1. Unit 5 – Sound

2. Sound What is wrong with the following clip?
Sound waves are vibrations that take place in what medium?
If you said “air”, you are only partially correct
Sound can travel in solids, liquids, and gasses
Sound waves are longitudinal, and are spherical

3. Sound
What is pitch?
Frequency corresponds to pitch – high frequency sound waves give you high pitches
Demonstration – frequency generator hooked up to oscilloscope and amplifier
Other than pitch, what can this show?
AM vs. FM radio :: what does AM and FM stand for?

4. Sound Human range of hearing – 20Hz to 20,000Hz
Infrasonic – below 20 Hz – ex) elephants, whales
Ultrasonic – above 20,000Hz – ex) dog whistle
Demonstrate with WAVEPORT
Older people can’t hear extreme frequencies because their eardrums become less flexible!

5. Sound is a longitudinal wave
Just like the compression wave with a slinky, sound has compressions and rarefactions
Compressions – dense regions of the medium (typically air)
Rarefactions – sparsely populated regions

6. Homework Read pages 381 – 384 Answer “Check yourself” on page 384
Questions to be completed for tomorrow:
Pg , #1-5

7. Speed of Sound
What is the rule for knowing how far away a lightning strike is?
Can you figure out how far away a strike is if you hear the thunder 10 seconds later?
V = d /t ; v=340m/s, t =10s
Distance = 3400m, ~2 miles!

8. Reflection of Sound Echo – when a sound wave reaches a new medium
*some energy is reflected (echo), while some is transmitted through the medium
The time delay of an echo is the extra distance divided by the speed of sound
FACT – the human ear cannot distinguish an echo from the original sound if the delay is less than 1/10th a second
FIND OUT – how far away must a wall be in order to hear the echo?

9. Reflection of Sound Echoes are very useful in nature
SONAR – SOund Navigation And Ranging
Submarines most popularly use this, but so do fishermen!
Whales, dolphins, and bats use it to “see” their surroundings

10. Natural frequency
If you hit a stop sign pole, or if you hit a chain link fence, they make different sounds
This is because they vibrate differently
Based on an object’s elasticity and shape, each object has it’s own natural frequency
An object’s natural frequency can be a combination of frequencies

11. Natural Frequency
Resonance – increasing of amplitude that happens when an object is given vibrations at its natural frequency
Ex) kicking your legs on a swing – timing has to be perfect
Ex) shattering a wine glass with sound
Ex) a bridge collapse when a large group of people walk in step with the natural frequency

12. Interference When waves combine, just like the slinky demo
Demonstrate with stereo speakers (note – it is not as effective inside due to reflection off of walls)
Constructive interference – loud sounds
Destructive interference – soft or no sounds
Ex) noise cancelling head sets

13. Homework
Read 386 – 393
Problems – p396, #9-13, 24-29

14. Doppler Effect What does a car engine sound like as it races by?
Doppler Effect explained
Doppler Effect visualized
Doppler effect – change in observed frequency when the observer is in motion relative to the source

15. Sound Intensity vs Loudness
Sound intensity – the power per unit area that a sound wave makes
Loudness – subjective way of qualifying the intensity of sound
 our ears our sensitive to specific frequencies, so though loudness and sound intensity are related, they aren’t the same

16. Sound Intensity vs. Loudness
Sound intensity is measured in Watts per meters squared (W/m^2)
Loudness is measured in decibels (dB)
Decibels are logarithmic – this means every time sound intensity is 10 times larger, the dB level is only 10 greater (difference between multiplication and addition)

17. Qualities of sound The 3 - Pitch, Volume, Timbre
Pitch – the frequency of a sound wave
Volume – the amplitude of a sound wave
Timbre – the quality that allows people to distinguish different types of sounds (i.e. saxophone from flute)
Common descriptors – “pure vs. rich”; “warm vs. cold”, “ dull vs. brilliant”

18. Sound
Almost all musical instruments are a combination of some harmonics
Demonstrate on waveport combinations of waves

19. Beats
Beats are produced when 2 waves intefere at different frequencies
Fbeat = f1 – f2
Demonstrate on waveport
If I had a frequency of 500 and 503, what would the period between beats be?

20. Air columns Given any air column:
If the end is closed, there will be a node
If the end is open, there will be an anti-node
Open ended air column (both ends open)
This is the shortest possible wave:
It’s ½ of a wavelength

21. Air Columns Closed air columns (one end closed)
¼ of a wavelength fits for the fundamental frequency
Overtones – higher frequencies that resonant in the given air column
This happens as long as the end conditions are met (node for closed end, anti-node for open)