| Materials Testing:
Diffusion and Scattering
Before we can discuss the subjects of scattering and diffusion we must understand how the terms are defined in the standards that are used to measure them. Specular reflections are those reflections that obey the tenants of Snell's Law. The angle of the specular reflection is always equal to the angle of incidence of the source energy.
Scattered energy is defined as the energy left over after subtracting specular energy loss from total energy loss (see Scattering Figure 1). Scattering is a measurement of random surface roughness not structured or large scale reflectors. The definition removes all surfaces that have a structural depth exceeding 1/16 of the sample size or surface size. Examples of this are large geometric reflectors such as pyramidals, cylindricals or structured seating areas. Basically all of these constructs are made up of surfaces that have large specular reflection surfaces and are exempt from this definition.
What is the "Scattering Coefficient":
The "Scattering Coefficient" is a parameter that is very much like the "Absorption Coeffcient". It describes the loss of energy in a total energy field and is defined as the resultant of subtracting the specular reflected energy from the total reflected energy. The energy loss is then divided by the area of the sample and becomes a "coefficient".
How is the "Scattering Coefficient" used?
Simulation programs used for architectural acoustics use coefficients to statistically calculate sound energy left in a room after a defined period of time. Up until the last few years, the programs used only absorption coefficients to calculate the total absorption of energy in a room. Lately there has been an effort to include "scattering" in the calculations to simulate the loss of energy as well as the randomizing of reflections from the surfaces of the "room". The absorption parameter is used to calculate the reduction of energy in the room. Scattering can also be used to do the same thing but is also used to indicate the percentage of energy that is randomly reflected back into the room versus the energy reflected in a specular way.
Absorption had well known test procedures that were later standardized to create the parameter, "absorption coefficient", but scattering coefficients were inserted into the programs and "tweaked" by users based on "experience" This method was better than nothing as long as measurements were not available and standards had not been developed.
This ended with the adoption of ISO-17497-1
ISO17497-1
The standard, ISO17497-1, was developed in Europe. It was based, primarily, on research conducted by Vorlander and Mommertz using a free field method. This was later converted to a method that used a reverberation room instead. Initially all research and measurements were done using scale models of the scattering surface. NWAA Labs was the first research and testing facility to develop a "FULL SCALE" method to perform scattering tests. There were numerous initial problems converting the scale model method to full scale but Ron Sauro made breakthroughs developing "corrected" procedures to get consistent and repeatable measurements of the full scale samples.
Clients like Armstrong, Real Acoustix and Kinetics came to us for full scale measurements and using reverberation rooms of medium volume, results were obtained. Many customers did not really understand the data but accepted it. Even though NWAA Labs normally uses the standard test procedures described for testing, Ron is constantly trying to improve or create better ways of measuring relevant acoustic parameters. After moving our laboratory from Califormia to our present location in Elma, Washington, we set up the same exact system in our new much larger facilities intended to measure low frequncies in a more accurate way. We installed the identical equipment, software and procedures and the same samples in our new facilities and proceeded to run the same exact tests. We had already qualified the new reverberation rooms using ASTM and ISO standards. The results from our new tests did NOT confirm the tests previously performed at the other facility. We performed the tests again and again and each time the data returned was completely random. This forced us to go back into a research mode and find out WHY we were not able to get answers that matched the previous data.
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