Introduction to

Sound Waves

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Basic wave properties:

wavelength

crest

trough

amplitude

Word bank:Trough CrestAmplitude Wavelength

Scientists Discover Most Relaxing Tune Ever

Compressional wave vs. Transverse wave

Sound waves are compressional waves because oscillations occur in the same direction that the way travels.

Transverse waves have motion have oscillations (green arrow) perpendicular to their direction of motion (blue arrow). Light and water waves are good examples.

Frequency is the amount of waves that pass a point in a given amount of time.

Time

wavesofNumber Frequency

Another example of frequency:

Frequency is measured in Hertz which means “per second”. We don’t have to even be measuring waves.

If the polar bears above were walking in the direcetion of the green arrow and passing the red arrow 3 times each second, you could say they are traveling with a frequency of _____ Hz.3

velocity = frequency x wavelength

= 2mHz 3f

velocity = 2 m x 3 waves/second = 6 m/s

If we know the frequency of a wave and the length of the wave, we can find its velocity. fv

Using symbols:

How fast does sound move?

http://www.gtcocalcomp.com/InterWriteBackgrounds/football_field.JPGhttp://www.gcsescience.com/xe7solid.gif

350 meters

Sound travels at about 350 meters per second through the air. It will travel faster through a medium with molecules that are closer together. For example, a solid concrete wall.

Sound travels at 1440 meters in water where the molecules are closer together.

Solid Gas

How does a sound wave move?

Your voice, an instrument or anything that moving back and forth causes a vibration here.

The vibration travels through the air as a series of compressed molecules and separated molecules.

Sound needs a medium (“stuff”) to travel through. Sound waves cannot travel in a vacuum.

Could you really hear in the spaceships fighting in outer space? Why or why not?

Pitch is another way of saying frequency when describing sound waves.

If a sound source gives off a high pitch, it has a high frequency.

If a sound source gives off a lower pitch, it has a low frequency.

http://www.phys.unsw.edu.au/~jw/hearing.html

Standing waves:Musical Instruments and Baseball Bats

Standing waves in instruments result from vibrating string or vibrating columns of air.

http://www.kettering.edu/~drussell/Demos/batvibes.htmlhttp://www.yamaha.co.jp/english/product/winds/product/wood/bassoon/img/bassoon.jpghttp://www.mathcs.duq.edu/~iben/

Standing waves from musical instruments

The velocity of a standing wave (for example, a guitar string) won’t change unless the thickness changes. That is why the 6 strings of a guitar are different thicknesses (or densities).

You can see in the velocity formula that the frequency of the sound will change if you have a longer wavelength or a change in the velocity.

v

f

Nodes don’t move.

Standing waves of air in woodwind instruments depend in the tube ends are open or not. Notice that neither tube has a full wavelength.

Anti-nodes move up and down.

Flute

Blowing across the flute opening causes turbulence in the air inside which results in standing waves.

Violin

The bow vibrates the violin string to certain notes.

Timpini

A percussion instrument, it creates a vibration in the air when it is hit.

http://www.mathcs.duq.edu/~iben/http://www.cs.dartmouth.edu/~wbc/instmnts/violin-1.jpghttp://ccgworld.com/tcollins/cards/flute.jpghttp://www.cnn.com/interactive/career/0104/ny.phil.percussion/story.timpani.jpg http://www.philtulga.com/harmonics.html

Doppler EffectAs a sound source moves past someone who is listening to it, the frequency of the sound can change.

Quick Experiment:What does a vibrating tuning fork sound like as it passes your ear?

Sound waves become further apart so the frequency changes.

Sound waves which are closer together make a higher frequency.

a) Bug buzzing and sitting still

b) Bug flying to the right and buzzing

Doppler Shifting Song—Beginning to 1:36 is about wave properties and the Doppler Effect.

Breaking the Sound BarrierA source of sound that is not moving will give off spherical sound waves, which move outward from the source in all directions (see the green arrows). A beeping fire alarm on a wall is a good example.

When the source is moving, like a fire truck with the siren going, the sound waves begin to “pile up” in the direction the truck is moving (moving in the direction of the red arrow).

http://www.kettering.edu/~drussell/Demos/doppler/doppler.html

If the source is moving at the speed of sound, all of the sound waves have created a “wall of sound” in front of the airplane. You won’t hear any noise from the plane until many waves hit you at the same time, with a “sonic boom”.

Wall of sound waves

http://www.youtube.com/watch?v=-d9A2oq1N38Slide 16http://www.youtube.com/watch?v=GL0hHTC-hu8

Sound Barrier

http://www.kettering.edu/~drussell/Demos/doppler/mach1.htmlhttp://www.youtube.com/watch?v=GvtAElaDVz8

Waves carry energyIf a wave has a lot of energy, it will have a large amplitude. Loud sound waves have a large amplitude.

http://jan.ucc.nau.edu/~lv5/

How can we prove the tuning fork is carrying energy?

Resonance:Every solid object has a natural frequency. When objects vibrate at this frequency, energy will build up and the amplitude of the wave will get bigger. The people who built the Tacoma Narrows Bridge below didn’t think about that but they soon found that the wind blowing across the bridge made it vibrate at its resonant frequency. The wave’s energy built up to the point the bridge collapsed.

An example of resonance:If you push someone on a swing, you must push them at a certain frequency (for example, once every 2 seconds). If you push them too quickly or slowly, they won’t swing higher and higher.

Mythbustershttp://www.youtube.com/watch?v=O9FrMkhQoA4&list=PL87265FD83AA5D8DD

Tacoma Bridgehttp://www.youtube.com/watch?v=3mclp9QmCGs