PS 1-3-08

Waves and Sound

New Website address

• www.Elyceum.wikispaces.com

Time until exams

• About a week of class time towards new material

• Waves, sound, light, and maybe color

• Some graded exercise on next Wednesday

Mid-year exam

• Review next Thursday and Friday

• Find your old tests

Waves

• Interference

• Constructive interference

• Destructive interference

• Relationship between type of interference and amplitude size

Interference and bubbles

• Constructive and destructive interference creates rainbow effect on bubbles

Interference and sound

• Keeping a piano in tune

• Dead Spots in a room

Standing waves

• Occurs where at least one end of wave is fixed

• Relationship between incoming wave and reflected wave

Standing waves in guitar

Nodes

• Place along medium of complete destructive interference

• Crest of incoming cancels trough of reflected (or vice versa)

Antinodes

• Place of maximum vibration

• Constructive interference

Tacoma Narrows Bridge

• Wind created a standing wave with in structure of bridge

Homework

• Page 478

• Problems 1-6

Sound

• Type of wave

• Speed of Sound

• 331m/s @ 0°C to 386 @ 25C

• 1490-1530 C in water

• 3800-5000 C

Loudness

• Depends on the energy in the sound wave

• Intensity = rate of energy transmission through a given area of the medium

• Greater Intensity Louder sound

Measure of Loudness

• Relative intensity

• Compares the intensity of the sound with the intensity of the quietest sound that a person can hear

• Measured in decibels

Decibel scale

• 0 dB threshold of hearing

• 30 dB whispering

• 50dB normal conversation

• 70dB vacuum cleaner

• 90 dB lawnmower

• 120 dB threshold of pain

• 150 dB nearby jet airplane

http://www.fotosearch.com/comp/phd/phd528/two-women-whispering-into-mans-ears-close-up-~-200023765-001.jpg

Pitch

• Not related to loudness

• Related to the frequency of wave

• Humans range of hearing range from– 20 HZ to 20,000 HZ

Sound beyond human range

• Lower than 20 Hz Infrasound

• Higher than 20,000 Hz ultrasound

• Dogs can hear to 46,000 Hz

• Dolphins can hear to 150,000 Hz

Echos

• What is needed to create an echo?

Places you here an echo

Ultrasound

• Ultrasonic waves are generated and moves away from the source

• They hit an object and reflect back to the source

• The time it takes for the wave to return is recorded and used to find the distance object is from source D = v t

Example of ultrasound use

Why use ultrasonic waves?

• Focused into more narrow beams

• Directed more easily

SONAR

• Sound, Navigation, and RAnging

• Uses acoustic signals and echo returns to locate objects