Answer:
14.48 ft
Explanation:
The relation between the location of the focus (c), the vertex on the major axis (a) and the vertex on the minor axis (b) with respect the center is:
b² = a² - c²
From the question:
c = 4 ft
a= 30/2 = 15 ft
Replacing into the equation:
b² = 15² - 4²
b = √209
b = 14.48 ft
So, he should build the whisper chamber at 14.48 ft out from the center along the minor axis