What effect does the Duck Velocity have on the waves seen by the observer? Towards the boat:

Away from the boat:

Same as the boat:

Not sure if this would answer your question but heres what i found: When the duck and boat move towards each other, the boat sees a higher frequency of waves from the duck. When they move away from one another, the boat sees a lower frequency of waves from the duck.

640 • .20=128
640 +128= 768