Longitudinal wave

requires a medium

(cannot travel in a vacuum)

Is known as

ACOUSTICS

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 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?

Waves travel fastest in solids, slowest in 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)

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

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

More commonly used is the decibel (dB)

= 0.1 Bell

(in dB) = 10 log(I/I0)

I is the intensity ( A2)

I0 = 1.0 x 10 -10 W/m2 the “threshold of hearing”

Jet plane at 30 m

Threshold of pain

Indoor rock concert

Auto interior

Street traffic

Conversation

Whisper

Rustle 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

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

Infrasonic: Infrasonic: below 20 Hz (below our hearing)

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

Ultrasound can be used to create internal images of the human bodyEx. 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 hearing Damage to ear resulting in hearing lossloss

StressStress

Caused by destructive interference

Result – hardly any or no sound

Bad for concert halls – designers be careful!

Used to locate underwater objects

and distances.

***Reflection**

Apparent change in frequency (pitch) Apparent change in frequency (pitch) of a sound from a moving source.of a 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 = velocity of source, v = velocity of soundsound

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

*Moving toward *Moving toward increasesincreases the the pitchpitch

*Moving away decreases the *Moving away decreases the pitchpitch

*Think of sirens*Think of sirens

Amazing Water & Sound Experiment #2[hd720].mp4