Q's: Damping&Filling/enclosure/braces/ohms


New member
Username: Parad0x

Post Number: 1
Registered: Apr-05
I have got a few questions actually, and I need some help with them before I finish building my enclosure for my 15" sub.

1) Damping/Filling materials? What are the names of all of these products that I can use, starting with best effectiveness? I understand polyester fill (also known as Polyfill, am I right about this one?, tar spray, fiberglass batting?). I might need this because of two possible factors, my box that i have almost finished building might be playing unwated higher Hz's, and the box enclosure might be too small.

2) Now, i need to know if I will be able to power this subwoofer by bridging two of the channels of this amplifier? Its reguarding ohms, if you guys can explain what the difference is, and when do I need to worry about ohms when it comes to wiring amps+subs.

Amplifier specs:
Low & High Level Inputs
4 Ohm Stereo Stable
2 Ohm Mono Stable
Gain Control

2 x 75 W Rms
2 x 150 W Max
40 Hz Low Pass Crossover

4 ohm
SVC (Its got only one +/- Terminal)

In the specs it says 2ohm Mono? I take it, this is the bridged setting? If indead it is 2 ohms when bridged, sub being 4 ohms, would this be dangerous? If so, does this mean i can only use stereo mode, and only use one channel (while leaving out the second channel?)

2) Okay, my enclosure is it too small? My desire is to have final bass between 35-70Hz. My enclosure is a sealed enclosure, cu ft. about 1.5, I did the calculations roughly after I had cut my wood, which gave me a QTC of 1.5. The website had said this would give me a boomier sound (What frequency does this mean??) If somebody could the QTC for me that would be great as well, and tell me what my QTC is and what frequency range it will be playing at, and what I will need to do to decrease the QTC to get the desired 35-70Hz range.

Cu Ft. 1.5
Fs 24.77
Qms 5.224
Vas 10.06
QTS 0.485

3) Its reguarding bracing the box.. Box material is 3/4" MDF wood, and the box internal dimensions are 18.5"H x 18.5"W x 8.5"D. The subwoofer is placed on the Front piece (18.5"H x 18.5"W). I have put 4 triangle braces on the 4 side pieces (top, bottom, right, left). And I am thinking about doing two things, which I need you guys to tell me if its worth while to do or not.

A) Slap on another 18.5" x 18.5" MDF piece on to the front place, to make it a double stack of MDF wood for the front, where the Subwoofer will be mounted too, making it a final 1.5" MDF piece for the front.

B) Put 4 dowels on the 4 corners of the inside of the enclosure (I would center the dowels towards the middle as close as possible making sure it doesnt' touch the sub), to brace the front piece (where the sub mounts to) against the back? Is this a good or bad move? I read so much about reflective frequencies, that I am worried about this move.

New member
Username: Parad0x

Post Number: 4
Registered: Apr-05
Come on people, its' been like 2 days since I posted this.. I check every few hours to find out nobodys helped me a bit even.

If I can get a few answeres to my questions that would be better then none. If you cant understand what I wrote, i can clarify it better, just let me know what's up.

Silver Member
Username: Rzarector

Coquitlam, Bc Canada

Post Number: 276
Registered: Dec-04
il give it a shot .. first off about the amplifier it seems like a very low wattage to drive a 15" woofer but if the wattage is that low then u probably dont even need bracing for the box..as for the connections if your amp is not bridgeable and a bridge it you will ruin it and maybe the sub(i think) the connections are very important as u can ruin your equipment..try get someone else to help you with that one "isaac" knows alot about amplifiers on here.. a boomy sounding box is a sort of echoing base that isnt very crisp... i just made a 3.8 cu ft box for my 15 .. its very boomy and sounds like shite imo but it sure hits the lows good .. the is a box building program u can punch all those numbers into and it will tell you how the sub will perform and such its called winisd or something .. lots of people on here use it so im sure someone knows it and can tell you. hope that helped u a bit

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 7608
Registered: Dec-03
1) Damping/Filling materials?

polyfill dacron. $2/bag or so from walmart. known as pillow stuffing. add 1 pound per cubic foot of enclosure volume. this gives the box the acoustic appearance of being 10% larger than it actually is.

"Amplifier specs:
Low & High Level Inputs
4 Ohm Stereo Stable
2 Ohm Mono Stable"

got this backward. it's 2 ohm stereo, 4 ohm mono stable.

"2 x 75 W Rms
2 x 150 W Max"

forget max power ratings. pretend they don't exist.
75x2 @ how many ohms? 4 ohms? if so that'll be around 250 watts x 1 @ 4 ohms bridged for the sub.
not nearly enough power for most 15" subs. look for a larger amplifier if you want decent output and don't try turning the amp gain up to compensate for the lack of power. you'll just end up blowing your sub and having to buy both a new sub and a new amp.

"In the specs it says 2ohm Mono? I take it, this is the bridged setting? If indead it is 2 ohms when bridged, sub being 4 ohms, would this be dangerous? If so, does this mean i can only use stereo mode, and only use one channel (while leaving out the second channel?)"

the amp will see half of the actual load when it's bridged. this is why amps are only stable to twice the minimum stereo load when they are bridged. The amp is 4 ohm stable when bridged. If you saw otherwise, it was a misprint. Trust me. Speaking as an electrical engineer, you'll never, EVER see an amplifier that is stable at a lower load bridged than when running unbridged.
that being said, a single 4 ohm speaker on a bridged amp is a fine combo, as long as the amp is big enough for the job.

for a 15" sealed sub, I'd go 2.5cu ft. you want to crossover the sub at about 60Hz. let rolloff take care of 60-80Hz. you also want a flat response to at least 20Hz or lower, as cabin gain will compensate for any rolloff in that direction.
that being said, here's the math you can use:

Optimum volume for sealed enclosure (cubic ft):

You may substitute any Qtc between 0.50 and 1.50 in place of 0.70 in both equations (both must have same value) to experiment with enclosure size. Qtc of 0.70 is generally considered an optimum alignment, with very good transient response, low cut-off frequency, and flattest response to the cut-off - See Qtc.

Note: You must always choose a Qtc higher than the driver's Qts!

Find alpha: X = (0.70 / Qts)^2 - 1

Then calculate enclosure volume: Vb = Vas / X

System resonant frequency: Fcb = 0.70 / Qts ( Fs)

To find the theoretical cut-off frequency, use the following chart to find the F3 factor:

0.50 = 1.55
0.60 = 1.21
0.70 = 1.00
0.80 = 0.90
0.90 = 0.83
1.00 = 0.79
1.10 = 0.76
1.20 = 0.74
1.30 = 0.72
1.40 = 0.71

Then: F3 = Fc x (F3 Factor)

I'd double the front panel if this is a high output sub and/or a very heavy sub (example are my RE subs that are 12" and weigh 60lbs each)
I think your bracing internally is already sufficient. remember to add the displacement of the bracing using geometric formulas for volume of a cylinder when you figure internal volume of the box. you also need to add the driver displacement.

New member
Username: Parad0x

Post Number: 8
Registered: Apr-05
Thanks for the responses guys mark/wolf, yours was really informative though wolf. I'll head over to walmart and ask them for that. I guess its' just pieces of what looks like wool? I presume I just staple/glue this stuff to a few sides, inside the box. As for the amplifier, I think its already half broken because I think the XO on it doesnt work as it plays music/lyrics through the sub with the LPF on so I'll replace the amplifier too.

Its kinda too late with building another enclosure for this sub, but great information. Im gonna save it for future reference when building my next box soon enough, I'll just hope for the best with the polyfill dacron. I Played the sub yesterday for the first time, my lil brother upstairs thought there was an earthquake happening as he ran outside the house for safety LoL ;)

Gold Member
Username: Glasswolf

NorthWest, Michigan USA

Post Number: 7635
Registered: Dec-03
just put the polyfill loosely in the box.
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