Spectrum analyzer
A measuring tool that shows how much energy a signal has in each frequency band — in audio, almost always an FFT redrawn several times a second.
Where a spectrogram shows frequency content over time, an analyzer shows a single moment: a curve with frequency along the x-axis and level up the y-axis. Under the hood the signal is cut into blocks, multiplied by a window function and fed through an FFT. Block size is a trade-off — large blocks give fine resolution in the bass but react slowly, small blocks the reverse.
The classic trap is that pink noise, which sounds even, appears to slope down towards the treble on a plain FFT display. That is why many analyzers tilt their view. Live sound still uses the older form, a third-octave RTA, together with pink noise to tune a room. And every mixing manual carries the same sensible warning: the analyzer tells you what is there, not whether it sounds good.