Interference & Sound Aim: How do waves behave when they interact with each other?

Superposition Superposition occurs when two or more pulses/waves encounter each other in the same medium These pulses/waves interfere with each other and could cause two types of INTERFERENCE – CONSTRUCTIVE INTERFERENCE – DESTRUCTIVE INTERFERENCE

Constructive Interference Constructive Interference occurs when two points in a pulse/wave add up, resulting in an INCREASE in amplitude

Destructive Interference Destructive Interference occurs when two points in a pulse/wave either DECREASE the amplitude or cancel each other out

Example #1 How will these two pulses interact with each other? A A B B A+BA+B

A+B B A A B

A View form the Top When viewed from above, a wave source makes circular patterns that radiate outward, like a pebble dropped in water

A View from the Top When viewed from above, a wave source makes circular patterns, like a pebble dropped in water crest (wave front) trough At this point, two TROUGHS meet to CONSTRUCTIVELY interfere At this point, two CRESTS meet to CONSTRUCTIVELY interfere At this point, a CRESTS and TROUGH meet to DESTRUCTIVELY interfere Ripple Tank Sim

Standing Wave Waves that occur in the SAME MEDIUM that have the SAME FREQUENCY & AMPLITUDE, but travel in the OPPOSITE DIRECTIONS will create a STANDING WAVE. A standing wave contains a pattern of NODES & ANTINODES

Examples How many nodes & antinodes are in these standing waves?

Examples How many nodes & antinodes are in these standing waves?

Examples How many nodes & antinodes are in these standing waves? The number of antinodes is always ONE LESS than the number of nodes

Sound Sound is a LONGITUDINAL (mechanical) wave – REQUIRES a medium Sound travels FASTER in DENSER MEDIUMS At STP sound travels at 331m/s in air – STP (Standard Temperature & Pressure) Temperature = 0°C = 273 Kelvin Pressure = 1 atmosphere (atm) = 101.3x10 3 Pascals (Pa) Sound travels FASTER at HIGHER TEMPERATURES Changing the AMPLITUDE of sound changes its VOLUME Changing the FREQUENCY of sound changes the PITCH (NOTE)

Example #2 A tourist yells into the Grand Canyon from the Skywalk and it takes 1.2 seconds for the tourist to hear her echo. How far away is nearest rock face? (Assume STP)

Resonance All objects have a NATURAL FREQUENCY, the frequency at which the object will vibrate When a periodic force is applied to the object and if that force matches the object’s natural frequency, the object will vibrate or RESONATE This phenomena is called RESONANCE If resonance occurs for an extended period of time, the object’s vibration could increase due to constructive interference and cause the object to become damaged or destroyed

Resonance Examples

Doppler Effect All sound sources emit waves in all directions that are equal in wavelength, frequency, and amplitude However, if the source of the sound or an observer of the sound move, there is an APPARENT change in the sound’s FREQUENCY

Doppler Effect Examples

Summary Describe Constructive Interference Describe Destructive Interference How is a standing wave produced? How fast does sound travel at STP? Describe Resonance Describe the Doppler Effect

Doppler Diagrams Stationary Sound Source Both observers hear the same wavelength and frequency

Doppler Diagrams Sound Source moving to the right Red observer hears a “lower” frequency due to the larger wavelength Blue observer hears a “higher” frequency due to the shorter wavelength

Doppler Diagrams Sound Source moving at the speed of sound to the right (Mach 1) Red observer hears a “lower” frequency due to the larger wavelength Blue observer hears a “higher” frequency due to the shorter wavelength

Doppler Diagrams Sound Source moving faster then the speed of sound (Super Sonic) This observer won’t hear the sound until the source has already passed her