Suppression of acoustic feedback by frequency shifting : An approach for howl suppression in a public address system

In this master thesis a way to suppress acoustic feedback in a public address system, has been studied. The setup is typically a microphone, amplifier and loudspeaker(s) used in a confined space, coupled with the solution (TFS) in this thesis as an external circuit between the microphone and the amplifier. The circuit is utilizing a small frequency shift of the sound, to allow the gain to be turned up a few more decibels, before the infamous howling sound takes over. An analogue hardware variant of the circuit described by Martin Hartley Jones was studied, developed and modernized, using current available components. The original circuit by Hartley Jones was greatly altered to implement possibilities for more comprehensive testing scenarios regarding the phase shift frequency. In addition, modifications were made to make the circuit able to handle balanced signals directly from a microphone. This resulted in a completely new quadrature oscillator section, as well as a new preamplifier section. The testing was performed with three different setups, and the possibility of higher gains before feedback occurs, were confirmed in all three scenarios. Even though the results indicate the possibility of approximately 6 dBA higher sound pressure levels, more testing is required to get explicit accurate numbers