Free online spectrum analyzer

How to read a spectrum analyzer

A spectrum analyzer (spelled analyser in the UK), also called a frequency analyzer or audio analyzer, shows how much energy a sound has at each frequency. The left side is the low end, the right side is the high end and the height is the level in decibels. Bass notes and kick drums sit on the left, vocals and guitars in the middle, cymbals and air on the right. The axis is logarithmic, like hearing, so every octave takes the same width.

Music does not look flat. Real sounds hold more energy in the lows, so an untilted spectrum slopes down to the right. The tilt control lays the curve over by 3 or 4.5 dB per octave around 1 kHz, a convention many analyzers use. With 4.5, a typical well-balanced mix sits roughly level, and a bump or a hole stands out. The numbers under the cursor are the real values, without the tilt.

What the controls do

A raw spectrum is jumpy. Octave smoothing averages the energy over a fraction of an octave. At 1/3 octave you see the broad shape, the way a graphic equalizer would, at 1/6 octave you see a bit more, and at 1/12 octave you keep most of the detail. Off shows every band as it is.

The peak line keeps the highest level of every band and lets it fall slowly, with the decay of the peak meters in IEC 60268-18, so you can see the top of a loud passage. Hold freezes it until you change the setting. Speed sets how fast the curve itself falls after a peak: fast follows every transient, slow gives a calmer picture.

Reading the spectrogram

A spectrogram, also called an audio spectrogram or sound spectrogram, turns the spectrum into a picture over time. Time runs from left to right, frequency from bottom to top and the level is the colour. Notes show up as horizontal lines, drums as vertical stripes and noise as a haze. The live spectrogram scrolls with the sound. The spectrogram of the whole song under it draws the entire file at once, so you can see the shape of a song, the build of a chorus, a filter sweep or a gap.

A spectrogram is also a quality check. A hard cut near 16 kHz in a file that should be full range is often the mark of a lossy MP3 that was converted to WAV, and a thin steady line at 50 or 60 Hz is mains hum. The colours go from dark for quiet to bright for loud, over a range of 100 dB.

What the stereo phase scope shows

The phase scope plots the left channel against the right, turned by 45 degrees. A mono signal draws a vertical line and a wide stereo mix fills the shape around it. A signal that leans towards the horizontal is out of phase and will lose its bass when someone plays your mix in mono, on a phone speaker or a club system.

Next to the scope, the correlation meter goes from +1 for mono to −1 for opposite phase, and the balance dot shows whether the mix leans left or right. If a microphone is mono, the scope draws a straight vertical line, because the two channels are the same.

Finding problems with a spectrum analyzer

A curve is not a verdict, and every pair of ears is different, but there are usual places to look. Rumble below 30 Hz eats headroom and can often be cut. Mud collects between 200 and 400 Hz when too many instruments pile up there, and boxiness lives around 400 to 800 Hz. Harshness sits between 2 and 5 kHz, where the ear is most sensitive, and sibilance between 5 and 9 kHz. Air and sparkle live above 10 kHz.

Look for one narrow, tall spike against a smooth curve, which can be a resonance or a note that rings. Compare the shape with a reference track you love, played at the same loudness, and move your mix towards it a little at a time. Match the loudness before you compare, because a louder sound always looks fuller.

Using the microphone

Press the microphone button and the analyzer shows what the microphone hears: a guitar amp, a voice, a room, a speaker playing a sweep. We ask the browser to switch off echo cancellation, noise suppression and automatic gain, which would change the picture. The sound is analyzed on your device, nothing is recorded or sent anywhere, and it is never played back, so there is no feedback.

A phone or laptop microphone is not a measurement microphone. Use it to compare and to find problems, not to calibrate a room.

Spectrum analysis for teachers and students

Seeing a sound helps people learn to hear it. Teachers use a spectrum analyzer to show what an equalizer does, why a bass and a kick collide, what a high-pass filter removes and what a vowel looks like. Students can drop a song they love and see the shape that makes it sound the way it does, then drop their own mix next to it.

How it works

The live view is real time, like an audio spectrum analyzer in a studio. Two Fourier transforms stand behind the curve: one of 2048 points for the mids and highs and one of 16,384 points for the bass, so low notes are resolved finely. They are joined around 1 kHz and shown on 1024 log-spaced bands with Hann windows. A full-scale sine reads 0 dB. The meters are the OBRA METER cores that draw every meter in OBRA Audio Mastering. The spectrogram of the whole file uses a transform of 4096 points and a gentle tilt of 3 dB per octave. Everything runs in your browser.