Spek: seeing what is inside an audio file
What it does
Spek is a spectrogram viewer. It reads an audio file and draws frequency against time, with brightness representing energy, so the shape of the sound becomes visible. The most common use is verification: a file encoded at a low bitrate shows a hard horizontal ceiling where the encoder discarded high frequencies, and that ceiling is visible at a glance. Spek handles the usual formats through FFmpeg libraries, computes the transform at a resolution you choose, and lets you save the resulting image. It is a small open source program released under the GNU General Public License, with builds for Windows, macOS and Linux.
The workflow
Drag a file onto the window or use the open dialog, and the spectrogram appears with frequency on the vertical axis and time on the horizontal one. Zoom and pan to inspect a region, and switch the frequency scale between linear and logarithmic depending on whether you care about high-frequency detail or musical pitch relationships. Changing the FFT window size alters the balance between time and frequency sharpness, which is the single most useful control when investigating a short event. Use the save command to export the image for a report, a forum post or documentation.
Practical settings and limits
Small windows produce blurry vertical smearing in time, while very large windows blur transients; a mid-range setting suits most inspection work. The colour palette is fixed in the main view, so screenshots are directly comparable between files, which is useful when documenting a batch. Spek only analyses: it does not play audio, edit it, or repair it, and it will not tell you definitively whether a file is authentic, only what the spectrum shows. Opening a very long file at a large window size takes noticeable time and memory.
Who should choose something else
Anyone who needs to listen, edit or repair audio should use an editor such as ocenaudio or Wavosaur. Users who want measurement rather than pictures, including level, loudness and frequency response, should look at dedicated analysis software, and mastering engineers will want a metering suite rather than a viewer. Spek is best understood as a quick, honest magnifying glass for the frequency content of a file.
- Best for
- Anyone checking whether a download really is lossless, or examining the spectral content of a recording without a full analysis suite.
- Good to know
- Spek only displays spectra; it cannot play, edit or repair the file it analyses.
How to get started
- Download the Windows installer from the official Spek page or its release page and run it.
- Launch Spek and drag an audio file onto the window, or use the open command.
- Adjust the FFT window size until the detail you care about is clear.
- Switch between linear and logarithmic frequency scales to read musical content more easily.
- Save the image if you need to document what the file contains.
Questions & answers
What does a flat ceiling in the spectrogram mean?
It usually indicates lossy encoding: the encoder discarded everything above a cutoff frequency, which is typical of low bitrate MP3 or AAC files.
Can Spek tell me if a file is lossless?
It shows evidence rather than proof. A full spectrum up to the Nyquist frequency is consistent with lossless audio, but it is not a definitive authenticity test.
Does Spek play audio?
No. It analyses and displays files only, so use an editor when you need to listen or make changes.
