How do you use fibreglass fill in a sub-woofer box?

[WarrenM]

I have a prefabricated sealed 1.4 cu ft MDF sub-woofer box which I have edge-braced, and lined with mass-loaded-vinyl floated on closed-cell-foam-rubber. With the additional internal bulk of the bracing and MLV, the size of the box ends up being about 1.37 cu ft. The sub which is to be inserted, optimally requires a 1.3 cu ft sealed enclosure, but the graph of its theoretical performance at 1.4 cu ft is practically identical to the graph at 1.3 cu ft (which corresponds to my understanding that a sealed enclosure of up to 20% greater than specified will work about as well for almost all subs).

Unfortunately that totally exhausts my understanding of the theory and practice of building a sub enclosure.

So here are my questions:

* What is fibreglass fill supposed to do?
* How much do you use?
* Is there a formula to correlate the weight or volume of fill to the various sub-woofer parameters and the performance characteristics of the final enclosure?

Thanks in anticipation

Warren in Sydney, Oz

[mikey72]

If u really wanna get that serious, measure out the back wall of the box on the inside to find out the face area.

Say if u need to fill in 50cm3 of space and your face area was 10cmx5cm. You would need a 1cm thick piece of board to fill in the area.

It's really just algebra AxBxC=D or WidthxHeightxDepth=Volume. All you need to do is solve for C.

Then just glue the board to the back of the box!

[trism]

What program are you using to model the response of the sub? Im assuming youre mounting it conventionally not inverted? If so, have you taken into account the volume that will be taken up by the basket and motor of the sub? Depending on the sub, this can get up near 0.4 cuft.

The fill is supposed to replicate the effects of having a larger box by affecting the dampening on the rear of the cone, and therefore affecting the limits of the suspension/spider, and essentially "tricking" the sub into thinking its in a bigger box. Most people just add enough to fill the box without packing it in tightly.

I think most people say it will effectively add about 0.3 cuft to the box

[mikey72]

I still think my drunken mathematics equation will work :D

[trism]

But Mikey, your method is usefully for reducing the internal volume. To do this, all you need to do is add stuff to the inside of the box. Phone books, bricks, small children. Anything will do because they are aall taking up space.

However, the matting is used to achieve the complete opposite: making the box seem bigger.

Sent from my custard cannon like Peter North.

[redefine]

fibreglass fill is a kind of dampening. when the sub kicks, a sound wave exits the front AND the back of the speaker, problem is at certain frequencies the rear wave reflects off the back of the enclosure and interferes with the free moving of the sub. this is why really bassy subs are huge (also more air it moves the bassier) and it also creates spikes and dips in the frequency response sometimes the reflected wave makes the frequency louder (standing wave/in phase) sometimes it makes it quieter (out of phase), fibreglass fill helps reduce the reflected wave and makes the box "seem" bigger to the sub driver, making your bass response smoother.

thats the general case anyway, problem with speakers is that the box has a resonance, the speaker coil has a resonance, the speaker cone has a resonance, the amp has a resonance, the acoustic area your playing it in has a resonance, so often designed boxes to suit a speaker will cancel out peaks in resonance, and its not always better to have a bigger box, especially for budget speakers, but if your going budget you might not necesarily care about a smooth bass response :p

honestly in a car, your not gonna get a perfectly smooth high fidelity bass, the way cars are made mean that theyre absolutely terrible acoustic chambers, so just put some in, bass speakers tend to fill most of the box with wadding, but its still relatively loose.

[WarrenM]

fibreglass fill is a kind of dampening. when the sub kicks, a sound wave exits the front AND the back of the speaker, problem is at certain frequencies the rear wave reflects off the back of the enclosure and interferes with the free moving of the sub. this is why really bassy subs are huge (also more air it moves the bassier) and it also creates spikes and dips in the frequency response sometimes the reflected wave makes the frequency louder (standing wave/in phase) sometimes it makes it quieter (out of phase), fibreglass fill helps reduce the reflected wave and makes the box "seem" bigger to the sub driver, making your bass response smoother.

thats the general case anyway, problem with speakers is that the box has a resonance, the speaker coil has a resonance, the speaker cone has a resonance, the amp has a resonance, the acoustic area your playing it in has a resonance, so often designed boxes to suit a speaker will cancel out peaks in resonance, and its not always better to have a bigger box, especially for budget speakers, but if your going budget you might not necesarily care about a smooth bass response :p

honestly in a car, your not gonna get a perfectly smooth high fidelity bass, the way cars are made mean that theyre absolutely terrible acoustic chambers, so just put some in, bass speakers tend to fill most of the box with wadding, but its still relatively loose.

I've come to the conclusion that fibreglass fill will be of no benefit to me for two reasons - the box is already slightly too large (not enough to significantly affect its frequency response); and I am already using Mass Loaded Vinyl over the inside of the box to massively dampen all back reflections.

I haven't listened to the box in my car yet - it goes in next week - but the combined effect of the MLV and the high quality driver I am using is that even midrange sounds very convincing through the sub, added to my office sound system.

[tron07]

Hi Warren,
Didnt really go thru this thread, but here is my 2 cents, hopes it helps.
If you find your box is too big, just put some MDF blocks inside. Put a telephone book or 2 inside to test and find which one suits you, then cut and measure MDF blocks and stick em inside the box.

I would design the box to be slightly smaller then trick it, want to save on boot space.
I used to put pollyfill as well as acoustic sponge or cascade deflex pad at the back of the box. I find it sounds nicer, and not produce heart thumping bass but softer and more rounder type of bass. Not sure how to describe how it sounds. For polyfill, just go find a polyfill pillow, rip it up and stuff it in the box. Less itchy and doesnt cut compare to fibreglass fill (but it could be similar to the fiberglass fill you are using??) I spend like a month time tuning the box, making sure it dont leak, trying deflex pad, stuffing and removing polyfill, etc

[tron07]

Forgot to add, calculations is just a start to get you going.... after that, its all listening and testing, see how the sound corrospond to the frequency response of the car. You might get fantastic calculation or linear measurement, etc but if it clash with the frequency respnose of the car you will get poor sound.