1. Example 6
A siren from a police car has a frequency of 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.

2. Going Faster Moving at the Speed of Sound Moving Faster than Sound
Moving Source
Mach I
Supersonic
Pressure wave, piled up at nose
Shock wave of constructive interference

3. Sonic Boom When the wave of sound passes you – Sonic Boom!
Do you see the plane or hear it first?

4. Sonic Boom High pressure Low pressure
Moisture condenses in low pressure region
High pressure
Low pressure

5. Wave Interference SUPERPOSITION ! Wave interaction, described by
Superposition –constructive or destructive combination of waves if present at the same place, same time

6. When Two Waves Meet… If two waves combine...
Constructive Interference – Result is a larger amplitude
Destructive Interference – Result is a smaller amplitude
Waves pass through each other afterwards.

7. Interference by Position
If you have the same sound but from two different speakers…
Constructive Interference– Waves must be a whole-number wavelength apart
Destructive Interference– Waves must be a half wavelength apart

8. Constructive/Destructive Interference – Applications
As the story goes, this man…
…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…
Ten years later, the Bose
Corporation introduced its first
noise-cancelling headphones to
the market

9. Constructive/Destructive Interference – Noise Cancelling Headphones
Two Kinds of Noise-Cancelling Headphones
Passive - Uses sound-absorbing materials to block sound
Active – Uses destructive interference to cancel out outside noise
High-density, sound absorbing foam