Silly Physics question -- compression, Hz, Dopplar effect, etc.?

Question

Robert is standing beside a parade, listening to the sound of a brass band. The speed of the sound in air is 330 metres/second. The band marches past Robert and off into the distance at a speed of 2.0 metres/second. The tuba is playing a note with a frequency of 96Hz. What frequency does Robert hear?

And please, refrain from being rude or vague. I don't even need the answer -- I'd much prefer someone explain this to me. In all other similar problems, they used the term "1000 compressions". How do I know how many compressions are made in this problem? Try not telling me how "simple" or "elementary" this problem is -- just answer the damn question. If you don't know what you're talking about, please just move along. For anyone wondering why this is in the Singles and Dating section, it's because this is (sadly) one of the more popular boards on Y!Answers.

Thank you!!!

Answer 1

The Doppler pitch shift is all about wavelength. Sound travels at a constant speed C relative to the air. The sound emitted by a moving source has a wavelength, in the forward direction, that is shorter than is perceived by the moving source itself. The reason is that each new cycle starts further along the sound path than the previous cycle did. (The emitter is "chasing" the forward-moving sound wave.) The period T of one cycle = 1/emitted frequency = 1/F0. If T elapses between each emitted cycle and the source moves at velocity V, the wavelength λ = (C-V)*T. For sound moving in the backward direction λ = (C+V)*T. A stationary observer, with the sound coming at him at velocity C, receives one cycle in time λ/C, thus hears a frequency C/λ, or F0*C/(C-V) in the forward direction and F0*C/(C+V) in the backward direction.
There. I didn't swear or stray from the subject. It's a good thing you also put this in the Physics section, because physicists are either married or not interested in dating!