✔ 最佳答案
From the CRO trace, there is no path difference at B, because it is just opposite A, the mid point between the two speakers.
Hence, at X, there is a path difference of (1/2) wavelength, because the distaance from crest to trough is half a wavelength.
At the next trough at the right side of X, there is a path difference of (1/2) + 1 = 1-1/2 wavelength. It is because the distance from trough to trough is a wavelength.
Therefore, at Y, there is a path difference of (1-1/2)+(1/2) = 2 wavelengths.
2009-04-06 22:08:32 補充：
note: [1-1/2] denotes "one and a half"
