Sound and Wave Interactions

Sound“If a tree falls in a forest …?”

• What is sound?– Longitudinal wave, particles transfer

energy

– Mechanical wave, must have a medium• Bell in a vacuum

Sound and Amplitude

• Amplitude: effects the loudness (volume) of sound– Decibel (db) measures volume

Direction of wave travel

Sound and frequency

• Frequency: determines “pitch” you hear– How “high” or “low” the sound seems– Gizmo on frequency– Ultrasound: above human hearing

• Dog whistles– Infrasound: below human hearing

• whales

Sound and Frequency

– Resonance: natural frequency of an object causes it to vibrate

• Glass of water• Speaker/strobe• Bridge Collapse

– Doppler Effect: sound waves change pitch as they move

• Sound waves pushed together in front of moving object (higher frequency)

– sound of Doppler• Doppler Radar

• Wave interactions video

Speed of Sound

• The speed of sound depends on 3 things1. Elasticity: ability for medium to “bounce back”

• More elastic means faster sounds– Solids are most elastic, liquids less, and gases not very

elastic at all

2. Density: how many particles are in a given space• [HOWEVER, more density means slower sounds within the

same phase of matter (more dense solids will be slower than less dense solids, etc.)]

3. Temperature: higher kinetic energy (higher temps) = faster sound travel.

– Speed of Sound video

What happens when waves meet?

• Reflections • The bouncing back of wave as it hits

another object• Law of reflection: “angle in = angle out”

– Echoes– Echolocation

» Sonar DVD

• Diffraction–Waves bend and spread out

around an object• Example: sound carries around a corner or over a hill (see diagram) from where it was made

Interference

• When two waves meet:– Constructive interference: a bigger wave is

produced as the crests meet– Destructive interference: waves of equal

amplitude meet with the crest of one and the trough of the other to cancel each other out• Active art on interference

• Gizmo on Sound Interference