[填空题]
Two speakers emit identical sound waves. A sound sensor moves along the line $XY$. As the sound sensor moves it detects locations where there is almost no sound. Some distances between the speakers and points $A$ and $B$ are given on the diagram.
1.Explain why there are quiet locations.
2.The speed of sound is $340\,ms^{−1}$. Point $O$ is where the highest intensity is received by the sound sensor. Point $A$ is the first quiet spot as the sensor moves from $O$ to $X$ and point $B$ is the second quiet spot as the sensor moves from $O$ to $Y$.
(1)Show that the frequency of the sound is approximately $140\,Hz$. f ≈ Hz
(2)Calculate the distance $∣S_1−B∣$, distance between Speaker 2 and point $B$. $∣S_1−B∣$ = m
