Seeing sound

A spectrogram shows which frequencies are present in a sound at each moment: time runs left to right, low notes at the bottom, high notes at the top, brighter where a frequency is louder. Puzzle makers draw shapes, words, even whole pictures into a sound so they only appear here.

Choose the musical (logarithmic) scale to spread out the low notes, and raise the contrast to bring out a faint pattern. The steady diagonal bands of an SSTV image show up clearly. To actually decode an SSTV picture, play the sound into an SSTV app, though seeing it here tells you it is there.