Mechanical Waves

1
Mechanical Waves
Two types Longitudinaldirection of wave travel
is along direction of vibrating
material Transversedirection of wave travel
perpendicular to direction of vibrating material
2
Sound Waves
Sound is a Mechanical Wave cannot transmit
through a vacuum, require a medium in order to
transport energy such as air molecules solid
objects water
3
Sound is Longitudinal
Compressions molecules are condensed/compressed
Rarefactions molecules are spread out
4
Frequency and Period
Frequency number of to-and-fro vibrations the
object makes in a given time (usually 1
second)unit is hertz (Hz) 1 vibration/1 second
1 Hz (vibration also called a cycle) Period
time it takes for a complete vibration Frequency
1/period Period 1/frequency
5
Sound Detected by Oscilloscope
Oscilloscope turns sound into a visual wave Can
measure frequency and period
6
Frequency
Humans can hear frequencies of 20-20,000
Hz Infrasound is lt 20 Hz Ultrasound is gt20,000
Hz Dogs can hear frequencies of 50-45,000
Hz Dolphins can hear frequencies up to 200,000 Hz
7
Determining Frequency
1 sec
0 sec
8
Sample Problem Toothbrush
An electric toothbrush completes 90 cycles every
second. What is (a) its frequency and (b) its
period?
Frequency 90 cycles/1 second 90 Hz Period
1/90 second
9
Sound Detected by Ear Drum

• Sensations of frequency often called pitch
• Vibrating molecules cause ear drum to vibrate
• Ear drum vibrates hammer, anvil, stirrup
• Amplified by fluid in cochlea
• Detected by tiny hairs connected to nerves

10
Music Terms
Pitch - frequency Loudness- amplitude (what does
an amp do?) Amplitude-Pressure