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Questions and Ideas for my hybrid Fiberglass/MDF enclosure


Yahooster

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Sup peeps, new to the forum but not new to forums in general. After a couple weeks of researching multiple different aspects of box fabrication I still have a few unanswered questions. 

I plan on building a hybrid fiberglass rear and bottom, mdf/birch front/top enclosure for my 2012 wrx. The main reason for this post is because I've found very little info on fiberglass ported enclosures online. I'm pretty confident in my abilities to make one of these enclosures even though it may be a little difficult (from what I've read). 

I'm going to be installing a 10" RF Punch P3. I'm currently mocking up a design with cardboard.

abknl3.jpg

The most feasible option in my mind is a 3" round PVC port that has to be just under 9" according to a couple online calcs. The final box volume I'm aiming for is going to be around 1.6 to 1.3 cubic feet, and I am aiming to tuning it to 35 Hz.

A couple port exit ideas.

2pqqkrc.jpg

14ybd35.jpg

OR


25qw6cw.jpg


I've read that having that 90 degree bend is going to hurt airflow. I'd like this exit because in the second design, the airflow from the port would most likely cause unwanted noise/vibrations from my privacy cover that would partially cover the port. I like the second design because it would be an easier install and I could just buy an aeroport and not have to flare the ends of the pvc after the 90 and making it flush and all that...


I haven't been to the store to see if a 3" 90 bend would actually fit in this space yet. 

24wxbip.jpg


I guess the main purpose of this thread is to bounce some ideas around and see what's feasible.

Few notes:

1. Couldn't really find too much info on 3" vs 4" PVC port. I know (based on the calcs) If I jump to a 4" port the length basically doubles which I'd prefer not to do because I don't think I have the space. Anyone have input as to if the 3" port is going to have noise? 

2. I have found a couple threads about JL Audio making a flex port which was a length of corrugated tubing with flared ports. Looks like they discontinued the product. Some people were saying the corrugation would disrupt the airflow even more than 90 degree bends. Not sure about the accuracy of any of this due to the lack of info on the subject. I was thinking of basically doing the same thing with some 3" rubber tubing with a smooth bore if I could find some, similar to radiator tubing.

3. Per RF specs a 1.4 cubic foot ported box tuned to 40 Hz is recommend. I plan on staying in that cubic foot range but I would like to try to get more output on the low end hence me aiming for 35 Hz. Any input on this? From what I've read most people usually tune lower than the manufacturers specs but usually don't tune lower than 30 Hz. I figured my lil 10 should be fine around 35 Hz :laugh:

2prxnc4.jpg

3. I would like to use some left over 3/4 plywood (which I'm pretty sure it's birch but it might be maple) for the front and top panels. I've read baltic birch is fine to use but haven't read much up on maple. Don't remember what species my plywood is so if I get enough comments saying maple is trash then I won't take the gamble and just go buy some MDF... 

I still haven't really found a clear answer on how to bond fiberglass to MDF/Birch. Most say the resin bonds well to MDF (not sure about birch). But when I place the MDF on top of the glass base, should I screw the fiberglass to the MDF from the bottom? Predrilling of course as to not crack the glass. I'll use some liquid nails or construction epoxy as well just cause I'm paranoid. And of course going to lay more glass on the joint inside the box. I can see myself throwing everything in the trash if I get everything together and the MDF ends up separating from the fiberglass. :suicide:

Example pic:

sf9uzs.jpg


Well I guess that's it for now. Sorry for the long post, just haven't really found anything online regarding my one off project. 

Any input would be appreciated!

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How much power are you going to be running?  The problem you may run into with a 3" port is it won't be big enough and you will get port noise/compression, it completely depends on how much power you will be using though. 

"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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1 hour ago, Triticum Agricolam said:

How much power are you going to be running?  The problem you may run into with a 3" port is it won't be big enough and you will get port noise/compression, it completely depends on how much power you will be using though. 

It's going to be powered by a RF Prime 500X1D. The birthsheet said 650 RMS. ? Going to be running at 2 ohms. You think the sub will be able to handle all 650 watts or will I have to turn it down a bit?

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9 hours ago, Yahooster said:

It's going to be powered by a RF Prime 500X1D. The birthsheet said 650 RMS. 1f60e.png Going to be running at 2 ohms. You think the sub will be able to handle all 650 watts or will I have to turn it down a bit?

The sub will handle 650 watts no problem.  With quality subs you can generally exceed the RMS rating by 50 - 100% as long as you are sending the sub clean output.  A 3" port however will not like 650 watts at all.  In fact a 4" port is really a bit too small for that amount of power as well.  

To help ya out, here is a port area calculator I created.  If you put in the box specs and input power you will be running it will give you recommendations for port area:

https://goo.gl/STAv4p

"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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49 minutes ago, Triticum Agricolam said:

The sub will handle 650 watts no problem.  With quality subs you can generally exceed the RMS rating by 50 - 100% as long as you are sending the sub clean output.  A 3" port however will not like 650 watts at all.  In fact a 4" port is really a bit too small for that amount of power as well.  

To help ya out, here is a port area calculator I created.  If you put in the box specs and input power you will be running it will give you recommendations for port area:

https://goo.gl/STAv4p

Thanks for the calc.

I might be looking over something here but when I input 1.5 cubic feet, 650 watts, and 35 Hz the optimal port area is 27 square inches. The area for a 4" circle is about 12.5 Sq In. RF recommend enclosure has one 4" port but that's less than half for the optimal port area per your calc. I also looked around and read that 12 to 16 Sq In per Cu Ft is the rule of thumb. Kinda confused on how to fit a big port in my little box. ??

 

I would need like a 6" diameter port but the length would need to be about 37"!

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5 hours ago, Yahooster said:

Thanks for the calc.

I might be looking over something here but when I input 1.5 cubic feet, 650 watts, and 35 Hz the optimal port area is 27 square inches. The area for a 4" circle is about 12.5 Sq In. RF recommend enclosure has one 4" port but that's less than half for the optimal port area per your calc. I also looked around and read that 12 to 16 Sq In per Cu Ft is the rule of thumb. Kinda confused on how to fit a big port in my little box. 1f914.png1f914.png

 

I would need like a 6" diameter port but the length would need to be about 37"!

Yeah, unfortunately car audio is the world of misinformation.  

I'll tackle the "12 to 16 per cube" issue first.  This rule has been around forever, but its flawed because it only takes one parameter into account (box size).  How much port area you need is VERY dependant on not only box size but also input power and tuning frequency.  As long as you are tuning to the low 30 Hz region, and are using pretty low power the "12 to 16 per cube" will work, but there are too many instances where it will not and your situation is one of them.  A 4" port is only about 9 sq in per cube anyway, so you don't even have enough by that standard.
 

As far as Rockford recommending a 4" port, they aren't the only manufacturer that undersizes their ports, Alpine (as well as others) are notorious for doing this.  I can only assume they do this because it makes for a smaller and easier to construct box and subs that fit in smaller boxes sell better.  Its is NOT going to get you the best performance.  RF's recommendation of a 40 hz tuning is also not exactly keeping up with the times either.

The goal when sizing a port is to make sure its large enough that port velocity won't get so high as to cause excessive port noise or port compression (which robs you of output).  I like to keep things under 32 m/sec for aero ports and 30 m/sec for slot ports, those number are absolute max.  Ideally I like to see under 22 m/sec peak port velocity.  You can use modeling software (such as WinISD) to determine your peak port velocity, the calculator I sent you does a decent job though and will get you close if you don't want to mess around with modelling software.  

I plugged your numbers into WinISD, for your 1.4 cu ft box, tuned to 35 Hz, on 650 watts, you need about 20 sq in to keep port velocity under 30 m/sec and about 27 sq in to keep it under 22 m/sec.

"Nothing prevents people from knowing the truth more than the belief they already know it."
"Making bass is easy, making music is the hard part."

Builds:

U7qkMTL.jpg  LgPgE9w.jpg  Od2G3u1.jpg  xMyLoO1.jpg  9pAlXUK.jpg

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26 minutes ago, Triticum Agricolam said:

Yeah, unfortunately car audio is the world of misinformation.  

I'll tackle the "12 to 16 per cube" issue first.  This rule has been around forever, but its flawed because it only takes one parameter into account (box size).  How much port area you need is VERY dependant on not only box size but also input power and tuning frequency.  As long as you are tuning to the low 30 Hz region, and are using pretty low power the "12 to 16 per cube" will work, but there are too many instances where it will not and your situation is one of them.  A 4" port is only about 9 sq in per cube anyway, so you don't even have enough by that standard.
 

As far as Rockford recommending a 4" port, they aren't the only manufacturer that undersizes their ports, Alpine (as well as others) are notorious for doing this.  I can only assume they do this because it makes for a smaller and easier to construct box and subs that fit in smaller boxes sell better.  Its is NOT going to get you the best performance.  RF's recommendation of a 40 hz tuning is also not exactly keeping up with the times either.

The goal when sizing a port is to make sure its large enough that port velocity won't get so high as to cause excessive port noise or port compression (which robs you of output).  I like to keep things under 32 m/sec for aero ports and 30 m/sec for slot ports, those number are absolute max.  Ideally I like to see under 22 m/sec peak port velocity.  You can use modeling software (such as WinISD) to determine your peak port velocity, the calculator I sent you does a decent job though and will get you close if you don't want to mess around with modelling software.  

I plugged your numbers into WinISD, for your 1.4 cu ft box, tuned to 35 Hz, on 650 watts, you need about 20 sq in to keep port velocity under 30 m/sec and about 27 sq in to keep it under 22 m/sec.

Thanks for taking the time to explain. This really makes me want to scrap the whole idea of going ported and just make a sealed enclosure like everyone else that does the fiberglass corner builds...Kinda disappointing...

Instead of the measly 3" x 9" port I was planning on having I'd have to fit anywhere from a 5" x 25" port that gives me 19.6 sq in, to a 6" x 37" port that gives me 28.2 sq in of port area. The 5" port would occupy another .28 cu ft and the 6" port .61 cu ft. And .61 cu ft is what RF recommends for their sealed enclosure for this sub.

What to do what to do.?

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If you don't have the room to spare for a properly sized port, i'd just go with a sealed enclosure. Although you won't get as much output from a sealed, you would rather have a proper sealed box than an improper ported box. You could always try tuning a bit higher to help reduce the length needed for the port. 

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I would scrap the corner box idea altogether and just wall it off with (4) 15s. But thats just me

Chevy Sonic Wall (4) 15's on 10k Build Log

https://www.stevemeadedesigns.com/board/topic/209412-2015-chevy-sonicwall4-15s10klithiumlarge-case-hairpin-us-alts/

(4) Fi SP4 15's | (2) Ampere 5Ks | Active! | Arc KS600.2 & KS125.2mini | CDT Silk Domes | Silver Flute 8s | 240Ah JY | US Alts

Jeep Wrangler JK (4) 8's in a 4th Order Build Log

https://www.stevemeadedesigns.com/board/topic/209841-shadow-2016-jeep-wrangler-jk4th-order4-8sdc-35klarge-case-hairpin-us-alts/

IG @walledsonic

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On 11/20/2017 at 5:30 PM, DLHgn said:

If you don't have the room to spare for a properly sized port, i'd just go with a sealed enclosure. Although you won't get as much output from a sealed, you would rather have a proper sealed box than an improper ported box. You could always try tuning a bit higher to help reduce the length needed for the port. 

That's what I'm going to do. Never knew about port area, kinda sucks but I just wanted to get the most out of my sub. 

On 11/20/2017 at 6:10 PM, WalledSonic said:

I would scrap the corner box idea altogether and just wall it off with (4) 15s. But thats just me

?? Wish I could man.

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