yeah, thank you very much ! This is exactly the answer 
I think some modification of the tone doc is needed to include the fact the frequency range is 31Hz - 65.5kHz with the built-in tone() function.
If we need a wider frequency range, starting from 1Hz, we must use the Tone library from the following link :
http://code.google.com/p/rogue-code/wiki/ToneLibraryDocumentation