1. Warm-Up: Doppler Effect #1
You and a friend are out on the lake, enjoying summer. You are driving along the coast, when you hear music playing from one of the docks. A particular frequency of 320 Hz sounds from the speaker. What is the frequency that you observed if you are moving 16 m/s and are already past the dock?
fo = 305 Hz

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. Example 6
The figure above shows two wave pulses that are approaching each other. Which of the following best shows the shape of the resultant pulse when the centers of the pulses, points P and Q coincide?

8. Example 7
The pulses below are moving towards each other through a medium at one grid square per second. Use the graph below to draw the resulting shape of the two at 1, 2, 3, and 4 seconds later.

9. 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

10. Interference by Position
Path difference of ½ λ leads to destructive interference
Destructive
Constructive

11. Constructive/Destructive Interference – Applications

12. 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

13. The ‘Beats’ If two sounds are only slightly off in terms of frequency
Produce a periodic rise and fall of amplitude (volume)
Throbbing Sound = Beats

14. The Beats #‘beats’ = how far apart the two frequencies are Ex.
Tuning Fork 1: f = 440 Hz
Tuning Fork 2: f =
*Beat Frequency of 2 Hz?

15. Example 8
A guitar string produces 4 beats per second when tuned with a 350 Hz tuning fork and 9 beats per second when tuned with a 355 Hz tuning fork. What is the actual frequency of the guitar?