Audio Quality Checker

Why files lie

Why Audio Files Lie About Their Quality

A file extension is a label, not a guarantee. Anyone can take a 128 kbps MP3, run it through a converter, and hand you a .flac that is larger, slower to download, and no better than the MP3 it came from. In DJ pools, private trackers, and reposts from Soulseek or YouTube, this happens constantly: a track gets ripped as lossy, re-encoded a few times as it is shared, and eventually re-wrapped as FLAC or WAV so it looks premium. The audio inside is still capped, and on a club system that missing top end and those stacked artifacts are audible.

You cannot hear the difference reliably in headphones on a noisy train, and you cannot see it in the file size. The one place the truth survives every re-wrap is the frequency spectrum.

How it works

How Cutoff Detection Works

Lossy encoders save space by discarding the highest frequencies, which the ear notices least. The result is a low-pass: above a certain frequency, the energy drops to nothing. The checker runs a hann-windowed FFT across the whole track, averages the spectrum, and walks down from the Nyquist limit to find the highest frequency where real energy resumes. It also measures how sharp that edge is: a lossy codec leaves a cliff, while natural high-frequency decay is gradual. A sharp cliff well below the Nyquist limit is the lossy fingerprint.

Here is the honest part. A cutoff proves that some ancestor of the file was lossy. It does notprove the file's provenance, and full bandwidth does not prove a file was never lossy. Some legitimate masters are deliberately low-passed, and many older recordings, analog transfers, and vintage samples simply never had energy up to 22 kHz. That is why a band-limited file with a lossy extension gets flagged as a likely transcode, while the same shape in an old recording is only reported as a cutoff. Read the verdict as strong evidence, weighed against what you know about the track, not as a certificate of guilt.

Full report

Beyond the Cutoff

The cutoff catches lossy audio wearing a lossless extension, but that is not the only way a file lies. Two more checks run on every track. Bit depth: the tool measures the effective resolution of the samples, so a 16-bit recording padded out to a fake 24-bit file is exposed no matter what the header says. Sample rate: a genuine 96 kHz file has real content above 22 kHz, while one upsampled from 44.1 kHz is empty up there, which flags it as upsampled hi-res. When it is still an MP3, the checker also reads the encoder tag in the bitstream, the LAME low-pass and CBR or VBR mode, a signed confession that corroborates the spectrum.

Then it reads what actually decides how a track holds up on a big system. True peak in dBTP catches inter-sample overs that clip through the club limiter even when the file reads under zero. Integrated loudness in LUFS lets you gain-match tracks the way streaming platforms do. Dynamic range shows how hard a master was squashed. Stereo phase warns when out-of-phase content will cancel bass and vocals the moment the system sums to mono. Each reading comes with a plain-language note, and the whole report is evidence to weigh, never a bare pass or fail.

Reference

Cutoff Frequency to Bitrate Reference

These are the typical low-pass ceilings for common encoders at 44.1 kHz. They are guidelines, not exact constants: encoder settings, tools, and source material shift them by a few hundred Hz.

Bad file?

What to Do With a Bad File

If a track you rely on comes back as a likely transcode, there is no software fix: the detail is gone and nothing can restore it. Re-buy the track from a source that sells real files, such as Bandcamp or Beatport, and re-check the download here to confirm it is clean. Do not try to rescue it by converting it to FLAC. As the MP3 to FLAC page warns, converting lossy audio to a lossless format does not restore quality; it just produces the exact fake FLAC this tool is built to catch.

To eyeball the cutoff yourself, open the same track in the Spectrogram Analyzer. To pull key and tempo while you audit your library, use the Song Key & BPM Finder.

Frequently Asked Questions