# How can you hear?

### Vibrations from source to detector—that's how we hear.

How can you hear the radio playing in all parts of the house or the bacon sizzling or the notes from the guitarist at a concert? The guitarist is up there on the stage and you are in the audience. What happens to allow you to hear the sounds from the guitar, which is at least 20 metre away?

To hear the guitarist there are two connections in the 20 metre journey:

something vibrating → a connecting material → you

The start, middle and end have special names:

source → medium → detector

For the guitarist:

• the source is the guitar
• the medium is the air
• the detector is you

You need all three to hear any sound. If there are no particles of matter between source and detector then the vibrations can't get to the detector.

### From source to detector

All sources of sound have the same kind of to-and-fro motion. All sources of sound vibrate.

Here are some sources: voice—vibration of vocal cords sitar music—vibration of a string trumpet music—vibration of lips at the mouthpiece.

The vibrating source moves to-and-fro. This moves the medium to-and-fro, copying the motions of the source. Later, the detector vibrates as it is set in to-and-fro motion by the medium. Explaining hearing is always the story of this journey.

### Density varies as the vibration travels

See the patterns of high- and low- air density.

The to-and-fro motion of the source pushes and pulls the air, squeezing and stretching the block of air next to the source. Later this to-and-fro motion travels away from the source as one block of air affects the next block of air. In this way, the vibrations travel out from the source through the surrounding air.

### What sound is

TeacherOne{But what exactly is the sound?}

TeacherTwo{It's what you hear, with your ears.}

The cone moving to-and-fro is not the sound. The cone is the source of the vibrations.

The detector is not the sound. The detector picks up the vibrations.

The to-and-fro motions in the medium are not the sound. These link a source to a detector.