Sound

1
Sound
2
Sound

• It travels in waves
• Waves are longitudinal
• Waves are areas of high and low air pressure
  (compression and rarefaction).
• Sound waves require a medium.

3
Speed of Sound

• Sound travels in air at approx. 343 m/s
• Velocity of sound depends on temperature of air.
• Velocity of sound is different in different
  media.
• How does the speed of sound in air compare to the
  speed of sound in water?

4
Frequency

• The number of waves that pass in one second
• Symbol f
• Measured in waves per second or cycles per
  second, sec-1, or Hertz (Hz)
• Frequency determines pitch
• How is frequency related to wavelength?

5
Frequency

• High frequency low frequency

6
Timbre

• The sound quality, because sound is made up of a
  combination of waves
• Can be observed in sound spectrum

7
Playing with the oscilloscope

• C\Program Files\Zelscope\Zelscope\zelscope.exe

8
Harmonics

• Sound is usually produced at several frequencies
  together
• This creates the timbre of the instrument
• Physics Videos MPG's

9
Harmonics

• Harmonic Series of Standing Waves on a Vibrating
  String (same as pipe open at both ends)
• fnnv/2L where n1,2,3

10
Harmonic Series of a Pipe Closed at One End

• Example is the resonance tube we used in lab.
  Another example is an organ pipe.
• Node at one end and anti-node at the other end.
• Only odd harmonics are present
• fnnv/4L where n1,3,5

11
Open vs. Closed Pipes
12
Doppler Effect

• Frequency changes if source or observer is moving
• Ex. Listening to an ambulance siren as it
  approaches and then passes you
• Homework Go play in traffic
• The Doppler Effect and Sonic Booms

13
Beat frequency

• Can be heard when frequencies are very similar.
• Sometimes used to tune instruments.
• beat frequencyf2-f1

14
Loudness

• Loudness depends primarily on the amplitude of
  the pressure wave
• Measured in decibels
• Humans perceive pressure variations from 2 x 10-5
  Pa to 20 Pa

15
Decibels

• Because of large range, sound intensity is
  measured on a logarithmic scale, called sound
  level.
• A ten-fold increase in intensity is an increase
  of 10 dB.