1. Sound
Chapter 12

2. If we had no Sound… No Speech – Rassi’s lectures
No Noise – no car horns, no buzzers for sports, no alarm clocks
No Music – no radio, no bands, no MP3 players, no CDs

3. Sound waves – what are they?
Sound waves are longitudinal waves
Sound sources push against medium creating more dense and less dense pockets of the material.
more dense = compression;
less dense = rarefaction

4. Speed of Sound
Sound travels at 343 m/s in the air at 20°C – it will travel faster through warmer air, slower through cooler air
Sound travels faster through denser objects, air < water < rubber < iron
Chart pg 410

5. Sound behaves like a wave
Sound reflects off of hard objects, we call the reflections echoes.
Bats, navy use echoes
Sound waves interfere and create nodes that are ‘dead spots’ where you don’t hear anything
Sound obeys the v = λf equation

6. Pitch – a particular musical tone
Pitch is directly related to frequency – Middle C is 262 Hz, the E above middle C is 327 Hz.
Pitches below 20 Hz = Infrasonic
Pitches above Hz = Ultrasonic
The ceiling of what we can hear drops as we get older, by 70 most people can not hear above Hz

7. Measuring Loudness Loudness comes from amplitude of the sound waves
Intensity = Power/area
Intensity = P/(4πr2)
We can hear a very large scale of amplitudes ranging over orders of magnitude

8. Loudness - Decibels
We measure loudness on a sound scale measured in Decibels
logarithmic scale -
This means that a doubling of the decibel level corresponds to a 10 times greater intensity
We generally perceive a 10 decibel increase in sound as twice as loud

9. Loudness of different sounds – pg 417
Faintest sound you can hear 0 dB
Whispers 20 dB
Moderate conversations 50 dB
Traffic noise 80 dB
Fire Engine 100 dB
Painful, dangerous if continuous exposure – airplanes, heavy machinery, rock concerts 120 dB and up – ear protection

10. Isn’t It Resonating
Resonance occurs when forced vibrations = Natural Frequency (inc. in amplitude)
Not restricted to wave motion

11. How the Ear works Outer ear collects the sound and sends it in
Middle ear has eardrum & 3 bones the anvil, hammer, and stirrup
Inner ear – cochlea; fluid filled, hairs in cochlea vibrate with sound, sending nerve signals

12. Feel the Effect Doppler Effect Frequency change due to motion
V is constant for a given medium
Frequency and wavelength change
Play Song with PP

13. Doppler Effect Moving toward – higher frequency – lower wavelength
Moving away – lower frequency – higher wavelength

14. Doppler Effect on light -
Blue Shift – Increase in frequency
Red Shift – Decrease in frequency

15. Shift up or down? f = (vw+vr) f0 (vw+vs)
vr velocity of receiver (toward source = positive)
vs velocity of source (away from receiver = positive)
f0 frequency produced
f frequency received

16. Closed Pipes Rules for pipe resonance:
end
Smallest fraction of standing wave that fits is: ¼=Length
Largest  -> smallest frequency – called Fundamental frequency (f0)
Other wave fractions that fit: ¾; (5/4); (7/4);…
Frequencies for these are called harmonics
H1 = 3f0; H2 = 5f0; H3 = 7f0; H4 = 9f0…

17. Open Pipes Same rules: Node@closed end/Antinode@open end
Smallest fraction of standing wave that fits is ½ = Length
Largest  -> smallest frequency – called Fundamental frequency (f0)
Other wave fractions that fit: ; (3/2); 2; (5/2)…
Frequencies for these are called harmonics
H1 = 2f0; H2 = 3f0; H3 = 4f0; H4 = 5f0…

18. Instruments - types What vibrates in each? Examples of each?
Percussion
String
Wind
Brass
Reed

19. What sounds good? Dissonance – Pitches that sound bad together
Consonance – Pitches that sound good together – also called a chord in musical terms
Whole number ratios of the frequencies create consonance

20. Chords and Octaves
Frequencies are in a ratio of 1:2 are an Octave apart
A ratio of 4:5 is called a major third; 3:4 is a perfect fourth; 2:3 is a perfect fifth
Combine major third and perfect fifth: 4:5:6 and you have a major chord

21. Beats Ever heard a tone get louder, then softer repeatedly?
This is called a beat – it results from the interference of 2 tones very close in frequency

22. Beats II
The Beat has a frequency too, how often it moves from loud to soft in a second
Freqbeat = |FreqA – FreqB|
Beats below 7Hz we hear as a change in loudness

23. Bow Waves
What happens when an object’s speed is faster than the speed of waves in that medium?
The wave crests overlap and “pile” up
Berry the bug – draw circles

24. Shock Waves
Shock Wave – Like a bow wave, but its a 3-D cone produced in the air by a supersonic aircraft
The air “piles up” – jet moves faster than sound waves
Waves pile up & create a cone
Extends back like a boat wake

25. Shockwaves Bug in the water Bow waves on ships Sonic booms
Concord, Thunder
Bull whip