Chiara Visentin

A numerical investigation of the Fick's law of diffusion in room acoustics

In this paper the validity of the Ficks law of diffusion in room acoustics is investigated in the stationary state. The Ficks law, underlying the room-acoustics diffusion model, assumes a proportionality relationship between the local sound intensity and the energy density gradient, the proportionality constant being the so-called diffusion coefficient. This relationship, based on an analogy with the behavior of real particles in a scattering medium, is assessed by using a numerical tool simulating the actual dynamics of sound particles in a room. Two types of room geometries are considered: rooms with proportionate dimensions and long rooms. Concerning proportionate rooms the numerical analysis highlights the presence of weak variations of the reverberant energy density, generating an intensity vector pattern which is shown to be correctly described by the theoretical Ficks law and homogeneous diffusion. Conversely, inside long rooms, an estimate of the local value of the diffusion coefficient is carried out, showing that the reverberant sound field is well described by a spatially varying diffusion coefficient (non-homogeneous diffusion). The rate of increase of the estimated diffusion coefficient depends on the cross-sectional area of the room and on the boundaries absorption coefficient.

An Italian matrix sentence test for the evaluation of speech intelligibility in noise.

Objective: Development of an Italian matrix sentence test for the assessment of speech intelligibility in noise. Design: The development of the test included the selection, recording, optimization with level adjustment, and evaluation of speech material. The training effect was assessed adaptively during the evaluation measurements with six lists of 20 sentences, using open- and closed-set response formats. Reference data were established for normal-hearing listeners with adaptive measurements. Equivalence of the test lists was investigated using the open-set response format at three signal-to-noise ratios (SNRs). Study sample: A total of 55 normal-hearing Italian mother-tongue listeners. Results: The evaluation measurements at fixed SNRs resulted in a mean speech reception threshold (SRT) of − 7.3 ± 0.2 dB SNR and slope of 13.3 ± 1.2 %/dB. The major training effect of 1.5 dB was observed for the first two consecutive measurements. Mean SRTs of − 6.7 ± 0.7 dB SNR and − 7.4 ± 0.7 dB SNR were found from the third to the sixth adaptive measurement for open- and closed-set test response formats, respectively. Conclusions: A good agreement has been found between the SRTs and slope and those of other matrix tests. Since sentences are difficult to memorize, the Italian matrix test is suitable for repeated measurements.

Permutation Approaches for Stochastic Ordering

In many application problems, when dealing with comparisons between two or more groups, the classical parametric inferential statistical methods are used, although in real problems the quite stringent assumptions required by such methods are rarely satisfied. In particular a parametric approach to the test on ordering of C > 2 populations is very difficult. In order to tackle this problem two alternative methods are proposed in the present paper. Both the methods consist in permutation combination based tests: the first is supposed to be more powerful and it is suitable when the main goal of the study is related to the global ordering of the populations; the second is useful when the interest is in the pairwise comparisons between the populations.

An experimental evaluation of the impact of scattering on sound field diffusivity.

This work provides a systematic experimental evaluation of the impact of scattering on sound field diffusivity in a proportionate medium-small sized room. A scale model is set up in many ways by increasing the amount of scattering, and detailing measurements of the reverberation time in each case. With the aid of statistical tools, the role of scattering in the process of achieving a diffuse sound field from initially non-diffuse conditions is outlined and a set of reference scattering threshold values is derived. It is found that the same values ensure the validity of the Sabine formula when corrections are adopted in its application. Reverberation time is also predicted in non-diffuse conditions by the Nilsson approach, and its performance is systematically compared with measurements. The Nilsson method was a better predictor of reverberation time under non-diffuse conditions than classical reverberation time formulas. However, for diffuse sound fields, the same method tended to diverge from measured values. An application using more realistic room conditions is developed together with computer simulations. The results outline that there is limited benefit to using computer-aided design models instead of simple formulas to predict reverberation time for non-diffuse sound fields in proportionate medium-small sized rooms.

Dynamic mechanical response of foams for noise control

In this work, the viscoelastic behavior of open cell polyurethane foams used in noise control applications is investigated through dynamic mechanical analysis in compression. Several levels of static strains superimposed on a small dynamic one were considered in order to assess the effect of material non-linearity on the mechanical response. Further, a wide range of frequencies and temperatures was explored. For each static strain a different master curve for conservative component of complex modulus, E’, could be determined. Interestingly, the loss factor was the same at all static strains, indicating that the relative contribution of energy dissipation and conservation is unaffected by the static strain. Moreover, shift factors (and thus the bulk material relaxation times) turned out to be independent on static strain level. These results suggest that the non-linearity of the foam is linked to the change in foam structure with strain rather than to a non-linear behavior of the viscoelastic constituent m...

On the relationship between a short-term objective metric and listening efficiency data for different noise types

This study aims to compare the distinct effects of a steady-state (SSN) and a fluctuating (ICRA) masker on speech reception performance. SNR, reverberation and masker type were combined as to create several acoustic scenarios; matrixed-word listening tests in the Italian language were proposed to a panel of young adults with normal hearing, collecting data on intelligibility scores (IS) and response time (RT). The listening conditions were objectively qualified with the short-term metric STIr, defined as the average of the STI values calculated over short time-windows, whose duration reflects the typical phoneme length. The results showed that for a given STIr, both maskers yield the same IS, being the fluctuation benefit already accounted for by the objective metric. The slope of the STIr-IS function only depends on the speech material. Anyway, the fluctuating masker calls for an increased amount of cognitive resources to be deployed in the speech reception process, traced by a statistically significant ...

Listening efficiency in real and simulated university classrooms

The present study examines how reverberation, background noise level and type, affect both speech reception performance and the perceived effort of students in a university classroom. The classroom has a volume of 198 m3 and a reverberation time in occupied conditions of 0.6 s, complying with the target value of the DIN 18041 standard. Diagnostic Rhyme Tests (DRT) in the Italian language were proposed to a group of 26 normal-hearing young adults: half of them native (Italian), the other half non-native (German) speakers. Data on speech intelligibility (SI) and response time (RT) were collected. The two quantities were combined in the joint metric of listening efficiency, effectively describing the interplay of perceptual and cognitive processes in speech reception performance. The experiment took firstly place in situ, where the distinct effects a speech-shaped stationary noise and a fluctuating (ICRA) noise with the same short-term STI were determined. Afterwards, acoustic simulations of the classroom se...

Speech reception in fluctuating noise: Which is the benefit?

Speech intelligibility in a fluctuating noise is widely acknowledged to be higher than in a stationary noise with the same long-term level, as listeners take advantage of the information in the masker dips. Anyway, it is demonstrated in a companion paper (On the relationship between a short-term objective metric and listening efficiency data for different noise types, Proc. of Acoustics’17) that when the two noises are described with a short-term metric, able to properly follow their run-time course, the same intelligibility is found in both maskers. In fluctuating (ICRA) noise the accuracy result is reached involving a greater amount of cognitive resources and the background noise is perceived as more effortful. In this study the same methodology, based on the joint assessment of accuracy results and cognitive load, is applied in two real life scenarios (a canteen and a cafe), where speech reception is impaired by diffuse, non- informative, fluctuating maskers. Impulse responses and noise samples were co...

Experimental analysis of the relationship between reverberant acoustic intensity and energy density inside long rooms.

In this paper, the validity of the Ficks law of diffusion in room acoustics is experimentally investigated inside long rooms. The room-acoustics diffusion model relies on Ficks law stating a proportionality relationship between sound intensity and energy density gradient inside a room through a constant diffusion coefficient. This relationship is investigated in the stationary state for the particular case of long rooms with different amounts of boundary scattering. Measurements were performed inside a 1:16 scale model, using a p-u sound intensity probe (calibrated with digital filters) to collect concurrent data in terms of sound pressure and axial velocity components. Then for each receiver position, sound intensity and energy density gradient were derived. The results show that inside long rooms the diffusion coefficient is not a constant but increases with the distance from the source with a slope depending on the scattering coefficient of the walls. Numerical simulations of the enclosures were performed too by using a sound particle-tracing code; a substantial agreement with the experimental findings is observed. The results imply that for such long enclosures, the diffusion model should consider a space-varying diffusion coefficient to be more consistent with real phenomena.

A numerical and experimental validation of the room acoustics diffusion theory inside long rooms

The paper focuses on the validation of the recently proposed room-acoustics diffusion theory by means of numerical simulations and experimental measurements. The analysis aims to verify the equation underlying the theory (Fick’s law of diffusion) which relates the energy density gradient and the sound intensity inside a room through a constant diffusion coefficient. In this work, the acoustic quantities are numerically/experimentally derived under stationary conditions, and their ratio is employed to estimate the effective value of the diffusion coefficient inside long rooms. The numerical study was carried out with particle-tracing simulations. The measurements were performed with a Microflown® three-dimensional sound intensity probe inside a 1:16 scale model of a long room, varying the absorption and the scattering properties at the boundaries. A comparison between numerical and experimental results is carried out with a least-square algorithm, showing a fair agreement between the diffusion coefficients estimated with the two methods. The results lead to the conclusion that the reverberant sound field inside long rooms can be described by a non-homogeneous diffusion process: the local diffusion coefficient is not a constant inside the room but increases with the distance from the source and depends on the acoustical properties of the room boundaries.