ItalianoEnglish
Which frequencies are in the room, right now. It opens analysing: no settings to visit first, no start button — just the spectrum moving with whatever the microphone hears.
A logarithmic axis: an octave is as wide low down as high up. 
A hand clap is still readable a moment later. 
Freeze and study it — and while it is frozen the app says so.
What it does
- A logarithmic axis, the way the ear hears. An octave takes the same width low down as high up, instead of squeezing every bass note into two pixels — the flaw in nearly every phone analyser.
- Colour says the frequency, height says the level. Two things at a glance instead of one.
- Peak marks that fall slowly. A hand clap is still readable a moment after it ended, instead of being a frame nobody sees.
- The loudest frequency spelled out, in Hz or kHz.
- Freeze the picture to study it — and while it is frozen the app says so, because what you see is no longer the room.
- In English and Italian, following the phone's language.
dBFS, not the decibels of the room
The level you see is in dBFS, relative to the converter's full scale: it says how loud a frequency is compared to the others, not how many decibels there are in the air. If that is what you want, that is a sound level meter and a different app. Confusing the two scales is the quickest way to get a room wrong, and Spectrum would rather say so.
The FFT is written by hand, and checked
Underneath is a Fourier transform written from scratch, with automated tests comparing it against synthesised tones of known frequency before every release. An analyser is believed or it is decoration: if the bands move convincingly but the numbers are wrong, the app is worse than useless.
Your data stays yours
Spectrum does not record: the sound is analysed in the phone's memory and thrown away, and no file is ever written. No account, no location, no server of ours. Two permissions — the microphone, and Internet for the ads.
How it is paid for
A banner at the bottom, away from the controls, and that is all: no subscription, no feature held hostage.