People are familiar with the acoustic feedback phenomenon, which results in a loud sound that is heard when a musician plays an electric instrument directly into a speaker. Acoustic feedback occurs when a source and a receiver are connected both acoustically through the propagation medium and electrically through an amplifier, such that the amplified received signal is continuously re-emitted by the source. The acoustic feedback can be initiated from a continuous sine wave. When the emitter and the receiver are in phase, resonance is obtained, which appears to be highly sensitive to any fluctuation of the propagation medium. Another procedure consists in initiating the acoustic feedback from a continuous loop of ambient noise. It then generates an unstable self-sustained feedback oscillator (SFO) that is tested here as a method for monitoring temperature fluctuations of a shallow-water oceanic environment. The goal of the present study is to reproduce and study the SFO at the laboratory scale in an ultrasonic waveguide. The experimental results demonstrate the potential applications of the SFO for the detection of a target in the framework of the acoustic-barrier problem in shallow-water acoustics.