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Mechanical Waves and Sound


Waves and Energy - Swain County Schools ... Ch 17 – PowerPoint PPT presentation

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Title: Mechanical Waves and Sound

Mechanical Waves and Sound
  • Ch 17

There are Different Types of Waves
  • The two basic types of waves are
  • Mechanical waves examples-sound, water waves, a
    pulse traveling on a spring, earthquakes, a
    people wave in a football stadium, etc.
  • Electromagnetic waves examples-visible light,
    radiowaves, infrared waves, etc.
  • For now, were going to focus on Mechanical Waves

What are mechanical waves?
  • A mechanical wave is a disturbance in matter that
    carries energy from one place to another.
  • The material through which a wave travels is
    called a medium.
  • Mechanical waves require a medium to travel
    through. Solids, liquids, and gases all can act
    as mediums. They can not travel through a vacuum
    (like space)

  • A mechanical wave is created when a source of
    energy causes a vibration to travel through a
  • In a wave pool, the waves carry energy across the
    pool. You can see the effects of a wave's energy
    when the wave lifts people in the water.

Types of Mechanical Waves
  • There are three main types of mechanical waves
  • transverse waves
  • longitudinal waves
  • surface waves.
  • Mechanical waves are classified into these 3
    types by the way they move through a medium.

Transverse Waves
  • When you shake one end of a rope up and down, the
    vibration causes a wave.
  • Vibrates in a direction perpendicular to the
    direction the wave travels.

Direction wave travels
Direction wave vibrates
Longitudinal/Compression Waves
  • A longitudinal wave is a wave in which the
    vibration of the medium is parallel to the
    direction the wave travels.

An area where the particles are spaced close
together is a compression. An area where the
particles are spread out is a rarefaction.
Direction of wave and vibration
Surface Waves
  • A surface wave is a wave that travels along a
    surface separating two media.
  • Ocean waves are the most familiar kind of surface

Properties of a Wave
  • Amplitude (A) Maximum displacement of particle
    of the medium from its equilibrium point. The
    bigger the amplitude, the more energy the wave
    carries. Measured from crest/peak to rest
    position OR from rest position to trough
  • Wavelength (?) For a transverse wave, wavelength
    is measured between adjacent crests or between
    adjacent troughs. For a longitudinal wave,
    wavelength is the distance between adjacent
    compressions or rarefactions.

Crest / Peak
Rest/equilibrium position
  • The more energy a wave has, the greater its

Low amplitude low energy
High amplitude high energy
More Wave Terms
  • Period (T) Time it takes consecutive crests (or
    troughs) to pass a given point, i.e., the time
    required for one full cycle of the wave to pass
    by. Calculated by T 1 / f.
  • Frequency (f ) The number of cycles passing by
    in a given time. The SI unit for frequency is
    the Hertz (Hz), which is one cycle per second.

Frequency 1.0 hertz One cycle per second
  • A wave vibrating at one cycle per second has a
    frequency of 1.0 Hz.
  • A wave vibrating at two cycles per second has a
    frequency of 2.0 Hz.

Frequency 2.0 hertz Two cycles per second
  • Wave speed (v) How fast the wave is moving.
    Speed depends on the medium. Calculated by v ?
  • The speed of a wave can change if it enters a new
    medium, or if variables such as pressure and
    temperature change.

Wave Terms and Concepts
  • Reflection occurs when a wave bounces off a
    surface that it cannot pass through.
  • Reflection does not change the speed or frequency
    of a wave, but the wave can be flipped upside

  • Refraction is the bending of a wave as it enters
    a new medium at an angle.
  • Refraction occurs because one side of the wave
    moves more slowly than the other side.
  • Example A lawnmower turns when it is pushed at
    an angle from the grass onto the gravel.
  • The wheel on the gravel slows down, but the other
    wheel is still moving at a faster speed on the

  • Diffraction is the bending of a wave as it moves
    around an obstacle or passes through a narrow
  • A wave diffracts more if its wavelength is large
    compared to the size of an opening or obstacle.
  • This wave diffracts, or spreads out, after it
    passes through a narrow opening.
  • Diffraction also occurs when a wave encounters
    an obstacle.

  • Interference occurs when two or more waves
    overlap and combine together.
  • Two types of interference are constructive
    interference and destructive interference.
  • When waves collide, they can occupy the same
    region of space and then continue on.
  • Constructive interference occurs when two or more
    waves combine to produce a wave with a larger
  • Destructive interference occurs when two or more
    waves combine to produce a wave with a smaller

Constructive Interference
  • Two waves that are in phase (have equal
    frequencies with crests and troughs that line up)
  • When these waves meet and overlap the result is a
    wave with an increased amplitude.

Destructive Interference
  • Two waves that are out of phase (do not have
    equal frequencies with crests and troughs that
    line up)
  • When a crest meets a trough, the result is a wave
    with a reduced amplitude.

  • A standing wave is a wave that appears to stay in
    one placeit does not seem to move through the
  • A node is a point on a standing wave that has no
    displacement from the rest position. At the
    nodes, there is complete destructive interference
    between the incoming and reflected waves.
  • An antinode is a point where a crest or trough
    occurs midway between two nodes.

Sound Waves
Sound is a longitudinal or compression wave.
Sometimes called a pressure wave.
If a sound wave is moving from left to right
through air, then particles of air will be
displaced both rightward and leftward as the
energy of the sound wave passes through it.
The motion of the particles are parallel to the
direction of the energy transport longitudinal
Speed of Sound
  • It takes time for sound to travel from place to
  • In general, sound waves travel fastest in solids,
    slower in liquids, and slowest in gases.
  • Particles in a solid tend to be closer together
    than particles in a liquid or a gas.

Sound Intensity
  • Intensity is the rate at which a waves energy
    flows through a given area.
  • Sound intensity depends on both the waves
    amplitude and the distance from the sound source.
  • The decibel (dB) is a unit that compares the
    intensity of different sounds.
  • For every 10-decibel
  • increase, the sound
  • intensity increases tenfold.

  • Lengthy exposure to sounds more intense than 90
    decibels can cause hearing damage.

Did you know that MP3 players such as iPods can
reach the 120-decibel level when they are at
maximum volume?
Sound Loudness
  • Loudness is a physical response to the intensity
    of sound, modified by physical factors.
  • The loudness depends on sound intensity.
  • Loudness also depends on factors such as the
    health of your ears and how your brain interprets
    sound waves.

Frequency Pitch
  • The frequency of a sound wave depends on how fast
    the source of the sound is vibrating.
  • Pitch is the frequency of a sound as you perceive
  • High-frequency sounds have a high pitch, and
    low-frequency sounds have a low pitch.
  • Pitch also depends on other factors such as your
    age and the health of your ears.

The Doppler Effect
  • As a source of sound approaches, an observer
    hears a higher frequency. When the sound source
    moves away, the observer hears a lower frequency.
  • The Doppler effect is a change in sound frequency
    caused by motion of the sound source, motion of
    the listener, or both.

Dial 911