A Spek alternative that runs on your iPhone

Spek has been the default answer to "is this file a transcode?" for years: free, open source, and it draws you a spectrogram in about a second. The catch has always been that it runs on a computer, and a lot of music now arrives on the phone and never leaves it.

Last updated 16 September 2026

What Spek does, and why people trust it

Spek decodes an audio file and draws its spectrum over time — frequency up the side, time across the bottom, loudness as colour. You look at the top of the image. If the energy stops dead at a flat horizontal line, something imposed that limit, and lossy encoders are the usual suspect.

It is a good tool precisely because it does not pretend to judge for you. It hands you the picture. Anyone arguing about a file can point at the same image and make their case, which is why forum threads have used Spek screenshots as evidence for over a decade.

Spek runs on Windows, macOS and Linux. There is no iOS version, and asking for one slightly misses the point: reading a spectrogram by eye on a 6-inch screen is not a pleasant job.

What actually has to change on a phone

Porting the interface would be the wrong move. Three things are different:

The screen is small, so the verdict has to come first

On a desktop you scan an image and form an opinion. On a phone the useful order is reversed: say what you found, then let the person open the evidence if they care. The picture still has to be there — a verdict with no evidence behind it is just an opinion with a nicer font — but it cannot be the first thing you have to interpret.

Files arrive continuously, not in batches

Desktop workflow is "I downloaded 40 albums, let me check them". Phone workflow is "I bought this one album on the train". Analysis that happens automatically as files come in fits that better than a scan you have to remember to run.

Battery and heat are real constraints

An FFT over a full album is not free. On iOS the sane approach is Apple's Accelerate framework, which uses the CPU's vector units, and to sample sections of the track rather than transform every second of it at full resolution.

Reading a spectrogram yourself, in one minute

Worth knowing whichever tool you use:

• A hard flat ceiling around 20kHz suggests a 320 kbps MP3 source; nearer 16kHz suggests something like 128 kbps.
• A ceiling well below what the format allows matters more than the ceiling itself. A 96kHz file can carry content to 48kHz. If it stops at 16kHz, the extra sample rate is carrying nothing.
• A ceiling that never moves is the strongest signal. Real recordings breathe — a cymbal reaches higher than a quiet passage. An encoder's limit sits in exactly the same place from the first second to the last.
• An early roll-off is not proof. Old masters, quiet acoustic recordings and analogue transfers can genuinely hold nothing above 17kHz. This is where cut-off-only tools produce false accusations.

Tools, side by side

Prices as of September 2026. Spek remains the best place to start if you are on a computer and want to learn to read this yourself — it costs nothing and it teaches you what the shapes mean.