Active control of multi-dimensional random sound in ducts
Previous work has demonstrated how active control may be applied to the control of random noise in ducts. These implementations, however, have been restricted to frequencies where only plane waves are propagating in the duct. In spite of this, the need for this technology at low frequencies has progressed to the point where commercial products that apply these concepts are currently available. Extending the frequency range of this technology requires the extension of current single channel controllers to multi-variate control systems as well as addressing the problems inherent in controlling higher order modes. The application of active control in the multi-dimensional propagation of random noise in waveguides is examined. An adaptive system is implemented using measured system frequency response functions. Experimental results are presented illustrating attained suppressions of 15 to 30 dB for random noise propagating in multiple modes.
Active systems for sound attenuation in ducts
Passive attenuation of sound in ducts is related to wavelength, and therefore requires large silencers at low frequencies. Active methods using auxiliary sound sources placed in the duct wall provide an economical solution to the problem. To achieve high attenuation, adaptive electronic control has to be applied. The microphone in the upper portion of the duct provides the time function of the controlled signal. The 'zero' microphone has to be strategically located to eliminate the adverse effects of evanescent waves, radiated by the secondary sources. Reflections from the duct termination or the duct discontinuities can cause instability of the system. While suitable attenuations under the first mode cutoff frequency have been obtained, the cross-modes in the duct require different microphone configurations than the low-frequency plane wave field.<<ETX>>
Capturing Crime in the Antipodes: Colonist Cultural Representation of Indigeneity
This chapter adopts a Southern criminological lens, which allows the authors to explore how colonialism has been constructed and depicted in museums within both regional and urban Australia. By studying spatial and historical silences in Australian museums, this chapter offers a consistent frame to chronologically chart Anglo-Australian depictions of Indigeneity, imperialism and violence wielded by the state. As this chapter will argue, museums can often reflect Westernized (white) conceptualizations about Indigeneity and the state violence involved in colonization, which actively re-enforces popular views and influences sociopolitical visions of identity, the state and justice.