What a "fake" FLAC is
FLAC is lossless: decode it and you get back exactly the samples that went in. But if what went in was a decoded MP3, the FLAC faithfully preserves an MP3's worth of quality in a file five times the size. The audio isn't damaged further; it's just not the lossless original it claims to be.
The tell: a frequency ceiling
MP3 and AAC encoders save space by discarding sound you're least likely to notice, and the first thing to go is the very top of the treble. Above a cutoff that depends on bitrate and encoder settings, the encoder stores nothing at all. Decode the file and that missing band stays missing, so a spectrogram of the "lossless" file shows a flat, straight ceiling running the length of the track.
| Where the content stops | What it usually means |
|---|---|
| About 16 kHz | MP3 at 128 kbps, a very common source of fakes |
| About 17–19 kHz | MP3 or AAC around 160–256 kbps |
| About 19.5–20 kHz | 320 kbps MP3, or a genuine file with little top end; hard to call |
| Up to 21–22 kHz, fading gently | Consistent with a genuine 44.1 kHz lossless file |
Exact figures vary between encoders and settings, so read the table as a rule of thumb. What matters most is the shape: lossy encoders leave a sharp edge, while real recordings tend to roll off gradually or carry faint noise right to the top.
Step by step
- Open the file in a spectrogram. WAVR's spectrum analyzer runs in your browser and shows 20 Hz up to half the sample rate.
- Look at the top of the picture across the whole track, not just one moment. Quiet passages naturally have less treble; a fake has the same ceiling in loud choruses too.
- Read the cutoff. WAVR's fake FLAC checker measures it for you and reports how steep the drop is.
- Rule out false alarms before deciding (see below).
False alarms
- Old analogue sources. Tapes, older vinyl transfers and early recordings often have little above 15 kHz, but the roll-off is gradual, not a cliff.
- Deliberate low-pass filters. Lo-fi, some hip-hop and many vocal recordings are filtered on purpose.
- Low sample rates. A 32 kHz file can't hold anything above 16 kHz. That's the format, not a fake.
- Very high-bitrate sources. A 320 kbps MP3 can reach close to 20 kHz and may pass the check. No spectral test can rule this out completely.
That's why WAVR's checker words its result as "likely", "possibly" or "no lossy cutoff found", and never as proof. Use it to decide which files deserve a closer listen or a better source.
What to do with a fake
You can't restore what the MP3 removed. If you have a fake in your library, either find a genuine source or convert it to a good MP3 or M4A and reclaim the space; you'll lose nothing you still had. For DJ use, a fake FLAC is no better than the MP3 it came from.