Index of content:
Volume 130, Issue 6, December 2011
- ARCHITECTURAL ACOUSTICS 
Enhancing the absorption coefficient of a backed rigid frame porous layer by embedding circular periodic inclusions130(2011); http://dx.doi.org/10.1121/1.3652865View Description Hide Description
The acoustic properties of a porous sheet of medium static air flow resistivity (around ), in which a periodic set of circular inclusions is embedded and which is backed by a rigid plate, are investigated. The inclusions and porous skeleton are assumed motionless. Such a structure behaves like a multi-component diffraction grating. Numerical results show that this structure presents a quasi-total (close to unity) absorption peak below the quarter-wavelength resonance of the porous sheet in absence of inclusions. This result is explained by the excitation of a complex trapped mode. When more than one inclusion per spatial period is considered, additional quasi-total absorption peaks are observed. The numerical results, as calculated with the help of the mode-matching method described in this paper, agree with those calculated using a finite element method.
Estimating the direct-to-reverberant energy ratio from the coherence between coincident pressure and particle velocity130(2011); http://dx.doi.org/10.1121/1.3658446View Description Hide Description
An analytical expression for the relationship between the direct-to-reverberant energy ratio (DRR) and the coherence estimation function between coincident pressure and particle velocity component is derived. The analytical solution is first validated with simulated room impulse responses and then used to estimate the DRR in five octave bands for several receiver positions measured in a total of 11 rooms of vastly different sizes and acoustic characteristics. The accuracy is evaluated by comparison with the DRR estimated directly from the room impulse response. The difference is typically 5 dB. For two rooms, the variation of the DRR estimate with source-to-receiver position is also shown. The method is blind in the sense that it is virtually independent of the signal generated by a single sound source.
130(2011); http://dx.doi.org/10.1121/1.3652880View Description Hide Description
An anechoic wind tunnel dedicated to fan self-noise studies has been designed and constructed at the von Karman Institute The multi-chamber, mass flow driven design allows for all fan performance characteristics,aerodynamic quantities (e.g., wake turbulence measurements), and acoustic properties to be assessed in the same facility with the same conditions. The acoustic chamber performance is assessed using the optimum reference method and found to be within the ISO 3745 standards down to 150 Hz for pure tone and broadband source mechanisms. The additional influence of installation effects of an aerodynamic inlet was found to create a scattered sound field only near the source location, while still providing good anechoic results at more distant sound pressuremeasurement positions. It was found to have inflow properties, span-wise uniformity, and low turbulence intensity, consistent with those desired for fan self-noise studies.