Нормальный слух и звуковоспроизводящее оборудование.

Better hearing with a hearing impairment?
In a blind test conducted by heise c’t, only one person was able to tell apart MP3 at 256 kBit/s from uncompressed music – this person had impaired hearing. How’s that even possible? The explanation lies in something called the masking effect. If the bass is very loud, it’s often hard to hear the soft mid-tones as they’re masked by the bass. So to reduce the amount of data, mids are omitted in MP3 compression. However, if a hearing impairment means you’re unable to hear the bass or hear it much less intensely, there’s no masking effect. Accordingly, you’d hear the mids and would also notice if they’d been removed from an MP3 file.