it depends on the power source.
If you are using an AC horn, the buzzing caused by rapid switching of the power source is controlled by the AC current. Since standard household current is 60 Hz, that means the hammer in the horn strikes the sounder plate 60 times per second.
That’s not all–the sound the buzzer makes also depends on the mechanism. While they still move at 60 Hz, the size of the sounder, the design of it, the shape, and the mechanism type will change the sound. While it’s still moving at 60 times per second, it just does not sound like it. On the 4040 and the 4050 (and I think also the 4030) the mechanism is two metal plates that clap together quickly. On the Tork Alert you mentioned above, it’s basically the AC version of a 9838 and uses a hammer that strikes a plate.
The 4050, 4040, and 4030 all make a 60 Hz buzz because of what I described above.
If you are using a DC horn, you now need a pair of contacts that are opened and closed by the magnet, wired so when the magnet is powered, the contacts open, cutting power to the magnet, which releases the contacts, completing the circuit again. The pitch of the alarm depends on the distance the contacts have to move (farther distance means slower) and the higher the time between strikes is, the lower the pitch of the horn.
The 9833, 9838, and 9806 have different pitches because of the position of their switching mechanisms. Oh and it may not look like it but the pitch on the 9833 and the 9838 can be changed easily.
With the smoke detectors, it seems that some of them run purely on DC power, with their AC inputs running through a rectifier. It also seems like others run completely on AC current.
Sources: electricity books, experimentation and observation.