Supersonic bending waves radiate sound in strongly favored directions

Evidence

At high enough frequency, the bending waves become faster than the speed of sound in air, reducing short-circuiting. The price paid is that the sound tends to be launched in strongly favored directions, as we now show, using a “trick” to deduce the sound radiated by a supersonic sinusoidally undulating surface. — Eric J. Heller, Why You Hear What You Hear- An…, 682