Sound & WavesMrs. Wolfe

Recall the two types of waves…

• Transverse– Particles move up and down

• Longitudinal/Compression– Particles move side to side

• What kind of wave is light?

• What kinds of waves are compression?

• Let’s find out!

Time for a Demo

• If a smack a metal rod, it vibrates.

• What does that do to the air around the rod?

• These “vibrations” are a series of compressions.

• And you HEAR something, right?! SOUND!

So SOUND is a longitudinal wave!

• If that’s true, then….

• I should be able to hear a sound anytime I vibrate an object that has air around it.

• How else can you make something vibrate rather than hitting it?

• Heat it up! DEMO TIME!

What’s going on here?

• Heat is energy.• The atoms in the metal are

getting excited.• They start to vibrate.• This compresses air around

the atoms.• Like a domino effect,

eventually the air around your eardrum will vibrate and you’ll HEAR something!

Sound waves• Sound travels to your ear through the air. It

travels as a COMPRESSION wave. The sound waves act as vibrations as they strike your eardrum, signaling your brain about different sounds.

Go to http://health.howstuffworks.com/hearing6.h

tm

Scroll down and watch the movie!

If there were no air, would you still hear?

• Go to:

• If there’s no air, then there’s nothing to vibrate your eardrum.

• So you can’t HEAR.

Let’s pretend that you’re out in space.

• There’s very little air in space. It’s mostly a vacuum.

• Pretend you smack a metal rod against your space ship and it starts to vibrate.

• Hypothetically, what could you do in order to hear it?

So, sound requires a medium in order to travel.

• Why is it that light can move through a vacuum just fine and sound can’t?

• Because sound is a COMPRESSION wave!

Time for a Lab!

Time for another DEMO!

• Use a CBL to graph a sound wave.

• Draw 1 high and 1 low frequency wave.

• How do the sounds compare?

1 Hertz = 1 vibration per second

So Frequency Determines PITCH!• Humans can hear

frequencies between 20 Hz and 20,000 Hz. Anything higher or lower pitched is inaudible to us.

• Elephants can hear lower than 5 Hz.

• Dogs can hear up to 50,000 Hz and bats can hear up to 100,000 Hz!

What about LOUD and SOFT?

• Do the DEMO again with a LOUD and SOFT sound.

• How do the waves compare?

• Amplitude is measured in Decibels (Db).

Decibel Scale

• Anything over 85 Db can harm your hearing by vibrating hair cells loose.

Read this article! (Handout!)

Review:Which wave sounds higher-pitched?

Rank these Waves in order from lowest to highest pitch.

Which Wave Sounds LOUDER?Rank them in order from loudest to softest!

Time for a Lab!

One final “note” on Sound

• Since sound is made by temporarily compressing the air, the sound you hear depends on how much the air has stretched back out when you hear it.

• Let’s take a look at:

http://library.thinkquest.org/19537/java/Doppler/html

• As sound approaches you, the compressions become MORE compressed, increasing the frequency and pitch.

• As sound moves away from you, the compressions stretch out, decreasing frequency and pitch.

This is called the Doppler Effect!

This object is moving FASTER than its sound waves, so it has broken the sound barrier….a SONIC BOOM!

How does the pitch of the sound heard by the two boys compare?

• Go to:• www.seed.slb.com/en/

scictr/lab/doppler/train.htm

• Then, try:• www.colorado.edu/

physics/2000/applets/doppler.html

Time for a lab! Let’s head outside!

• HONK 4 PHYSICS!!

• Maybe we can hear the Doppler Effect!