1700987413
参考图7-72，t时刻B位于P0，与S相距r0，t＋dt时刻B位于P，与S相距r，有
1700987414
1700987415
1700987416
1700987417
1700987418
1700987419
1700987420
1700987421
图 7-72
1700987422
1700987423
t时刻过B0的波阵面为∑0，t＋dt时刻此波阵面延展成∑面，两者间距为udt．dt时间内扫过B的波列长度为
1700987424
1700987425
1700987426
1700987427
1700987428
这一波列长度包含的全振动次数为
1700987429
1700987430
1700987431
1700987432
1700987433
接收频率便是
1700987434
1700987435
1700987436
1700987437
1700987438
（2）设vS≠0，vB＝0．
1700987439
1700987440
参考图7-73，设t＝0时刻S的振动状态于t时刻到达B，则有
1700987441
1700987442
r0＝ut 或 t＝r0/u．
1700987443
1700987444
1700987445
1700987446
1700987447
图 7-73
1700987448
1700987449
再设t＝dt0时刻S的振动状态于t＋dt时刻到达B，则有
1700987450
1700987451
1700987452
1700987453
1700987454
得
1700987455
1700987456
1700987457
1700987458
1700987459
dt0内S的全振动次数为
1700987460
1700987461
1700987462