Problem 'o the day

Joke

Types of Waves

Interference & Standing Waves

Resonance

Speed of Sound

Doppler Effect

Problem 'o the day:

The source of all wave motion is:

a) wave pattern.

b) harmonic object.

c) oscillating object.

d) region of variable high and low pressure.

e) any of these.

Joke

Types of Waves 

Some questions about waves:

What are the sources of waves

Typically waves arise from oscillating objects, things that are going back and forth... and back and forth. An example is a vibrating guitar string, which is the source of a sound wave.

Do waves involve the transport of material? If not, what IS transported

Waves do NOT involve the net transport of material in the direction of wave travel. The air that supports a sound wave doesn't travel from the speaker to the listener, for example. What is transported? ENERGY!

What do waves look like

In general there are two types of waves: transverse and longitudinal.

So the movement of waves involves displacement (real or imagined) of tranport medium (caveat: in the case of electromagnetic waves, there doesn't need to be a "transport medium--" they can travel through empty space).

Transverse waves:

• medium is displaced perpendicular to the direction the wave travels.

• Examples: guitar strings, seismic "shear" waves, water waves, light.

Longitudinal waves:

• medium is displaced (but NOT transported) in the direction of the wave.

• Examples: sound waves, seismic "P" waves.

How do we otherwise describe waves

We describe waves in terms of these concepts:

wavelength:

period:

frequency:

...... by the way:

...... so which has the higher frequency?

speed:

And there's a relationship between a wave's speed, wavelength and frequency.... (uh, let's see, speed is wavelength's illegitimate child by frequency's brother....)

No, its:

So, what am I looking at here

Interference & Standing Waves

You may have noticed that multiple water waves can "occupy the same space." That is, a short, narrow wave can ride on a large swell in the ocean.

We call this phenomenon interference. Waves can superpose (add together) on one another. Then interesting things can happen. Consider two waves of equal wavelength and frequency. Further, both waves interfere, and both start at exactly the same time. The result will be a wave of twice the amplitude. This is called constructive interference.

Now consider shifting the starting point of one of those waves by 1/2 wavelength. The sum of these two waves will be zero. This is called destructive interference. If these two waves are sound waves, you won't hear anything!

Let's try this out with the old stereo!

Speed of Sound

Sound travels at different speeds through different materials. Why

Why was Tonto always putting his ear to the ground

The speed of sound in 20^oC air is about 340 m/s (0.21 mi/s). Is it faster or slower in warmer air? Why

Why does sound bend? Why do I hear the train so loudly some nights

What causes sound to reflect

Comparison of speed of sound in air to speed of light.

How far away was that lightning

Resonance

What happens when an oscillator is driven? More importantly, what happens when it is driven with just the right frequency?

How do you use resonance every day

What happens to resonance frequencies when the length of the resonator changes? What happens when the speed of sound changes? Why do you sound like Donald Duck if you breathe helium?

Build your own clarinet from a straw!

Doppler Effect

Have you ever waited for a train to cross and notice how the sound of its "whistle" changes?

This is called the doppler effect. It is caused because the sound source, the train in this case, is first moving towards you and then away from you.

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