A WAV file is not always what it claims to be. If you download a track labeled .wav from a music store, you assume it's lossless — uncompressed PCM audio, bit-for-bit identical to the original master. Sometimes it is. Sometimes it's an MP3 that someone converted to WAV, either by accident or to make it look higher quality than it is. The file size looks right. The bit depth looks right. Your media player reads it fine. But the audio has already been degraded, and no amount of re-encoding recovers what lossy compression discarded. This is called fake lossless audio. It's common enough in DJ libraries and audiophile collections that I built a macOS app — Spectro — to detect it automatically. This post is about how the detection works. Why lossy compression is detectable MP3, AAC, and OGG compress audio by permanently discarding frequency information that psychoacoustic models consider inaudible — mostly high-frequency content above a certain threshold. A 128kbps MP3 typically cuts off around 16 kHz.…