Manifold Subwoofer

[moniker]

I have a need for subwoofers in the ceiling.

I was planning on using the RS315HF in an Acoustic Suspension mode but think it might not be a good idea to have the driver facing down due to gravity.

So I came up with a concept of a manifold to direct the sound downward as in the attached pic.

Is it ok to do it this way? Any warnings or suggestions?

Thanks

[---k---]

Have you heard of an infinite baffle?

http://home.comcast.net/~infinitelybaffled/

[Gir]

If your going to have it up in the ceiling, why not make an infinite baffle out of it? Other than that it should work. You may want to think about two opposing drivers to cancel out physical vibrations in the ceiling joists.

[moniker]

Yes I know about IB designs. But in this situation, all HT sounds must be minimized outside the HT rooom. The backwave from the subs has to be enclosed and stopped from affecting other rooms through the ceilings. :M

I have used the Room Mode Calculator from FRD and got equivical results. I am not sure if one ceiling source is better than more than one or which locations make the most sense. I have tried the program with mirror imaged setups and get different results. That should not be so if the speakers are modeled as point sources. Does the program require "aiming" the speakers in a certain direction? :huh:

I am not sure how much driver volume displacement I need for the room that is 19' W 15'6" L and 12' H. That is about 3,500 cub ft. For example, if I want 105 db at 30 Hz 10' away from the manifold, what displacement do I need from the drivers to support a 3,500 cub ft room? :huh:

Is there a calculator anywhere for SPL vs room volume vs speaker displacement vs frequency vs distance for a manifold or plenum design? :huh:

Thanks :T

[Dennis H]

For 105dB in a 3500 cu.ft room, below the lowest room mode where the room is being pressurized, you need about 7 liters peak-peak displacement or 3.5 liters 1-way. That's about 5 of the RSS315.

[moniker]

Dennis, thanks for the suggestions. :T

Hope its ok to ask all these questions. I want to feel confident about putting the thing together cause after construction there will be no chance for modification. :roll:

Is it fair to say that for any frequency below the lowest room mode you need 1 liter of Vd (one way) for every 1,000 cub ft of room volume to achieve 105 db spl?

Is there a site that explains the derivation of this rule?

Thanks :T

[Dennis H]

Is it fair to say that for any frequency below the lowest room mode you need 1 liter of Vd (one way) for every 1,000 cub ft of room volume to achieve 105 db spl?

Is there a site that explains the derivation of this rule?

Yup it's the ratio of speaker displacement to room volume that matters. It comes from the definition of dB SPL which is a certain pressure in Pascals which converts to a certain pressure change in atmospheres or the ratio of how much the driver moving changes the room volume. The formula works out to:

dB SPL = 194 + 20*log(Vdriver/Vroom)

where Vdriver is the 1-way Vd. Vdriver and Vroom need to be in the same units, e.g. 1 cu.ft. = 28.3 liters.

PS, I worked the formula out on my own and wasn't sure about it until I saw Linkwitz uses the same one buried in one of his spreadsheets so I knew it was okay.

[moniker]

Denis,

Thanks very much for the equation. :T

It is interesting that below the lowest room mode the equation does not involve frequency but in free space, half space, quarter space and eighth space the equations are frequency dependent. Why is that so?

Thanks :T

[Dennis H]

Well, the equation is a bit of a simplification because it assumes the room is airtight. If the room were perfectly sealed you could pressurize the room at DC by hooking a battery up to the drivers. Below the lowest mode, the room is too small for waves to fully form so it's acting more like a pressurized container. One way to look at it is room gain cancels out the driver's normal rolloff with frequency. Real rooms aren't perfectly sealed but the equation still gives you a rule of thumb for the minimum number of woofers you need.

[moniker]

Thanks for the info.