Chapter 26Sound

Conceptual PhysicsHewitt, 1999

Bloom High School

26.1 The Origin of SoundAll sound produced by vibration of an object

Strings in instrument, reed in a sax, etc.

Pitch- how our anatomy interprets frequency (tone or note)Human range 20-20,000 Hertz

Infrasonic- frequencies <20 HertzElephants, whales, earthquakes

Ultrasonic- frequencies >20,000 HertzPlastic welding, jewelry cleaning, mixing

26.2 Sound in AirSound travels in longitudinal wavesCompression- molecules pushed together

represent a crestRarefraction- molecules pulled apart represent a

trough

Rubens TubesUses a flammable gas in a metal tube to show how

frequency and wavelength are related to compressions and rarefractions

http://www.youtube.com/watch?v=FOYXomUFyb8

26.3 Media that Transmit SoundSound needs something to compress to transmit energy

Gases- air, usuallyLiquids- water (whale and dolphin communications)Solids- earthquakes, railroad tracks

Sound cannot travel in spaceEver hear the Death Star explode?

26.4 Speed of SoundSpeed of sound is dependant on how close the

molecules areThe closer they are, the faster the speed

Solids- fastest Liquids- fast Gases- slowest

Temperature dependant- the faster the molecules are moving, the faster the speed of sound

v=330m/s + (T)(0.6m/°Cs)Speed of sound in air, T=temperature in °Cv=d/t (still and always)

26.5 LoudnessLoudness- how our

anatomy interprets amplitude (volume)

Decibel (dB)- 1/10th of a belEach level 10 times as

loud as the prior level

26.6 Forced VibrationsForced vibrations- a non-vibrating object that

vibrates as a result of other vibrationsTuning fork alone vs. tuning fork on a tabletop

26.7 Natural FrequencyNatural frequency- depends on material,

elasticity of object, shape of objectSmall brass bell vs. large brass bellBaseball bat vs. wrench dropped on the floorChampagne glass

28.8 ResonanceResonance- when the frequency of a forced vibration

matches a natural vibrationKicking your legs while swinging to go higherIncrease in amplitude (loudness)

Crest overlaps with crestTacoma Narrows Bridge Collapse

26.9 InterferenceInterference- waves from two separate sources

overlapping

Constructive interference- the crest of one wave overlaps with the crest of another waveIncreases amplitude (loudness)

Destructive interference- the crest of one wave overlaps with the trough of another waveDecreases amplitude (loudness)

http://www.falstad.com/interference/

26.10 BeatsBeats- the periodic variation in loudness

Caused by the interference of two slightly different frequencies

www.sciencejoywagon.com/explrsci/media/tonebeat.htm

Used to tune instruments

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