1. Warm up 1. Differentiate between infrasonic and ultrasonic.
2. Compare and contrast a rarefaction and a compression.
3. What is the source of all sounds?
Homework
Read and notes pg – 26.9
Supplement pg.

2. SOUND
Chapter 26

3. THE ORIGIN OF SOUND
All sounds are produced by the vibrations of objects
A.k.a a disturbance
Waves are created by some form of a disturbance or vibration

4. THE ORIGIN OF SOUND
As the particles are moved from their rest position, they exert a force on the adjacent particles
Then transfer their kinetic energy
Thus sound energy travels outward from the source.

5. Mechanical wave
Mechanical waves require a medium in order to transport their energy
Examples:
Slinky waves, water waves, stadium waves and sound waves
Sound waves are incapable of traveling through a vacuum

6. Wave propagation
creating a pressure disturbance consisting of an alternating pattern of compressions and rarefactions

7. Wave propagation For example:
a vibrating guitar string forces surrounding air molecules to be compressed and expanded
The amount of energy transferred to the medium is equal to the amplitude squared of the wave

8. Pressure Wave? In terms of pressure what is a sound wave composed of?
Illustration of the pressure-time fluctuations
Do not conclude that sound is a transverse wave which has crests and troughs

9. Longitudinal Wave
Definition?

10. Longitudinal Wave parts
RAREFACTION
a disturbance in air (or matter) in which the pressure is lowered
The transfer of a low pressure air front
Simulated guitar string

11. Longitudinal Wave parts
COMPRESSION
A pulse of compressed air
a disturbance in air (or matter) in which the pressure is increased
The transfer of a high pressure air front

12. Bobby’s hummer is a mess. There are papers piled up on the seats
Bobby’s hummer is a mess! There are papers piled up on the seats. Whitney, decides to straighten up.
Whitney quickly and forcefully opens the door. Predict what will happen to the papers
The air in the car will be rarefied and the papers will go flying.
Bobby sees this, gets very upset. He forcefully closes the door. Now, predict what will happen to the papers
The air in the Hummer will be compressed, and the papers will go flying again!

13. Why are sound waves longitudinal waves?
How is wavelength measured?

14. MEDIA THAT TRANSMIT SOUND
Sound can travel through:
Air
Solids
Liquids
Sound cannot travel through…
A vacuum
There is nothing to compress & expand

15. The two main factors affecting the speed of sound…
The medium
The temperature
Minor factors
Humidity
Air pressure

16. Density & the Speed of sound
The speed of sound is not always the same
It is easier for sound waves to go through solids than through liquids because…
The closer the molecules are to each other the tighter their bonds,
the less time it takes for them to pass the sound to each other

17. Density & the Speed of sound
Material
Speed of Sound
Rubber
60 m/s
Air at 40oC
355 m/s
Air at 20oC
343 m/s
Lead
1210 m/s
Gold
3240 m/s
Glass
4540 m/s
Copper
4600 m/s
Aluminum
6320 m/s
Density affects the speed of sound in a material
Density describes the mass of a substance per volume

18. Temperature and the speed of sound
Heat, like sound, is a form of kinetic energy
higher temperatures have more energy, therefore they vibrate faster
molecules vibrating faster means, sound waves can travel more quickly
If no temperature is stated we will use 340 m/s

19. If no temperature is stated we will use 340 m/s
Speed of Sound
The speed of sound in dry air is given approximately by
V sound in air = ( * Tc ) m/s
@ 0o C speed of sound is approx. 331 m/s or 742 mi/hr
@ 20oC speed of sound is approx. 343 m/s or 769 mi/hr
If no temperature is stated we will use 340 m/s

20. THE SPEED OF SOUND
The speed of sound is slower than the speed of light
Speed of Light = 3.00 x 108 m/s

21. How far away is the storm?
If you see a lightning flash and five seconds later you hear the thunder how far away is the storm?
Assume speed of sound to be 340 m/sec or .21 miles/sec
If v = d / t then d = v * t
d = 340 m/sec * 5.0 sec
d = 1700 meters away
d = 0.21 miles/sec * 5.0 sec
d = 1.1 miles away
Extra practice: Suppose the delay is 8 seconds.

22. Frequency vs. Speed Frequency refers to the…
number of vibrations that an individual particle makes in a specific period of time
Frequency refers to how often a wave passes through a certain point
Speed refers to how fast a wave passes through the point.

23. PITCH & FREQUENCY Pitch – impression of frequency
Some might say the two are synonymous
High-pitched vs. Low-Pitched

24. NATURAL FREQUENCY
All objects have a natural frequency or set of frequencies at which they vibrate
A natural frequency is one at which minimum energy is required to force vibrations
Natural frequency depends on factors such as the density, elasticity and shape of an object

25. FORCED VIBRATION
Forced vibration is when one object is connected to a larger one,
forcing its sound vibrations on the larger surface
The resulting sound is relatively louder

26. RESONANCE
Resonance – when the frequency of a forced vibration of an object matches the object’s natural frequency, there is a dramatic increase in amplitude.
To resonate, objects need a force to pull them back to their starting position
For ex: pumping a swing in rhythm with its natural frequency produces larger amplitudes.

27. Energy Transport and the Amplitude of a Wave
A wave transports energy along a medium without transporting matter
The amount of energy carried by a wave is related to the amplitude of the wave

28. Intensity
The amount of energy which is transported past a given area of the medium per unit of time
The greater the amplitude,
the greater the rate at which energy is transported, and therefore the more intense the sound wave

29. LOUDNESS Loudness is a physiological sensation sensed in the brain
It is subjective, but relates to sound intensity
Intensity of sound is measured with the decibel (dB)
0 dB is the threshold of hearing for a normal ear
An increase of 10 dB means that it increase by a factor of 10
10 dB = 10 x 1 dB
20 dB = 10 x 10 dB
30 dB = 10 x 10 x 10 dB

30. Decibel quiz 1. The decibel level that causes pain 1. (130)dB
2. The decibel level that can break bones in the ear
2. (150)dB
3. The decibel level that is 1000 times more powerful than 10 dB
3. (30)dB

31. Common dB
levels

32. Human Hearing Thresholds
Infrasonic
frequencies below 20 hertz
Ultrasonic
frequencies above 20,000 hertz
We cannot hear these sounds

33. Decibel History
A bel (symbol B) is a unit of measure of ratios
Deci (symbol d) is the metric prefix meaning ten
Invented by engineers of the Bell Telephone Laboratory, it was originally called the transmission unit or TU
Renamed in in honor of the laboratory's founder and telecommunications pioneer Alexander Graham Bell

34. Reflection of Sound
Sound reflects or bounces from all surfaces-walls, ceiling, floor, furniture, and people
An echo is reflected sound
Sound and light follow the same rules about reflections

35. Reflection of Sound Acoustics
the study of the reflective properties upon surfaces
Reverberations
Persistence of a sound, as in a echo , due to multiple reflections

36. Reflection of Sound
Reflection of sound in a room makes it sound lively and full
In a concert hall, highly reflective surfaces are often placed behind and above the stage to direct sound out to an audience

37. Reflection of Sound
When sound reflects off a special curved surface called a parabola
it will bounce out in a straight line no matter where it originally hits
Many stages are designed as parabolas so the sound will go directly into the audience, instead of bouncing around on stage

38. Example: Parabolic Stage Location: HOLLYWOOD BOWL

39. Reflection of Sound
If the parabola is closed off by another curved surface, it is called an ellipse
Sound will travel from one focus to the other, no matter where it strikes the wall
A whispering gallery is designed as an ellipse

40. Example: Ellipse Design
In here if you and a friend stand in opposite corners, as if being punished, you can carry on a whispered conversation that others can’t hear
Located in Grand Central Station

41. Refraction or (bending) of Sound
Sound waves are refracted or bent when parts of a wave front travel at different speeds
this happens in uneven winds or…
when sound is traveling through air of uneven temperatures

42. Refraction of Sound
On a warm day the waves tend to bend away from the warm ground…
making it appear that the sound does not carry well
On a cold day the speed of sound is reduced on the ground…
causing the waves to bend towards the earth, carrying the sound longer distances

43. Pop Quiz
How does sound reach you when are in a different room than the source?
Refraction and Reflection!

45. Pop Quiz

46. INTERFERENCE Waves can be made to interfere with each other.
When the crests of one wave overlap those of another wave, there is a constructive interference and increase in amplitude.
When the crests of one wave overlap the overlap the troughs of another, there is a destructive interference and a decrease in amplitude.

47. INTERFERENCE AND SOUND
Interference effects loudness
If one is equally distant from two speakers simultaneously triggering identical sound waves, the sound is louder because the waves add.
Destructive interference is usually not a problem, and there is enough reflection to fill in canceled spots.
“Dead spots” are present in poorly designed theaters and gyms.

49. BEATS Periodic variation in the loudness of sound is called a beat.
When two tones of slightly different frequency are sounded together, a fluctuation in the loudness of the combined sounds is heard.
Beats can occur with any kind of wave.