A spectrogram is a picture of sound. It shows time from left to right, frequency from bottom (low) to top (high) and loudness as brightness or colour. Once you can read one, you can "see" birdsong, spot a hum in a recording, or tell rain from traffic without pressing play. Every field recording page on OttoAnna shows a spectrogram, and you can make your own with the live spectrogram tool.
The basic shapes
- A thin horizontal line – a pure, steady tone, like a whistle or a tuning fork.
- A curved or sloping line – a tone that changes pitch: a rising whistle, a siren, a bird's slur.
- A stack of parallel lines – a harmonic sound: a voice, a violin, a crow. The lowest line is the fundamental; the others are harmonics at 2×, 3×, 4× the frequency.
- A vertical stripe – a short, sharp transient: a click, a footstep, a drum hit, a woodpecker's tap.
- A broad, fuzzy band – noise: wind, rain, a waterfall, traffic, a hiss. Noise spreads energy across many frequencies at once.
Reading a bird song
Birdsong is ideal for spectrograms. A chiffchaff's song appears as a row of near-identical marks alternating between two pitches; a blackbird's as rich, curving phrases ending in a scratchy twitter; a wren's as a rapid machine-gun of vertical marks. Many birders identify species from spectrograms in apps – try it with the recordings on our bird pages.
Watch for:
- Frequency range – small birds sing high (3–8 kHz), large birds and owls low.
- Rhythm and repetition – trills, repeated notes, pauses.
- Shape of each note – up-slurs, down-slurs, buzzy bands.
Reading a soundscape
A city recording usually shows a dense, low band (traffic below 1 kHz), interrupted by vertical lines (footsteps, doors), harmonic stacks (voices) and perhaps a horn. A forest at dawn shows little energy below 1 kHz and many bird marks above 2 kHz. Ecologists use these patterns to measure biophony (animal sound), geophony (wind, water) and anthrophony (human noise) – see soundscape ecology.
Settings that change the picture
Spectrograms are made with the Fourier transform over short windows of audio. Long windows give sharp frequency detail but blur fast events; short windows show timing precisely but smear frequencies. A logarithmic frequency axis (like our tool's) gives low frequencies more room, matching how we hear; a linear axis is common in bioacoustics software.
Try it
Open the spectrogram tool, allow the microphone and whistle a rising note, then say "aah" – you will see a line, then a stack of harmonics. Clap: a vertical stripe. Exhale through your teeth: noise. You have just read your first spectrogram.
Keep exploring