There is a way to check if your audio files really are the quality they say they are.
A low quality MP3 file (e.g. 128 kbps) can be upscaled to a high quality file such as 320 kbps. The upscaled version tricks you into thinking you have some top notch audio but it only takes up more space, there is no quality increase. It’s just a low quality version packaged as something better.
As an example I’ve taken the last 2 minutes of the track Starseeds. I’ve used the LAME MP3 encoder with the high quality 320 kbps setting to encode the original WAV file, this is the result of the spectogram:
The Y-axis shows the frequency range, meaning the range of audio signals from low to high. The maximum frequency stays around 20 kHz. Humans can perceive a frequency range between 20 Hz and 20 kHz. By the way, almost no one can tell the difference between a WAV file and it’s 320 kbps MP3 compressed version, as several tests have shown.
Lowering the MP3 setting to 192 kpbs not only reduces the file size and quality, but also removes more of the higher frequencies:
If we lower the MP3 setting further to 128 kbps, you can see that almost no frequency above 15 kHz exists anymore:
If you listen to this version, you can hear that – besides the MP3 artifacts, such as a metallic garble – there are less higher frequencies. You can especially notice this by the cymbal around 00:03, which lacks high end frequencies compared to the 320 kbps version. The ‘whoosh’ of the cymbal sounds more dull and muddy.
128 kbps MP3
320 kbps MP3
If we would re-encode the MP3 file from 128 kbps to a 320 kbps, the frequencies above 15 kHz would remain absent. When in doubt about your 320 kbps MP3 file, just load it into Spek and check the high frequency range in the spectogram. If almost everything above 15 kHz is cut off, you can be pretty sure it was upscaled.