Majority of current investigations focus on attenuation of tonal frequencies by active and passive techniques, as well as local noise cancellation at the point of the recipient. However, the anticipated drastic progress on noise reduction can likely stem from sound cancellation at the source. Forming equal-amplitude and opposite-phase pressure waves to the noise, the sound emanating from the system can be negated. Thus far, the technological issue hindering the practical implementation of this approach has been absence of devices, which can be mounted on the entire noise generating surface without affecting the intended operation. Although the moving-coil loudspeaker has seen the most scientific development over the past 150 years, other forms of sound reproduction exist. In particular, thermophones utilize periodic Joule heating of an electrically conductive body to create surface temperature fluctuations, which are then converted into pressure waves by the thermo-acoustic effect. To date, a comprehensive model, which captures the exact mechanism of heat transfer in such devices, does not exist. Therefore, we have been working on a semi-analytical solution that couples the dual-phase-lag hyperbolic heat conduction problem with the ballistic transport on the surface. Considering the wave nature of conduction in small timescales, our model predicts the existence of thermal shocks, thermal resonances, and thermal interference patterns. Based on these estimates, we have engineered thin solid and epoxy media, stretched it between copper electrodes and excited with a combination of direct and alternating currents. Applying this formation on top of a
conventional loudspeaker, our method has achieved acoustic cloaking by 500-fold reduction of the generated sound pressure levels. The next stage of the project focuses on reducing the aero-acoustic noise generated by the rotor-stator interaction in a small-scale ducted fan engine, where the thermophone will be deposited on the stator.
