I just built two small Cabs for D-130,s ???

Gary Steele · Post by **Gary Steele** » 18 Nov 2000 10:15 pm

I got all the specs from JBL. But can you over do the Fiberglass insulation? Does the speaker know how thick the Insulation is? They are small cabs 17X17X14 deep I have a bunch in there But i need to know if the speaker thinks the Cabinet is as small as the insulation. Anyone know forsure? Thanks much. Gary

Jack Stoner · Post by **Jack Stoner** » 19 Nov 2000 4:16 am

The specs for the cabinet should have taken into consideration the insulation. I think the standard R4 that's used in walls is sufficient for all the sides except the speaker mounting board. If you have it ported correctly it should work fine.

Jay Ganz · Post by **Jay Ganz** » 19 Nov 2000 9:23 am

Hey Gary,

I've built a bunch of speaker cabinets in the
past 20 yrs. or so. I follow the rule that
I've seen used in most fully enclosed &
ported cabinets. About an inch or two of
insulation on the back, one side, & either
the top or the bottom. So that's 3 sides
of the interior. Too much insulation soaks
up too much sound. Are your cabinets closed
or open back? If they're open, then of course, you don't need any!

Donny Hinson · Post by **Donny Hinson** » 19 Nov 2000 3:53 pm

No more than a couple of inches is required in cabinets as small as yours. 1 1/2" to 2" should be fine. If you were using medium-density foam rubber, 1" would be fine.

Al Marcus · Post by **Al Marcus** » 19 Nov 2000 8:20 pm

Gary-
How much does one of your cabinets weigh??..al

Gary Steele · Post by **Gary Steele** » 22 Nov 2000 1:32 am

Al, They aren't done yet. Will let you know.

Terry Downs · Post by **Terry Downs** » 22 Nov 2000 11:13 am

Gary,
I don't think anyone answered your question.
<BLOCKQUOTE><font size="1" face="Verdana, Arial, Helvetica">quote:</font><HR><SMALL> But i need to know if the speaker thinks the Cabinet is as small as the insulation. Anyone know
forsure? </SMALL><HR></BLOCKQUOTE>
Insulation filling will actually ADD to the effective volume "seen" by the driver. This seems counterintuitive since the insulation takes up space. This exists because of the thermodynamics within the cabinet. An unfilled cabinet is an adiabatic thermodynamic process. When the driver compresses the air, both the pressure and temperature are raised. If you fill the cabinet with fiberglass insulation, the fiberglass will absorb the heat created by the air compression. Now the thermal mass of the fiberglass keeps the temperature constant. The equilibrium state is the same with or without compression. You now have an isothermal system. Adding the thermal mass lowers the resonance of the box, so a smaller box can be used to achieve the same resonance. I have never measured and verified this, but some technical papers will claim that you can increase the apparent size of the box by 15% with dacron and up to 18% for fiberglass.

The main reason why you use insulation in a box is to reduce standing waves. Reflections of sound off walls and features of a box will mix with other waves. They will add or subtract depending on phase and time delay. A spherical enclosure would be the worst since surfaces are equidistant. A square box has a lot of standing waves. This family of standing waves adds to the sound of the box, especially in the midrange region. You don't want the box creating sounds of its own. The standing waves can also excite resonances of the panels, making additional noise. Some speaker manufacturers add bracing in the cabinet to break up the standing waves. You can completely fill the box with fiberglass if you like. Thiele documented that damping the vent (port) with filling will reduce efficiency, so keep it out of the port if your design has one. Foam products will accomplish suppression of the standing waves if they have acoustical absorption properties, but they will not have the thermal properties like the fiberglass. This is why foam is not recommended.

So the answer to your question is, the speaker will think it is in a slightly bigger box with the insulation. I would recommend assembling your box with out the filling first and listen to it. If you are building two boxes, assemble one with and one without. The box without filling will sound louder, but the extra volume is the added coloration caused by the standing waves. I think you want to hear the beautiful sound that your steel guitar created, not sympathetic vibrations of a wooden box. I hope this helps.

------------------
Terry Downs
http://nightshift.net
terry@nightshift.net

Jim Smith · Post by **Jim Smith** » 22 Nov 2000 11:44 am

That certainly clears it up for me!

Gary Steele · Post by **Gary Steele** » 22 Nov 2000 6:48 pm

Right on Jim. TERRY that is a super response. Guys please tell your little secrets so everyone can do better. If we share things It will come back to us. Thanks!
Gary

KENNY KRUPNICK · Post by **KENNY KRUPNICK** » 23 Nov 2000 12:08 am

In reading this ,it does make sense.With out the insulation,I guess you could say it's like a mini echo chamber inside the speaker enclosure.

Gary Steele · Post by **Gary Steele** » 23 Nov 2000 8:15 am

Close this up bob. The speakers sound good and are closed up now and i want to quit talking and use these Babes I mean JABES. Well i guess they can be called either name. They are little but their loud, AND HEAVY. Made out of MDF board. THEY SOUND GREAT!!