Applicability of Radiowave Anechoic Chambers for Acoustic Free-Field Measurements on the Example of the Chamber at ITMO University

Abstract

Acoustic anechoic chambers allow free-field measurements required for the verification of effects under investigation and for the characterization of developing devices. However, the construction of an anechoic chamber is a labor-intensive process that requires a lot of resources, which is why these facilities are rather rare. An analogous statement can be made about radiowave chambers. At the same time, it is known that materials for radiowave absorption might absorb acoustic waves as well, and it can be expected that some chambers can be utilized for both electromagnetic and acoustic free-field measurements. This work examines the feasibility of the radiowave anechoic chamber of ITMO University (Saint-Petersburg, Russia) for acoustic measurements. The acoustic properties of the chamber coatings are estimated via measurements and numerical calculations. Characterization of sound pressure level, background noise level, and reverberation time is performed in accordance with the ISO 3745:2012 standard. The key conclusion is that the chamber can be considered as an acoustic anechoic chamber, but only for specific frequency ranges and distances from the source, which depend on the measurement directions.

Figures (6)

• Figure 1: Considered system. (а) Photo of the anechoic chamber as well as photo and the schematic image of a pyramid element of the absorbing coating. (b) Measurement scheme with marked points at which the sound pressure level (red dots), background noise (orange dots) and reverberation time (green dots) are measured. The measurement directions are marked by indexes I -- IV.
• Figure 2: Acoustic properties of the coatings. (а) Measured absorption coefficient of the coating material and its approximation via Delany-Bazley model. The sample represents a disk with the radius $55mm$ and the thickness $25mm$. (b) Approximation of the absorption coefficient using different empirical models. (c) Schematic illustration of the numerical model for calculation of absorption coefficient spectra of pyramid elements. Floquet periodicity conditions are imposed on the boundaries of the computational domain lying in $xy$ and $xz$ planes. (d) Numerically calculated absorption coefficient for the disk and pyramid samples characterized by the material with the flow resistivity $3437Pa.s/m\squared$ in the framework of the Delany-Bazley model.
• Figure 3: Background noise level. Spectra of the averaged equivalent sound pressure levels (SPL) of background noise and the equivalent SPL of the utilized noise source. Measurements are carried out at the center frequencies of the third-octave bands. Shaded are indicates the spectral range from $250$ to $5000Hz$.
• Figure 4: Measured reverberation time. The figure shows averaged reverberation time in the $1/3$-octave frequency band. Shaded area indicates the spread of values.
• Figure 5: Measured sound pressure levels. Sound pressure level (SPL) spectra as functions of the distance from the source for different measurement directions.