Chapter 12 Sound and Light

1
Chapter 12 Sound and Light

• All sound waves are longitudinal
• They all require a medium
• The speed of sound is determined by the density
  of the medium and air density is affected by
  temperature, altitude, and humidity.
• Room temperature and sea level 760 miles per hour
  or 346 m/sec.

2
Speed of sound in various mediums

• Air (0 C) 331 m/s
• Air (25 C) 346 m/s
• Air (100 C) 386 m/s
• Water 1490 m/s
• Sea water 1530 m/s
• Copper solid 3813 m/s
• Iron 5000 m/s
• Solid rubber 54 m/s

3
Speed of sound in a medium

• Some materials transfer the energy more easily
  than other materials.
• Sound travels through solids faster than liquids
  and through liquids faster than gases!
• If the solid inhibits the sound transfer it can
  be used for sound proofing rubber, cork, fiber
  tiles.

4
Loudness

• Loudness is determined by energy,
• Energy is determined by amplitude
• Distance also affects loudness!
• Intensity describes the loudness of a sound from
  a particular distance from the source.
• A sound with twice the intensity doesnt sound
  twice as loud, it sounds 10X louder.

5
Relative Intensity Scale

• Determined by comparing the intensity of a sound
  with the quietest sound a person can hear
  (threshold of hearing).
• The decibel (dB) unit is a relative scale.
• The quietest sounds humans can hear is 0
  decibels, a 10 dB sound is twice as loud.
• A sound of 120 dB is at the threshold of pain.
• Louder than 120 dB sustained result in permanent
  deafness and headaches.

6
Pitch

• The frequency of sound determines the pitch the
  higher the frequency the higher the pitch the
  shorter the wavelength, the shorter the period.
• High pitches are created by something vibrating
  rapidly these are generally produced by smaller
  instruments.
• Trained musicians can hear as little a difference
  as 2 Hz

7
Hearing Thresholds

• The limit of human hearing 20 Hz to 20,000 Hz.
• Infrasound sound range below the human
  threshold. (lower than 20 Hz)
• Elephant 6Hz to 12 000 Hz
• Ultrasound sound range above the human
  threshold. (above 20 000Hz)
• Dog 40 Hz to 40 000 Hz,
• Cat to 45 Hz 64 000 Hz
• Dolphin 70 Hz to 150 000 Hz
• Bat 2000 Hz to 110 000 Hz
• Mouse 1 000 Hz to 91 000 Hz
• Goldfish 20 Hz to 3 000 Hz

8
Music Instruments

• All musical instruments use standing waves to
  produce sound.
• A standing wave involves a primary wave and a
  reflected wave which interfere with one another.
• In a standing wave, the two ends of the string
  become nodes and the string vibrates in between
  them.

9
Standing Waves

• This site shows how standing waves are created.
• http//id.mind.net/zona/mstm/physics/waves/standi
  ngWaves/standingWaveDiagrams1/StandingWaveDiagrams
  1.html

10
Standing waves demonstrated in a Lab

• This site shows standing waves of various
  harmonics.
• http//www.pkwy.k12.mo.us/west/teachers/sci_videos
  /StandingWaves.mov

11
Tacoma Narrows

• November 7, 1940 near Tacoma Washington, across
  the Puget Sound stood an engineering marvel,
  until a few months after its completion - the
  wind found its natural frequency and created a
  standing wavethat lead to complete collapse.
• http//www.civeng.carleton.ca/Exhibits/Tacoma_Narr
  ows/

12
Musical instruments

• The primary standing wave on a string instrument
  has a wavelength twice the length of the string.
  This is the fundamental frequency.
• In a flute, the standing wave occurs in the
  column of air inside the flute.
• The wavelength and frequency of the standing wave
  is changed by opening and closing holes that
  change the length of the air column.
• On percussion, standing waves form on the drum
  head.

13
Standing waves on a speaker

• The rice collects in the nodes and moves away
  from the vibrating part of the speaker called the
  antinodes. Changing the frequency changes the
  locations of the nodes. The node areas of the
  speaker take on various geometric patterns.
• http//www.youtube.com/watch?vnO0bSSXmr1A

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Harmonics

• A tuning fork produces a pure tone of only one
  pitch or frequency.
• A clarinet produces sounds at several pitches
  because its air column vibrates at its
  fundamental frequency and a some whole number
  multiples of that frequency or harmonics. This
  produces a complex wave with its own unique
  resulting from the intensity of different
  harmonics in each note from the clarinet.
• Every instrument has its characteristic sound
  quality from its own mixture of harmonics.

15
Harmonics
16
Resonance

• Instruments use resonance to amplify sound.
• The vibration of one object causes the vibration
  of another object.
• The body of the guitar will vibrate at its
  specific natural frequencies.
• The vibrating string causes the body of the
  guitar to vibrate at a natural frequency. When
  the string and guitar body vibrate at the same
  frequency, this is resonance, and when it occurs
  the sound is amplified.

17
Sonar
Ultra Sound is transmitted until it strikes
another surface and reflects back To the source.
The amount of time between when it was
transmitted and Received back allows locating
objects distance and direction from the source.
distant
18
Ultrasound Imaging

• With high speed digital computing we are able to
  observe developing fetuses. This one is 23 weeks.
• Ultrasound has wavelength of less than 1 nm and a
  frequency of 15 M Hz.