How to play Two Speakers
- Choose a test and read the note, the path difference, or the speaker wiring on the card.
- Use λ = v ÷ f with v = 340 m/s, quiet at Δd = (n + ½)·λ and loud at Δd = n·λ.
- Type your prediction and press Start. The microphone walks away from the middle.
- Compare the orange mark with the quiet and loud spots, in the room and in the loudness graph.
Interference of sound
When two waves meet, they add up: where crest meets crest they make a bigger wave, where crest meets trough they cancel. Two speakers playing the same note fill a room with a fixed pattern of loud and quiet spots. What decides it is the path difference Δd: how much farther the sound travels from one speaker than from the other. If Δd is 0, λ, 2λ… the waves arrive in step and it is loud; if Δd is ½λ, 1½λ… they arrive out of step and cancel. Walking across the room, you hear the sound swell and fade. Wire one speaker backwards and every loud spot becomes quiet and every quiet spot loud. Noise-cancelling headphones use the same idea: they play a copy of the noise turned upside down, so the two cancel at your ear. Light interferes too, which is what colours a soap bubble.
A focused learning model
Sound travels at 340 m/s. The speakers are 3 m apart and the microphone walks along a line 4 m in front of them. The waves are drawn slowed down to one vibration a second; real ones vibrate hundreds of times a second. Echoes from the walls and the fading of sound with distance are left out, so the quiet spots are completely silent. Answers within 2% are right.