Sound Reflection from Building Facades

Abstract

This report describes a method for measuring the reflection coefficient in situ for perpendicular sound incidence. The impulse response is measured in front of a surface. In this experiment the surface is an outdoor building facade. A freefield impulse response is subtracted from the measured impulse response, canceling the direct impulse in the measured response. This results in an impulse response for the reflected sound. To avoid parasitic reflections, the reflected impulse response is multiplied with a time window of suitable size. The frequency response of the direct and reflected impulse response is obtained by fast Fourier transform of the windowed time signals. The reflection coefficient is calculated by dividing the absolute value of the frequency response of the reflected signal with the absolute value of the frequency response of the freefield signal. The absorption coefficient is found by subtracting the reflection coefficient from one, and average over the desired frequency bands. Here one third octave frequency bands are used.The method proves to be valid for measuring the reflection coefficient of building facades in a frequency range from 250 Hz to 2500 Hz. No interference effects are found in this frequency range.