Active Control of Energy Density in a Mock Cabin

Acoustic energy density’s dependence on particle velocity as well as acoustic pressure makes it a good candidate for an error signal in an adaptive noise control algorithm. It has been shown that minimizing acoustic energy density often provides more global attenuation of an enclosed sound field than minimizing squared acoustic pressure. Nodal regions in three-dimensional enclosed sound fields comprise entire planes when looking at squared pressure and lines when looking at energy density. This means that for most locations in the sound field, an energy density sensor will have a much lower probability of being at a nodal position than a pressure sensor for the natural modes of the enclosure. Active noise control has been applied to tractors in order to reduce operator noise exposure. The research discussed in this paper applies active minimization of acoustic energy density to the problem of noise attenuation in enclosed tractor cabins. Previous approaches to this problem which based themselves on minimization of squared pressure achieved attenuation in a relatively small area localized around the operator’s head and required that the error sensors be placed near the operator’s ears, thus restricting the operators range of movement to either side. However, the results discussed later in this paper were obtained with an energy density sensor located directly above the operator’s head against the ceiling of the cab.