In his seminal paper1, Albert Einstein introduced the concept of stimulated emission of electromagnetic radiation that led to the invention of masers and lasers. Ever since, there has been a propensity to regard stimulated and spontaneous emission as quantum-mechanical effects. However, these concepts have clear classical (nonquantum) analogues that we demonstrate in experiments with ultrasound. We have designed an electromechanical system consisting of ultrasonic oscillators that emit both spontaneously and by stimulated emission like atoms in an optical laser. The set of nominally independent oscillators lock to each other and produce a high-intensity coherent field. In the absence of stimulated emission, they produce a weak and incoherent field..
