Though resonance has an effect on sound: neither seeing nor hearing a product will exclude the possibility of resonance. You'd need to hook a velocimeter to the side.
I disagree with the cost analysis. Granite (counter-top thickness) can be bought for $40 ft^2. If those are 1'x1'x2' (which they are clearly smaller than), that would still only require 20ft^2 of granite.... about $800. Your tweeters are $260 ($131/ea), I can't find the mids or anything like them. I would think that material would lack sufficient damping to make a good mid: but if I assume you are correct about the granite being the most expensive part: that would make it $800 at most.
So $1860 +crossover costs (I won't estimate) in parts.
Interestingly: There are several companies building out of cast marble (I have a pair of Green Mountains around here somewhere) for similar reasons; and at lower costs. Come by and we can measure the cabinet resonance on my Europa's.
RE: Granite
Granite is one of the strongest naturally occurring materials on earth, measuring an eight out of ten on Mohs hardness scale. It is an important building stone and is extremely resistant to weathering. To give you an idea of granite’s strength, diamonds, the hardest natural material, are required to cut, shape and polish granite. Marble is relatively soft when compared with granite, which explains its use in sculptures since classical time. It scores a six out of ten for hardness. It is still suitable for use in the home, but more care is required to avoid chipping, staining or scratching. Joel Foust will be assisting me in measuring these speakers but given the track record RBH has with their upper echelon products like this, and what I am hearing from my listening tests, I expect no surprises NOR do I detect any audible resonances. I ran a slow sine-sweep through them at high power and the cabinet was completely free of mechanical vibrations which I can rarely say for a loudspeaker at any price point.
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RE: Cost Analysis
You disagree with my cost analysis b/c you've never actually built a product or done the R&D to develop one. It's easy to sit back and analyze raw material costs without factoring in research and labor. I've been on both sides of the fence for years so I appreciate everything that is involved for product development and know it's not as easy as it may look to an outside audio enthusiast without the experience.
Here is a little background on the Granite Enclosures for these speakers that I learned from dealing with RBH's Chief Designer:
First, depending on the quality of Granite, it can range from $40 to 200 per SQ ft.
For the first pair of granite enclosures, RBH walked into a granite supplier in China and paid about $40 for each enclosure. They were just a rectangular box and functional, but there was no they could sell anything like that because of the poor seams/ fit and finish. Although, they were probably as good if not better than the average DIY person could come up with. The problem with DIY estimates is that they never take into account the R&D and true production cost (i.e.what do you have to pay the guy to program and run the CNC router,) overhead for company, marketing to show the product at shows and advertise, brochures, etc.
The enclosures RBH are making now are made here in Utah and actually quite tricky to make given the shape of the cabinet. So tricky in fact, that most of the stone fabricators they approached refused to do them at any price. Why? Because they are way more hassle than a counter top which they sell fabricated by the square foot. It took RBH several years to find the right fabrication facility for their cabinets.
The $15K price of the speakers includes matching stands which requires over 30 square feet of stone. Also, they are offering these speaker in custom finish options, so that means the price the stone can vary substantially. The speakers that were sent to me for review were a grade 4 stone (1 being the lowest and 5 being the highest). Again far more costly than $40 SQ ft, not even factoring in labor costs.