03-12-08 - sound

SOUNDSOUND a range of a range of compressioncompression

wavewavefrequencies to which thefrequencies to which thehuman earhuman ear is sensitive is sensitive

TheThe audio spectrumaudio spectrumextends from approximatelyextends from approximately

20 Hz20 Hz toto 20,000 Hz20,000 Hz..

Sounds are produced bySounds are produced byvibrating mattervibrating matter

1. reedsreeds

2. stringsstrings

3. membranesmembranes

4. air columnsair columns

Sound is a Sound is a mechanical wavemechanical wave (longitudinal). (longitudinal). It will It will notnot travel through a vacuum. travel through a vacuum.

Sounds possess the Sounds possess the characteristicscharacteristics

and and propertiesproperties that thatare common to allare common to all

waves.waves.

Just like all longitudinal (compression)Just like all longitudinal (compression)waves, sound waves possess awaves, sound waves possess a

velocityvelocity, , frequencyfrequency, , wavelengthwavelength,,phasephase, , periodperiod, and , and amplitudeamplitude..

Sound waves also Sound waves also reflectreflect, , refractrefract,,diffractdiffract, and , and interfereinterfere..

The velocity of sound in air The velocity of sound in air dependsdepends

on the air temperature. The speed on the air temperature. The speed ofof

sound in dry air is sound in dry air is 331.5 m/s331.5 m/s at at 0 0 ººCC.. This speedThis speed

increasesincreaseswith with

temperature: temperature: about about 0.6 m/s0.6 m/sfor every 1 for every 1 ººC C increase in increase in

temperature.temperature.

Sound generally travels Sound generally travels fastestfastest

in solids and slowest in in solids and slowest in gases,gases,

but there are some but there are some exceptions.exceptions.

Medium Velocity (m/s) Medium Velocity Medium Velocity (m/s) Medium Velocity (m/s)(m/s)

Air 330 Carbon dioxide 260Air 330 Carbon dioxide 260

Helium 930 Hydrogen 1270Helium 930 Hydrogen 1270

Oxygen 320 Water 1460Oxygen 320 Water 1460

Sea water 1520 Mercury 1450Sea water 1520 Mercury 1450

Glass 5500 Granite 5950Glass 5500 Granite 5950

Lead 1230 Pine wood 3320Lead 1230 Pine wood 3320

Copper 3800 Aluminium 5100Copper 3800 Aluminium 5100Sound waves travel faster in solids and liquidsThe speed of sound in solids (earth) is 3,350 m/sThe speed of sound in liquids (water) is 1,525 m/s

The human ear relatesThe human ear relatesamplitudeamplitude to to

loudnessloudnessandand

frequencyfrequency to topitchpitch..

Listen to various sound frequencies Listen to various sound frequencies here and mixtures of sound waves here.and mixtures of sound waves here.

Click Click here to make your own sound waves.You should hear that frequencyYou should hear that frequency

relates to pitch and amplitude relatesrelates to pitch and amplitude relatesto loudness (for a given frequency).to loudness (for a given frequency).

Sound waves refract.Sound waves refract.

Click Click here to view a simulationof the refraction of sound of the refraction of sound

waves.waves.

The The interferenceinterference of sound of soundwaves can cause “beats”waves can cause “beats”

Click Click here and here to run computersimulations of interfering sound wavessimulations of interfering sound waves

that result in discernable beats.that result in discernable beats.

View interference “beats” View interference “beats” here and here.

What are evidences of What are evidences of reflectionreflectionand the and the diffractiondiffraction of sound? of sound?

All objects have a naturalAll objects have a natural

frequency of vibration.frequency of vibration.

ResonanceResonance - the inducing- the inducingof vibrations of a naturalof vibrations of a naturalrate by a vibrating sourcerate by a vibrating source

having the same frequencyhaving the same frequency

““sympathetic vibrations”sympathetic vibrations”

Famous Bridge Collapses:Famous Bridge Collapses:Evidences of Resonance?Evidences of Resonance?

Tacoma Narrows link Tacoma Narrows link Others linkOthers link