All traditional methods for measuring the sound absorption coefficient are based on assumptions about the overall sound field affecting the test material. Laboratory measurements in a controlled sound environment with a known and predictable sound field. However, outside the laboratory, the sound field can be difficult to control, unpredictable and variable depending on sound sources, for example. The ability of a material to absorb sound depends on the sound source and the environment around it. Acoustic absorption is not a material property alone!

Acoustic engineers know how much a material absorbs normal or random incident sound waves, measured in the laboratory, but not for oblique incident sound waves in situ, for example. In essence, acoustic engineers now lack ways to measure the effectiveness of absorption materials where they are applied.

The sound absorption coefficient is defined as the ratio of active to incident sound power with respect to a surface. In traditional measurements, sound pressure and sound intensity are measured in a controlled sound field. This assumes that the material being tested will perform similarly in an anechoic environment and in the field. That may not be the case. Instead of taking the global sound field into account, the local-plane-wave method takes the local sound field into account. And it assumes that the normal component of the sound field at each point can be described by an incident and a reflected plane wave. If we assume local plane waves near the surface, the effective in-situ absorption coefficient can be derived from sound field measurements. Therefore, we don’t take the material to a laboratory, we take the Sonocat to the actual sound field.