Bullduck, I agree with Williaty that debating the specifics about the Belaire compressor derails your "show me your compressor DB" survey a bit.
On the other hand, since the Belaire claims to be a "quieted" model, and is one of the few to market an actual DB rating, I can see why you brought it up.
I'll add to the confusion (and keep your thread alive) by pointing out another reciprocating compressor manufacturer (Ingersoll Rand) that did the same type of sound enclosure trick 11 years ago with their TS Series line of reciprocating two stage compressors. These looked on the outside a lot like their Rotary Screw line (Now called UP6 series), but the TS series was actually a recip with a twin cylinder pump of the shared head Siamese type, called an "enblock" design (NOT a V cylinder type T30).
Ingersoll Rand didn't just put a box around the motor and enblock pump. They engineered an entirely arrangement, using a squirrel cage centrifugal blower instead of just a flywheel fan. This blower is very different from the typical way reciprocated compressors move air across themselves, and was very key to totally enclosing the business end of the TS Series compressors.
Another key differentiator integrated a miniature air cooled aftercooler coil that sat INBETWEEN the motor and the pump in a TRANSVERSE direction, not behind the traditional flywheel in a parallel direction as is typical of most reciprocating compressors.
The pressure switch, pressure guage, and pressure relief valve were also enclosed and incorporated into the front face of the housing into an "instrumentation panel". This made the unit look very similar to the Rotary Screw line up, at half the price.
The enclosure was metal, with inch thick high density open cell foam lining all interior surfaces, including the baffle that separated the motor from the pump and aftercooler.
The enclosed TS5/SS5 through TS15 compressor by IR was one of the very few RECIPROCATING compressors that I am aware of other than the Belaire and some Champion models to publish the db sound level measurements in their marketing materials. (Ignoring that guy in Canada who sells so called made in USA compressors on ebay who changed his name from mimicking the goodwill of a long standing American truck transmission and axle company to a colder sounding name... but like always, uses Chinese import pumps and types more than I do).
Ingersoll Rand's published db measurements for the enclosed vs. unenclosed version of same TS Series compressor:
5 HP , 17 CFM
83 db UN-enclosed
69 db Enclosed
7.5 HP, 25 CFM
83 db Un-enclosed
70 db Enclosed
Source: Ingersoll Rand: December 28, 2002
So, how much does this 14 db drop cost?
It doesn't matter. Talk about "quiet"... Ingersoll Rand "quietly" ceased making these soon after introducing them. The entire TS line is not available at all through IR, and only the NON-Enclosed versions of the TS series lasted through the retail channels such as Sears, Tractor Supply, Northern Tool, and Grainger. No "enclosures" or fully packaged options are available.
Also, the TS7.5 was dropped, leaving only the TS5 and the TS3 (3HP, 10 CFM). Again, no enclosure.
I say all this to point out the fact that IR spent a lot of time and money "thinking INSIDE the box" to make this special foam lined enclosure with the centrifugal blower system. And soon afterwards, it was discontinued. To me, that says something. I'm not sure WHAT it says, but it says something.
In the meantime, the Type 30 design has been chugging along now for 85 years and counting, with tweaks here and there. IR's 75th anniversary edition of the Type 30 was produced in 2003. There is something to be said there too... about what is tried and true.
I'd be cautious about that shroud that Belair has got on their compressor. What kind of heat load does that shroud induce?
Finally, Champion has a shroud for some of their reciprocating compressors in their "Evolution" line as well as their high dollar oil free line. I can't get Champion's website to work well on my computer right now, so you can look them up more easily on your own. Champion advertises a sound level as low as 66 for one of their oil lubricated reciprocating compressors in the Evolution line. I do not know the technical details of how Champion achieved (or measured) this.
We should email their tech department a link to a cell phone app we all agree on, and have them test. (Rest easy Willaity... I'm only joking. Sort of. Hmmm, now that I think about it, wouldn't that be a riot to ask, just to see if they would?)
You could make power arc measurements with the $20 meter, but even then the environment is going to introduce so much error it's still completely useless to compare.
