A.2165.6Hz B.2000Hz C.1857.9Hz D.1955.2Hz
一列火車(chē)以速度U向頻率為f的靜止聲源運(yùn)動(dòng),波速為C,火車(chē)的運(yùn)動(dòng)速度U為(設(shè)△f=f'- f0≠0):()
A.AB.BC.CD.D
A.f1=f2=f0; B.f1逐步升高,f2逐步降低; C.f1>f0>f2; D.f1<f0,f2> f0