Bitrate

The concept of resolution cannot be applied to compressed audio, since the software doing the compression may adjust the resolution to suit the content of the sound.^2.2 A passage played by a full symphony orchestra, for example, would require more bits than a passage played by a solo oboe, simply because the sound is more complex. For compressed files, instead of measuring bitdepth we measure the bitrate, or the number of bits used to store a second of sound. Bitrates are measured in thousands of bits per second (kbps), and as with resolution, the higher the bitrate, the better the sound. The bitrate for a typical MP3 file is 128 kbps.^2.3

Here is a useful formula to calculate the size of a compressed audio file: size in megabytes = number of seconds x bitrate in kbps / 8,388.608 (the number of kilobits in a megabyte). Using this formula, one minute of audio compressed at 128 kbps would have a size of .9155 MB (60 seconds $\times$ 128 kbps / 8,388.608).