1. Natural frequency is the frequency at which a system oscillates when disturbed, also called its fundamental frequency. Resonance occurs when an object is vibrated at its natural frequency, amplifying the amplitude of the wave, or when a second object vibrates along with the first, amplifying the energy. 2. Standing waves are waves that are trapped in one location, appearing as a single wave but made up of many. Sound waves are longitudinal pressure waves that travel through air, with crests of high pressure and troughs of low pressure. 3. The pitch of a sound depends on its frequency, with higher frequencies having higher pitches. Loudness depends on the amplitude of the sound wave,

1. Harmonics & Sound

2. What is Natural Frequency?
• The frequency a system oscillates when
disturbed
• Also called fundamental frequency
• All things in the universe have, some have
more than one

3. What is Resonance?
Happens 2 ways
1. Object is vibrated at its natural frequency
– When this occurs, the amplitude of the wave is
maximized
2. Resonance – a 2nd object vibrates along with
1st object and amplifies energy (makes larger)

4. • Wine Glass video
• Tacoma Bridge video

5. Standing Waves
• A wave that is trapped in one spot
– A single wave looks like many

6. • In any wave there are nodes and antinodes
– Nodes: places of NO amplitude
– Antinodes: places of greatest amplitude

7. • 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

8. • 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!

9. 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

11. 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

12. • 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

13. 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

14. • Some common sounds and their pitch:

15. • 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

16. 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

17. • 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

18. 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