Coherent longitudinal phonons at 9.0 GHz in X‐cut quartz rods have been detected by means of high‐speed superconducting bolometers. This type of detector is responsive to ultrasonic intensity and is insensitive to the ultrasonic phase distribution over the endface of the quartz rod, which often disguises the true nature of the echo‐decay envelope when piezoelectric detection of microwave phonons is employed. These bolometers are also capable of detecting the incoherent phonons produced in the dissipation and thermalizing of the coherent microwave phonons.