How to play Sound in Slow Motion
- Choose a test and read the frequency or the delay on the card. Sound travels at 340 m/s.
- Use λ = v ÷ f and T = 1 ÷ f for notes, and d = v·t for thunder and echoes.
- Type your prediction and press Start. Notes play slowed right down; thunder and echoes in real time.
- Compare the orange marks with the squeezes in the air, or with where the storm or the cliff turns out to be.
Sound is a wave
When a speaker cone or a guitar string vibrates, it pushes the air next to it, squeezing it, then pulls back, leaving it stretched. Each squeeze pushes on the air beyond, and so the squeezes travel outwards, at about 340 m/s in air. The air itself goes nowhere: each bit only jiggles back and forth along the direction the sound travels. That makes sound a longitudinal wave, unlike a wave on a rope, where the rope moves sideways. The frequency is how many squeezes leave each second; it sets the pitch, and people hear from about 20 Hz to 20000 Hz. The period T = 1 ÷ f is the time between squeezes, and the wavelength λ = v ÷ f is the distance between them. Low notes have long waves, metres long; the highest notes have waves shorter than your finger. Because light is almost instant, the delay before thunder is the time the sound took to reach you. An echo is sound bouncing back, so it travels the distance twice.
A focused learning model
Sound travels at 340 m/s, as in air at about 15 °C, and nothing is lost on the way. Notes are slowed down by their own frequency, so one vibration lasts one second on screen; the air is drawn as dots whose back-and-forth motion is much enlarged. The thunder and the echo play in real time; the flash is treated as arriving instantly, which is true to within a few millionths of a second. The cliff reflects all of the sound. Answers within 2% are right.