In-situ sound absorption measurements
All current methods to measure the sound absorption coefficient rely on assumptions for the global sound field impinging on the material under investigation. Measurements performed in laboratory (such as impedance tube or reverberant room) are in a controlled sound environment where the sound field is known and predictable. However outside the lab the sound field can be hard to control, unpredictable and changing depending on sound sources for example. The capacity of a material to absorb sound, depends on the sound source as well as on its environment. Acoustic engineers know how much a material absorbs normal or random incident sound waves, measured in a laboratory, but not for oblique incident sound waves in situ for example. Essentially, acoustic engineers now lack means of measuring the efficiency of the absorption materials where they are applied.
The sound absorption coefficient is defined as the ratio between the active and the incident sound power in relation to a surface S. In classic measurements the sound pressure and sound intensity are measured in a control sound field, with the assumption that the material under test will behave the same way in an anechoic environment and in-situ. It might not be the case. Instead of considering the global sound field the local-plane-wave method takes the local sound field into consideration and assumes that the normal component of the sound field in each point can be described by an incident and a reflected plane wave. If we assume local plane waves near the surface S, the effective in-situ absorption coefficient can be derived from sound field measurements. We therefore do not need to bring the material to a laboratory, we bring the measurement equipment to the actual sound field.