1.For any of the piano sound samples, why does the signal go positive, then negative, and back again?

2. In the first sample, carefully examine the fourth of the 6 notes that were played.There is a very clear pattern of oscillations, but it is more complex than a simple pure sine wave that would be one frequency.How many frequencies do you think would be needed to explain what you see in the plot of amplitude versus time? Here is an example from that file taken around 10.35 seconds into the record.Explain your answer.