1. Sound
Ch 14

2. Producing a Sound Wave Sound waves Caused by vibrations
Require a medium in order to propagate
Are longitudinal waves

3. Characteristics of Sound Waves
Diffraction: All waves bend when they pass around the edges of an obstacle

4. Characteristics of Sound Waves
Diffraction

5. Characteristics of Sound Waves
Diffraction

6. Characteristics of Sound Waves
Infrasonic
Not audible, below 20 Hz
volcanoes, avalanches, earthquakes and meteorites. The eruption of the Fuego volcano in Guatemala produced infrasonic sound in excess of 120 decibels in the range below 10Hz.
vocalizations of elephants were measured to have frequencies as low as 14 Hz which were detectable at a range of 10 km. Observations of elephant behavior suggests that they responded to the waves through the ground before they heard them in the air

7. Characteristics of Sound Waves
Ultrasonic
above the frequencies of audible sound, over 20,000 Hz
communication and navigation by bats, dolphins, etc…
Ultrasound imaging or sonography is often used in medicine, testing of products and structures, cleaning, mixing, and to accelerate chemical processes.
Animals (e.g. bats and porpoises) use ultrasound for locating prey and obstacles.

8. Characteristics of Sound Waves

9. Characteristics of Sound Waves
Sound level is measured in decibels (dB)

10. Characteristics of Sound Waves

12. Characteristics of Sound Waves

13. Characteristics of Sound Waves
Piezoelectric effect
Conversion of mechanical energy to electrical energy
A reversible reaction

14. Characteristics of Sound Waves
X rays vs ultrasound as a diagnostic tool

15. Characteristics of Sound Waves
X rays vs ultrasound as a diagnostic tool

16. The Speed of Sound
The speed of a sound wave in a fluid depends upon the fluid’s compressibility and inertia
The speed of sound also depends upon the temperature of the medium
In air
Vs = speed of sound in air at zero Celsius

17. Energy and intensity of sound waves
Intensity: the rate at which energy flows through the surface
E = energy
A = area
P = power
Units: W/m2

18. Energy and intensity of sound waves
Intensity level (decibel level)
Relative intensity of sound, compared to the threshold of hearing 1.0 x W/m2

20. Energy and intensity of sound waves
3 Waves\Doppler Effect.ppt

21. Earthquakes

22. pH

23. Intensity in Decibels
Relative intensity of a sound is intensity level or decibel level (β)
Units decibels (dB)
The intensity of a given sound is compared to the faintest sound detectable (I0) (1.0 x W/m2)
Notice that this a logarithmic scale, multiplied by ten

24. Intensity in Decibels
Notice that this a logarithmic scale, multiplied by ten

26. Interference of Sound Waves
What is wave interference?

27. Wave anatomy Nodes and Antinodes
Node: no displacement from neutral position
Antinode: maximum displacement from neutral position (crests, troughs)
Nodes: destructive interference
Antinodes: constructive interference

28. Interference of Sound Waves

29. Interference of Sound Waves
If two speakers are hooked up to the same sound source producing a monotone sound, then a sound interference pattern can be observed within the room.

30. Interference of Sound Waves

31. Interference of Sound Waves
Path Difference (PD) = r1 – r2
For antinodes, constructive interference
PD = nλ
n = 0, 1, 2, 3…
(n) nodal line number

32. Interference of Sound Waves
Path Difference (PD) = r1 – r2
For nodes, destructive interference
PD = (n + ½) λ
n = 0, 1, 2, 3…

33. Interference of Sound Waves

34. Interference of Sound Waves
Two point sources, 3.0 cm apart, are generating periodic waves in phase. A point on the third antinodal line of the wave pattern is 10.0 cm from one source and 8.0 cm from the other source. What is the wavelength?
PD =
2.0 cm
Are the conditions constructive or destructive?
Therefore PD =
and λ =
λ = 2.0 cm / 3 = .67 cm

35. Interference of Sound Waves

36. Standing Waves
String
All points on the string vibrate with the same frequency, but with different amplitudes
The ends of the string must have nodes, but the total number of nodes and antinodes can vary

37. Standing Waves
String
All points on the string vibrate with the same frequency, but with different amplitudes
The ends of the string must have nodes, but the total number of nodes and antinodes can vary

38. Standing Waves
String
The lowest frequency of vibration is the fundamental frequency or the first harmonic.

39. Standing Waves
String
The next, higher frequency is the second harmonic

40. Standing Waves
String
The next, higher frequency is the third harmonic

41. Standing Waves
String
What is the pattern of frequency and harmonics?

42. Standing Waves
String
What is the pattern of frequency and harmonics?

43. Standing Waves String Wavelength of a standing wave
The frequency of a standing wave can also be calculated

44. Standing Waves
String
The wave equation can be helpful when working on these types of problems

45. Standing Waves
String
The wave equation can be helpful when working on these types of problems

46. Forced vibrations and resonance
Objects have natural frequencies that they vibrate at
Pendulums
Wine glasses
Bridges
Buildings…

47. Forced vibrations and resonance
A vibrating object can stimulate vibration in another object
The initial vibration must be at the second object’s natural frequency

48. Standing waves in air columns
Describe a standing wave on a string
1st Harmonic
2nd Harmonic
3rd Harmonic

49. Standing waves in air columns

50. Standing waves in air columns
An air column (or pipe) can be open at both ends or closed at one end

53. Standing waves in air columns
Open vs closed end piped instruments

54. Standing waves in air columns