FLAC is a lossless format, but nothing stops someone from converting a 128 kbps MP3 into FLAC. The file grows to FLAC size, the player shows a lossless badge, and the sound is still MP3 quality. These files are often called fake lossless, upconverted or transcoded files.
File size, bitrate and the format name cannot reveal this. What gives it away is the frequency content: lossy encoders throw away the highest frequencies, and that loss stays visible after conversion.
Why the frequency cutoff gives it away
To save space, MP3 and AAC encoders apply a lowpass filter that removes content above a certain frequency. The lower the bitrate, the lower the cutoff. A genuine CD-quality recording usually has content up to about 20 to 22 kHz, while a 128 kbps MP3 is cut at about 16 kHz.
On a spectrogram, a transcoded file shows a sharp horizontal shelf: plenty of energy up to the cutoff, then almost nothing above it, for the whole length of the track.
Typical cutoffs of lossy encoders
| Source | Typical cutoff | What you see |
|---|---|---|
| MP3 128 kbps | ≈ 16 kHz | Hard shelf at 16 kHz |
| MP3 192 kbps | ≈ 18–19 kHz | Shelf just below 19 kHz |
| MP3 320 kbps / AAC 256 kbps | ≈ 19.5–20 kHz | Shelf near 20 kHz, harder to spot |
| Genuine CD lossless | ≈ 20–22 kHz | Content fades naturally toward 22 kHz |
How to check a file in under a minute
- Open the SoundBench Quality Analyzer and drop the FLAC or WAV file onto it. Nothing is uploaded.
- Read the verdict and the detected cutoff frequency.
- Look at the average spectrum: a vertical cliff well below the top of the range is the fingerprint of lossy compression.
- Check the spectrogram over time. A real recording's top end moves with the music; a transcode has a flat ceiling from start to finish.
When a low cutoff is not a fake
Some genuine recordings have little high-frequency content: old tapes, some live recordings, deliberately dark mixes or masters made with a lowpass filter. A gentle, uneven roll-off is usually natural. A perfectly straight brick wall at a typical encoder frequency is the strong sign of a transcode.
Fake hi-res is a related problem: a 24-bit or 96 kHz file made from a CD source shows nothing above 22 kHz, or only the lowest 16 bits in use. The analyzer reports both cases.
Frequently asked questions
Can a fake FLAC be fixed?
No. The detail removed by the MP3 encoder is gone. The best you can do is find a genuine lossless source or keep the original MP3, which is smaller and sounds the same.
Is a 320 kbps transcode easy to spot?
It is harder, because the cutoff is near 20 kHz. Look for a perfectly flat shelf around 19.5 to 20 kHz across the whole track.
Does the check work for WAV and ALAC too?
Yes. Any lossless container can hold transcoded audio, and the spectrum analysis works the same way.
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