A general wave decomposition formula for the measurement of normal incidence sound transmission loss in impedance tube

Abstract

Two types of general methods can be found in the literature for the determination of the normal incidence sound transmission loss (nSTL) of acoustical elements. The first one is based on the transfer matrix (TM) approach, and the second one is based on the wavefield decomposition (WD) theory. From all the techniques proposed in the literature, the general TM methods (two-load or two-source location) are the only methods yielding the exact nSTL of an acoustical element without any assumptions on its symmetry and on the termination (i.e., the load). Except for the case of an anechoic termination, there is no method based on the WD theory which yields exact nSTL. This paper presents a general WD method to measure the exact nSTL of an acoustical element without any assumptions on its symmetry and on the termination. Similar to general TM methods for non-symmetrical elements, four microphones and two loads will be required. As a first validation of the method, symmetrical and non-symmetrical porous materials are investigated. Results are discussed and compared with some existing methods and with the classical two-load method. A perfect agreement is found with the classical two-load method.

Acoustic contributions of a sound absorbing blanket placed in a double panel structure: Absorption versus transmission

“…This confirms that the reflection coefficient of the rear surface of the material r 2 , is different from the one of the front surface r 1 , as mentioned by Salissou and Panneton in Ref. 20. Figure 3͑b͒ shows that, in the case of this multilayer blanket, the contributions of TL u and TL d are of the same amplitude above 200 Hz.…”

“…͑6͒͒ can also be used to estimate the normal incidence sound transmission loss of a double panel structure filled with a multilayer sound absorbing blanket from the measurements of the surface density of the two panels and the measurements of the transmission and reflection coefficients of the multilayer blanket TL m , r 1 w and r 2 . [18][19][20][21] In this case, the measurement of all the non-acoustic properties ͑i.e., porosity, static airflow resistivity, tortuosity, viscous and thermal characteristic lengths͒ of each component of the multilayer filling the double panel structure and required generally in the models is not necessary. However, it is important that the experimenter ensures that the multilayer blanket behaves as an equivalent fluid ͑rigid or limp͒ inside the impedance tube with no contribution of the frame elastic behavior and no leakage effects.…”

Normal Incidence of Sound Transmission Loss from Perforated Plates with Micro and Macro Size Holes

“…The effect of perforation is also plotted to validate the measured data with (38). Based on the diameter of the tube, the result is only valid from around 300 Hz [16]. Good agreement with small discrepancy of less than 1 dB can be seen for the STL except below 500 Hz where the measured results overestimate the theory.…”

“…This is due to a large resistive force which restricts the motion of the air inside the holes. At frequency well below the natural frequency that is, ≪ 0 , and where the loss factor is typically much less than unity, from (16) and (18) this yields = ≈ − / ; the mechanical impedance is primarily determined by the elastic stiffness. As the resistive part dominates the hole impedance ℎ, ≫ ℎ, , from (20) and (21), the transmission loss in (29) can then be written as…”

“…The experiment employed the signal processing technique proposed by Salissou and Panneton [16]. This method requires two different loads for termination.…”