Air Absorption Error in Room Acoustical Modeling

Most statistical and ray-tracing computer models take into account the absorption of sound by air to estimate the reverberation time. Extensive research by many scientists lead to the standardized calculation model for pure tone air absorption. The phenomenon was discovered from a room acoustical point of view by Sabine, while the further development of the calculation model took place in the fields of physics and environmental noise. As a result, several parameters and units are used for the same phenomenon. However, air absorption is calculated for pure tones, while room acoustics calculations are performed in frequency bands. Most computer models use the centre-frequency of the normalised frequency bands to calculate the air absorption by the pure tone method. Reverberation measurements in frequency bands under laboratory and practical conditions show that errors larger than the Just Noticeable Difference result in calculating the air absorption by this ‘centre-frequency method’. No literature was found that provides an accurate air absorption calculation for frequency bands in relation to the reverberation time without the use of iteration. An existing rule of thumb is validated for application to room acoustics.