A fake FLAC checker looks past the .flac extension. Drop one file or a whole album above: this page builds a spectrogram in your browser and looks for the frequency cutoff lossy encoders leave behind. Files never go to a server. This tool is part of the audio tools on TechOpt.io.
How this fake FLAC checker reads a spectrogram
A genuine lossless encode of a CD still has energy up toward 22.05 kHz (Nyquist at 44.1 kHz), usually with a gentle fade rather than a hard shelf. MP3 and AAC throw those highs away. Re-wrapping that audio as FLAC cannot put them back — you only get a larger file with the same brick wall. On a spectrogram the wall is a horizontal line: content below, dead air above. Typical shelves sit near 16 kHz (128 kbps MP3), 19 kHz (192 kbps), or 20–20.6 kHz (320 kbps / AAC). This page looks for that compact knee plus a flat empty band above it, and only then draws a cutoff overlay.
Fake hi-res and padded bit depth
A 96 kHz “studio master” that was upsampled from 44.1 kHz is empty above about 22 kHz. A 24-bit header whose low eight bits are always zero is 16-bit audio in a wider container. The checker flags both. Neither test uses tags: the FLAC format STREAMINFO block reports the advertised rate and depth, then the decoded samples are what count.
What this tool will not claim
A 320 kbps transcode with the usual ~20.5 kHz lowpass is in scope. If the encoder disabled that lowpass, energy can reach 22 kHz and the spectrogram will not show a wall — look at the plot. Vinyl rips, tape, and some classical masters that are already empty by ~16 kHz come back as inconclusive, not fake. A full-bandwidth mix is not called a fade just because 21 kHz is quieter than 5 kHz. Encoder Vorbis comments are easy to forge. The verdict is “likely,” not a courtroom stamp.
FAQ
What is a fake FLAC?
A fake FLAC is a valid .flac whose audio was transcoded from a lossy source such as MP3, AAC, or Opus. The container is lossless, so the file is larger, but the missing high frequencies never come back. The spectrogram still shows the encoder’s cutoff.
How does this fake FLAC checker detect a transcode?
It decodes the file in your browser, builds a spectrogram, and looks for a sharp frequency knee well below Nyquist with empty air above it — the brick wall MP3 and AAC encoders leave behind, including the 20–20.6 kHz shelf on many 320 kbps files. It also checks for upsampled hi-res (no energy above ~22 kHz on a 96 kHz file) and 16-bit samples padded into a 24-bit header.
Why is a 320 kbps MP3 hard to catch?
A 320 kbps MP3 often low-passes around 20–20.6 kHz, close to the 22.05 kHz Nyquist of a 44.1 kHz CD. That shelf is still a brick wall and this checker looks for it. The case that still slips through is a 320 kbps encode with the lowpass disabled, so energy reaches Nyquist — treat a “likely genuine” verdict then as a maybe, and look at the spectrogram.
Can a vinyl rip or old master look like a transcode?
Yes. Tape, vinyl, and some classical masters roll off high frequencies naturally, without a brick wall. This checker only calls that inconclusive when energy is already gone by about 16 kHz. A mix that still has content up toward Nyquist, even if the top is quieter than the mids, is treated as lossless. A hard horizontal shelf with dead air above it is the transcode signature.
Does this fake FLAC checker upload my file?
No. It runs entirely in your browser. Files are read locally and never sent to a server.
Does a FLAC encoder tag prove the file is lossless?
No. Vorbis comments such as ENCODER are easy to edit. A file tagged reference libFLAC can still be an MP3 wrapped in FLAC. Trust the spectrogram and cutoff, not the tags.
Can I check WAV or MP3 files too?
Yes. WAV and AIFF use the same spectrogram and cutoff checks. An MP3, AAC, or Ogg file is reported as a lossy source rather than a fake FLAC, and the plot still shows where that encoder cut.
Looking for the longer walkthrough? Read how to identify fake FLAC files, or browse the music production guides and the rest of the tools.