ABSTRACT
This chapter covers a variety of methods used to measure and characterize absorbers. Table 4.1 outlines the main techniques applied to existing materials and most used by acousticians. Indeed, for many, the only important measurement is that which gives the random incidence absorption coefficient in a reverberation chamber. While this may be the number that is needed for performance specifications in room design, other measurements are needed to understand, design, and model absorptive materials. For instance, the prediction of the random incidence absorption coefficient is problematic, and consequently, it is necessary to measure materials in a more controlled environment to allow direct comparison between theory and experiment. <target id="page_92" target-type="page">92</target>Principal measurement methods for the most common absorber properties https://www.niso.org/standards/z39-96/ns/oasis-exchange/table">
Apparatus
Measures
Sample Commonly used for
Commonly used for
Impedance tube
Normal incidence surface impedance, z, and absorption coefficient, α. Transmission coefficients for fabrics. Also can be used to extract porosity, tortuosity, and characteristic lengths via inverse methods.
Small
Absorber and prediction model development and evaluation
Two-microphone in hemi-anechoic
Surface impedance, z, and absorption coefficient, α for specific angles of incidence
Large
Research
Reverberation chamber
Random incidence absorption coefficient, α
Large
Performance specification
In situ
Surface impedance, z, for specific angles of incidence
–
Research
Flow resistivity rig
Flow resistivity, σ
Small
Absorber and prediction model development
