You don't have to measure it, strictly speaking. When you know the acoustic material properties of the surfaces well enough, you can compute an impulse response

You don't have to measure it, strictly speaking. When you know the acoustic material properties of the surfaces well enough, you can compute an impulse response with high enough accuracy (getting to one jnd and below). The tricky part is knowing the material properties. These are often just guesses as measuring them in situ is very difficult/impossible.