MP3 Audio Compression
MP3s are audio files compressed using lossy compression.
The lossy compression allows great savings in file size, with the average MP3 file being 90% smaller than an equivalent uncompressed audio file.
Like all lossy compressed files, savings in size are made by deleting data that the computer believes is redundant and will not be missed by the user.
MP3 audio compression reduces a file size through:
- Perceptual music shaping
- Reducing the audio bitrate
Perceptual Music Shaping
Perceptual music shaping refers to the process of removing inaudible sounds in order to make a file size smaller.
Inaudible sounds may include:
- Noises at frequencies that humans cannot hear
- Quiet sounds that cannot be heard over louder sounds
In audio files, the bitrate is the number of bits that need to be processed every second. This is measured in kilobits per second.
The bitrate is calculated by multiplying the sample rate by the bit depth and number of audio channels.
The bigger the bitrate, the better the sound quality, but the larger the file size.
The sample rate (measured in Hz or kHz) is the number of samples (snapshots in time) of sound that are recorded to represent an audio performance.
Taking more samples per second will result in a more accurate and better sounding audio file. However, increasing the sample rate increases the file size.
Bit depth is the number of bits of information recorded in each sample. MP3s with a high bit depth will contain a wider spectrum of frequencies, giving a more accurate recording of the audio performance. However, the higher the bit depth, the greater the file size.
Finding the sweet spot
Deciding how best to compress audio will depend on many factors.
Aggressive compression will allow you to squeeze more tunes onto your device, but what’s the point if they sound terrible?
On the other hand, what’s the point of downloading large, slightly compressed files, if you only intend to play them through cheap speakers and cannot tell the difference anyway?
When streaming audio, we want it to download quickly and sound amazing, but unfortunately we can’t have it all!
Somewhere in the middle though is an acceptable compromise and a compression “sweet spot”, ideal for that particular purpose.