This dissertation is concerned with the active control of local sound fields, as applied to an active headrest system. Using loudspeakers and microphones, an active headrest is capable of attenuating ambient noise and providing a comfortable acoustic environment for an occupant. A finite element (FE) model of an active headrest is built and analyzed such that the expected noise reduction levels could be quantified for various geometries as well as primary sound field conditions. Both plane wave and diffuse primary sound fields are considered and it is shown that the performance deteriorates for diffuse sound fields. It is then demonstrated that virtual sensing can greatly improve the spatial extent of the quiet zones as well as the attenuation levels. A prototype of the active headrest was constructed, with characteristics similar to those of the FE model, and tested in both anechoic and reverberant sound fields. Multichannel feedforward and feedback control architectures are implemented in real-time and it is shown that adaptive feedback systems are capable of attenuating band-limited disturbances. The spatial attenuation pattern surrounding the head is also measured by shifting the head to various positions and measuring the attenuation at the ears. Two virtual sensing techniques are compared in both feedback and feedforward architectures. The virtual microphone arrangement, which assumes that the primary sound field is equivalent at the physical and virtual locations, results in the best performance when used in a feedback system attenuating broadband disturbances. The remote microphone technique, which accounts for the transfer response between the physical and virtual locations, offers the best performance for tonal primary sound fields. In broadband sound fields, a causal relationship rarely exists between the physical and virtual microphones, resulting in poor performance. / PHD / Excessive noise and vibration levels in aircraft, rotorcraft, launch vehicles, and other aerospace vehicles may create harsh acoustic environments inside the vehicle. In some extreme cases, military applications being a prime example, hearing damage can occur due to the high noise levels associated with certain vehicles. Noise canceling headsets have been proven an effective solution to this problem, although in certain instances their use may not be safe or feasible. In this work, an active noise canceling headrest, or active headrest, is explored as an alternative solution to noise canceling headphones/headsets. An active headrest uses microphones and loudspeakers, typically located non-intrusively behind the head of the seat occupant, to reduce the ambient noise levels in the vicinity of the head and create a comfortable acoustic environment. A thorough investigation of the viability of such a system in a practical vehicle is assessed through the use of theoretical analysis, finite element modeling, and real-time performance experiments. Performance predictions generated using the finite element model were verified by performing real-time experiments, thus providing a level of confidence in additional predictions for alternative headrest geometries and configurations. Factors such as loudspeaker and microphone placement, head movements away from the nominal position, primary acoustic field characteristics, and choice of control strategy are all found to heavily influence the performance of an active headrest. Real-time experiments were performed in anechoic and reverberant sound fields and it is found that the noise canceling capability of the active headrest worsens in reverberant sound fields as compared to free field conditions.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/94626 |
| Date | 25 April 2018 |
| Creators | Bean, Jacob Jon |
| Contributors | Aerospace and Ocean Engineering, Woolsey, Craig A., Fuller, Christopher R., Schiller, Noah Harrison, Philen, Michael K., Sultan, Cornel |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
Page generated in 0.0019 seconds