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>>
New Low-Profile Applicators for Local Heating of Tissues
A new type of low-profile applicator is described giving efficient power transfer to tissue with low leakage. It consists of a resonant element spaced from a ground plane by low-loss material having high relative permittivity and some relative permeability. This combination is covered by similar material which can be placed in direct contact with the tissue to be heated, or separated from it by a water bolus as required. Laboratory measurements using phantom materials show that these applicators are well matched and produce well defined and relatively uniform heating profiles. Applicators having maximum dimensions of 3-15 cm covering the frequency range 1000-100 MHz, respectively, are described. Penetration depths of at least 4 cm at 100-200 MHz are achieved. Tests on anesthetised pigs and some clinical assessment have confirmed the usefulness of this compact low-frequency type of applicator and have shown that performance compares favorably with that of other recently reported types. The design can be optimized for treatment of specific sites, and suitable choice of dimensions and electrical characteristics of the layer material allow the low-profile concept to be extended over a wide range of operating frequencies.