Frank has put the insulation on the ceiling, and sent me his latest REW measurements, so here's some more "before" and "after" pairs, where "before" is with just the rear corner treatment in, and "after" is with the ceiling insulation in too:
WATERFALLS:
Frank-REW-WF-20-500--Rear-Corner.png
Frank-REW-WF-20-500--Ceiling-Insul.png
You can see the very nice effect that this is having on the modes at 113 Hz, 177 Hz, 205 Hz, and 232 Hz. Those have pretty much gone now, as well as general low end smoothing.
This is a real "slap in the face" for those folks who insist that a couple of inches of insulation has no effect below about 400 Hz or so! Much to their chagrin, the result here is glaringly beautiful: That's some pretty major changes, especially at 113 Hz... and all Frank did here was to put insulation in between the ceiling joists. Just a few inches thick, nothing more, and nothing special: just ordinary insulation.
Got any photos of that Frank, to permanently silence the noisy "insulation doesn't work down low" mob? Showing what you did, and how thick it is?
(For those who don't know what I'm referring to here: there's a vocal bunch of detractors of the benefits of thin porous absorption, who loudly voice their ignorance all over the internet, insisting that insulation can only be effective if it is 1/4 wavelength thick. According to them Frank would need insulation that is two and a half feet thick to have any effect on 113 Hz (the wavelength is 10 feet, so the quarter wave is 2.5 feet). Even worse, what Frank did here also had a nice effect on the problem at 86 Hz, where the Wavelength is 13.1 feet, so you'd need 3 and a quarter feet, according to the silly folks. But you can clearly see that just 8" of insulation is having a very, very nice effect, even down at these low frequencies. 8" is just 6% of ten feet, and 5% of 13 feet, which proves Andre's theory that good porous absorbers is still effective down to 3.5% of the wavelength for random incident sound, and 7% for normally incident sound. And if you look closely, you'll see that this treatment even had some effect on 74Hz. Not a lot, but it's there. The wavelength for 74 Hz is 15.25 feet, so according to the nuts, he would have needed nearly four feet thick insulation to do that...
8" is 4.3% of that, still within the 3.5% theoretical range...
--- Sorry for the rant, Frank, and for hijacking your thread to do it, but this is one of my pet peeves! People spouting off about stuff they know absolutely nothing about. I just wanted to mention that, since your carefully done tests highlight their ignorance so beautifully! The ONLY thing you did was to put the insulation in, and the results clearly show that it works very decently.)
SPECTROGRAMS:
Frank-REW-SP-20-500--Rear-Corner.png
Frank-REW-SP-20-500--Ceiling-Insul.png
You can see the same thing here, and also something more important: The SBIR null at 144 Hz is filling in nicely too. I highlighted that with the cursor. (I'm pretty sure that is SBIR, not modal, since there's also a phase shift going on at that point.)
(You have to be careful when looking at this pair, since REW has automatically adjusted the scale between the two! Because the overall level went down, REW modified the color scale and intensity, so it looks like the low end got louder, but it didn't. REW just shifted the scale so that the highest level is now at 89 dB, instead of 91 dB before. So refer to the key color-strip on the right side of the graph for clarification.
FREQUENCY RESPONSE:
Frank-REW-FR-20-500--Rear-Corner.png
Frank-REW-FR-20-500--Ceiling-insul.png
Nice changes, smoothing out neatly.... The 144 Hz issue is highlighted there, with good improvement.
RT-60:
Frank-REW-RT-20-20k--Rear-Corner-B.png
Frank-REW-RT-20-20k--Ceiling-Insul-B.png
The decay times are coming down, especially in the high.mids and highs. But I'm a little concerned that we might be going too far in the high end: the very top end is already down to about 230 ms, with plenty of treatment still remaining. So well need to do something about that.
So, Frank, you'll need to get some very thin plastic, around 1 mil, and cover SOME of that insulation in the ceiling, but not all of it. I'd suggest maybe 50% coverage. The type of plastic you need is the very thin stuff, that painters use as drop cloth to protect furniture while they paint. You can buy it in hardware stores (Home Depot, etc.). Cut off reasonably broad strips, maybe 8" or so wide and as long as you need to span between joists, then leave 8" open spaces between them. Don't make them too wide, or they'll be reflecting too much. Also, don't make them too narrow, or they won't do their job. It's not too critical: anywhere around 7" to 9" wide is fine.
So put those up on the ceiling, and try another REW test. There won't actually be very much difference in the graphs with this test at this point, since there's still way to much other stuff going in, that is going to be masking the effect, but it needs to be done now, before moving on to the rest.
- Stuart -
I forgot to mention it, Sorry Stuart... So we have 3.5" of rockwool and 1" of fiberglass duct liner.
