Goal: To understand sound

Objectives:1) To learn about Sound waves

2) To understand the Speed of sound3) To explore Reflections (echo, echo,

echo)4) To examine Refraction

5) To understand The energy of sound

6) To learn about Natural Frequency7) To learn about Resonance

8) To examine some Hollywood follies

Sound waves

• What type of wave is a sound wave?

Compression

• Since sound is the compression of a material, sound is a Longitudinal wave (compression wave).

• You compress the material (in our case air) at some rate (so you compress the air some # of times per second), and this comes out as a pitch.

• This is usually created from the in and out vibrations of an object which causes the compression of air.

• The pitch is how often the object vibrates.

Speed of sound

• You see a flash of lightning in the distance.

• The light gets to you almost instantly.

• When will you hear the lightning strike “ka-boom”?

Speed of Sound

• In air at room temperature sound travels about 340 m/s.

• So, if the lightning strike is about 1 mile away it will take about 4.5 seconds for you to hear the “ka-boom”.

• However, the speed of sound depends on temperature. At 32 F the speed is 330 m/s.

• It also depends on the medium. In water the speed of sound can be 1500 m/s.

• So, why do you think it is like this?

What determines the speed of sound?

• Since sound is a compression wave the speed of sound is how fast you can move that compression wave.

• Warmer air has molecules that move faster, so the wave moves from one molecule to the next faster.

• Water molecules in liquid move faster than air and are closer together, so the sound waves move faster.

• What do you think the speed of sound in outer space is?

Echo, Echo, Echo, Hello, Hello, Hello

• Like any other waves, sound waves reflect.

• So, when you design a theatre you have to take into account what sounds you want to reflect, and what you want absorbed.

• This is called acoustics.

Refraction

• Refraction means a bending.• Whenever the sound speed changes you get a

refraction of the waves.• Since the speed of sound is different in warm air

than it is in cold air, if you have two different layers of air and send a sound wave between the layers, the direction of the sound wave changes.

• In the ocean that means that your sonar won’t measure exactly the spot that you think, and there will be blind spots you can’t get to.

Ultrasound

• High frequency pulses reflect differently against different materials, and differently on the outside vs. inside of organs, so you can make a map of the inside without using more dangerous X-rays.

Energy

• Sound waves have energy.

• A large enough “boom” can be felt, and not just heard.

• The energy of sound is given as a decibel level – which is an exponential scale.

• If the sound has enough energy, it can harm your eardrums.

• If even stronger it can break windows.

Natural frequency

• Much like the string or spring we say last week has frequencies that will generate standing waves, any object has frequencies which they will naturally vibrate.

• These are Natural frequencies.• Yes, if you vibrate something enough at its

natural frequency, such as glass, it CAN break.

What determines the frequency

• Material – different materials have different sound speeds, and therefore different abilities to propagate sound.

• Size – the smaller the object, the faster it vibrates, and visa versa.

Resonance

• If you force a vibration to occur at the natural frequency this causes “resonance”.

• Think of this like a swing.

• You push the swing at the right time it goes higher.

• This makes the amplitude of the vibration to increase! Oops…

Catastrophic resonance

• Sometimes resonance is a VERY bad thing.

• In my home state of Washington in 1940 a bridge nicknamed Galliping Gerty (the Tacoma-Narrows bridge) started swaying at resonance.

• This destroyed the bridge!

The stupid stuff Hollywood does

• A spaceship explodes at the end of a Hollywood movie.

• What do you see?

More stupid stuff

• If they can’t hear you scream in space (unless you have a radio), then why does Hollywood think that engines make a whoosh noise?

• With nothing to compress, there is NO sound in space!!!

Conclusion

• Sound is a compression wave

• Like properties of waves, sound obeys all of those properties, and does NOT travel in space.

• Sound can reflect and resonate.