1. Chapter 26: Sound

2. Do You Hear What I Hear
Sound produced by vibrations of material objects
How is a sound wave produced in a:
Violin?
Saxophone?
Flute?
Your Voice?

3. Sound Travel Sound disturbances travel through mediums
Sound Properties
Pitch describes frequency of sound
Pitches below 20 Hz = Infrasonic
Pitches above Hz = Ultrasonic

4. Sound Properties
As a source vibrates, a series of compressions and rarefactions (a disturbance in air where pressure is lowered) is produced.

5. Let’s Find a Happy Medium
Sound can travel
Faster in denser materials
Solids > Liquids > Gases
Sounds cannot travel in a vacuum
no medium; nothing to compress and expand

6. Watch Your Speed Speed of sound Dependent on
Slower than the speed of light (ex: you hear thunder after you see a flash of lightning)
Speed of Sound at 0ºC = 330 m/s
Dependent on
Temperature
Increases 0.60 m/s for every 1o C above 0
Elasticity of the medium

7. Keep the Noise Down Sound intensity can be measured
proportional to square of the amplitude I α A2
Measured in Decibels
Sound loudness subjective

8. You Can’t Force Me
Forced vibrations are vibrations from one object that cause a nearby object to vibrate
Examples: Music Box

9. It’s All Natural Natural frequency
Frequency at which object will vibrate
Frequency that requires least amount of energy to continue vibrations
Depends on elasticity and shape of object

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

11. Resonance examples

12. Don’t Interfere Sound waves can be constructive or destructive
Affects the loudness of sound
Destructive Examples
“Dead Spots”
Antinoise technology
ex: Jackhammer

13. Skip to the Beat
Beats
Caused by interference of two tones of slight frequency differences sounded together
Freqbeat = IFreqA–FreqBI
Humans can’t detect Beat frequencies over 7 Hertz
What is the beat frequency when a 262 Hz and 266 Hz tuning fork are sounded together?

14. 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…

15. 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…