Ideal point source for modeling room acoustics

When analyzing the characteristics of sound—be it in a concert hall, a doctor's office, or a city street—acousticians can't always have unfettered access to the soundscape. So they build scale models and adjust the sounds' frequencies and amplitudes accordingly. A broadband, omnidirectional source of sound is very useful to modelers, and electrical sparks have been used to that end for many years. But the waveforms that emanate from electrodes are not only directional but vary from spark to spark in unpredictable ways. In addition, the electrodes' presence can complicate the sound propagation. So a group of acousticians at Aalto University and Helsinki University, both in Finland, came up with a solution that has been effective for studying shock wave propagation: They used a laser-induced pressure pulse (LIPP). When a point in space is heated to thousands of degrees by a focused laser, a local dielectric breakdown in air gives rise to an electrodeless spark that sends out a pressure wave—the LIPP. In their version, the acousticians focus a pulsed laser and send it into the scale model through an acoustically opaque glass window. Projected into the enclosed space, the LIPP has all the hallmarks of an ideal acoustical point source: It produces a lot of sound while being small, massless, omnidirectional, and broadband. (J. G. Bolaños et al., J. Acoust. Soc. Am. 133, EL221, 2013.)—Stephen G. Benka