Understanding Frequency Shelves in Digital Audio
Lossy compression algorithms like MP3 and AAC discard high-frequency audio data that the human ear struggles to perceive in noisy environments. A genuine 320 kbps MP3 preserves content up to approximately 20 kHz. In contrast, 128 kbps or 192 kbps files aggressively low-pass filter audio at 16 kHz to save bandwidth.
If you download a track labeled as FLAC or WAV that terminates abruptly at 16 kHz or 18 kHz with a flat, unnatural cutoff shelf, the file is a "transcode" — a low-bitrate lossy file re-encoded into a lossless container.