difference. If two sounds are made with greatly differing amplitudes, we often cannot detect the lower volume sound. For example, if you are talking to someone on the sidewalk and an ambulance drives past with its siren on, you will not be able to hear the other person talking, even though they may actually be speaking.

Masking

The process of masking is when our brain tunes out certain sounds. When we are in a crowded room with many people talking and other noises, we are able to easily hold a conversation with someone because our brain ignores the extraneous sounds. That is not true with hearing aids. Consequently, a common complaint about them is that they pick up and amplify all sounds, indiscriminately.

Masking and MP3

MP3 is a form of digital audio encoding which allows for much smaller files. Those sounds which are determined to be inaudible to most people are reduced in accuracy. The sounds we can hear are then reproduced efficiently. Although, technically, an MP3 recording is not an accurate reproduction of the original, what we perceive sounds as though it is.