Beat Notes reproduces the following figures.
Do this:

Figure 3-3

1. Set Fc to 200, Delf to 20 and both Amplitude A and Amplitude B to 1.
2. Set Plot Time to 100m.
3. You now see Figure 3-3. Where are the nulls?
4. Click Play as Sound.

Figure 3-4

1. Adjust Delf to 9 Hz.
2. What happens to the plot? Where are the nulls?
3. Listen to it. How does its sound compare to Figure 3-3's?
4. Click Play an Example. Adjust the parameters so your sound sound's like the example.

Figure 3-5

1. Adjust Delf, Amplitude A and Amplitude B to get Figure 3-5.
2. Hint: A and B will have different amplitudes.

Figure 3-6

1. You should be able to figure out this one.