Mechanical Waves and Sound

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Mechanical Waves and Sound

• Ch. 17
• Physical Science

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Mechanical WavesProperties of Mechanical Waves

• 17.1 / 17.2
• Physical Science CIA

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• What are mechanical waves?
• What do you think waves carry?
• Can you see waves? Examples?

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Mechanical Waves

• Mechanical waves are disturbances in matter that
  carry energy from one place to another.
• Usually require matter through which to travel
• The matter a wave travels through is called a
  medium.
• Medium can be a solid, liquid, or gas
• Some waves can travel through spacewith no
  medium!

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How are mechanical waves created?
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Creation of mechanical waves

• Need a source of energy!
• That energy causes a vibration to travel through
  the medium

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Types of Mechanical Waves

• Transverse
• A wave that causes the medium to vibrate at right
  angles to the direction of the wave

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Parts of a transverse wave
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Types of Mechanical Waves, ctd.

• Transverse
• Longitudinal
• A wave in which the vibration of the medium is
  parallel to the direction the wave travels

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Parts of a longitudinal wave
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Remember!

• A wave doesnt move the mediumits just energy
  traveling through the medium!

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Transverse and Longitudinal WaveWhich is which?
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Types of Mechanical Waves, ctd.

• Transverse
• Longitudinal
• Surface
• A wave that
• travels along a
• surface
• separating two
• media

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Period vs Frequency

• Period (T) The time it takes for one cycle
  ( how long?)
• Measured in seconds
• Frequency (f) The number of cycles in a given
  time ( how many?)
• Measured in Hertz (Hz)
• Frequency is the inverse of the Period
• f 1 / T

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Speed of a wave

• We find speed by distance divided by time.
• The same holds true for waves!
• Speed of a wave Wavelength/Period
• Speed of a wave WavelengthFrequency

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Speed of a Wave

• The speed of a wave is constant within a medium.
• The speed can change when a wave enters a new
  medium
• All waves of the same type travel at the same
  speed
• This means wavelength is inversely proportional
  to frequency!
• If the wavelength increases the frequency has to
  decrease!
• If the wavelength decreases the frequency has to
  increase!

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Practice Problem 1

• A wave in a spring has a wavelength of 0.1 meters
  and a period of 0.2 seconds. What is the speed
  of the wave?
• L
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• S

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Practice Problem 2

• Find the wavelength of a wave in a rope that has
  a frequency of 2.0 Hz and a speed of 0.4
  meters/second.
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Behavior of Waves

• 17.3 Physical Science CIA

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What happens when

• A wave meets a hard surface like a wall?
• A wave enters a new medium?
• A wave moves around an obstacle?
• A wave meets another wave?

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A wave meets a hard surface like a wall?

• Reflection
• A wave bounces off a surface that it can not pass
  through
• Reflection does not change the speed or frequency
  of the wave, BUT the wave can be flipped upside
  down!

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How?
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A wave enters a new medium?

• Refraction
• The bending of a wave as it enters a new medium
• The wave bends because as it enters a new medium
  it either speeds up or slows down!

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Refraction
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Refraction
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A wave moves around an obstacle?

• Diffraction the bending of a wave as it moves
  around an obstacle or passes through a narrow
  opening

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Diffraction
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A wave meets another wave?

• Interference when two or more waves overlap and
  combine together.
• Constructive
• Destructive

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Constructive Interference

• Two or more waves combine to produce a wave with
  a larger displacement

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Destructive Interference

• Two or more waves combine to produce a wave with
  a smaller displacement

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Standing Waves

• Occur because of interference!
• A standing wave is a wave that appears to stay in
  one place.

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Parts of a Standing wave

• Node Where there is no displacement of the
  medium in a standing wave
• Antinode Where there is maximum displacement of
  the medium in a standing wave

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How do standing waves fit on a string?

• Standing waves only form if a half a wavelength
  or a multiple of half a wavelength fits exactly
  into the length of a vibrating string

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Sound and Hearing

• 17.4 Physical Science CIA

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Properties of Sound Waves

• What type of waves are sound waves?

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Properties, ctd.

• Sounds behave in certain ways because of the
  properties of sound waves. Some properties are
• Speed
• Intensity Loudness
• Frequency Pitch

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Speed

• Speed of sound 342 m/s
  THATS 765 mph!
• Sound speed varies, depending on medium
• Travel fastest in solids, slowest in gases
• Travel fastest in the most dense media
• Speed of Sound animation

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Intensity

• Intensity the rate at which a waves energy
  flows through an area
• Sound intensity depends on
• Amplitude
• Distance from source
• Measured in decibels (dB)

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Loudness

• Subjective! (This means it depends on the person
  who is hearing it.)
• Loudness is a personal, physical response to the
  intensity of sound.
• As intensity increases, so does loudness, but
  loudness also depends on the listeners ears and
  brain.

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Frequency Pitch

• Frequency of a sound wave depends on how fast the
  source of the sound is vibrating.
• Pitch is how we hear frequency of sound waves
• Pitch depends on frequencyhigh frequency sounds
  are high pitched, and low frequency sounds are
  low pitched.
• Pitch also depends on age and health

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Frequency and Pitch animation
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Ultrasound

• Most people hear sounds between 20 and 20,000 Hz.
  
• Infrasound sound at frequencies lower than
  people usually hear
• Ultrasound sound at frequencies higher than
  people usually hear
• Used in technologies such as sonar and ultrasound
  imaging

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Ultrasound, ctd.

• Sonar a technique used to determine the
  distance to an object under water.
• Ultrasound medical technique used to take
  pictures of different organs (or a fetus!)

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The Doppler Effect

• Where have you heard of the term Doppler?
• Doppler Effect a change in sound frequency
  (pitch) caused by the motion of the sound source,
  the listener, or both
• Why do we observe this?
• Doppler Effect animation

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Hearing and the Ear

• Your ear has a membrane that vibrates when sound
  waves hit itwhat is this membrane?
• The Ear animation

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Hearing the Ear

• Ear consists of 3 main parts
• Outer Ear gathers and focuses sound
• Middle Ear receives and amplifies vibrations
• Inner Ear uses nerve endings to sense
  vibrations and send signals to the brain

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Reproduction of Sound

• What are some ways that sound is stored?
• How do you think sound is stored, and then played
  back?

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Reproduction of Sound

• To record sound waves must be converted into
  electronic signals that can be stored
• To reproduce electronic signals are converted
  back into sound waves

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Music

• Musical instruments change their pitch by
  changing the frequency of the waves they produce
  how do they do this?
• Reflection animation