wow....that's fascinating that 99% of the ambient noise is in the sub-200Hz region. would be neat if a measurement could be made at say, 2am in the morning (when there is minimal traffic) to definitely confirm it is noise from cars/traffic flow; but I fully understand after your Herculean labours, midnight vigils are not exactly what the doctor ordered.
Hey I already mentioned, I'm no Hercules!!
Yeah, a check on the freq spectrum in the wee hrs of the morn is next. I suspect it is the traffic noise, as well as any residual noise from the 2 fridges and one freezer in the house. If any of the compressors in those machines are going, it transmits some very low freq vibrations into the frame of the house, which is picked up by the mic. The ACO Pacific mic is absolutely flat, btw, to below 10 Hz... probably not such a good thing for my needs!!
It's been suggested by a helpful SPCR reader (who happens to be in the business of anechoic chamber design) that the concrete slab which lies below the carpet and underlay is a source of low frequency noise "flanking" -- it doesn't come through the walls and acoustic batting but up through the floor. With the very low freq noise from traffic noise 1.5 blocks away, I expect this is the main noise path. It's probably conducted through the ground, the cement slab and into the room. I don't hear it because the noise is still at a fairly low level, and human hearing sensitivity falls away steeply below about 100Hz... but the mic's sensitivity doesn't.
The questions are:
1) Would a floating floor by itself (without floating walls & ceiling) eliminate most of the low freg noise transmission? I hypothesize yes -- if most of the low freq noise is via conduction, it enters the house through the floor, causing up/down movements. Those same vibrations in the walls cannot cause anywhere near the same level of noise as in the floor because it's in a perpendicular plane. It might cause a fair bit of noise in the ceiling tho, as that's parallel to the floor, but how much of the vibration is conducted up to the ceiling? Probably not as much as the floor.
-- and --
2) since all of SPCR's noise measurements are done with A-weighting, and there's no reason to change this, is the elimination of the low freq noise worth pursuing?
If the answer to 1) is yes, then I'd go ahead and install a floating floor. It's not that difficult or expensive, at least not the way I'd go about it. My basic approach:
a. Remove all the wall blue fill & frames,
b. loosely put down a 1.5~2" thick layer of high density foam, or even better -- closed cell foam* -- over the entire floor.
c. lay two overlapping layers of medite board over the entire floor, probably each with 5/8" thickness, and join the two layers using 1" screws. This MDF platform would become the new floating floor -- it would touch nothing except the foam beneath it, and extend to about 1" from the walls. That edge would be covered up by the blue fill which goes up against the walls.
Someone w/ experience or greater knowledge -- perhaps my helpful angel from anecholand -- needs to tell me whether this floating floor is worth doing.
*Why closed cell foam? It would be recycled from all the packing material in boxes that come with product samples, especially cases. It's very uniform -- the density of all the packing closed cell foam seems to be the same and ~1.5" thickness is very common. I've done a quick and dirty test on how much resilience it has under pressure: A wood board under 1' sq was placed atop a similar amount of the foam, and I stood on it placing my ~165lb weight on it. The compression was about 1/4", and it still felt quite resilient when I bounced my weight on it by flexing my knees up/down quickly. Now, the MDF floor I described above would have a weight of approximately 300 lbs and cover an area of 120 sq ft -- less than 3 lbs per sq foot, compared to the 165 lbs/ft I applied by standing on a sq foot.
The only questions about closed cell foam is whether there's enough here to cover the whole floor, and how effectively it stops mechanical conduction of low freq vibration. I know from extensive experience with trying to stop low freq conduction from HDDs (in the other direction) that open cell foam is much superior to closed cell foam. But this may be due to the relatively higher resilience (softness) of the open cell foam, which is appropriate for the low mass of a HDD. Some experimentation is probably needed to check this.