Flanking noise on brick walls

[RussinParis]

I have three exterior walls and one interior wall, all of which are built of cinder block and are about 20cm thick. All are plaster both sides. I was thinking of adding a second plasterboard wall (metal frame with insulation and two layers of plasterboard) on the exterior walls, but leaving the interior wall. this is a basement that is partially above ground.

I won't be floating the floor (just use those Auralex blocks for a drum/amp riser), and don't think I can afford to double the ceiling (2.21m/7.4ft) because of the loss of height. The ceiling is about 24cm thick, with reinforced concrete beams and cinder blocks, plastered.

I'm not concerned about transmission noise in the house, mostly from the outside; we're only about 10kms from an airport.

How much noise is going to come through that interior wall and the ceiling if I don't double it?

Thanks!

Russ

I'll see if I can include the current floor plan I'm working on. The interior wall is the bottom one in the diagram.

[knightfly]

Airplane noise tends to be for the most part pretty low frequency; so if those blocks are HOLLOW, you already have the most efficient use of mass (other than decoupling inner mass from outer mass) - best results for LF noise are gotten with a mass-air-mass barrier, and hollow blocks act very much like a stud wall with paneling on both sides. - so for best increase in performance, placing more mass AGAINST one or both surfaces WITHOUT more air gap works best.

If you put another air space (with or without insulation) you create a triple leaf wall, which usually has better attenuation at midrange but MUCH poorer attenuation at LOW frequencies.

There are 'way too many variables in building methods for anyone to answer your questions about the ceiling, since it also depends on the rest of the building, methods of attachment, porosity of materials, etc -

Have you actually taken noise samples within the room, so you have at least a spectrum analysis of the noise you're trying to stop? Without that, you're not just "tilting at windmills", but imaginary ones at that... Steve

[RussinParis]

Ok, I purchased a Apex220 measurement microphone (equivalent to Behringer ECM8000) and set it up in the (more or less empty) room. There's no divider wall at the moment, just a long, low, narrow room. I put it on a normal metal mic stand with a boom at about ear height and about where the middle of the tracking room would be. The (loosely fitting, single leaf) garage doors were closed, as was the (hollow) door to the other part of the house.

To try and "calibrate", I set a Mackie speaker in front of it and ran 100hz signal through it at a medium volume and with my trusty Radio Shack db meter (set on "A" weighting) measured 64db right beside the mic.

I adjusted the gain so that the spectrum analyzer (SignalScope from www.faberacoustical.com) indicated "0" at 100hz.

Then I waited for an airplane, took a sample, and then took a sample when there was no "extra" outside noise (this was about 6:00pm, so there were a few birds, but no vehicles, pedestrians or wind).

I'm attaching the two plots, showing only up to 6k hz. In case they don't come out too clear, the "airplane" plot showed the loudest frequency at 46.875hz, signal level of -26, and the "no airplane" showed the same frequency, with a level of -29 dbV.

I played with it for about 40 mins, and saw a high of about -19, always at very low frequencies (35-45).

Does any of this seems valid?

As they are both log scales, can I subtract the dbV from the acoustic measurement? So that, at say, 200hz, where the scope measured -46dbV, this meant it was at 18db inside? (64db - 46)

This is the first time I've used a spectrum analyzer. Let me know if there's something I should change.

Thanks,

Russ

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[RussinParis]

Well, before finding out what I should be doing for the walls (doubling with insulation or not), I'm going ahead with the garage door which needs to be done in any case, and for which I don't have huge choices.

Plan is to construct two-door, two-leaf door with 23mm+19mm MDF on the inside, 110mm of insulation, and 23mm of MDF on the outside, with sills on all four sides and seals. To get the 110mm depth, I'll be using two 55mm boards. All glued and screwed and hopefully calked (when I find acoustic calking in France...)

I'll first reinforce the existing frame (although I can hang on the existing door, and I weight over 100kgs, so I think it should be ok) and beef up the hinges.

I'll post some pics as well; when I was trying to figure out what to do I didn't find any pics of anyone trying to keep their garage door, so maybe it'll help some one.

Hopefully by the time this is done I'll know what to do for the inside walls.

This may also give me a more realistic picture of how bad the airplane noise problem is, because the existing door is far from air tight, so I think most of what I'm measuring is coming through there.

Russ

[knightfly]

That sounds like a good way, as you said those problems will need fixing anyway - it's 4 am here and I'm about to crash, but here's maybe a bit of help on garage door ideas -

http://www.johnlsayers.com/phpBB2/viewt ... &start=105

Check out starting about the 3rd post down on the page... Steve