400hzs
 

On early intercoms the band was 500 to 3500 cycles (no hzs in those days). To stop the leakage of noise from the A/C power to the I/c 400 c/s was used. Greater than 3500 and the loses became too high.
All before my time but I remember am old crusty one telling me.

Two 737 BBJ's that I've worked on had 240V 50Hz single phase outlets throughout the cabin. They had two inverters mounted in the stab compartment.

From memory it becomes a problem if try and run a 400Hz device off 50/60Hz rather than the other way around.