Multiple Small Subs - Geddes Approach

[gedlee]

Markus

The modal density at 300 Hz is enormous. Modes are not an issue.

[markus76]

a) Depends on the room size, b) I said "up to" and c) don't forget about boundary effects.

[cap'n todd]

Originally posted by gedlee


Quite true, it is tough to visualize because the wavelength is so long, but there is energy transport and time delay and that to me is a wave.

John and Earl, all true. I was speaking only of the 0th mode though, not the total soundfield. i suppose that is a dangerous, and probably not useful thing to try to do.

[pos]

Originally posted by gedlee



A vented sub cannot excite the oth mode even in a sealed room, but a closed box (monopole) can. You asked about "hearing" and I doubt that anybody could "hear" the 0th mode, but you cann feel it. A car is a classic example. The pressure that you feel on your ears in a noisey car is indeed the 0th mode. It has been found that this pressure, while you don't "hear" it, results in a lot of fatigue and discomfort. Cars that don't have it are more 'comfortable".

Thanks a lot for your answer.

I am a little bit confused with the term 0th mode. I should not have use it myself

Is this phenomenon true for any frequency below the lowest mode the room "supports" ?
I mean, in a small room if the lowest mode is at 40Hz then nothing can be heard under that frequency, only felt, and then only with a sealed sub in a sealed room?

[gedlee]

Originally posted by pos
I mean, in a small room if the lowest mode is at 40Hz then nothing can be heard under that frequency, only felt, and then only with a sealed sub in a sealed room?

This is darn near true and basically true for all "Practical" purposes. If the source is not a monopole then you cannot get sound below the first mode. If it is a monopole then the amount of sound will be directly dependent on how much leakage the space has - and the source strength or course.

[Pan]

Originally posted by pos

Is this phenomenon true for any frequency below the lowest mode the room "supports" ?
I mean, in a small room if the lowest mode is at 40Hz then nothing can be heard under that frequency, only felt, and then only with a sealed sub in a sealed room? [/B]

You can produce sound, hear it and feel it below the lowest room mode, not only with closed box woofers but even with bassreflex or dipole for example.


/Peter

[soongsc]

Originally posted by gedlee



This is darn near true and basically true for all "Practical" purposes. If the source is not a monopole then you cannot get sound below the first mode. If it is a monopole then the amount of sound will be directly dependent on how much leakage the space has - and the source strength or course.

This is one reason why it's beneficial to prevent modes from establishing.

[TRADERXFAN]

Originally posted by gedlee
Markus

That works, but I don't see where "custom" materials are required. Just use two sheets of dry wall, 1/2".

Fill the area between the joists with fiberglass. Attach the RC-1, horizontal as shown. Hang the first sheet and glue (liguid nails)and screw it to the RC-1, but don't use any more screws than you need to. Then glue the second sheet to the first. Today I would use 2-part soft polyurethane thickened with micro-ballons. This stuff never hardens and hold extremely well. This is spread evenly on the drywall surface (lay it on the floor). You can use liquid nails, but this gets expensive and is a pain to smooth out. Then attach it to the hung drywall board with screws to hold it. When setup, remove the outermost screws. The drywall needs to be held off the floor until complete, but just use spacers and then remove them. When done fill all remaining cracks with spray foam (Great Stuff).

It doesn't get any cheaper or easier than that.

I am a bit confused...
Were these instructions specifically to adding a cld layer of drywall resting on a rc1 channels mounted to existing drywalled wall of a room?

Please excuse my ignorance as I have no construction knowledge, but what is the "joist"? I wikipedia'd this and it said it was something that is in regard to the ceiling/roof. So where is the fiberglass supposed to go? Could you please describe in very simple terms?

I didn't think there would be any area to fill with fiberglass if the room was already built. Unless its between the new layer of drywall and the old layer?

As an alternative to the liquid nails, I was looking at Green Glue. Mainly for comparison on cost and VOC. I found that the Green Glue company site has posted some comparisons for CLD constructions versus other methods.

I want to point out that they specifically recommend AGAINST implementing a rc1 channel over an existing wall when trying to target low frequency absorbtion.
http://www.greengluecompany.com/upgr...stingWalls.php

More direct, look at the bottom of this link for conclusion
http://www.greengluecompany.com/unde...TripleLeaf.php



-Tony

[markus76]

Originally posted by TRADERXFAN
I want to point out that they specifically recommend AGAINST implementing a rc1 channel over an existing wall when trying to target low frequency absorbtion.
http://www.greengluecompany.com/upgr...stingWalls.php

Here's data from BBC:
http://downloads.bbc.co.uk/rd/pubs/reports/1995-06.pdf

Best, Markus

[gedlee]

Originally posted by TRADERXFAN


I am a bit confused...
Were these instructions specifically to adding a cld layer of drywall resting on a rc1 channels mounted to existing drywalled wall of a room?

Please excuse my ignorance as I have no construction knowledge, but what is the "joist"? I wikipedia'd this and it said it was something that is in regard to the ceiling/roof. So where is the fiberglass supposed to go? Could you please describe in very simple terms?

I didn't think there would be any area to fill with fiberglass if the room was already built. Unless its between the new layer of drywall and the old layer?

As an alternative to the liquid nails, I was looking at Green Glue. Mainly for comparison on cost and VOC. I found that the Green Glue company site has posted some comparisons for CLD constructions versus other methods.

I want to point out that they specifically recommend AGAINST implementing a rc1 channel over an existing wall when trying to target low frequency absorbtion.

More direct, look at the bottom of this link for conclusion

-Tony

Tony

Yes, I was confusing. They are studs not joists. And if there is already a wall there then the studs aren't available to put in fiberglass.

But lets keep one very important thing in mind. I am talking about LF absorption NOT sound isolation. All the articles above are dealing with sound issolation and absorption and issolation are not the same things.

A far as hanging RC-1 on an existing wall I think that Green Glue is oversimplifying and overstating the situation. They are basically correct in most of what they say, except that there is no way that RC-1 would be worse, just not as good as striping down the wall to the studs and putting it on those - FOR SOUND ISSOLATION! BUT, I would suggest that the RC-1 on the existing wall would act as a very good sound absorber, even if it wasn't the best sound isolator.

The situation is very complex and to talk details you have to understand the details. I don't agree with Grenn Glues comment to avoid using RC-1 directly on a wall as a really BAD thing. That is just not true. There might be better ways to do, but they are NOT as easy to do. Striping down a wall is a pain, just adding a layer with RC-1 is not hard at all.