Hopefully one in each corner of each side is enough. With a bit of luck I can get the location by interpolating between the values read there.
If you are mounting “a number of” piezo sensors, you need to keep in mind that there are a limited number of input pins available on the Arduino Duemilanove.
Yes, I will probably need the Arduino Mega with its 16 analog inputs…
Also, the top of a table tennis table is pretty sturdy. I’m not sure that you are going to be able to differentiate which sensor is nearest the ball. If I remember correctly, a table tennis ball is pretty light. I’m not sure how much movement it will cause in a 3/4 plywood table top.
Actually I am thinking about those that work like contact microphones (pictured here: http://en.wikipedia.org/wiki/Contact_microphone). They are very cheap and very sensitive.
This also makes me a bit worried: I fear the sound will transfer too well through the table so the sensor readings gets too similar. Testing will tell, I guess.