1. Chapter 13 - Sound
13.1 Sound Waves

2. The Production of Sound Waves

3. The Production of Sound Waves
Compression: the region of a longitudinal wave in which the density and pressure are greater than normal
Rarefaction: the region of a longitudinal wave in which the density and pressure are less than normal
These compressions and rarefactions expand and spread out in all directions (like ripples in water)

4. The Production of Sound Waves

5. Characteristics of Sound Waves
The average human ear can hear frequencies between 20 and 20,000 Hz.
Below 20Hz are called infrasonic waves
Above 20,000 Hz are called ultrasonic waves
Can produce images (i.e. ultrasound)
f = 10 Mhz, v = 1500m/s, wavelength=v/f = 1.5mm
Reflected sound waves are converted into an electric signal, which forms an image on a fluorescent screen.

6. Characteristics of Sound Waves
Frequency determines pitch - the perceived highness or lowness of a sound.

7. Speed of Sound Depends on medium Also depends on temperature
Travels faster through solids, than through gasses.
Depends on the transfer of motion from particle to another particle.
In Solids, molecules are closer together
Also depends on temperature
At higher temperatures, gas particles collide more frequently
In liquids and solids, particles are close enough together that change in speed due to temperature is less noticeable

8. Speed of Sound

9. Propagation of Sound Waves
Sound waves spread out in all directions (in all 3 dimensions)
Such sound waves are approximately spherical

10. Propagation of Sound Waves

11. The Doppler Effect
When an ambulance passes with sirens on, the pitch will be higher as it approaches you and lower as it moves away
The frequency is staying the same, but the pitch is changing

12. The Doppler Effect
The wave fronts reach observer A more often than observer B because of the relative motion of the car
The frequency heard by observer A is higher than the frequency heard by observer B

13. HW Assignment
Section 13-1: Concept Review

14. 13.2 - Sound intensity and resonance
Chapter 13 - Sound
Sound intensity and resonance

15. Sound Intensity When you play the piano Hammer strikes wire
Wire vibrates
Causes soundboard to vibrate
Causes a force on the air molecules
Kinetic energy is converted to sound waves

16. Sound Intensity
Sound intensity is the rate at which energy flows through a unit area of the plane wave
Power is the rate of energy transfer
Intensity can be described in terms of power
SI unit: W/m2

17. Sound Intensity
Intensity decreases as the distance from the source (r) increases
Same amount of energy spread over a larger area

18. Intensity and Frequency
Human Hearing depends both on frequency and intensity

19. Relative Intensity Intensity determines loudness (volume)
Volume is not directly proportional to intensity
Sensation of loudness is approximately logarithmic
The decibel level is a more direct indication of loudness as perceived by the human ear
Relative intensity, determined by relating the intensity of a sound wave to the intensity at the threshold of hearing

20. Relative Intensity
When intensity is multiplied by 10, 10dB are added to the decibel level
10dB increase equates to sound being twice as loud

21. Forced Vibrations Vibrating strings cause bridge to vibrate
Bridge causes the guitar’s body to vibrate
These forced vibrations are called sympathetic vibrations
Guitar body cause the vibration to be transferred to the air more quickly
Larger surface area

22. Resonance
In Figure 13.11, if a blue pendulum is set into motion, the others will also move
However, the other blue pendulum will oscillate with a much larger amplitude than the red and green
Because the natural frequency matches the frequency of the first blue pendulum
Every guitar string will vibrate at a certain frequency
If a sound is produced with the same frequency as one of the strings, that string will also vibrate

23. The Human Ear The basilar membrane has different natural
Frequencies at different positions

24. Chapter 13 - Sound
Harmonics

25. Standing Waves on a Vibrating String
Musical instruments, usually consist of many standing waves together, with different wavelengths and frequencies even though you hear a single pitch
Ends of the string will always be the nodes
In the simplest vibration, the center of the string experiences the most displacement
This frequency of this vibration is called the fundamental frequency

26. Fundamental frequency or first harmonic
The Harmonic Series
Fundamental frequency or first harmonic
Wavelength is equal to twice the string length
Second harmonic
Wavelength is equal to the string length

27. Standing Waves on a Vibrating String
When a guitar player presses down on a string at any point, that point becomes a node

28. Standing Waves in an Air Column
Harmonic series in an organ pipe depends on whether the reflecting end of the pipe is open or closed.
If open - that end becomes and antinode
If closed - that end becomes a node

29. Standing waves in an Air Column
The Fundamental frequency can be changed by changing the vibrating air column

30. Standing Waves in an Air Column
Only odd harmonics will be present

31. Standing Waves in an Air Column
Trumpets, saxophones and clarinets are similar to a pipe closed at one end
Trumpets: Player’s mouth closes one end
Saxophones and clarinets: reed closes one end
Fundamental frequency formula does not directly apply to these instruments
Deviations from the cylindrical shape of a pipe affect the harmonic series

32. Harmonics account for sound quality, or timbre
Each instrument has its own characteristic mixture of harmonics at varying intensities
Tuning fork vibrates only at its fundamental, resulting in a sine wave
Other instruments are more complex because they consist of many harmonics at different intensities

33. Harmonics account for sound quality, or timbre

34. Harmonics account for sound quality, or timbre
The mixture of harmonics produces the characteristic sound of an instrument : timbre
Fuller sound than a tuning fork

35. Fundamental Frequency determines pitch
In musical instruments, the fundamental frequency determines pitch
Other harmonics are sometimes referred to as overtones
An frequency of the thirteenth note is twice the frequency of the first note

36. Fundamental Frequency determines pitch

37. Beats
When two waves differ slightly in frequency, they interfere and the pattern that results is an alternation between loudness and softness - Beat
Out of phase: complete destructive interference
In Phase - complete constructive interference

38. Beats