[填空题]
A speaker emits sound waves toward a boundary between dry, cool air and moist, hot air. The speed of sound in the dry air is $340\,ms^{−1}$ and speed of sound in the moist air is $375\,ms^{−1}$. Three wavefronts enter the boundary with an angle of $51^{\circ}C$ as shown.
1.Draw lines to show wavefronts in the moist air.
2.For the sound traveling in the moist air, calculate the angle between the wavefronts and the boundary. θ = $^{\circ}C$
3.Now the moist air region has been replaced with another medium $X$. Determine the minimum speed of sound in the medium $X$ so sound cannot enter. v = $ms^{-1}$
