Sound is a longitudinal wave that needs a medium like air to travel through. It propagates through the air by oscillating pressure between high and low amounts. Sound travels through air at 340 m/s but can travel faster through solids. The properties of sound waves include frequency, wavelength, loudness and pitch. Loudness depends on amplitude while pitch depends on frequency. The Doppler effect alters the frequency and wavelength of sound waves when the source or observer is moving.

1. Sound

2. • Sound are longitudinal waves
– Also called pressure waves
– created as air pressure oscillates between high
and low pressure
– On a graph:
• Crests show
high pressure
• Troughs show
low pressure

3. • Sound needs a medium to travel through
– Needs air molecules to vibrate in order for the
energy to get passed along
– Space is silent! No air molecules in space!

4. Propagate
• All ways waves travel from a single point
– Energy decreases with distance from starting point

5. Acoustics
• Get dead-spots
– Destructive Interference
– Areas with no or low sound
– Ways to fix
1. Speakers placed in back of auditorium
2. Room built to prevent dead-spots

7. Loudness
• The high and low pressure of sound
– More energy = louder sound
– Sounds too loud can lead to deafness
• Unit - decibels (dB)
– The decibel scale is a logarithmic scale
• Every +10 dB change means twice as loud
• Every -10 dB change means half as loud

8. • Some common sounds and their loudness:
10 – 15 dB
A quiet whisper, 3 feet away
30 dB
A house in the country
65 dB
Ordinary conversation, 3 feet away
70 dB
City traffic
90 – 95 dB
Sustained exposure may result in hearing loss
115 dB
Rock concert
94 dB
iPod, sound ½ way up
120 dB
Pain threshold

9. Pitch
• Frequency of sound
– Higher frequencies have higher pitches
– Lower frequencies have lower pitches
– Humans hear frequencies from 20 Hz to 20,000 Hz
• Infrasonic – below human hearing
– Elephants and submarines use
– Communicate long distances
• Ultrasonic – above human hearing
– Bats use to detect food and objects (echolocation)
– Ex) dog whistles

10. • Some common sounds and their pitch:

11. • As we get older, we lose our ability to hear
higher frequencies
– Most adults cannot hear frequencies over 15,000 Hz
– Children can often hear to 20,000 Hz

12. Speed of sound:
• 340 m/s (or 660 mph)
• Changes with:
– Hotter (faster) gases conduct sound faster
– Sounds travel faster in solids
• because the pressure between particles is higher

13. • If object passes the speed of
sound a sonic boom is created
Ex) Thunder, bull whips, and bullets
• Supersonic - objects travel faster
than the speed of sound

14. Doppler Effect
• If object is moving relative to an observer, the
sound waves are altered
– The frequency and wavelength are changed
Ex) Passing sirens
Ex) Light used in weather radar