# How an AMT Speaker Works

By Yan Kurniawan

Topic: Art, Music & Film

[Open the interactive Jig](https://ask.jig.com/j/3swCtFxRIWWj)

## Guide

### 1. A tweeter that breathes

This is an AMT, short for Air Motion Transformer\. It is a tweeter: the part of a speaker that plays high sounds like cymbals and birdsong\. Instead of a cone, it squeezes air with a folded strip of film\.

Physicist Oskar Heil invented it, and speakers using it went on sale in the 1970s\.

### 2. Look inside

Let's slice the model open\. Strong magnets sit on each side, and rows of steel bars guide their pull across the middle\. In the gap between the bars hangs the folded film\.

### 3. An accordion of film

The film is folded back and forth like an accordion, and each fold is deeper than it is wide\. A thin metal track printed on the film winds over every fold\.

The real film is about 0\.013 mm thick, thinner than a human hair\.

### 4. Music becomes a push

The music arrives as an electric current\. It runs up one fold and down the next, inside the magnets' field\. Current in a magnetic field feels a push, so neighbouring folds are pushed in opposite directions\.

### 5. Squeeze and open

Folds squeeze together, then spring apart\. When two folds close, they squirt air out of the gap between them\. When they open, air rushes back in\. Slide to try a lower or higher note\.

Shown very slowly here\. For a high note the folds swing thousands of times every second\.

### 6. Small move, fast air

Watch one fold and one puff of air\. The fold moves only a little, but the air squeezed out moves about 4 to 5 times faster\. That speed boost is the 'transformer' in the name\.

### 7. Sound both ways

The film is open at the front and the back, so sound leaves in both directions\. Because the opening is tall and narrow, the sound spreads widely from side to side\.

Speakers that send sound out front and back like this are called dipoles\.

### 8. Part of a team

A real AMT is only about the size of your palm\. It handles the high notes, while a bigger cone called a woofer plays the low ones\. Together they fill the room with music\.