Anything that vibrates generates a sound! (unless it’s in a vacuum)

The Way Sound Travels

What type of wave is sound?

longitudinal

How does sound get from one place to another?

By pushing (compressions) and pulling (rarefactions) air molecules.

What determines the speed of sound?

The medium its traveling in & temperature.

Speed of SoundWhere will sound travel fastest?

Metal

Water Air

Solids & Liquids are generally good conductors of sound. Much better than air!

Besides the medium, what other factor determines how fast sound will travel?

Air Temperature!!!

Temperature & Speed

Temp = speed of soundAt O°C = 331 m/s

At room temp = 340 m/s

temp = Faster molecules = collisions

Will it travel in a vacuum?Absolutely not!

Speed of Sound Equation

vsound = 331 + .6(T)

m/s = 331 + .6(°C)

In air, at normal pressure…

Example Problem #1• An air tube is 125.0ºC. How fast will

a sound travel in the tube?

vsound = 331 + .6(T)

• v = 331 + .6(125.0ºC)

• v = 406 m/s

– Note: Use 3 SF because 331 is rounded off.

Example Problem #2• An air tube at 25.0ºC is 100. m long.

 How long will it take for a sound wave to travel from one end to the other?

– First, find the velocity of the sound at 25.0ºC:• v = 331 + .6(25.0ºC) • v = 346 m/s

– Second, use the v = d/t equation:• 346 = 100./t• t = .289 s

Compressions vs. Rarefactions• Compressions: PUSHING air molecules.

– Above normal air pressure.

• Rarefactions: PULLING air molecules. – Below normal air pressure.

Compression

Rarefaction

Intensity vs. Loudness• Intensity: The POWER of the waves.

– Objective-doesn’t change for different people– Measured in W/m2

– Equation- I = P/A

intensity = power/area

W/m2 = Watt/square meter

• Loudness: Sensed by the brain. Result of intensity & distance.

– Subjective- can change for different people– Measured in dB (decibels)…after Alexander Graham Bell

Typical Sound IntensitiesSource Intensity (W/m2)

Pin Dropping 1.0 x 10-12

Rustling Leaves 1.0 x 10-11

Whisper 1.0 x 10-10

Conversation 1.0 x 10-6

Loud Music 1.0 x 10-4

Loud Car Horn 3.0 x 10-3

Shout 1.0 x 10-2

Rock Concert 1.0

Yell in the Ear 1.0

Jet Engine 1.0 x 102

This is the lowest frequency produced by the instrument which can produce resonance.

• When the frequency

of a forced vibration

matches the object’s natural frequency.

Because the 40 mph winds happened to resonate with the nat. freq. of the bridge, the amplitude increased so much it collapsed!

It was only 4 months old!!!

Interference

Dark lines show sound cancellation.

Constructive Interference

Destructive Interference

Creating Beats• Two slightly mismatched tuning forks are

sounded together.

• They have different frequencies.

• Sometimes they will be in step, other times, they won’t be.

• In step=louder Out of step=softer

Why We Hear Different Sounds

Different sounds are produced using

different frequencies &different wavelengths!

What wave characteristics do high pitched sounds have? They have a high frequency and a short wavelength!

VOLUME

What part of a wave corresponds to loudness?

Amplitude!

Any Questions?