# Sound Stage Waves

A high\-school physics lesson on mechanical waves and sound: pluck and tune a giant stage string, see its waves slowed down, find standing waves and make a wine glass resonate\.

By Numa

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/jDoUaIL0C36U)

## Transcript

### 1. A wave you can hear

A wave carries energy without carrying the material along\. A plucked string sends one wave along itself and another through the air: sound, shown by the cream ripples\. Grab the string and pull it sideways\.

The motion is slowed 200 times so you can see it\. The sound plays at its real speed\.

### 2. Pull farther, play louder

Amplitude is how far the string moves from its resting line\. Predict which pluck sounds louder, then try both buttons or pull the string yourself\. A bigger amplitude makes a louder note, but the pitch stays the same\.

### 3. Tighten the string, raise the pitch

Frequency is how many times the string vibrates each second, measured in hertz\. Drag the tuning peg clockwise to tighten the string and raise the pitch\. The ripples crowd closer, because the wavelength, the distance between crests, gets shorter\.

Turn the peg counterclockwise to loosen the string\.

### 4. Speed equals frequency times wavelength

Wave speed equals frequency times wavelength\. Sound in air keeps about the same speed, so when the frequency goes up, the wavelength must get shorter\. Predict what happens when the frequency doubles, then choose 440 Hz\.

### 5. Standing waves and harmonics

Waves bounce off both ends of the string and overlap into a standing wave\. Nodes stay still, and antinodes swing the most\. Slide the felt pad to the middle for 2 loops, or a third of the way along for 3 loops\. Anywhere else, the sound is choked\.

These loop patterns are called harmonics\.

### 6. Make the glass sing

Every object has natural frequencies where it vibrates most easily\. Push at that frequency and the vibration keeps growing\. This is resonance\. Tune the string to the crystal glass's 392 Hz and watch it sing, crack and shatter\.

In 2005, singer Jaime Vendera shattered a crystal glass with his unamplified voice on MythBusters\.

### 7. Why soldiers break step on bridges

Bridges have natural frequencies too\. In 1831, marching soldiers reportedly shook England's Broughton Suspension Bridge until it collapsed, because each step pushed in time with its swing\. Press Break step: scattered footsteps no longer push in time, and the swaying dies away\.