Absorption characteristics of fibrous material covered with perforated facing and film

Abstract

We propose a method for predicting the absorption characteristics of a fibrous material, that is glass wool, covered with a perforated facing and an impermeable film, typically used for noise barriers. The method is based on Ingard and Bolt's model. It accounts for interactions among perforated facing, film and fibrous materials. The interaction occurs in areas where they are close to each other. That area was determined empirically as the coverage. The coverage is approximately 10 mm for a perforated facing with a 0.2 open area ratio. In the coverage, the perforated facing increases the acoustic impedance of film and fibrous material according to distance. The fibrous material causes acoustic resistance to the film when the film contacts the fibrous material. The formulae for their acoustic impedances were derived from many results of acoustic impedance measured using an impedance tube. The end correction of holes of the perforated facing was modified using the relationship between the measured values of resonance frequency for Helmholtz resonators with the perforated plate and their open area ratio. Results predicted by this method agree well with measured results obtained in most instances.

Analytical Expression for Sound Transmission Loss Calculation: An Improvement to the Existing Method after Singh and Katra

“…Autospectrum of these captured signals is then used for TL calculation purposes. From (2), one could write the expression for the sound pressure level on both surfaces of the test sample, for x = 0 and x = d, as follows:…”

“…Researchers have developed methods and measurement techniques for these purposes. Such measurements simply categorized into two main groups: first, the standing waves method by using impedance tube with single microphone and single tone sound source; this method is inefficient due to its time consuming [1,2]; second, the spectral decomposition-based measurement, including a two-microphone impedance tube [3][4][5] and its four microphones improvement for TL measurement purposes [6][7][8]; the second group uses random noise instead of single tone sound source which is more efficient compared to the first group.…”

Improvement of the Delany-Bazley and Miki models for fibrous sound-absorbing materials

“…(2) are expressed as Eqs. (5) and (6), where 0 is the density of air, c 0 is the speed of sound in air, f is the frequency, and is the airflow resistivity. The coefficients of ( f =), a, c, p and r, and the degrees of ( f =), b, d, q and s, are shown in Table 1.…”

“…Delany and Bazley, and Miki have presented expressions that predict the acoustical properties of porous soundabsorbing materials only from their airflow resistivity [1,2]. These expressions are widely applied to fibrous materials at present [3][4][5].…”

Generation of uncertainty envelopes for thin acoustic screens with uncertain parameters

“…These composite structures usually involve a small number of foam/fibrous layers with carefully chosen properties and thicknesses in order to meet specific absorption objectives [1]. To protect the panel and sometimes fine tune its properties, thin films can be added on the free surface [8,9]. Because of the films' airflow resistivity is often high they are known to play a prominent role in the behaviour of the overall systems [10][11][12].…”