Re: Re: Re: Re: Re: DC mode
Todd,
Lumped parameter analysis is based on the assumption that the wave length is large compared to any room dimensions, therefore pressure variations can be neglected because they are small. It doesn't say they don't exist. It doesn't change the real physics. There is always a wave. But most of the time we can ignore it. However, in the case of the DBA we have a time scale in the problem, the delay of the rear woofer array, that is on the same order of magnitude as the propagation time associated with length. Thus while the pressure may remain nearly constant in the room, we still have to correctly account for the fact that any movement of the front woofer launches a disturbance that propagates to the rear wall over a time of L/c. The fact that the change in pressure over that time increment may be minuscule doesn't mean it happens instantaneously.
If you look at a simulation of the DBA for the ideal rigid room with no losses what you find is that the pressure at low frequency is nearly uniform everywhere, but when you look at the phase at a given low frequency it varies with position due to the propagation delay.
It similar to a dipole. If you don't account for the delay between sources you get zero pressure every where instead of a pressure that drops off a 6dB/octave with decreasing frequency.
cap'n todd said:
Todd,
Lumped parameter analysis is based on the assumption that the wave length is large compared to any room dimensions, therefore pressure variations can be neglected because they are small. It doesn't say they don't exist. It doesn't change the real physics. There is always a wave. But most of the time we can ignore it. However, in the case of the DBA we have a time scale in the problem, the delay of the rear woofer array, that is on the same order of magnitude as the propagation time associated with length. Thus while the pressure may remain nearly constant in the room, we still have to correctly account for the fact that any movement of the front woofer launches a disturbance that propagates to the rear wall over a time of L/c. The fact that the change in pressure over that time increment may be minuscule doesn't mean it happens instantaneously.
If you look at a simulation of the DBA for the ideal rigid room with no losses what you find is that the pressure at low frequency is nearly uniform everywhere, but when you look at the phase at a given low frequency it varies with position due to the propagation delay.
It similar to a dipole. If you don't account for the delay between sources you get zero pressure every where instead of a pressure that drops off a 6dB/octave with decreasing frequency.
pos said:
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".
At my request, the developer of SoundEasy has provided me with a special version which allows FEM simulations of DBAs. So, I will be able to simulate DBAs in both rectangular rooms and room with of more general shapes build from rectangular blocks. (L shaped rooms, rooms opening to other rooms....)
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.
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.
gedlee said:
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?
pos said:
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.
gedlee said:
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/upgradingExistingWalls.php
More direct, look at the bottom of this link for conclusion
http://www.greengluecompany.com/understandingTripleLeaf.php
-Tony
TRADERXFAN said:
Here's data from BBC:
http://downloads.bbc.co.uk/rd/pubs/reports/1995-06.pdf
Best, Markus
TRADERXFAN said:
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.