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Introduction to Waves and Sound

Introduction to Waves and Sound. Chapters 14 and 15. Periodic Motion. Motion that occurs in a regular cycle. Period – the time it takes to complete one cycle (T) Amplitude – the maximum distance the object moves from equilibrium

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Introduction to Waves and Sound

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  1. Introduction to Waves and Sound Chapters 14 and 15

  2. Periodic Motion • Motion that occurs in a regular cycle. • Period – the time it takes to complete one cycle (T) • Amplitude – the maximum distance the object moves from equilibrium • Can occur back and forth (like a pendulum) or up and down (like a weight suspended from a spring)

  3. Resonance • An increase in amplitude that occurs when small forces are applied at regular intervals to a vibrating or oscillating object. • Some examples include: • Pumping your legs while you swing. • Jumping on a trampoline. • Bridges collapsing when soldiers cross in unison.

  4. Wave Types • Wave – a disturbance that carries energy through matter or space • Transverse • Wave vibrates perpendicular to the direction of motion • Ex. Light waves • Longitudinal • Waves vibrate along the direction of motion • Ex. Sound waves • Surface waves • A combination of both other wave types • Ex. Water waves

  5. Measuring Waves • Speed – displacement divided by time (v) • Wavelength – distance between identical parts of a wave (λ) • Phase – particles that have the same displacement and the same velocity • Frequency – how many oscillations occur in 1 s (f)

  6. Diagram of a Wave • Transverse • Longitudinal crest amplitude wavelength rarefaction trough compression

  7. Wave Behavior • Amplitude measures energy • 2 x the amplitude is 4x the energy • Medium affects speed: • Temperature • Depth • Tension • Density • When the medium changes, wave speed changes • This results in part of the wave being transmitted, and part being reflected back into the original medium • The reflected wave will be inverted if the new medium has a slower speed than the original

  8. Superposition • Occurs when more than one wave is in the same place at the same time. • Results in wave interference • 2 types: • Constructive – in phase waves create a new wave with larger amplitude • Destructive – out of phase waves create a new wave with less amplitude • Standing wave – special type of interference where parts are exactly in phase (antinodes) and parts are exactly out of phase (nodes), resulting in a wave that appears not to move • Faster vibrations result in more nodes (harmonics)

  9. When Waves Hit a Barrier • Normal – a line perpendicular to the barrier • Reflection • The wave returns back into the medium it came from • Law of Reflection – the angle of incidence is equal to the angle of reflection as measured from the normal • Refraction • The wave changes direction (gets bent) because of a change in speed

  10. Sound Waves • A longitudinal wave caused by something vibrating causing molecules (usually in the air) to vibrate. • Typical sound wave speed: 340 m/s • Changes in warmer temps or in more solid materials • Must be in a medium to travel (no sound in space)

  11. Hearing • Vibrating air molecules cause the ear drum to vibrate, in turn causing bones in the ear to vibrate. • Next, the vibration gets transferred to tiny hairs in the inner ear. • Losing these hairs is a typical reason for hearing loss • These hairs trigger nerve impulses which send messages to the brain to process the sound. • Humans can detect and process sounds between 20-16,000 Hz typically

  12. Perception of Sound • Pitch – how high or low a sound is • Caused by the frequency of sound • Loudness – how strong a sound is • Caused mostly by the amplitude • Measured in decibels (dB) • 10 dB is barely audible to humans, 110 dB is the average rock concert • Long term exposure to sounds around 100 dB causes permanent hearing loss

  13. Doppler Effect • A change in a sound’s apparent frequency due to motion of the source or receiver. • Red shift – frequency decreases because the source is getting further away • Blue shift – frequency increases because the source is getting closer • The faster the motion, the more dramatic the effect • Used in animals (like bats and dolphins) and in medicine (like sonograms)

  14. Interference in Sound Waves • Resonance • An increase in sound produced occurs when forced vibration equals the natural vibration (constructive interference occurs) • Affected by length of air column, size of vibrating surface, etc • Beats • Alternating periods of constructive and destructive interference that result in a throbbing effect

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