44 kHz and 48 kHz are sampling frequencies for digital audio recording. In a recording at 44 kHz, the sound signal on each channel is represented by 44 000 numbers, the samples, per second, each number being coded usually on 16 bits. So the bit rate of an uncompressed stereo recording is
2 channel x 44 000 samples per channel and per second x 16 bit = 1.4 million of bits per second, that is 1.4 Mb/s.
The size of a recording file is the product of the bit rate and the duration. For instance, the size of a 5 minutes uncompressed stereo recording as above is 1,400,000 bits per second x 300 seconds = 420,000,000 bits = 52,500,000 bytes = 52.5 MB because 1 byte = 8 bits.
Hence after MP3 compression at a bit rate of 128 000 bits/s, that is 128 kb/s, this recording will be 1,400,000 / 128,000 times, that is about 11 times, smaller. Indeed, its size will be 128,000 bits per second x 300 seconds = 38,400,000 bits = 4,800,000 bytes = 4.8 MB.
Didier - can you say that in English, please?
Seriously, thank you so much for the explanation. For some reason, this is still confusing to me. But I'm going to save your words in one of my files so that I can read it a couple dozen more times. Maybe one day it'll sink in.