MP3 (in full version Moving Picture Expert Group-1/2 Audio Layer 3) is a lossy audio compression algorithm, developed by the MPEG group, capable of drastically reducing the amount of data needed to store a sound, while still be a reproduction. acceptably faithful to the original uncompressed file.
Data compression is carried out by eliminating all those sounds that are not perceived by the human auditory system. In fact, recordings in wav format contain much more information than our ear is capable of distinguishing: by removing too much information, it is possible to obtain size reductions of more than 10 times (compared to uncompressed wav format).
In practice, the mp3 format exploits the limits of our auditory system, limits that depend so much on the structure of our ear (not capable of perceiving frequencies above 20 kHz, but particularly sensitive to the intermediate range that goes from 700 Hz to around 6000 Hz) and the ways our brain perceives and interprets sounds. These perceptual limits are studied by psychoacoustics, a sector of psychology that deals with the perception of sounds.
A particularly important and exploited effect in mp3 compression is masking (masking), in which the perception (or less) of a sound is influenced by the presence of another sound. For example, conversation at a bus stop can become completely impossible if a noisy bus approaches: a weaker sound is “masked”, that is, made inaudible by the presence of a louder sound. Masking is all the more effective the closer the two sounds are to frequencies, since our auditory system is unable to discriminate sounds with very close frequencies.
As we have mentioned, mp3 compression is lossy type, that is, it causes irrecoverable data loss and consequently quality degradation. Mp3 encoding allows you to choose how much compression the compression should have, keeping in mind that higher compressions will always correspond to lower sound quality. The quality of mp3 compression is adjusted by a parameter called bitrate that specifies how many kbits the mp3 file should use for each second of recorded audio. There is also the possibility to vary the bit rate within a single mp3 file, using higher bit rates for some parts and lower bit rates for others (for example you can use a lower bit rate for the silent parts or that contain few instruments and use a higher bit rate for more complex parts).
The usable bit rate values are many (from 8 to 320 kbit / s) but 128 kbit / s is considered a good compromise between compression and quality. Knowing the bit rate, it is easy to calculate the size of an mp3 file, since it is enough to multiply this value by the duration of the recording.
