phy問題 (2題 共20分)

1. In a laboratory exercise, the wavelength of a 40 000 Hz ulurasound wave is measured to be 0.868cm. Find the air temperature.
聲速為：
v = fλ
= 40000x 0.00868
= 347.2ms-1
已知聲速和溫度T的關係
v = 331 + 0.6T
347.2 = 331 + 0.6T
T = 27℃

2. A person stands directly in front of two speakers that are emitting the same pure tone. The person then over to one side until no sound is heard. At that point, the person is 7 m from one of the speakers and 7.2 m from the other. What is the frequency of the tone being emitted?
相消干涉時波程差為 (7.2 – 7) = 0.5λ，所以
λ = 0.4m
若波速如上題則
v = fλ
347.2 = f x 0.4
f = 868 Hz