Raffaele Dragonetti

Measurement of the resistivity of porous materials with an alternating air-flow method

Air-flow resistivity is a main parameter governing the acoustic behavior of porous materials for sound absorption. The international standard ISO 9053 specifies two different methods to measure the air-flow resistivity, namely a steady-state air-flow method and an alternating air-flow method. The latter is realized by the measurement of the sound pressure at 2 Hz in a small rigid volume closed partially by the test sample. This cavity is excited with a known volume-velocity sound source implemented often with a motor-driven piston oscillating with prescribed area and displacement magnitude. Measurements at 2 Hz require special instrumentation and care. The authors suggest an alternating air-flow method based on the ratio of sound pressures measured at frequencies higher than 2 Hz inside two cavities coupled through a conventional loudspeaker. The basic method showed that the imaginary part of the sound pressure ratio is useful for the evaluation of the air-flow resistance. Criteria are discussed about the choice of a frequency range suitable to perform simplified calculations with respect to the basic method. These criteria depend on the sample thickness, its nonacoustic parameters, and the measurement apparatus as well. The proposed measurement method was tested successfully with various types of acoustic materials.

Assessment of Large-Scale Action Plans to Reduce External Background Noise in School Buildings

In this paper the authors report on a simplified procedure, developed in a Geographical Information System (GIS) environment, for the localization of school buildings characterized by high indoor background noise levels due to the outside noise sources. The noise sources include road, rail, and air traffic and other human activities (playgrounds, commercial activities, etc). Starting with the knowledge of noise pollution levels in the urban areas where the educational premises are sited, indoor background noise levels are obtained by adding two corrective terms; the first term takes into account for road configuration sound attenuation and the second one for façade sound insulation. For each educational building, a priority index is assigned on the basis of fixed objective parameters. The developed procedure has been applied to case studies of school buildings in Naples. The procedure promises a practical, low cost tool for municipal administrations to assess critical educational buildings and to plan noise reduction measures.

Statistical considerations on stationary-random noise propagating through a solid medium

In this paper the noise propagation through a solid medium is considered to study how the parameters of the statistical model change. In particular the external and internal sound fields of a box are taken into account. It is experimentally shown that the variance of the real and the imaginary parts of the external stationary random noise change differently while going through the box walls. The generalized Rayleigh distribution is shown to be more suitable to model the noise inside the cavity instead of the classical Rayleigh distribution. Starting from this outcome it has been studied how this noise affects a signal. It is known that a signal modulus affected by stationary random noise can be described by using the Rice distribution if the real and imaginary parts of the noise are statistically independent and identically distributed with zero mean and equal variances. Otherwise the less popular generalized Rice distribution must be used. The main differences of using the Rice or the generalized Rice distributions are shown by considering phasors both with constant and variable phases. In this paper it is shown that the bias estimation depends on the phase of the phasor if the variances of the real and imaginary parts are different and a numerical quantification is given. The attained results are useful when performing measurements of the sound pressure field due to a sound source inside a cavity affected by external noise.

The role of stack non acoustic parameters on thermodynamic performance of standing wave devices

Introduction The stack is the core of an thermoacoustic standing wave device. In thermoacoustic engines, heat pumps and refrigerators the geometric characteristics of the stack influence the thermodynamic performance. Until now the stack is studied by using viscous and thermal functions that vary with the shape of the pore. However more advantageous general functions can be taken into account to study a complex cross-section of pores and also the tortuosity. These functions involve classic non-acoustic parameters such as the air-flow resistivity, the tortuosity, the thermal and viscous characteristic length. Aim of this work is to introduce these functions in the classical thermoacoustic theory and to study theoretically the effect on the performance of a thermoacoustic engines.