Best way to design a subwoofer enclosure

[soundNERD]

I built a subwoofer enclosure, with the amps mounted inside it next to the speaker, and would like to know which configuration will produce the loudest sound with the least power:

Currently every opening is sealed with sealer foam. The sound is decent, but not when the volume is cranked. I noticed that when the back cover is removed, the sound is almost twice as loud as with it closed. The speaker also moves more and easier. I thought about the following changes and wanted to know which works best:

1. Leave it the way it is
2. Install a 2" port on the bottom left and right of the front
3. Install the same ports, but on the top
4. Something else? Maybe a different sized port, etc.

Thanks for the help, Mike

 

[SkinnyBoy]

Throw the box in the bin and start again.. this time designing the box properly....

Sorry. I'm in a bad mood...

but that is the answer...

 

[navin]

if you want to salvage teh box (if it is already finished) I would try putting ports on the rear. if that is too difficult try an aperiodic vent (all you need is a 4" dia x 2" tube filled with opencell foam).

do you have T/S specs of the driver? have you measured the driver? often the publsied specs are off by a lot.

a port is usually a port the location wont change much. yes some designs couple the output from the port to room boundraies (rear or floor) it serves the double purpose of making port noise less audible.

when you say "The sound is decent, but not when the volume is cranked" what do you mean? it there a lot of boom? when yo mean louder is the bass quality better too? what music do you listen to?

 

[AndersZ]

Leave the back off and and use an equaliser to raise the response by 12dB between 32 and 124 Hz (5dB/oct for 2 octaves). Then you've made a quick and dirty dipole woofer. The reason for it to play twice as loud is the dipole radiation (sum of front and back waves) and the easier cone movements because there is no air compressing behind the driver. When you feel like it you could chop up your woofer and make it a H-baffle and eventually use the Linkwitz proto woofer eq.

For more info you could check Sigfried Linkwitz's dipole prototypes on the Phoenix section of his site http://www.linkwitzab.com or my page
http://www.t.kth.se/andzak54 for more info on how I'm doing the dipoles (digital crossover/eq on the soundcard).

Good luck

Anders

 

[navin]

are dipoles twice as loud as monopoles?

if so then how come one sees so few dipoles esp since it aleviates teh problem of calculating box volume?

if dipoles are twice as loud why the boost?

i have no exp. with dipoles but would love to learn. linkwitz presumes i know more than i do.

 

[AndersZ]

They have twice the sensitivity at high frequencies (see linkwitz design models), but they cancel themeselves at lower frequencies and as a result the maximum SPL is lower than for a driver in a box, if the box allows the driver to have a flat response. SL has made an excel spreadsheet that let's you calculate the maximum SPL with a given driver (cone area and cone travel are required) and baffle width.

To sum up: Dipoles can be made to have a flat response to "any" frequency but is limited by it's volume displacement capability. So, a phoenix style woofer with two 12" drivers standing on the floor can play 86dB at 25Hz, or with two of them 98dB which should be enough for most rooms (equal to a subway train passing by at one meter)

Anders

 

[soundNERD]

I really would like to keep my current case. It is made nicely out of cherry wood. What I mean is that when the volume is turned up, the sound output isn't as high as I would hope. It distorts before it reaches a level I would like. I noticed that removing the back cover made the sound better, so I assumed that a port would too. All thr circuitry is mounted along the right side of the case, so that leaves plenty of room for the port. Will this increase the sound out of it as much as I would like? I also like the idea of using a port because the amp would be kept cooler, and air leakes in it won't affect anything. So, can a port help me, or would it just be adding more to the case?

 

[navin]

They have twice the sensitivity at high frequencies...but they cancel themeselves at lower frequencies and as a result the maximum SPL is lower than for a driver in a box...Dipoles can be made to have a flat response to "any" frequency but is limited by it's volume displacement capability...two 12" drivers standing on the floor can play 86dB at 25Hz, or with two of them 98dB which should be enough for most rooms (equal to a subway train passing by at one meter)
Anders

thanks for the simplification. SL is too high brow for me. just one clarification....yuo sid 2 x 12" can make 86db at 25Hz or with 2 of them 98db does this mean one needs 4 x 12" for 98db? How does thi compare to a well designed TL or sealed box? I am only considering dipoles as there are claims that it reduces boom due to standing waves.

i am thinking of making a small dipole using 2 x 8" with the box having a front and sides but no back. i also hope to make the sides curved as per the latest trend so the rear will effectivly be only 4" wide not 10" as the front. does this make sense.

I really would like to keep my current case. It is made nicely out of cherry wood.....I also like the idea of using a port because the amp would be kept cooler, and air leakes in it won't affect anything. So, can a port help me, or would it just be adding more to the case?

from what little i know a port and box must be designed from scratch to work with a woofer it take more work to desing a ported box than a sealed box or even a TL. you have to be extremely lucky to plug a port in a sealed box design an get anything that is accurate. I hence feel that maybe you box is over damped and a aperiodic port will reduce the air pressue with involving port - box dynamics (fb, vb, etc...).

many years ago dynaudio (today SS also does this) sold little plastic vents filled with foam or fiber wool and called them aperiodc vents. I thknk you can do the same by cutting a 3-4" dia hole in the wood and filling the hole with open cell foam. if you want a more resisteive aperiodic vent you can insert a 2-4" long port in the hole and fill that with open cell foam.

as far as keeping the amp cool jus tmake sure the heatsink is available for ventillation. th rest of teh amp can be kept "indoors".

hope i am right and hope this helps.

 

[AndersZ]

does this mean one needs 4 x 12" for 98db

Yes, you could either make it a pile of 4 drivers or two "boxes" with two drivers. However, there is a point two have two separate woofers as explained by David Griesinger and SL on his Surround page.

By making 'em with no back reduces the peak that can be seen (and heard!) around the frequency where the sensitivity equals that of a monopole, the Fequal in SL's models. Fequal is determined by the distance from the front of a driver to the back. By making the sides deeper the woofer can go deeper but you shouldn't use a H-baffle above that frequency Curved sides should work just fine, it's just the thing I'm going to do next with my dipoles. Two 8" works for a speaker to reach 50Hz or so.

How does thi compare to a well designed TL or sealed box?

A monopole with a single driver sitting on the floor (same premises as the dipole I mentioned, but with one driver of the same kind) can put out 94dB/25Hz if the box has a -3dB point at 13Hz or so.

So for the woofers the befefits are no bass boom, natural bass, small footprint, good frequency response and better dispersion while the downside is less SPL and the cost of more drivers (although cheaper could be used for almost the same performance).

Hope this helps,

Anders

 

[navin]

i was thinking of making sides that are about 12" deep

i intend to use teh woofer below 80Hz (sub out of my AV amp)

I get a lot of boom and it irritate me so that i dont use the sub at all now

BTW what if I just leave the back of teh speaker open. the sides are 14-16" deep anyway will that be a dipole too?

 

[AndersZ]

If you leave it open it will be a dipole too, it's just that without equalisation the frequency response drops. However, with sides that are 14" deep the Fequal will be fairly low so it could be used without eq if a non flat response is acceptable.

It's all up to you!

Anders

 

[navin]

thanks.