Assessing Sound Field Diffuseness in Direct Field Acoustic Tests Using Fractional Octave Band Coherence

Measurement of the spatial coherence of high-intensity sound fields has received considerable attention, particularly when comparing the sound fields created during direct and reverberant field acoustic test methods to the ideal diffuse acoustic field. Assessment of the sound field diffuseness by sampling the pressure response at two points and calculating spatial coherence from the constituent auto and cross-spectra estimates obtained using Welch's method appears to be widely used, with varying results due to methods of application, computation, and parametric representation. A review of coherence quantities is provided here, including the complex components of the coherence and the magnitude-squared coherence. Numerical simulations are used to demonstrate the effect of spectral resolution on the coherence estimates. Standard ANSI S1.11 fractional octave band analyses are leveraged to promote conformity in sound field coherence measurements and improve clarity by reducing variance through additional band averaging. The use of quadrature coherence to indicate directional bias in sound fields is also explored. The approaches described here are subsequently applied to experimental data from a recent test campaign involving direct and reverberant field acoustic testing. The motivation is to promote conformity and best practices in sound field diffuseness assessment among high-intensity direct and reverberant field acoustic test practitioners through the use of ANSI S1.11 fractional octave band coherence quantities and signal processing best practices.