Fibreglass boxing and optimal sub enclosure size

[someonestolecc]

I'm toying with the idea of making a custom box in fibreglass for my boot. I'm comfortable with my grasp of the fibreglassing however I'm concerned about sound quality.

Basically it will be for a 10" PPI single driver. I've had a few ideas of how I could install it nicely but I'm wondering if there are any basic guidelines of what I should and shouldn't do.

1- need to determine the optimal size as currently my sub is very boomy and produces great bass for hip hop, but does not produce enough responsive bass when I listen to hardcore dance music. I'd like to correct this and determine the optimal size for the enclosure.

2- having worked out the enclosure, I'm wondering does shape matter? One of the setups I've been considering is to have the box fit under my strut brace, but this would mean it doesn't have much clearance behind the drive on the inside of the box and I'm wondering if this is bad.

3- does it matter about sound bouncing around inside the enclosure? this relates to the question 2.

4- should I be pointing my sub into the cabin, out the back of the car, sideways, reverse mounting?

Thanks.

[Chris Rogers]

download blaubox off the net and have a play with it. as long as you have the volume behind the sub correct it *should* work ( I'm not building the box hence *should*) may want to pop a bit of SD in it as well.

[someonestolecc]

Thanks for the tip I'll check it out.

I've read that SD increases the volume of the enclosure (for the soundwaves) is this a legit method for fine tuning the box after it is built?

Ta

[Chris Rogers]

dacron does. sd will stop it ringing IF it does at all.

[someonestolecc]

Thanks

[mokompri]

port length and diameter are very important for reverb the sub gives, this should be given by the program your using to calculate box volume with, i personally use WSID, its quite good and lets you model the gains in different frequencies quite easily, also gives port length and diameter needed

[Fhrx]

Okay someonestolecc, I'll address your points in your original post one at a time.

1. Sadly you cannot simply chuck any sub into any enclosure. What you need to do is take into account how loud you want it, what you listen too, the available power on tap and most importantly, the Thiele / Small parameters of the sub driver. Once you have all these determined it is often easier to chuck them all into a program but if you want to email me I can explain how to do them all with a calculator.

2. If the enclosure is sealed than shape does not matter at all so long as there is sufficient room behind the PPI driver to let the pole cent flow air. If you don't leave sufficient space you risk overheating the sub because it cools through the vent. Now if the enclosure is ported then you also need to make sure you leave enough room for the air to flow freely from the cone backside to the port.

3. See question 2.

4. Which way should you aim your subwoofer? The aiming of subwoofers has been quite a topic of speculation for years now in car audio. Through experimenting many people have found that their subwoofers sound much better when aimed backwards. Many people realise that there seems to be much more bass with the boot open than with the boot closed. We've heard a great deal of strange and utterly incorrect theories to explain this phenomenon.

The main reason this phenomenon occurs is all about sound waves (direct and reflected) and more importantly the cancellation of these waves. The diagrams below assist in showing the sine waves and their phase relationships between the direct sound wave entering the car and the reflected wave that hits the back of the boot and reflects forward. Since the reflection is bounced into the listening area one can treat them much the same as having two sources.

In the above picture the vertical black line at the left of the picture is the boot rear panel (the beaver panel the tail lights are mounted on). This picture is an illustration of what happens when sound comes out of the front side of the subwoofer enclosure. Sound travels forward into the car (the red wave) and also backwards to reflect off of the back of the boot (the purple wave). Both the direct wave and reflected wave get to the listener but they are way out of phase causing enormous cancellation in the listening area. At this stage if you opened the boot the reflected wave would disappear and NOT reflect back into the car, thus resulting in no cancellation

In the picture above picture the speaker box has been aimed at the boot instead of the rear seat and you can clearly see the direct and reflected waves are not nearly as much out of phase as in the first example. This resulting in much better bass reproduction.

This picture above represents the subwoofer enclosure being moved to the rear of the trunk with the subwoofer aiming forward. The waves are a little closer to being in phase with each other.

In this picture (above) we're aiming the rear mounted subwoofer enclosure at the rear so the direct and reflected wave are very close to being in perfect phase from the start and hence they reinforcing each other quite well.

Note; these pictures are simulated using a 60Hz note with the rear of the box mounted approximately 3 feet from the back of the boot. Keep in mind we're only discussing the direct and rear reflected sound in an effort to try to simplify this. The reflecting sound waves in a car are much more complex than these drawings indicate but we must start simple before getting too carried away. This explanation should be a nice foundation for those of you who wish to study this phenomenon further. One other thing we should mention is that before people comment that this cannot be true because the interior of cars being small in relation to bass wavelengths, the full wavelength does not have to completely develop to be in or out of phase with its own reflected sound. The pictures above are showing a 60Hz wavelength and the bounce distance to reflect back out of phase a complete 180 degrees is just over 4 foot. At higher frequencies the distance is less (120Hz is 2.3ft for example).

Hope that sorts your questions.

[someonestolecc]

Thanks for the reply mokompri and Marty - very detailed thank you.

Thanks for going through point by point.

I do recall reading up on that sometime a while back and studying the diagrams you've posted - I think it may have been your article actually.

I've looked at blaubox and I can't seem to make sense of it. I mean, I can work it, I'm just not entirely sure of what it's telling me.

1. Sadly you cannot simply chuck any sub into any enclosure. What you need to do is take into account how loud you want it, what you listen too, the available power on tap and most importantly, the Thiele / Small parameters of the sub driver. Once you have all these determined it is often easier to chuck them all into a program but if you want to email me I can explain how to do them all with a calculator.

How loud do I want it - I like to have my eyes blur. I think I've mentioned this before to you but until I can afford my idmax and 1.400 I have to stick to this

What I listen to - hip hop and hardcore dance music (fast responsive bass). Unfortunately I think these two music styles are opposed when trying to build a system. As it stands the system is fine for hip hop. It's a joke when I try to play 150bpm hard hitting Euro hardcore and old school trance with 808's and 303's.

Available power on tap - Actually this is a strange one. It's being run by a noname amp that according to calculations (my mate who sold it to me measured the current it was drawing) it *should* be able to pull 400wrms or so. I'm not sure at what ohm but considering I paid $50 for it it can't be *that* good. As I understand it this simply means the enclosure can't be too small because the amp can't push/pull the woofer fast enough to be responsive.

The thiele/small parameters - What the? I can't seem to find the PPI driver on the net. Can I work this out somehow?

As far as I understand it, I need to make the enclosure smaller than it is at the moment, so that it's more responsive, but that assumes that my amp is powerful enough to drive the woofer properly.

The guy who sold me the sub+box said he made the box using calculations from a program. I'm scared that he's made it optimal already, but on the other hand I refuse to believe a sub could be so sloppy that it can't keep up with 120-150bpm music.

Plus I'm itching to try fibreglassing - seems I could make it lighter than the mdf in my boot currently, and reclaim my bootspace

Consequently any extra help (or spoonfeeding (it feels like)) greatly appreciated :innocent:

[Fhrx]

The PPI flat piston subwoofers are very good subs and they can sound very controlled and tight in the correct enclosure. Especially the 10" if it has sufficient power running it.

Acording to the info I have on the Skyline and the program I use (WinISD), the enclosure (if sealed) should be around 0.422cf to achieve a good sound quality Q (0.7).

But anywhere up to 0.9cf will work, the Q just goes down to 0.55.

[someonestolecc]

I'll check to see exactly what size woofer it is. I'm pretty sure it's a 10". How do I tell if it's the flat piston type?

I don't actually have a skyline it's in a cefiro - I can measure the boot if that helps?

Thanks again fhrx!

[Fhrx]

How do I tell if it's the flat piston type?

Because it will look like this:

[someonestolecc]

D'oh that's not it. Mine says PPI on the front I think.

Perhaps it was a look-alike brand it was sold to me as -

Precision Power 12" Subwoofers. They are inside a closed inclosure box

model: PP124

Good budget SQ Sub.  

size: 12"

Nominal Power Handling 300 Watt

Peak Power Handling 500 Watt

Impedance 4 ohm

Admittedly I didn't pay that much for it but I'm pretty sure it's the sub attached to this post.

[Fhrx]

No no, that sub is a PPI sub,. I was just assuming when you said flat piston you were talking about THE 'Flat Piston' PPI subs.

For one of those I get 0.977cf.

[someonestolecc]

Hay you mentioned flat piston subs first!! hehe

Thanks very much for the volume. Hopefully this will improve my response and at the very least if it's the same as it is now I'll be happy to have some more bootspace

Is there anything else I need to know before I start? I might give this a go in the next month or so as soon as I have some more money to buy fibreglass and paint etc and think about the design of mounting amps etc. When I do I will post up pics.

[Fhrx]

Hay you mentioned flat piston subs first!! hehe

True, my bad.

Is there anything else I need to know before I start? I might give this a go in the next month or so as soon as I have some more money to buy fibreglass and paint etc and think about the design of mounting amps etc. When I do I will post up pics.

I can offer a few pointers for fibreglassing. Making fibreglass and carbon fibre (including Kevlar) parts in not overly hard, it is just time consuming when you have to sand the thing to perfection.

I think one of the most important things to remember when creating fibreglass enclosure is preparation. Some things to consider are:

* What do you want the final enclosure to look like?

* Will you need a basic guide frame to hold the shape it up because the glass get heavy and sags once the resin in on it. Many people forget this.

* Have you double masked everywhere where it could potentially drip? I.e., not just under the enclosure itself. What about too and from the car?

* Have you considered ventilation. I've seen one or two installers knock themselves stupid quite a bit whilst trying to glass inside cars without ventilation.

* Make sure you have plenty of rags, presol and acetone etc on hand to save any potential spills. Better to assume the worst and have it close at hand rather than rushing around looking for it once the resin begins going off.

* Make sure you put quite a few layers on because high quality subs can push titanic amounts of air pressure when angry.

Can I assume you've already know all about the mixing process and protective gear like gloves etc?

Post picces as soon as you can!

[someonestolecc]

Thanks Marty that's great pointers.

I did not think about it sagging actually - I have noticed this. Is chicken wire a good mitigating strategy? I was thinking of taking the initial mould from the plug in 2 or more parts. Once I have the basic design rip the chicken wire out of it (I don't like the idea of unnessary parts being left on the inside) then put it all together with more fibreglass.

I'm familar with the mdf ring and dowel technique that's on the net so I'm trying that. Then stretch the fabric over it and keep glassing.

I don't usually use gloves, I might start. I have a respirator that's rated for asbestos so I'm assuming fibreglass particles aren't smaller than asbestos - otherwise I'm screwed.

How many layers do you suggest? I was thinking 5-10 light ones but I always don't want to make it too heavy.

Will I need to sand between fibreglass/resin applications if it dries? I've read this somewhere but it didn't seem necessary.

Thanks again. Pics will be up when I do it step by step for others

[Fhrx]

In answer to your questions champ:

1. Yeah chicken wire is fine. There are many different ways to get the shape because remember stretching cloth only works with certain shapes. E.g. try stretching cloth to get this shape:

2. As for strenght testing, we use a highly scientific, highly advanded, secretly developed motorised testing system on out enclosures made from glass; I take em out on the driverway and drive over em! :D If it takes that, then it's strong enough.:crazy:

3. Test for leaks and volume by slowly filling it with water one litre at a time.

4. An asbestos mask should be fine for when you're sanding it but I was referring to when you actually apply the resin; just be careful the fumes don't get to you.

5. When making the enclosure give each layer a light sand between coats just to remove little nicks and pricks sticking up.

[someonestolecc]

Awesome. Thanks. With that I should be armed with everything I need to know.

Need a part time installer who doesn't play any instruments?

I think I'll be okay with the chickenwire because the shape will more or less be rectangular but it need to go around the strut brace. It will also try to be semi stealth and very practical.

I was more concerned that if I didn't do it all in one peice it wouldn't got as well. Is it okay to make the initial moulds of the plug in more than one piece then join them up and re fit?

[Fhrx]

Post photos for us to see...

[someonestolecc]

Will do Marty - hopefully my assembled box will be ok .. Dono bout driving the car over it though