hi im still newbie in arduino , i wanna ask something
how many maximum frequency that can be generated by Arduino
ur answer will helpful to me
thanks 
Way above audible - more like RF.
Maybe 1MHz or 2MHz when using the internal timers. I'm not sure.
The Tone library can't go very high. It is ment for up to 20kHz.
The toneAC can go a lot higher: Google Code Archive - Long-term storage for Google Code Project Hosting.
This shows the limits for toneAC: Google Code Archive - Long-term storage for Google Code Project Hosting.
The toneAC uses fixed pins, because it uses a hardware timer.
thank u i think i will try u suggestion