I found a fast solenoid valve lying around (~1ms rise time) so I thought it would be fun to hook up to a compressed air line and use to generate some really loud bass. I hooked it up to a function generator and air supply only to find that it was disappointingly quiet (well not quiet, but nowhere near as loud as I was expecting for 80psi of overpressure). I did some thinking and reading on the topic and realised there is probably some impedance mismatch issue at play, which is a little puzzling. I found an old broken trombone and brazed on a fitting to the horn to connect it to the solenoid valve. It made the setup greatly louder, however most of the energy was in higher order harmonics (still, not much bass was generated). My impression is that acoustic horns are required to match the impedance for compression drivers because the driver membrane needs to generate a high pressure against significant resistance (high throat impedance). The gas then expands to a greater area and lower peak pressures (and higher velocities). Why is this also the case for compressed air? An explosion or other kinds of overpressure events generate loud sounds, yet my solenoid valve expanding directly into the atmosphere (sans horn) isn't so loud, especially at low frequencies. Is the function of the horn in this context to allow the air to expand in one direction, converting the high pressure into high velocity particles moving in one direction? Kind of like a nozzle on a rocket converting the disordered motion of high temperature, high pressure gases into lower pressure and temperature gases moving in one direction (axially, out of the nozzle). Is the perceived efficiency gain I am getting in my compressed air setup therefore not a result of impedance matching, but simply the result of a collimation and directional expansion of the compressed gas (a kind of second law efficiency gain)? I would like to know what the transfer function of my horn is (or any horn as a function of frequency) in this context, but I don't really know where to begin. Most of the derivations I have seen yield acoustic impedances, but I don't know how applicable that is here to a fluctuating source of compressed air instead of a reciprocating membrane (speaker/compression driver). Conventional theory suggests that throat impedance needs to be closely matched to driver impedance, but I have no driver in this scenario.