SOS Morse Code Sound: Hear It, Flash It, Signal It

The SOS Morse code sound is nine tones in one run: di-di-di-dah-dah-dah-di-di-dit, three short, three long, three short, with no pause between the letters. Press Play above to hear it at 10 WPM, Flash to see it as light, or WAV to download the sound for a class, a film scene or a project. The sections below cover what a correct SOS sounds like by ear, how long it lasts at each speed, the flash timing that certified distress lights use, and how to signal SOS in Morse code when all you have is a torch, a whistle or a wall.

A correct SOS sounds the same forwards and backwards: three quick tones, three held about three times as long, three quick again, and only then a stretch of silence. The symmetry is the point. Even if static or distance swallows half the run, the half you catch still identifies the signal.

Two mistakes are easy to hear. If there is a short gap after the third tone and again after the sixth, the sender spelled S, O and S as three separate letters instead of one continuous sequence. If the long tones are barely longer than the short ones, a listener may hear nine dots.

The pitch does not matter, the ratio does. This page plays a 600 Hz sine tone, which sits at the low end of the 600 to 1,000 Hz range most Morse audio uses; sound-effect libraries often pick 800 Hz. Whatever the tone, each dash must last three dots, and the gaps inside the signal must last one.

One SOS is 23 timing units: nine signals (3 + 9 + 3 units) and eight one-unit gaps between them. A unit lasts 1,200 ms divided by the speed in words per minute, so the whole signal takes 23 × 1.2 ÷ WPM seconds. The presets on this page cover the first three rows.

The duration line under the code updates when you change speed. If you record the WAV for a loop, add a gap of at least seven units after the signal so the repeats do not run together; at 10 WPM that is 0.84 seconds.

SOS is a prosign: a single Morse character that happens to look like three letters, written with a bar over the letters or as ⟨SOS⟩. Run any letters whose dots and dashes add up to ...---... together and you get the same signal; V7 (...- then --...) is the usual example, and early American Morse operators sometimes wrote it S5S because three dashes meant the digit 5 in their code.

That is also why SOS stands for nothing. "Save Our Souls" and "Save Our Ship" are memory aids attached after the pattern was chosen for being easy to send and hard to confuse. The spoken equivalent on voice radio is Mayday, and the earlier Marconi signal CQD was sent as three separate letters, which is one reason it was replaced.

The full story, from the 1906 Berlin convention to the Titanic, is on the Help in Morse code page, next to the spelled-out word.

Electric distress lights sold for boats in the US must flash SOS to a fixed schedule set by the Coast Guard in 46 CFR 161.013-7. The schedule keeps clear two-second gaps between the letters, unlike the run-together radio form; the rule gives no reason, so copy it as written. Both forms are read as SOS.

One cycle takes 14 seconds, about four signals a minute; a companion rule (46 CFR 161.013-9) requires three to five signals per minute even after six hours of continuous use. The light must produce the pattern automatically, is labeled for emergency use only, and counts as a night signal, so by day a flag or other signal is still needed.

If you flash by hand with a torch, this schedule is the one to copy: a count of one for a dot, three for a dash, a clear rest between letters, and a longer rest before you repeat. The Slow preset above (200 ms dots) is the nearest on this page to the certified 1/3-second flash, and the light flasher can send SOS on your screen or, on supported phones, through the camera flash.

Any signal you can start and stop will carry SOS: a torch, a whistle, a car horn, a mirror in sunlight, or knuckles on a pipe.