Sound

Sound is caused by vibrations which sets air molecules in motion

Crests correspond to compressions and troughs correspond to rarefactions. The x-axis represents time and y-axis represents density

Frequency Audible – what we hear (20 – 20000Hz) Infrasonic – less than 20Hz (Elephants) Ultrasonic – greater than 20000Hz (Dogs) – higher frequencies have smaller wavelengths so they can be absorbed by smaller objects – Can produce images Pitch -is determined by frequency – Higher f, higher pitch – High or lowness of a sound based on frequency of sound waves

Speed of Sound Depends on the medium – State of matter Travel fastest in solids due to density Travel slowest in gases due to density – Temperature High temperature means more motion due to more kinetic energy

-Sound travels in 3D -Assume sound waves are spherical -crest or dark lines of circle represent the compressions -lines between circles are rarefactions and represent the trough

Doppler Effect Stationary SourceMoving Source Doppler Effect - Apparent change in the object’s frequency due to the relative motion of the source and the observer -it is an APPARENT not real change. The source would not hear a change in frequency

Sound Intensity Intensity: W/m 2 Power: W r: distance from source (m)

Audible Sound Determined by frequency and intensity – Threshold of hearing I = 1.0 x W/m 2 – Threshold of pain I = 1.0 W/m 2 Middle can be heard at lower intensities Higher and lower frequencies can be heard if intensity is greater

Relative Intensity Decibel increases by 10 which corresponds to a 10 factor on intensity. – Ex) Decibel change of 30, means intensity changed by 1000 Intensity (W/m 2 )Decibel level (dB)Examples 1 x Threshold of hearing 1 x Rustling leaves 1 x Quiet whisper 1 x Whisper 1 x Mosquito buzzing 1 x Normal conversation 1 x Air conditioning at 6 m 1 x Vacuum cleaner 1 x Busy traffic, alarm clock 1 x Lawn mower 1 x Subway, power motor 1 x Auto horn at 1 m 1 x Threshold of pain 1 x Thunderclap, machine gun 1 x Nearby jet airplane

Tacoma Narrows Bridge (Link is also on my website) Resonance = when the natural frequency of a on object is met by a forced frequency on the object. -Forced frequency can be made by people, weather, or other factors

- Sound travels from Outer ear to middle ear to inner ear. -Sound travels from ear canal to the outer ear. Ear canal ends at eardrum which vibrates and transfers vibrations to the middle ear. -The 3 bones in middle ear then vibrate which then transfer the vibrations to the inner ear. -The cochlea in the inner ear has membranes of different frequencies at different positions. -These create impulses in different nerve fibers which are sent to the brain to interpret.

Standing Waves -Wave created when a wave is traveling from 2 different directions -Harmonic Series: series of frequencies that include fundamental and multiples of that fundamental

Open pipe has antinodes at both ends due to complete freedom Closed pipe (like instruments) has a fixed end on one side which represents the node. Open end is the antinode Keys on an instrument change column length which changes the harmonics (notes)

-Beats are formed by the interference of two waves of slightly different frequencies moving in the same direction -Difference in frequency = number of beats per second