1. Sound lab Digital piano Oscilloscope with speakers
Speaker into tuning box for resonance on scope
Speaker w/mic and tuning box on scope for beats ( )
Mic straight into speakers
Sounding board
Scope freq versus wavelength
Pitch versus freq (sin versus square wave)
Hydrogen fuel (fracture water) w/ resonance

2. CH 15 Sound

3. The nature of Sound Sound is a longitudinal wave
A sound wave is created by a vibrating object and is transmitted through a medium

4. Sound
When a diaphragm moves, it compresses the air molecules in front of it.
The compression causes the air pressure to rise. On
Off

5. Sound Waves Sound travels in waves known as “Longitudinal” Waves.
The picture below shows a Longitudinal wave formed in air molecules due to a sound.
The higher density areas are known as compressions.
The lower density areas are known as rarefactions.
When studying interference in sound waves, treat the compressions as crests and the rarefactions as troughs.

6. Sound
Sound waves move through air by creating variations in air pressure.
The air molecules collide, transmitting pressure oscillations.
Areas of high pressure are compression
Areas of low pressure are call Rarefactions

7. Distance between compressions is λ Sound travels in all directions
Wave front can be created by connecting similar point on the waves
Distance between compressions is λ
Sound travels in all directions

8. Note the Energy of the sound is now spread over each “growing” wave front.
The area of the wave front is πr2
The Intensity (I) drops by the area of a circle (πr2)

9. Pressure Amplitude
Loudness depends on the pressure amplitude of the wave
Sound Intensity I (Watts W/m2)
P Power
A Area
r Radius
Area of a sphere

10. Pressure Amplitude example
A bus generates sound intensity of 10μW/m2. A student barely hears the sound from pressure wave of 0.076W. How far away is he?

11. Pressure Amplitude Sound Intensity I (Watts W)
Sound Intensity level is measure in decibels (dB)
Io=1x10-12 W/m2 Threshold of human hearing

12. Intensity Level
The threshold for human hearing is 1E-12 W/m2. The average whisper is 1E-10 W/m2. How many dB louder is the whisper?

13. Intensity Level
For every 10x increase in Intensity, the Intensity Level increase 10dB.
Event
W / m2 dB
Threshold of hearing
1x10-12/ 1x10-12
Whisper
1x10-10 / 1x10-12 20
Classroom
1x10-7 / 1x10-12 50
Normal Conversation
1x10-6 / 1x10-12 60
City Street
1x10-5 / 1x10-12 70
Threshold of pain
1x10-0 / 1x10-12
120

14. Intensity
Two different stereo systems can produce a 45dB output and a 25dB output. What is the difference in their intensities?

15. Practice Problems WS
WS 15.2
#’s1-6

16. Resonance
A Phenomenon that occurs when the forced frequency on an object matches the object’s natural frequency, and a dramatic increase in amplitude results.

17. Tacoma narrows bridge video

18. Beat Frequency
Beats: A variation in amplitude (loudness) caused by the interference of two slightly different frequencies (tones).
Ex: A 330Hz and a 332Hz tuning forks are struck simultaneously. What is the beat frequency?

19. Beat Frequency Example
A student has 2 tuning forks. One is 349Hz, the other is an unknown frequency. Together they create a beat of 3Hz.
What are the possible frequencies?

20. Doppler Effect
Doppler Effect: the change in wavelength and frequency due to the motion of the source.
Police

21. Doppler Effect
Sound waves propagate out from the source in all direction.
If the source isn’t moving, the wavelengths are constant
Doppler Effect
Police
Sonic boom from real player library

22. Doppler Effect Source moving towards you: Source moving away from you:
Police

23. Doppler Effect Example
A train is approaching you at 31m/s and blows its whistle of 305hz.
What frequency do you hear?

24. Doppler Effect Example
A train is approaching you at 31m/s and blows its whistle of 305hz.
b) What frequency does your friend hear if the train has past him and continues moving away?

25. Expanding Galaxy

26. Practice Problems
Ws 15.2a
1-5
WS 15.3
1-4

27. Standing Waves Standing Wave: Node: Antinode:
Record these vocabulary terms and define their meaning using your text.

28. Standing Waves
Standing Wave: has stationary nodes and antinodes. It is the results of identical waves traveling in opposite direction.
Node: The medium is not displaced as the waves pass through
Antinode: The displacement caused by interfering waves is largest.

29. Standing Waves
In order for a standing wave to exist, there must be an identical wave traveling in the opposite direction
Standing wave demo

30. Standing Waves
Harmonics: Standing wave that consist of more than one pulse
4th Harmonic
1st Harmonic
2nd Harmonic

31. Standing waves
Fundamental Frequency: The lowest frequency that creates a standing wave in a given medium.
Harmonics (overtones): frequencies with integer multiples of the fundamental frequency.
These frequencies make up the harmonic series.

32. Harmonic Series in a string
Length L
1st Harmonic
2nd Harmonic
3rd Harmonic

33. Sound
End Ch 15