acoustics - Resonance in a 1 ft granite box

• I have a granite cube made using 6 slabs of granite 1 foot square and 1 inch thick. The top and bottom slabs have a 1 inch margin around the edge. The slabs are just set together, not notched or mortared or anything.



• I also have a concrete block with pipes running through it that generates a sonic pulse/compression wave from a valve closing. If this were a regular water system we'd all call it water hammer. This water hammer comes out the top of the block. It is assumed that this block is not going to self destruct from the water hammer. It pulses about 60 times per minute. There is a working example of this block that I have seen online, so I know how to build it.

Now what I'm trying to figure out is what would happen if I would set my granite cube on top of the concrete block. Would the sound reverberate around in the granite box until it blew it apart? Or would it somehow go through the block somewhere else? What would happen? Would it resonate at a certain frequency and produce a hum? Would it make a difference if I took away the bottom slab?

Answer

You state that the source produces a pulse per second. A hum, on the other hand, is a sound with a frequency of several hundreds of cycles per second.

If that source is particularly violent it may be able to get a sound going in a very good resonator.

(Example: I have a guitar, and I've noticed that when I sneeze and the guitar happens to lie right next to me sometimes one or two strings are audibly triggered.)

Anyway, to have a chance the resonator must be one that is very responsive. That is, the resonator must be shaped ideally for one particular frequency, so that all of the energy that it does aborb goes to that one frequency.

The cube that you describe is definitely a very poor resonator. I think you would have a hard time getting the sound from any source to resonate in it at all.

For comparison, bottles tend to have a narrow resonance response. You blow over the top of an open bottle, and when you hit the right angle of blowing just over it, and with the right airflow, you get a resonance in that bottle. Hum that same note to the bottle and your humming gets amplified. That amplification shows the resonator's responsiveness.

Summerising, to have a chance of producing some effect you need a resonator that is very, very responsive.