The Physics of Sounds

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Content:

W hat is Sound?Properties of a Sound W ave

Speed of SoundConstructive and Destructive InterferenceDiffractionThe Doppler EffectIntensity


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S ound:y V ibratory disturbances in a gas, liquid, or solid

medium.y sound is the vibration of any substancey our ears gather these vibrations and allow us to

interpret them.


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A s sound waves travelthrough the material,

each molecule hitsanother and returns to its

original position

result is that regions of themedium become alternately more dense , when they are

called condensations,

less dense, when they are called

rarefactions.


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y Sound waves are often depicted in graphs like the onebelow, where the x-axis is time and the y-axis is pressure or the density of the medium through whichthe sound is travelling.


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Wav elength


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Period:


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Amp litude:A decibel is a

scientific unit thatmeasures the

intensity of sounds.

The softest sound

that a human canhear is the zeropoint. W hen thesound is twice asloud, the decibel

level goes up by six.

Humans speaknormally at 60

decibels.


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Speed of soundy sound waves travel the slowest through gases,y faster through liquids, andy fastest through solids.y Temperature also affects the speed of sound.


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G as e sy In a gas, it is particularly important to know the

temperature.y

This is because at lower temperatures, moleculescollide more often, giving the sound wave morechances to move around rapidly.

y A t freezing (0 Celcius), sound travels through air at

331 meters per second (about 740 mph). But, at 20C,room temperature, sound travels at 343 meters persecond (767 mph).


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S olid sy Sound travels fastest through solids.y This is because molecules in a solid medium are much

closer together, allowing sound waves to travel morequickly through ity sound waves travel over 17 times faster through steel

than through air.y The exact speed of sound in steel is 5,960 meters per

second (13,332 mph)!y But, this is only for the majority of solids. The speed of

sound in all solids are not faster than in all liquids.


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Con s tructi v e a nd De s tructi v eInterference of Sound Wav e s :


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B e a t s These periodic variations in loudness

are called b eats .

In this situation you will hear the loudness

rise and fall 2 timesper second because

440-438=2.So, there is a beatfrequency of 2 Hz.


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Diffra ction: A n obstacle is no matchfor a sound wave; the wave simply bendsaround it

This bending of a waveis called diffraction

The sound outside of the room has varyingintensity depending on where you stand


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y If we let angle x be the location of the first minimum

intensity point on either side of the center,y W be the wavelength, andy D be the width of the doorway, the equation

. For a c ir c ular opening , the equation is slightly different


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The Intensity of Three-Dimensional W aves:These circles

are called w avefronts

The space inbetween wavefronts is the w avelength

Rays indicate themotion of a set of

wavefronts.The intensity level of sound ismeasured in deci b els (dB).

threshold of hearing is

0 dB

threshold of pain is120 dB


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Threshold of pain

Threshold of hearing

Threshold of hearing formajority people

A uditory area


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Typical speech sounds

Typical music sounds

A uditory area of thehuman ear


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T he Do pp ler Effect: y change in pitch


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End

The Physics of Sounds

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