Volume 135, Issue 5, May 2014
Index of content:
- NOISE: ITS EFFECTS AND CONTROL 
135(2014); http://dx.doi.org/10.1121/1.4869089View Description Hide Description
Splitter silencers are found in ventilation and gas turbine systems and consist of parallel baffles of porous material placed within a duct so that they split the mean gas flow. Theoretical investigations into dissipative splitter silencers have generally been limited to two dimensions and this limits the analysis to finding the silencer eigenmodes or, for a finite length silencer, to rectangular baffles only. In this article a numerical point collocation approach is used to extend theoretical predictions to three dimensions. This facilitates the analysis of more complex silencer designs such as “bar” silencers and theoretical predictions are validated by comparison with experimental measurements. The insertion loss of different silencer designs is evaluated and the performance of a bar silencer is compared to traditional designs for rectangular and circular ducts. It is shown that a bar silencer with a volume of material identical to an equivalent parallel baffle design delivers a significant improvement in insertion loss at higher frequencies, although this is at the expense of a small reduction in performance at low frequencies. It is also shown that under most circumstances it is possible to get good agreement between prediction and experiment even for relatively large Helmholtz numbers.
Thin broadband noise absorption through acoustic reactance control by electro-mechanical coupling without sensor135(2014); http://dx.doi.org/10.1121/1.4871189View Description Hide Description
Broadband noise with profound low-frequency profile is prevalent and difficult to be controlled mechanically. This study demonstrates effective broadband sound absorption by reducing the mechanical reactance of a loudspeaker using a shunt circuit through electro-mechanical coupling, which induces reactance with different signs from that of loudspeaker. An RLC shunt circuit is connected to the moving coil to provide an electrically induced mechanical impedance which counters the cavity stiffness at low frequencies and reduces the system inertia above the resonance frequency. A sound absorption coefficient well above 0.5 is demonstrated across frequencies between 150 and 1200 Hz. The performance of the proposed device is superior to existing passive absorbers of the same depth (60 mm), which has lower frequency limits of around 300 Hz. A passive noise absorber is further proposed by paralleling a micro-perforated panel with shunted loudspeaker which shows potentials in absorbing band-limit impulse noise.
135(2014); http://dx.doi.org/10.1121/1.4870481View Description Hide Description
This paper presents a model that predicts measured sound pressure levels using geospatial features such as topography, climate, hydrology, and anthropogenic activity. The model utilizes RANDOM FOREST, a tree-based machine learning algorithm, which does not incorporate a priori knowledge of source characteristics or propagation mechanics. The response data encompasses 270 000 h of acoustical measurements from 190 sites located in National Parks across the contiguous United States. The explanatory variables were derived from national geospatial data layers and cross validation procedures were used to evaluate model performance and identify variables with predictive power. Using the model, the effects of individual explanatory variables on sound pressure level were isolated and quantified to reveal systematic trends across environmental gradients. Model performance varies by the acoustical metric of interest; the seasonal L 50 can be predicted with a median absolute deviation of approximately 3 dB. The primary application for this model is to generalize point measurements to maps expressing spatial variation in ambient sound levels. An example of this mapping capability is presented for Zion National Park and Cedar Breaks National Monument in southwestern Utah.