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  Can occur back and forth (like a pendulum) or up and down (like a weight suspended from a spring)

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.

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

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)

Diagram of a Wave  Transverse  Longitudinal wavelength amplitude crest trough compression rarefaction

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

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)

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

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)

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

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

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)

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