Sound Waves

1
Sound Waves

• 13.4-13.6

2
Sound Travels in Longitudinal Waves

• Lets say the air is made up of tiny
  randomly-moving ping pong balls.
• If you vibrate a ping pong paddle in the middle
  of them, the balls then vibrate in sync with it.
• In some regions they are momentarily bunched up
  (compression) and in some they are spread apart
  (rarefaction).

3
Parts of a Sound Wave

• Compressions more dense area of the wave
• Rarefaction less dense area of the wave
• Wavelength distance between successive
  compressions or rarefactions

4
Pitch

• Pitch is our subjective impression about the
  frequency of sound.
• High pitch High frequency
• Low pitch Low frequency
• We can usually hear frequencies between 20 and
  20,000 Hz. This changes as we age.

5
Where does sound travel?

• Sound usually travels through air, but any
  elastic substance (solid, liquid, or gas) can
  transmit sound.
• Many solids and liquids conduct sound better than
  air.
• Examples
• Sound of a distant train approaching when placing
  your ear against the rail
• When swimming, have someone click two rocks
  together beneath the water while youre submerged.

6
Speed of Sound

• Did you ever wonder why you see lightning before
  you hear thunder?
• Or why you can see a person at a distance
  hammering before you hear it?
• This happens because sound takes time to travel
  from one location to another.
• It actually travels at about 330 m/s in dry air
  at 0ºC.

7
Speed of Sound

• What doesnt affect it?
• Loudness or softness
• High or low pitched
• What affects it?
• Wind conditions
• Temperature
• Humidity
• How do they affect it?
• Temperature faster in warm air
• Humidity faster in water vapor

8
Reflection

• We call the reflection of a sound wave an echo.
• Reflected sound in a room makes it sound lively
  and full.
• Being too reflective, makes things sound garbled.
• Being too absorbent keeps the sound level low and
  dull.
• In the design of an auditorium or concert hall,
  there has to be a balance between the two.

9
Refraction

• Sound waves bend when parts of the waves travel
  at different speeds, like when traveling through
  uneven winds or varying temperatures.
• Refraction is the bending of sound.
• Depending on the weather, sometimes sound doesnt
  carry well.

10
Refraction

• On warm day, air near the ground may be warmer
  than the air above, so the speed of sound near
  the ground increases.
• Therefore, sound waves tend to bend away from the
  ground, making it seem like it doesnt carry very
  well.

11
Reflection and Refraction

• Multiple reflections and refractions of
  ultrasonic waves are used by physicians to see
  into the body without X-rays!
• This is what we more commonly know as an
  ULTRASOUND!!
• The ultrasound echo technique is also use by bats
  and dolphins to locate objects around them.

12
Dolphins!!

• Sight is not very useful for dolphins in the
  murky and dark ocean, so sound becomes a very
  useful sense.
• Distance is sensed by the time delay between
  sending sound and receiving its echo.
• Direction is sensed by the differences in time it
  takes for the echo to reach each ear.
• Since a dolphin mainly eats fish and fish only
  hear at low frequencies, they dont realize they
  are being hunted!