A summary, after the past week's experimentation with the mostly treated room.
It now measures ~12 dBA pretty much any time of day or night.
There are some spikes in the response curve most likely related to reflections off the mic stand, the table, the wood frames for the blue fill, etc, but these are minor issues.
The acoustic isolation is not as good as hoped for. When it's reasonably quiet outside, it's perfectly quiet in there. This could be said before any of the treatment was done... but the ambient was some 2-4 dBA higher. When there's plane/car noise, it's at a much lower level than before, but still audible.
Would any of this noise come through in the recording?
A bit, with the quietest products, if you turn up the playback gain high enough. But if you follow the directions about how best to listen to them (which is to turn the volume to the point where the ambient turns inaudible), then no, you'd never hear it.
What about in SPL measurements?
Not really, not with A-weighting.
At the start of the construction, when I first discovered the dramatic effect of sealing up that window and doubling up the external wall, I asked whether this was good enough sound isolation for SPCR's needs -- and everything I've done with the room since then has been to try and answer this question.
The answer is both yes and no.
Yes -- it's probably good enough, given the objectives: Higher accuracy and greater ease in making recordings and measurements of quiet gear for SPCR.
No -- not if we want to be able to ignore
outside noise altogether. It probably could be made 10 dB quieter in the lower freq... maybe better, quiet enough to be a "serious" anechoic chamber.
What would this take?
Going back to the original design -- a room in a room -- but adding one more feature: a floating floor of 2-3 layers of 3/4" plywood on neoprene damping pucks. The walls would have to be built atop this floor, making no contact with any of the existing walls. The ceiling would bolt atop the walls w/o touching the existing ceiling. As mentioned in another thread, it's been done by a Canadian digital recording tech store and at least partly documented.
All this would mean undoing much of the work I've done. The nearly-600lbs of blue fill would have to go back into the garage, the wire suspension on the ceiling removed, etc. But this is relatively simple, compared to the original installation. Re-installation of the blue fill would go much more smoothly now that I've figured out how, and w/all the frames for the blue fill already built. Still it would mean the mess down here continues on, and even get worse with drywall dust being added to the mess, for at least a couple more weeks.
Will I do this? At present, I'm torn. PArt of me just wants to finish off what I've done already and get on with more and better reviews & articles. Another part wants to make the chamber the best that it can be -- and I'm half way there. Decisions, decisions.
Late last night, I had the mic in the room with the doors sealed shut, and studying the freq spectrum on the screen as I listened through headphones. This was to understand better the relationships between what I hear, what the mic picks up, and what shows up in the RTA screen. It would help identify the sounds that are getting through to the mic in that room.
It was very quiet, around midnight on a Sunday. The gain was turned up to max -- what I heard in the bass through the mic was louder than what I could hear with my bare ears, even though I was outside the treated room. Furthermore, because the rest of the freq range was so subdued, the low freq sounds came through much more clearly.
There were numerous little spikes in the sub-100Hz region, and I was trying to identify these sounds. Some were obvious, like a car a block a way, idling while dropping someone off, the entry door being closed a few houses away, etc. All these were still registering under 20 dB. But there was a series of rhythmic thumps, perhaps 15 in total with a slight jog in the middle, that I could not identify immediately... until I heard them again in a slightly different rhythm a few minutes later. The second set of thumps made me realize it was someone coming back down a staircase with a landing in the middle, after having gone up the stairs a few minutes before. Thinking through, it was probably the next door neighbor. The outer wall of that house starts about 10-12 feet from the lab room window. I have never heard neighbors walking up/down their stairs before.
I spy, SPCR style.