My last two free standing spas have and had pretty powerful blowers for the bubbles that come up from the foot and leg areas. The blower is the loudest motor of the four in the spa but not as loud as most of my household and shop vacs. The two jet pumps, at 5 HP each, are not nearly as loud as the blower. Of course these are all inside the enclosure but right under where one sits in the spa. The enclosure is pretty thin cedar planks with no insulation but the actual fiberglass spa has a lot of foam insulation on the underside side.
When in the spa the noise of the bubbles helps to drown out the blower noise. The bubbles are loud enough to make having a quiet conversation difficult even sitting next to someone. The blower motor noise seems much louder when standing outside of the spa. It is not as loud as my shop vacs, all of them, and it is not nearly as noisy about 10 ft away from the spa. At that distance the sound of the rumbling bubbles is louder than the motor, even with the insulated cover on. (The spa goes through an automatic short cycle with all the motors once a day just to clear and refresh the lines and pipes.) The blower motor is the highest pitch also. This blower motor was a new replacement last year. It didn't give any indication it was going like water pump motors usually do, like bearing noise. One day it worked the next day it didn't.
I can hardly hear the low flow circulation/filter pump even when I stick my head into the enclosure.
I wonder if you could put some foam insulation on the inside of the rock allowing for free air movement and heat dissipation.
BTW... my biggest, most powerful shop vac, 16 gallons, 6.25 HP is the quietest of all my household and shop vacuum cleaners and hand helds, except for the robot cleaners and the little Shark battery powdered vacuum (mostly brush action with low vacuum power but does a great job). The newest iRobot Roombas, the 5xx series, are pretty quiet, especially using the Aerovac bins, almost silent on soft surfaces (most of their noise comes from the locomotion and side brush motors), but the much more powerful "real" vacuum cleaning robots, Electrolux Trilobites, have pretty loud vacuum motors.
I don't think I've mentioned lately how much I love my army/pack/gang of cleaning 'bots and gadgets. :lol:
gg=alice