Unit 8 Waves and Sound

Simple Harmonic Motion Vibration - a disturbance in time. A periodic disturbance that occurs over time. –An object vibrating always tries to return to equilibrium simple harmonic motion Pendulum and mass on a spring and object moving in a circle fendt.de/ph11e/springpendulum.htmhttp:// fendt.de/ph11e/springpendulum.htm

Simple Harmonic Motion Simple Pendulum Variables T = Period g = Acceleration due to gravity L = Length of the pendulum

Waves A disturbance in time (vibration) that moves through space Waves are a transfer of Energy not mass

Medium The material which a wave moves through –Some waves require a medium some don’t Examples: air, water, earth, strings, springs etc.

Types of Waves Mechanical Waves: (require a medium) Sound, earthquake, water, waves on a string, waves on a spring Electromagnetic Waves (do not require medium) Visible light, radio waves, x-rays, microwaves, gamma rays, ultraviolet, infrared

Pulse A single vibration traveling through a medium wave-on-a-string

Wave measurements Amplitude (A) max displacement of wave (amount of energy in the wave) Wavelength (λ) the distance covered by a wave in one oscillation Period (T) Time for one wave to pass Frequency (f) number of waves created in one second

Wave speed Wave speed: depends only on medium Velocity of a wave is constant in same medium, regardless of wavelength or frequency Equation:

Transverse waves Waves what vibrate the medium perpendicular to the wave (energy) motion Ex) light, earthquake s-waves, water, t.html on.html

Longitudinal Wave Wave that vibrates the medium parallel to the direction of the energy (wave) motion Ex) sound, earthquake p-waves waves/wavemotion.htmlhttp:// waves/wavemotion.html

Phase In Phase: points on a wave that are moving in the same direction with the same amplitude 180 degrees Out of phase: points moving in opposite direction with opposite amplitude

Boundary A region between two mediums –Transmission – energy moves through the boundary into a new medium –Reflection – energy is bounced off the boundary back into the same medium –Absorption- energy is kept by the boundary (usually turned to heat)

Reflection Bouncing of a wave off a boundary back into the original medium Fixed boundary – easily vibrated medium to rigid medium Unfixed boundary – rigid medium to easily vibrated medium

Waves changing mediums When a wave is transmitted into a new medium –The wave will change speed –The wave will change wavelength –The wave may change direction –The wave will not change frequency Highest wave surfedHighest wave surfed 100ft = amplitude 50ft

Principal of Superposition The ability of two or more waves to exist in the same place at the same time. Waves pass through each other.

Constructive Interference Two waves in phase superimpose and add their amplitude’s together resulting in a larger wave.

Destructive Interference Two waves that are 180 o out of phase superimpose and add their amplitudes to result in a smaller wave

Diffraction -The ability of a wave to move around obstacles -Larger wavelength results in more diffraction Ripple tank

Small opening compared to the wavelength more diffraction

Large opening compared to the wavelength less diffraction

Sound Needs a medium Pitch- frequency of sound –Humans hear 20 Hz to 20,000 Hz Infrasound frequencies less than 20 Hz Ultrasound frequencies greater than 20,000 Hz Loudness- Amplitude (amount of energy) –Measured in Decibels (dB)

Decibel Scale Near total silence - 0 dB A whisper - 15 dB Normal conversation - 60 dB A lawnmower - 90 dB A car horn dB A rock concert or a jet engine dB A gunshot or firecracker dB

Speed of Sound vs. Speed of Light Speed of sound depends on medium –v air = 345m/s at room temperature –v air = 331m/s at 0 degrees C Equation: v air = 331m/s + 0.6(T) –v water = 1500m/s –v steel = 5000m/s Speed of light (c) = 3x10 8 m/s

Beats 2 waves with different frequencies interfere Moving interference pattern

Forced Vibration: An object is forced to vibrate waves hitting it. Can be caused at any frequency. Ex. Tuning fork on the table top.

Resonance A phenomenon where standing waves are created by energy being inputted into the medium at the Natural Frequency (Resonate Frequency) of the medium Natural Frequency: Frequency where the medium easily absorbs energy (creates a standing wave) or (Resonates)

Standing Waves When two waves superimpose that are 180 degrees out of phase, moving in opposite directions with the same wavelength and frequency they produce a standing wave interference pattern. The pattern is made up of alternating regions of constructive interference (antinodes) and destructive (nodes) interference

Types of standing waves Closed-Closed Pattern –Medium fixed at both ends Open-Open Pattern –Medium unfixed (open) at both ends Open-Closed Pattern –Medium fixed on one end and unfixed on opposite end.

1st harmonic L = ג /2 2 nd harmonic L = ג 3rd Harmonic L = 3/2 ג 4th Harmonic L = 2 ג L L NN NN N A A A L NN N AA N A L N NN AA N N AA

Open-Open Standing Wave 1st harmonic L = ג/2 2 nd harmonic L = ג 3rd Harmonic L = 3/2 ג 4th Harmonic L = 2 ג L N A A L A NN A A A L N N N A A A L A N N N A N AA A

Open – Closed Standing wave 1 st Harmonic L = λ/4 3 rd Harmonic L = ¾ λ 5 th Harmonic L = 5/4 λ 7 th Harmonic L = 7/4 λ L N N A L N A A L N NN A AA L N NN A N A A A

Example: Draw the 6 th harmonic of standing wave in a pipe open on both ends.

L = 3λ 1m = 3λ λ = ? λ =.33m v = f*λ 345m/s = f*(.33m) f = ? f = 1035Hz If the pipe is 1 meter long what is the wavelength of the wave? Find the frequency of the wave if the speed of the sound wave is 345 m/s.

Doppler Effect If the source of a wave and an observer are moving together the observed frequency of the wave will be higher and the wavelength shorter If the source of wave and an observer are moving apart the observed frequency of the wave will be lower and the wavelength longer

Doppler Shift With sound the observer hears a higher pitch if source and observe come together and a lower pitch if they move apart With light the observer will see a blue shift in the color if they are coming together and red shift if they are moving apart

Happy Birthday Melody C C D C F E Harmony A A B b Melody C C D C G F Harmony B b B b A Melody C C C A F E D Harmony F C B b Melody B b B b A F G F Harmony C A