Materials, Building Components and Structure Testing
ISO-354 & ASTM-C-423:
NWAA Labs tests for absorption coefficient using both the ISO-354 and the ASTM-C-423 standards. This is a technique that compares the reverberation times of an empty reverberation room and the same room with a sample in it. The reverberation times of the empty room are subtracted from the reverberation times of the full room and the resultant is converted to an absorption quantity expressed in Sabins or m squared. The quantity of absorption is then divided by the square area to produce an "Absorption Coefficient". The results are then delivered to the customer as a detailed report along with a chart of coefficients and a graphic display of the coefficient.
Ron Sauro has actively researched the acoustical parameters of materials that are used in construction. Absorption is one of those parameters and this research has produced a number of papers, studies and white papers on this subject. One of the problems researched was that some "Absorption Coefficients" exceeded 1.0. A "coefficient", by mathematical definition, cannot exceed 1.0 so the general feeling was that the measurement technique was flawed.
The "Absorption Coefficient" is calculated by dividing the absorption of the sample by the square area. When this value exceeds 1.0, we need to recognize that these coefficients are correct. What this really means is that the divider is not the correct parameter.
Many people explained that the results were affected by something called "edge effect". When these people were asked "What is edge effect?" no one seemed to have a real answer and pointed to papers going back to the 1930's. After studying these papers, Ron found no real answers to his question but many observations of "edge effect". Ron undertook a regime of testing comparing various configurations of a common materials with the idea of finding out what "edge effect" was. Why is this important?.
All architectural acoustics consultants use the coefficient to predict the reverberation time of rooms being designed. Absorption has consistantly been underestimated because of the restriction of the value of the coefficient. Consultants need a more accurate constant to be used as an absorption constant
You can read about the results here.