[GeeTwo]

Especially in the high frequencies, the primary sound you will hear from a pneumatics cylinder is the exhaust, as sound from air flow within the system will be attenuated twice, once as it goes from air to plastic or metal, and again as it goes back to air.

I suspect that the major part of the sound in most cases is during the initial decompression of the compressed air, before the piston has moved very far. For a given exhaust port size, a larger volume piston will create a longer sound with a more gradual reduction in pitch, a smaller piston a shorter, sharper sound. Sound duration is determined by volume.

Just as with a brass or woodwind instrument, The pitch of the radiated sound will depend primarily on the length of the run from the exhaust port to the end of the tubing; a shorter tube will produce a fundamental frequency with a higher pitch.

As you look at robots that make the sound you desire, focus on the exhaust end first, from the exhaust orifice outward to get the pitch, then on the cylinder size to get the duration/decay rate.