You're thinking of the decibels as measured from the exterior, but this likely refers to the decibels as measured at the point of constructive interference of t

You're thinking of the decibels as measured from the exterior, but this likely refers to the decibels as measured at the point of constructive interference of the waveform in the chamber - ie the method of action of this heat pump. Those decibels then turn into heat, not exterior sound. It's a really cool application of the ideal gas law: PV = nRT where the sound is really playing with the pressure (P) using sound so various parts of the chamber have different temperatures (T).