[Outlaw76]

I think my old wind up alarm clock worked at about that 7Hz. Ok, maybe closer to 3Hz, but more than enough to cause me pain.

Seriously though, a gong like device, or bell, being struck by a solenoid could operate very well at that frequency.

[eno_]

The problem is that the energy is in the bell ringing, not a 7hz tone. To really do a 7hz tone you would need a big pipe and a pump that could move a large amount of air into it. The saving grace is that is would not have to be very precise.