Hi @zhexingli,
{Edited}
To begin with, I think we all agree that there are 1375 output steps.
With the current version of the code, the number 20 carries two meanings:
A. threshold for identifying it as an activation signal and thus adding a chime
B. minimum time interval between two chimes.
We can set both of them to the same value (A = B), or have B > A. In the current version of the code, it uses A = B = 20.
If you experiment with the code by changing the number 20 into 75, no chime sound will be added to the first example immediately after, so 75 isn’t a good value for B (also supported by @balaji.ambresh’s comment) even if we wanted to cope with the description.
Therefore, we might keep A = B = 20, or change it to A = 20 and B = 75, and then make sure the text is consistent with the code.
The above is my comment, and since I didn’t make this part of the notebook, the course staff should have the final say of how to make what changes.
Your hypothesis would alter the meaning of output steps, however, it is a nice one because you won’t be able to propose any hypothesis if you have not thought about it, and I appreciate that 
Cheers,
Raymond