Ben's Answer

What affects the pitch of a sound?

The frequency of the wave.

Now, view Scene 1 of the demostration, the point on the left end of the ruler is a source of sound. It is much like the fire engine, except it's stationary, for now.

unpause the simulation, observe the sound wave travels in the space. After a few seconds, pause the simulation. you should have a screen that looks like this

The sound wave travels in all directions, but for this exercise, we are only interested in the part of wave travelling in the horizontal direction. what is the wavelength of this sound?

Measured from the ruler, it's 0.5m

Given  the speed of sound in air is 340m/s. what is the frequecy of this soundwave?

frequency = velocity/wavelength = 340/0.5=680Hz 

Now switch to scene 2. this is where we make the "fire engine" move. The point source is now given a horizontal speed. (in this case, we let it move at 40% of the speed of sound, which is way faster than a fire engine. This is so that it is easier to compare the difference) unpause and run the simulation for a few seconds; then pause it again so that you have a few cycles of waves to the left and right of the source. It should look something like this.

Observe the pattern carefully. Are there any difference in the pattern of the wave on either side of the source?

The wave to the right of the moving source appears to be more concentrated than the left.

Measure the respective wavelength of the sound on both side of the source, and calculate the frequencies.

Right side:                 wavelength=0.3m,          frequecy=1133Hz

Left side:                   wavelength=0.75m         frequency=453Hz            

How will the pitch of the sound be to the left and right of the moving source?

Since the right side has a higher frequency, it will have a higher pitch than the left side. 

Scene 3 is an automated version. Two detectors are placed on the left and right end of the measuring rule. the frequencies of the sound is detected and plotted on the graph. 

the point is initially at rest. unpause the simulation and wait until both detector recorded a wave, like this:

Pause the simulation and study the graph. what are the frequencies on the both detectors? how do they compare to your calculation in scene 1?

Change the source speed to 137.44m/s (the same value as scene 2). Unpause the simulation until change in frequency are observed on both detectors, then pause it again. The simulation should look something like this:

Study the new graph: what are the frequencies at both detectors? How does the simulation result compare to your answer in scene 2?

the frequencies are 1050Hz on the left and 450Hz on the right of the moving source. The values are close to the calculated value in scene 2 

What does the frequency tells you about the pitch of the sound?

Higher frequency sound wave has higher pitch. 

Watch the video again. Is your simulation result consistent with the phenomenon observed (heard) on the video? If not, why not?

It is not totally consistent. in the video, the pitch of the siren changes while in the simulation, the frequency does not change. There maybe a change in speed in the video that is not reflected in the simulation. 

To solve the final mystery, watch Scene 4, then make a brief explaination.

The frequency now changes. the reason behind the change is that the observer is not in the path of the moving source, therefore the relative speed of the source changes. that leads to the frequency change.