3D simulation · Waves and sound

Sound waves and the Doppler effect, in 3D

Name: Why a siren changes pitch
Uploaded: 2026-11-05
Description: Waves spreading through the air, and why a siren's pitch drops the moment it passes you.

Waves spreading through the air, and why a siren's pitch drops the moment it passes you. Not a drawing and not an AI guess: a simulation that runs the real numbers, narrated, with every moment landing on its word.

Make this video →Free to try · 11 languages · about 3 minutes

What the simulation shows

A drop on still water: circular waves spread out (the wave moves, the water just bobs).
Two sources: their waves overlap into a pattern of calm lines (crest meets trough) and doubled waves (crest meets crest).
A speaker pushing and pulling air molecules: bands of squeezed and stretched air travel away (sound).
The pitch doubles (440 to 880 Hz): the bands get closer together.
Louder: the squeezes get bigger, the same spacing.
A wall: the sound bounces back (an echo).
A moving source seen from above: its waves bunch up ahead (higher pitch) and stretch out behind (lower).
Faster than sound: the waves pile into a cone (a shock wave): the sonic boom.
A callout: sound travels at 343 metres a second in air.

The real numbers it runs on

The simulation is built on these facts, so the narration and the picture always agree:

A wave carries energy, not stuff: a ripple travels across a pond but the water only bobs up and down.
When two waves meet they add up: crest plus crest makes a bigger wave, crest plus trough cancels (interference); noise-cancelling headphones use this.
Sound is a wave of squeezed and stretched air (pressure) made by something vibrating.
Pitch is how many waves pass each second (frequency): the A above middle C is 440 per second (440 Hz); twice the frequency is an octave higher. People hear about 20 to 20,000 Hz.
Loudness is how big the pressure changes are; every 10 decibels sounds about twice as loud.
Sound travels about 343 metres a second in air (about a kilometre in three seconds), four times faster in water.
The Doppler effect: a moving source's waves bunch up ahead (higher pitch) and stretch out behind (lower), which is why a passing siren drops in pitch.
Faster than sound, the waves pile up into a cone, a shock wave: the sonic boom.

The script this video was made from

“Why a siren changes pitch”: the narration of the video above. Each highlighted moment in the simulation happens on the word that names it.

An ambulance races past and its siren suddenly drops in pitch. The siren didn't change. You did hear something real, though.

Sound

Sound is air being squeezed and stretched. The closer together the squeezes, the higher the note.

The squeeze

A moving siren chases its own waves. Ahead of it they bunch up, so you hear it higher. Behind, they stretch out, and it drops.

The limit

Go faster than sound itself and the waves pile into one shock wave. That's a sonic boom.

Make your own

Type a question, like “Why a siren changes pitch”, or your own angle on it.
ContentRoof picks this simulation, researches the facts and writes a short script, with every moment cued to a word.
A natural voice reads it in your language and the simulation plays in time with it. Change any line and it follows.
Download a vertical video for Shorts, TikTok and Reels, or use it in class.

Make this video →

Questions

Is this a real simulation or an AI-generated video?

A real simulation. Sound waves and the Doppler effect runs on the numbers listed above, so what you see is what actually happens, frame by frame. AI video generators guess what a scene should look like; they often get the science wrong.

Can I change what the narrator says?

Yes. Type your own question or idea and the AI writes a new script for this simulation, or edit any line yourself. Every moment of the simulation lands on the word that names it.

Which languages can it narrate in?

Eleven: English, Urdu, Hindi, Arabic, Spanish, French, Portuguese, German, Italian, Russian and Polish, with captions written properly (Urdu in Nastaliq, Arabic right to left).

Can I use it in class or post it on YouTube?

Yes. The video is yours to use in lessons, on YouTube Shorts, TikTok and Instagram Reels. The free plan makes 2 3D simulations a month; paid plans make more, in 1080p without the small mark.