Mechanical%20Waves%20and%20Sound

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Mechanical Waves and Sound

• Physical Science

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Mechanical WavesProperties of Mechanical Waves
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• What are mechanical waves?
• What do you think waves carry?
• Can you see waves? Examples?
• https//www.youtube.com/watch?vcfXzwh3KadE

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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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What type of wave is this?

1. Transverse, because the energy travels
   perpendicular to the direction of the wave
2. Longitudinal, because the energy travels
   perpendicular to the direction of the wave
3. Transverse, because it travels parallel to the
   direction of the wave
4. Longitudinal, because it travels parallel to the
   direction of the wave

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What type of wave is this?

• Transverse, because the energy travels
  perpendicular to the direction of the wave
• Longitudinal, because the energy travels
  perpendicular to the direction of the wave
• Transverse, because it travels parallel to the
  direction of the wave
• Longitudinal, because it travels parallel to the
  direction of the wave

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Types of Mechanical Waves, ctd.
What two media are separated in this picture?

• 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)

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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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Sound and Hearing
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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

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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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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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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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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