• A siren from a police car has a frequency of 500.0 Hz and is moving with a velocity of 45.0 m/s. You are driving on the other side of the road towards the police car with a velocity of 36.0 m/s. What is the observed frequency from the police car? Assume the velocity of sound as 343 m/s.

Example 6

Moving Source

Going FasterMoving at the Speed of Sound

Moving Faster than Sound

SupersonicMach I

Pressure wave, piled up at nose

Shock wave of constructive interference

• When the wave of sound passes you – Sonic Boom!

Sonic Boom

Do you see the plane or hear it first?

Sonic Boom

High pressureLow pressure

Moisture condenses in low pressure region

Wave Interference Wave interaction, described

by

SUPERPOSITION!

Superposition –constructive or destructive combination of waves if present at the same place, same time

• Constructive Interference – Result is a larger amplitude

When Two Waves Meet…

Destructive Interference – Result is a smaller amplitude

Waves pass through each other afterwards.

If two waves combine...

• Constructive Interference– Waves must be a whole-number wavelength apart

Interference by Position

Destructive Interference– Waves must be a half wavelength apart

If you have the same sound but from two different speakers…

Constructive/Destructive Interference – Applications

• As the story goes, this man…

Ten years later, the Bose Corporation introduced its first noise-cancelling headphones to the market

…was on an airplane – trying to concentrate during take-off, but couldn’t due to the noise of the plane.

So during the plane ride, this man came up with a design for something new…and…

…he owned a company that could do it…

• Two Kinds of Noise-Cancelling Headphones– Passive - Uses sound-absorbing materials to

block sound– Active – Uses destructive interference to

cancel out outside noise

Constructive/Destructive Interference – Noise Cancelling Headphones

High-density, sound absorbing foam