Estimation of room absorption of acoustic cones based on impedance tube measurements

Abstract

Acoustic cones are often the primary absorption treatment choice for constructing anechoic chambers. Reverberation chamber estimation of room absorption will most accurately represent the actual effect of three-dimensional treatments, such as cones. However, for low frequencies, large reverberation chambers are often required, which may not be available due to space and cost constraints. Impedance tubes measurements are more accurate than reverberation chamber measurements in terms of sound absorption coefficient. Conversely, due to the two-dimensional nature of impedance tubes, room absorption of three-dimensional objects is difficult to estimate. Absorption coefficient of the material is a function of not only surface area but also material thickness. This paper proposes a technique to estimate the room absorption of three-dimensional treatments based on measured absorption coefficient of different thickness treatment specimens by impedance tube. This extrapolation of impedance tube measured absorption coefficient to actual room absorption can be used to evaluate the effectiveness of absorptive acoustic room treatments, including effects of adhesives and air gaps, without the need of constructing the actual treatment prototypes. The validity of this impedance tube room absorption estimation was verified for a tetrahedral cone treatment using a large reverberation chamber room absorption measurement.