Normal Incidence Absorption

The normal incidence absorption coefficient is the fraction of energy absorbed when a plane wave hits a surface at normal incidence. This can be approximated for test purposes in a travelling wave tube (sometimes called an impedance tube or Kundt's tube) at frequencies below the first cross mode. For a typical 100 mm diameter tube this would be frequencies up to 1600 Hz.

This value can not exceed 1. It is a useful quantity for research and also for comparing materials, but is not useful in architectural or room acoustic calculations as the values do not agree with the apparent absorption as measured for instance by Sabines equation relating reverberation and absorption within a room.