longitudinal wave requires a medium (cannot travel in a vacuum such as space)

•Longitudinal wave

•requires a medium

(cannot travel in a vacuum such as space)

is known as

ACOUSTICSACOUSTICS

2. What causes sound?2. What causes sound?

• A sound is a vibration (rapid back and forth motion that creates a longitudinal wave– Remember this wave has

compressions and rarefactions

–The velocity of speed of sound depends on twos things:the medium it moves through and temperature

SolidSolid LiquidLiquid GasGas

•Fastest in solids, slowest in gases.

Air = 340 m/swater= 1440 m/ssteel = 5000 m/s

•Supersonic: faster than the speed of sound.

https://www.youtube.com/watch?v=gWGLAAYdbbc

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

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

increases?increases?

Can affect speed

Speeds of Sound at T of 20 CSpeeds of Sound at T of 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. Everything has it’s own natural frequency

High frequency = High pitch

Low frequency = Low pitch

Note: resonance (amplitude) is the strengthening of sound when combined with frequency

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

Large Amp. = Loud sound

Small Amp. = Soft sound

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

8a. Normal hearing 20Hz--20 000 Hz

8b. InfrasonicInfrasonic sounds are below our hearing (below 20 Hz)

8c. UltrasonicUltrasonic sounds are those above our hearing (above 20,000Hz) (ultrasound)

8d. Sound can damage hearing by damaging the fine hairs that vibrate

•Sound waves reflecting from hard

surfaces but spaced out so it is heard many

times over long distances

•Sound waves reflecting from hard surfaces

but faster or closer together

•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

Sound is a pressure wave

Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

Tuning fork creating a sound wave

Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

Guitar String creating a sound wave

Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

• 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 An equal mixture of all frequencies of soundfrequencies of sound

• 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 Caused by destructive interferenceinterference

• Result – hardly any or no soundResult – hardly any or no sound• Bad for concert halls – Bad for concert halls –

designers be careful!designers be careful!

use of sound waves to interpret

echoes for location of landforms,

prey, etc. by bats,

dolphins, submarines (SONAR),

etc.

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

Moving Moving towards towards increasesincreases the pitch the pitch

Moving Moving away away decreasesdecreases the pitch the pitch

https://i1.ytimg.com/vi/-wy5oX0AXv8/mqdefault.jpg