What's great about the chart Murph posted is that on the piano keyboard, it even shows (in gray), the extended bass keys found on the Austrian Boesendorfer piano, the lowest of which is around 16 Hz. The lower keys are rarely used by concert pianists. I have only one recording of French piano transcriptions in which the very low keys are used for a selection called "The Sunken Cathedral". The pianist is Carole Rosenberger and it may be still available.
The bottom string on a normal concert grand piano is at 28 Hz, correctly noted in the chart.
Regards,
Alan
