Sound is of course a type of mechanical wave, a longitudinal wave. It is studied separately since there are many phenomena, although not necessarily unique to sound waves, are relatable since we really on sound for communication.
1. Beats Explained
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When to sound waves of very similar frequencies interfere, the result is a periodic change in amplitude referred to as beats. The beat frequency is equal to the difference in the frequency of the two sound waves. 
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A really simple way to hear beats is to get together with another person with another phone. On your phone have a signal generator app installed. There are many, here been a free one.
Now have both play the same frequency, say 500Hz and stack them. Now increase or decrease one of the apps by 1 Hz at a time and you will here the beats.
Another detailed way to demonstrate is connect both to an oscilloscope. One for each input. The choose to add the two inputs .
Now have both play the same frequency, say 500Hz and stack them. Now increase or decrease one of the apps by 1 Hz at a time and you will here the beats.
Another detailed way to demonstrate is connect both to an oscilloscope. One for each input. The choose to add the two inputs .
2. Doppler shift  conceptual
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Most people have heard a siren passing by, only to hear a shift in the frequency of the siren. It's not because the siren is changing, rather, there is emotion involved of the siren
If there is relative motion between a source that produces a sound and the observer that hears the sound there will be a shift in frequency as perceived by the observer. This shift is referred to as a Doppler shift.
In the first video I demonstrate the Doppler shift by spinning a speaker around my head. In the second video I show an animation that demonstrates the Doppler shift in action.
If there is relative motion between a source that produces a sound and the observer that hears the sound there will be a shift in frequency as perceived by the observer. This shift is referred to as a Doppler shift.
In the first video I demonstrate the Doppler shift by spinning a speaker around my head. In the second video I show an animation that demonstrates the Doppler shift in action.


An in class demonstration

Using an online applet

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Set the sliders to adjust the speed of the sound source and the sound observer. Use the buttons to start or reset and to turn the sound on and off. By Tom Walsh
3. Doppler Explained  mathematical
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Now that we have a conceptual understanding of the Doppler shift, we now need to look at the mathematical modelling that relates the frequency with the relative velocity between source and observer. This video examines the mathematical model. Click the Going Deeper tab if you want to see how the formula is derived 
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Although not part of a standard high school course, its helpful to see how the Doppler effect formula comes about, so I show you here

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A Doppler effect worksheet
4. Resonance Explained
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Have you ever noticed that a swing has a natural frequency, its swings and a very specific rate. What happens if you apply energy at the same frequency, that is you push at the same frequency?
You will discover that you will be able to increase the amplitude with minimal effort. This is called resonance.. Using a wineglass and a speaker, I show you the principles behind resonance. 
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This is an interactive simulation intended to help students get a better feel for mechanical resonance. Using the given masses and spring constants, students can calculate the expected natural frequency of each spring's oscillation. When the outside oscillator matches any of these frequencies, that particular mass will go into a larger resonant oscillation. By Tom Walsh
5. What are standing waves?
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Standing wave seems to be a contradiction of terms. A wave should always be moving. But in fact standing wave is a situation in resonance, where a wave, reflecting off a surface is interacting with itself, resulting what we refer to as a standing wave.
Innocent in the first in a series of videos I discussed the principles of a transverse standing wave using a guitar and a string to demonstrate and explain. 
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Standing wave in string interactive developed in Geogebra by Tom Walsh
6. Standing Waves in Pipes
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Standing waves can also occur in sound. Many musical instruments rely on the principle of standing waves and resonance of sound.
In the second video I examine the standing waves in closed pipes, and explain the mathematical models associated with it. 
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Standing wave in pipe interactive developed in Geogebra by Tom Walsh
 pHET graphing animation  this interactive from the University of Colorado pHet team is a great way to demonstrate the relationship between motion and its graphical analysis. That why I used it in my video. At this time its Java based so will only work on PC/Mac
7. Standing Waves in 2 dimensions  Chladni plates
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In the previous examples we were looking at standing waves in one dimension. However standing waves can occur in two and three dimensions. Chladni plates provide a fascinating visual representation of standing waves in 2 dimensions, this video will discuss the principles of how it works.

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