Reverberation of ultrasound in an irregular cavity: Analysis of harmonic content at high excitation amplitudes

The reverberation of ultrasonic waves in cavities has numerous applications, including material characterization, high-quality focusing, and ultrasound image reconstruction. Most of these applications rely on sustaining long-duration waves inside the cavity, making the reverberation time a key parameter. In this work, we measure the reverberation time of ultrasound in a water-filled cavity with an irregular shape. The acoustic field generated by a short pulse at 1 MHz is analyzed under both low and high excitation levels. In the linear regime, reverberant signals are centered around the fundamental frequency, whereas higher excitation levels introduce nonlinear harmonics. We show that these harmonics decay exponentially and can be accurately described using a linear approach based on Sabine's law. The agreement between experimental data and the model confirms that the harmonics propagate independently. This enables the characterization of both wall absorption and frequency-dependent attenuation in water.