Chapter 26: sound

1
Chapter 26 sound
2
Do Now 3/13/09

• Sound waves are longitudinal waves. When sound
  waves occur, air molecules press together and
  push apart. Based on what you know about
  longitudinal waves and what they look like, draw
  a picture of what you think the air molecules in
  a sound wave look like.

3
Sound waves are Longitudinal
High pressure COMPRESSION
Low pressure RAREFACTION
4
Compression/Rarefaction
http//www.physicsclassroom.com/mmedia/waves/tfl.c
fm
5
Pitch

• High pitch sounds have high frequency waves
• Low pitch sounds have low frequency waves
• A person your age can normally hear pitches with
  frequencies from about 20 to 20,000 Hz.

6
Infrasonic/Ultrasonic Sound

• Sound waves with frequencies below 20 Hz are
  called infrasonic
• Sound waves with frequencies above 20,000 Hz are
  called ultrasonic
• We cannot hear infrasonic of ultrasonic sound
  waves, but many animals can.
• Bats, dogs, dolphins can hear ultrasonic sounds
  Elephants and giraffes can hear infrasonic
  sounds.

7
INFRASOUND infrasonic sound

• What are infrasonic sounds used for among humans
  AND among animals?
• Underline any interesting facts that you read
  from the article.

8
Do Now 3/16

• 1.) Describe the compressions and rarefactions of
  a sound wave.
• 2.) When a tuning forks prongs are apart, are
  compressions or rarefactions created? Why?
• 3.) What kind of animals can hear ultrasonic
  sounds? What kind of animals can hear infrasonic
  sounds?
• 4.) Does sound travel faster through steel or
  water?

9
Speed of sound

• Light travels faster than sound
• If you want to know how far away a storm is,
  count the number of seconds between when you see
  the lightning and hear the thunder. Then
  multiply the seconds by 340. This will tell you
  how far away (in meters) the storm is.
• How far away is a storm if you note a 3 second
  delay between a lightning flash and the sound of
  thunder?

10
Sound Intensity vs. Loudness

• Sound Intensity is different from loudness
• Sound intensity can be measured by a sound waves
  amplitude.
• sound intensity amplitude2
• But loudness is subjective. It is a sensation in
  the brain, so it varies with each person.
• Sound intensity is measured in decibels (dB)
  after Alexander Graham Bell

11
Forced Vibration

• When an object is made to vibrate by another
  vibrating object.
• An object will vibrate on its own, and the
  vibrations will produce a sound. But when that
  vibrating object is touching another object, the
  sound produced by the vibration intensifies and
  gets louder.
• Ex If you hold both ends of a string tight and
  pluck them, it will make a sound. But, if you
  attach those strings to a wooden box, like the
  body of a guitar, the sound is intensified. Not
  only are the strings vibrating, but also the body
  of the guitar, producing more sound.

12
Natural Frequency

• Natural frequency is the frequency that an object
  will naturally have when it vibrates.
• Natural frequency depends on the elasticity of an
  object (how stretchy an object is) or the shape
  of an object.
• A small bell has a higher pitched sound than a
  large bell, so a small bell has a higher natural
  frequency than a large bell.
• Some objects will have a set of frequencies (more
  than one natural frequency).

13
Resonance

• When the frequency of a forced vibration matches
  the natural frequency of an object a dramatic
  increase in amplitude occurs.
• In order for an object to resonate, it must
  contain enough energy to keep it vibrating.
• (A piece of paper is too light and doesnt have
  enough energy to keep vibrating, neither does a
  handkerchief.)
• Ex swinging on a swing, breaking a wine glass
  with your voice, Tacoma Narrows Bridge collapse

14
Do Now 3/17/09

• 1.) How far away is a storm if you note a 5
  second delay between a lightning flash and the
  sound of thunder?
• 2.) Give an example of a forced vibration
• 3.) Give an example of an object that has a
  natural frequency
• 4.) When does resonance occur?

15
Tacoma Narrows Bridge

• What caused the Tacoma Narrows Bridge to
  collapse?
• The bridge has a natural frequency. The wind
  caused forced vibrations in the bridge. The
  natural frequency of the bridge matched the
  frequency of the vibrations in the wind, which
  resulted in increased amplitude.
• The bridge swung back and forth, higher and
  higher (amplitude increased), until the bridge
  reached a point where it could not go any higher,
  so it collapsed.

16
In phase and out of phase

• In phase When the crests of one wave overlap
  with the crests of another, and the troughs
  overlap as well. (Constructive Interference)
• Out of phase When the crest of one wave overlaps
  the trough of another. (Destructive interference)

17
Interference of sound waves

• Sound waves (longitudinal waves) can
  constructively interfere
• When two identical sound waves interfere their
  compressions meet, and the result is an increase
  in pressure

The superposition of two IDENTICAL longitudinal
waves that are in phase will look like this
18
Interference of sound waves

• Sound waves (longitudinal waves) can
  destructively interfere
• When two identical sound waves interfere, the
  compressions and rarefactions cancel out, leaving
  molecules that are free to move.

The superposition of two IDENTICAL longitudinal
waves that are out of phase will look like this
19
Do Now 3/18

• 1.) What caused the Tacoma Narrows Bridge to
  collapse?
• 2.) Draw a picture of two identical longitudinal
  waves that are in phase.
• 3.) Draw a picture of two identical longitudinal
  waves that are out of phase.
• 4.) When two sound waves are in phase, is the
  sound louder or softer than when the two waves
  are out of phase?

20
Beats

• When two compressions of a sound wave meet, the
  sound is at a maximum (at its loudest).
• When a rarefaction and a compression meet, the
  sound is at a minimum.
• This variation in loudness is called beats.
• Two sound waves that are in phase then out of
  phase then in phase again and so on, produce
  beats.

21
Beat Frequency

• If one sound has a frequency of 200 Hz and
  another sound has a frequency of 204 Hz, the two
  sound waves will be in phase 4 times each second.
  (Constructive Interference will occur 4 times
  each second)
• The number of times that two waves are in phase
  is equal to the difference between their
  frequencies. 200 Hz and 204 Hz are in phase 4
  times each second. 204 200 4.
• Beat frequency is the number of times that two
  waves are in phase.