Assessment of desirable reflections and control of undesirable reflections in rooms are best accomplished if the reflecting surfaces are properly localized. Several measurement techniques exist to identify the incident direction of reflected sound, including the useful polar energy time curve (Polar ETC), which requires six cardioid impulse response measurements along the Cartesian axes. The purpose of this investigation is to quantify the incidence angle estimation error introduced into the Polar ETC by non-cardioid microphone directivities. The results demonstrate that errors may be minimized with a cardioid-family microphone possessing a certain range of directivities and by maximizing the measurement signal-to-noise ratio.

Information technology (IT) noise is very prevalent in today’s society. Active noise control (ANC) has shown promise in minimizing the effect of fan-induced IT noise on users. Much of the previous research has concentrated on axial cooling fans, such as those found in desktops and servers. This approach was based on the concept of minimizing radiated acoustic power in a model of the fan radiation, and using those results to determine appropriate nearfield locations for the error sensor(s). This paper describes modifications to this previous method to develop a modeling approach to implement active noise control with a centrifugal blower, such as those found in fan trays and laptop computers. This model has been used to predict tonal noise inside and outside the duct, as well as how to best develop an ANC system for such an idealized setup. Differences between the axial fan model and the centrifugal blower model are discussed, as well as some limiting assumptions for each model.

This paper describes the principle behind a high amplitude non-contact acoustic source based on the principle of time reversal (TR), a process to focus energy at a point in space. By doing the TR in an air filled, hollow cavity and using a laser vibrometer in the calibration of the system, a non-contact source may be created. This source is proven to be more energetic than an off the shelf focused ultrasound transducer. A scaled up version of the proposed source has the potential to allow nondestructive evaluation processes that require high amplitude, such as nonlinear elastic wave spectroscopy (NEWS) techniques.

Previous work by these authors in active control of axial fans suggests an approach that can be successful in applying active control to small centrifugal fans used in fan trays and laptop computers. The modeling and analysis strategies developed for axial fans were modified for use with centrifugal fans mounted in a rectangular exhaust duct. Experimental verification allowed for proper inclusion of damping in the model. By minimizing the sound power radiated from the duct, optimal error sensor placement was predicted. Experimental results verified the effectiveness of placing the error sensor at these locations. Using predicted control source and error sensor locations, the rectangular duct was replaced by a centrifugal fan and duct attached to a heat sink, with the total dimensions being the same as the previous rectangular duct. The experimental results indicate that significant global reduction of the radiated tonal fan noise can be achieved.

No abstract