A high-temperature and high-pressure speed of sound apparatus measures the speed of sound in a sample at high-temperature and high-pressure, the apparatus includes: a defined-length spacer that has a fixed length spacer portion; an acoustic reflector, such that a crucible is interposed between the acoustic reflector and a pressure vessel, the pressure vessel in which is disposed the crucible, the disposed defined-length spacer, the sample, an acoustic reflector, and an acoustic rod; and the acoustic rod on which is disposed an ultrasonic transducer.