A shot in the dark

[Domsz06]

i always make them on the smallest size and stuff, never had a problem.

[jleger98]

good to know.

you see the mythbusters where they made a 51" subwoofer in a car?

[Domsz06]

yeah I saw that, it was SICK!

[jleger98]

they screwed it up though. They didn't account for the horizontal travel of the rod on the drive shaft. They needed to ensure that the rod went up and down only. THEN you woulda seen some s**t

[Domsz06]

yeah, i laughed when I saw that. I love their show, but some of the things they do I'm like, you forgot do to this or that, it's funny! I miss the discovery channel.

[Domsz06]

engineering marvels was another one i really liked, it's so freaking cool watching them build some of the stuff they do

[spharis]

Polyfill causes negative effects on sound waves, and your driver actually underperforms when it is in a polyfilled enclosure....although it may sound better. It is an absorbtion material, usually used to dampen the standing waves in an enclosure. It "warms" the sound. A couple of inches of polyfill lining will help "fix" a poor box design.

Say you built a box and it is a perfect square, when the bass hits it sounds flat. Adding polyfill will cause the sound waves inside the box to become irregular, in effect regulating standing sound waves. A standing sound wave is one that cancels out due to an opposing wave. This is real common in hatchbacks, and trunks. It is also sometime used to control the Q value os the rolloff of the driver. A sealed enclosure works by using the backpressure of the box to regulate the drivers excursion. Adding polyfill helps dampen the excursion of the driver in cases where it flexes too much. It sort of pushes the response curve of the woofer per tone up or down because of the change in backpressure.

Found a picture....see how the return wave reflects back a mirror of the sending wave, this cancels each other out, the result of a perfect standing wave is silence.

[jleger98]

So if i have a sealed box of about 1.5 cu feet or so and a 10" SW, polyfil? or no?

[Domsz06]

sp

I had no idea, wow. Thanks for the info, I won't be adding any polyfill to my new boxes, wow and all these years......

[spharis]

So if i have a sealed box of about 1.5 cu feet or so and a 10" SW, polyfil? or no?

depends on how it sounds. If you feel it sounds flat, then you can try polyfill. You only need a few inches though unless you are trying to adjust for internal volume, and even then there are better methods than polyfill. Some people use polyfil to remove some enclosure volume, but this will also limit the backpressure on the driver, reducing its response curve.

Polyfill will in most cases tighten up the response somewhat. I get the biggest kick out of drummers who stuff their kick drum with pillows and poly......they could tighten the heads, and get the tighter response without the reduced Q values or rolloff.

sp

I had no idea, wow. Thanks for the info, I won't be adding any polyfill to my new boxes, wow and all these years......

Lining with a couple of inches is good in some case, but I would add it, listen, then remove it and listen to see if it was really needed.

[Domsz06]

yeah I'll have to look into that. Thanks though, that makes sense, I just never thought about it that way

does a box have to be square? I mean is it ok to make curved boxes?

Also when making a sealed box for 2 10's, do I need to separate them with a wall as well?

What about a ported box?

[jleger98]

cool

[Domsz06]

i love these threads, you can learn so much!

[spharis]

yeah I'll have to look into that. Thanks though, that makes sense, I just never thought about it that way

does a box have to be square? I mean is it ok to make curved boxes?

Also when making a sealed box for 2 10's, do I need to separate them with a wall as well?

What about a ported box?

Ported boxes are a whole new topic, but you should build a ported box around the tuning frequency you want.

Boxes can be any shape, but certain shapes are going to have adverse effects on the waves inside the box. I usually try to use right angles, or slight diagonals. Bose spends millions on developing enclosures to account for the sound waves inside the box.....there is too much science involved for me. It's easier to build what you want, then tune it up a little. It's going to be so loud it really won't matter that much in the end. The main thing is to get the driver to sound right.....IE not restircted or flat. Minor changes can fix this, like a small amount of polyfill, or adding bracing in the corners of the box.

The biggest killer of a good sounding sub is dead spots in the innner space (flat waves) and poor box materials/not enough rigidity. If the box flexes, the driver is not performing as it could, and obviously if the notes are cancelling internally, the backpressure is resricting the excursion......flattening the wave you hear.

Sealed boxes with a split is again used to help control standing waves. If you have a frequency with a 1 ft wavelength, and the drivers are 2 feet apart, then you will get a standing wave between them. A separation removes that chance. That said, I have heard amazing sound out of multiple drivers in a single space.

People who build awesome boxes have just built enough of them to know what works, or they got lucky. KNowing what works is usually the best method, but that involves alot of trial and error. I still don't know what works, but I try.

[jleger98]

I'm using 3/4" Poplar board. screws every 2 " and some glue on the seams, and sealed all the seams on the inside. I also added a "back plate" behind the speaker, so I got 1 1/2" there. seems pretty rigid.