•Longitudinal wave

•requires a medium

(cannot travel in a vacuum)

is known as

ACOUSTICS

Properties of SoundProperties of Sound

• A sound is a vibration• The vibrating causes the air molecules near the

movement to be forced closer. This is called compression

• As the vibration moves on, the density and air pressure becomes lower than normal and is called rarefaction

• Pressure wave – longitudinal• Frequency = pitch• v = 334 m/s in air at room temperature• Velocity is dependent upon the material

Do molecules move faster or slower Do molecules move faster or slower as temperature increases?as temperature increases?

So would sound travel faster or So would sound travel faster or slower as temperature increases?slower as temperature increases?

Can affect speed

Waves travel fastest in solids, slowest in Waves travel fastest in solids, slowest in gases.gases.

SolidSolid LiquidLiquidGasGas

•Fastest in solids, slowest in gases.

Air = 340 m/swater= 1440 m/ssteel = 5000 m/s•Supersonic: faster than the speed of sound.

•Sound travels faster in warm water than in cold water

•By measuring the time it takes for sound to travel a known distance through the ocean the average temperature of the water can be calculated = ATOC (acoustic thermometry of ocean climate)

Speeds of SoundSpeeds of Soundat T = 20 Cat T = 20 C

• Air Air • HeliumHelium• HydrogenHydrogen• WaterWater• Sea waterSea water• Iron/SteelIron/Steel• GlassGlass• AluminumAluminum

• 343 m/s• 1005 m/s• 1300 m/s• 1440 m/s• 1560 m/s• ≈5000 m/s• ≈ 4500 m/s• ≈ 5100 m/s

The highness or lowness of sound.

Depends on the frequency of sound waves.

High frequency = High pitch

Low frequency = Low pitch

•Also called LOUDNESS•Amount of energy•Depends on the amplitude of sound waves. (amplifier)

Large Amp. = Loud sound

Small Amp. = Soft sound

Intensity of SoundIntensity of Sound

• Unit is the “Bel”. Named after Alexander Unit is the “Bel”. Named after Alexander Graham Bell Graham Bell

• More commonly used is the decibel (dB)More commonly used is the decibel (dB)

= 0.1 Bel= 0.1 Bel (in dB) = 10 log(I/I(in dB) = 10 log(I/I00))

• I is the intensity ( I is the intensity ( AA22))• II0 0 = 1.0 x 10 = 1.0 x 10 -10-10 W/m W/m22 the “threshold of hearing”the “threshold of hearing”

Some Intensities (in dB)Some Intensities (in dB)

• Jet plane at 30 mJet plane at 30 m• Threshold of painThreshold of pain• Indoor rock concertIndoor rock concert• Auto interior Auto interior • Street trafficStreet traffic• Conversation Conversation • WhisperWhisper• Rustle of leavesRustle of leaves

• 140• 120• 120• 75• 70• 65• 1x10-10

• 1x10-11

•Sound waves reflecting from hard surfaces•Ex.: Multiple echo resulting from the direct sound AND the reflected sound

Reverberation vs Echo

Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

Near total silence - 0 dB A whisper - 15 dB Normal conversation - 60 dB Lawnmower - 90 dB Threshold of pain - 120 dB A rock concert or a jet engine - 120 dB Gunshot, firecracker - 140 dB

Near total silence - 0 dB A whisper - 15 dB Normal conversation - 60 dB Lawnmower - 90 dB Threshold of pain - 120 dB A rock concert or a jet engine - 120 dB Gunshot, firecracker - 140 dB

Measurement of loudness

Measurement of loudness

Sound is a pressure wave

Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

Ear

Human Ear 20Hz--20 000 HzHuman Ear 20Hz--20 000 Hz

Infrasonic: Infrasonic: below 20 Hz

(below our hearing)

Ultrasonic Sound: Ultrasonic Sound: above 20,000Hz (above our hearing)

Tuning fork creating a sound wave

Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

UltrasoundsUltrasounds

• Ultrasound can be used to create internal images of the human body– Ex. Pregnant woman gets a “picture” of

her unborn baby

• An equal mixture of all frequencies of An equal mixture of all frequencies of soundsound

• Used to calm stressUsed to calm stress

• Used in office buildingsUsed in office buildings

• Yep! There is such a thingYep! There is such a thing• Causes:Causes:

– Damage to ear resulting in Damage to ear resulting in hearing losshearing loss

– StressStress

• Caused by destructive interferenceCaused by destructive interference

• Result – hardly any or no soundResult – hardly any or no sound

• Bad for concert halls – designers be Bad for concert halls – designers be careful!careful!

Used to locate underwater objects and

distances.

***Reflection**

Doppler EffectDoppler Effect

• Apparent change in frequency (pitch) of a Apparent change in frequency (pitch) of a sound from a moving source.sound from a moving source.

• Source moving toward observer:Source moving toward observer:– f’ = f / (1-vf’ = f / (1-vss/v)/v)

• Source moving away from observer:Source moving away from observer:– f’ = f / (1+vf’ = f / (1+vss/v)/v)

f = frequency of source, f = frequency of source,

f’=frequency heard by observer, f’=frequency heard by observer,

vvss = velocity of source, v = velocity of sound = velocity of source, v = velocity of sound

Change in pitch due to Change in pitch due to motion.motion.

*Moving *Moving towards towards increasesincreases the the pitchpitch

*Moving *Moving awayaway decreases the decreases the pitchpitch

*Think of sirens*Think of sirens

Guitar String creating a sound wave

Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com