How to inject the maximum acoustic power into a block of granite? I know that we can use transducers that are glued to a surface to achieve this. If I want, for example, to have 200 watts of actual acoustic power in the audible range in the granite, is a transducer the easiest cheapest way? Are there other methods?
 A: Here is the challenge you have to overcome.
To actually deliver 200 watts of acoustic power into a solid block of granite requires that the transducer be impedance-matched to the load.
If you press on a piece of granite, it presses back really hard and experiences almost no compression in response. This makes it an ultrahigh impedance load, and if your transducer impedance is lower than this, most of the acoustic energy will bounce right off the block and not enter it. Your transducer might be producing 200 watts but 199 of that is reflected off the block; meanwhile you are still pumping 200 watts into the transducer, and it will soon blow up.
This effect rules out the use of any sort of moving-coil loudspeaker as a transducer because they are all designed to drive a very low impedance load: the air in front of the cone.
You are pretty much restricted to piezoelectric-type transducers, as used in very large ultrasonic welding machines, but even then you will need a custom-fabricated horn assembly which will match the impedance of the piezo transducer stack to that of the rock at the frequency of interest. This sort of setup will cost of order ~tens of thousands of dollars.
Note that typical piezoelectric ultrasonic welding machines operate at and above 40 kilohertz. To drive 200 watts into a chunk of rock in the audio range will require many, many transducers.
BTW, what exactly are you wanting to accomplish with such a setup?
A: *

*Use an industrial size vibrator motor bolted to the granite. The motor shaft has an eccentric mass on it which will vibrate at the rotational frequency of the motor. Not enough power? Use more than one.


*Use a plate compactor if the process can be conducted outdoors.


*Use a jackhammer, or its smaller cousin, an electric demolition hammer. If you don't want a hole in the granite, cut off the pointy end of the bit and weld a thick flat plate to spread the impact over a larger area.
I have no idea if any of these methods will deliver 200 watts of power to the granite, but the power will be in the acoustic range and the cost is low enough to try it and take measurements. Not to mention the fun factor.
