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Discussion Starter · #1 ·
Lets say I attempt the experiment of porting my IB subs in my trunk. If I place a long smaller tube on the two vents to the outside for heating/cooling venting so they still function, such as 1.25" by 12" or longer, would they not tune or tune under 10Hz? The ports for the subs will be into the cabin and something like two 4"x11" long PVC or so at 17Hz (though I might make a foam lined box instead to combat trunk noises). Or will they screw up my cabin ports anyway and I have to close them off?
 

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If I place a long smaller tube on the two vents to the outside for heating/cooling venting so they still function, such as 1.25" by 12" or longer, would they not tune or tune under 10Hz?
Technically yes, but losses and port saturation (which would happen very quickly at those frequencies with thin "ports" like that) would make any real effect negligible, you'd have more bass leakage at 10hz through the metal of the boot...
 

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Discussion Starter · #3 ·
This car seems to do its HVAC venting out the trunk, there are no vents in the doors. I left some venting open cabin to trunk, it has a convoluted path and seems to not hinder bass output. Is at base of rear glass in the deck. I may have to modify those too. The issue seems to be can I keep them operational without screwing up the tuning/effectiveness of the ports.

These subs drop off at 25Hz or so, and the model says ports would bump 30 up high and 20 as well. I'd have to EQ 30 down to get flat response. But yes its just a model with no cabin gain (that this car seems to not have) and its a trunk so the vent (or ports) is not going to produce what a box will...or may not work well at all who knows. IMO large vented boxes work differently than normal size ones anyway but I can't find any research to support that. I feel that the vent has reduced response, as the box nears Vas the vent does not unload the sub as a smaller box because the sub is near unloaded already, or does not affect the subs as much, something like that lol. Anyway, my only point here is to hopefully allow any 20Hz backwave to come back through the port. Even half the port output in the model would be great and serve my needs perfectly. Since they are IB now with too much output, I don't care if they unload or any of that. Of course they don't have enough output under 25, but I don't necessarily think it is an issue of the subs, I'm not sure, so this would be an experiment to find out. It may have to do with the cabin size.
 

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Just a thought, but if the subs have more than sufficient output presently, why not just bump the 25hz area with a little EQ? I have a big TL at home that only needs the littlest bump in low end EQ to really make a signifigant difference in how it sounds.

Another, practical, thought on the porting suitation, you may need a lot more port area then you would think, and the models indicate, to get an actual real world result, given the lossy nature of the boot as an enclosure.
 

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I agree that porting in this situation is probably not going to work. I have been researching ported enclosures, specifically the port and port area. I happened to ask John at AE. He gave me some very interesting advice. I will paste it below.

This may not help you specifically but I thought it was the best info I have found, where someone who knows tells in what ballpark you should be in. Cause the manufacturers of the woofers sure don't. 3" port for a 22mm xmax sub tuned to 30 hertz is not realistic.


This is my question.



I have been doing some reading regarding how to size the port area. Many websites give a rule of thumb of X number of inches per cubic foot of enclosure. Another had a (to me) very complex equation and stated that ports are to be designed for the woofer, not the enclosure. the problem I see is that to small and the normal problems occur, to big and your port takes up more room than enclosure let alone the pipe resonances.

Dmin = 2030(Vd2/Fb)1/4/Sqrt Np

This is the equation. There is an online calculator that seems to follow this equation fairly closely PORT Size Calculations and Formulas for WOOFER and Subwoofer BOXES

My question is what IS the correct method to follow when designing a port. One to avoid port noise when running the amount of power we all seem to, it seems that a lot of manufacturer recommended port sizes are way undersized and you are doomed to get noise and compression.

Just curious what your feelings are.


And this is his response:

While there are formulas, the best option is simply to model it. Audible distortion will occur once vent velocity gets over 10m/sec. Ideally you want to keep the vent velocity below that point at full power. It isn't always practical though. The point of no return is about 30m/sec where the air flow becomes turbulent. At that point you can't get any more output from the port and distortion skyrockets. In a reasonable world, shoot for 20m/sec at full power at tuning as a conservative goal. You'll notice how big the ports have to get for this especially with low tuning, which is why passive radiators become such a good option..

John
 

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shoestring, if i understand what your saying, you want to rebuild the rear deck to seal the trunk(stuff the rear window vents with material of your choice, then use two 1.25" inch pipes(sort of as a replacement) and two four inch pipes(as the true ports) to try to get two different tunings going on, the problem i see with that is that you have those big 15"s in a large enclosure which will definately reach resonance quick if you try to port them and 1600sq cm of cone and 1 inch ports equal huff and puff, heck i have some cheap 6.5"s here that make a 1" ported test box i have here chuff LOUD on maybe 10-20 watts playing bass-midbass. if you really wanted to port i would look at something more like 2 folger coffee cans connected by fiberglass, duck tape, liquid nails, whatever you want firing up through the rear deck similar to a blowthrough and create a vanity panel under the trunk hinges with lots of 3 inch holes cut trough it then cover that with grill cloth to "stealth" out the entire install, that would be sure to get you some low lows with low to no port noise!
 

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Discussion Starter · #7 ·
Very interesting, I certainly have no room for passive radiators. I have an amp rack under the deck and the 15s into the seat, I did a build thread under pyle 15s, not much on pyle comes up with a search here lol. Much of the deck is closed off with 3/4 ply to seal it.

The idea was to port the air vents so long they would never work, but would flow air for HVAC purposes. I don't play it that loud most of the time but a proper port would be working of course.

I'm trying to get under 25Hz, but when I play a tone down there the subs move a lot and I don't get much output. Could be because I am in a car, or the dB just drops that much...or both I don't know. The model does drop under 30 just like I get in the car. I boost 20 a little now, not much, it sounds pretty nice on the bottom end I can live with it but you know how that goes lol. I've read about how you can't get 20Hz in a car, maybe it is true but I do get something down there just not enough. Like HT, those sounds are mostly for effect and ambiance anyway. Maybe I should have tried 18" subs in there.

The pyles are Fs 20 and qts .7. The trunk is about 1.5 times Vas of each driver, more volume does not do much to the curve.
 

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The idea was to port the air vents so long they would never work, but would flow air for HVAC purposes. I don't play it that loud most of the time but a proper port would be working of course.

I'm trying to get under 25Hz, but when I play a tone down there the subs move a lot and I don't get much output. The pyles are Fs 20 and qts .7. The trunk is about 1.5 times Vas of each driver, more volume does not do much to the curve.
The first part of that statement will be the problem, ported boxes have to HAVE TO be air tight to get low, theres no way around that, and if those 1 inch ports are allowed to relieve any air pressure out of the trunk, there goes ALLLLL of your subbass, and the folgers cans are gonna be the port, 6 inch by about 14" deep going down to the trunk floor maybe you can couple them with some mdf rings and make it longer (then you rework your amp rack under the 6x9 openings), so your trunk is about 14.5 cu. that should be a really nice ebs/overdamped response
 

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Making 25hz loud in a car is hard. My current wooofers are 2 AV 15's tuned at 25hz via 4 passive radiators, also 15's. 25hz is definetly audible, but 30hz is ALOT louder by comparision, human hearing just drops off around there pretty sharply. You can feel your hair and clothes move on 25hz, but it's still far from audibly loud...
 

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The first part of that statement will be the problem, ported boxes have to HAVE TO be air tight to get low, theres no way around that, and if those 1 inch ports are allowed to relieve any air pressure out of the trunk, there goes ALLLLL of your subbass,
By about an octave above the tuning frequency of the vent ports(which are small enough and can be easily made long enough to put that frequency at something silly like 5hz) they really have no effect anymore, think about it, it's like saying that in a normal ported box, the driver won't react properly because there's a big hole in the box...when we know that at about an octave above tuning the sub, from it's perspective, is more or less in a sealed box, despite the big hole.

The vents won't steal pressure from the real ports, the real ports will steal pressure from the vents and will un-load the drivers at a frequency higher than the vent ports will need to do anything.

I'm trying to get under 25Hz, but when I play a tone down there the subs move a lot and I don't get much output. Could be because I am in a car, or the dB just drops that much...or both I don't know. The model does drop under 30 just like I get in the car. I boost 20 a little now, not much, it sounds pretty nice on the bottom end I can live with it but you know how that goes lol. I've read about how you can't get 20Hz in a car, maybe it is true but I do get something down there just not enough. Like HT, those sounds are mostly for effect and ambiance anyway. Maybe I should have tried 18" subs in there.
Anyone who says you can't get under (arbitary number) Hz in a car is wrong, by the time you get down to even 50hz in a car you are operating in a pure pressure/rarifacation mode, let alone down at 20hz, it's like saying room gain suddenly stops at 25hz.

That's not to say that 20hz is easy, you're looking at a BIG low tuned enclosure or lots of swept volume or both, I had 2 JL 15W-6's in a sealed enclosure in my little coupe years ago and they would do 10hz pretty well and just do 5hz, obviously well down on what 40-50hz was, but I recall measuring about 120db at 10hz (difficult to accuratly measure since harmonic distortion would trigger 30-40-50hz modes which would measure a lot higher)

It's possible in your situation that you are getting some panel resonance (even the entire boot lid acting like a passive radiator, bouncing on it's rubber seal, front/rear glass flexing in the seals) that is acting like a resonant absorber around 20hz.
 

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we know that at about an octave above tuning the sub, from it's perspective, is more or less in a sealed box, despite the big hole.

The vents won't steal pressure from the real ports, the real ports will steal pressure from the vents and will un-load the drivers at a frequency higher than the vent ports will need to do anything.
hmm, interesting way to describe it, which in theory looks right but we both build our own boxes so something smells fishy, there is no way a 50 hz soundwave from 2 15"s will come through those 1" ports with anything over a milli-volt without the tubes exploding, on the other hand you have to deal with 4 impedance spikes nows, all below 50 hz, i don't know how thats gonna sound but im gonna plot the two boxes on the same graph and then observe what happens with 400 watts each with my sub and then I'll be back. You definately stirred up the pot there volenti, you crazy aussie:D :laugh:
 

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wth, the plots said it will work but in my mind I get to thinking about dipole subwoofers. shoestring try it but do a 6x9 style mdf ported baffle thingy with the two ports(lol at my wording) you may be onto something here for all the IB guys. sure got my interest peaked!!!
 

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Discussion Starter · #14 · (Edited)
Here is my current setup, scroll down for the amp rack. I was thinking of ports or a slot port in the center towards the floor. The baffle is very sturdy the steel is boxed tubular all the way around and stout, the deck however does vibrate some at the rear. The front covered with ply does not. Just the same that should affect it more at high volumes, I just need SQ at moderate volumes I'm not worried about SQ when the car shakes. Most SQ levels the subs are not moving much at all, minimal vibration.

Interestingly enough it seems to have a little more bottom with a window open than all closed, just enough to notice. Not worried about under 20Hz that much at this point. I know I'll get some but 20 would be nice for now. I can hear a 25Hz tone ok it is down but not bad, below that by 23Hz it is gone mostly.

That is what I was figuring; the small ports would not be active at 20Hz or better so they would not affect anything. Sure the trunk will leak and all that it is not ideal, but like I said if I could get part of the output from the port I'd be smiling. I need that empty trunk at times, this is a great setup considering.

Seems like this would be the same thing as building a bandpass with ports tuned at two frequencies....would it work normally above the tuning of the lower tuned port?

Oops forgot the link
http://www.diymobileaudio.com/forum...5s-ib-baffle-into-seat-yes-plwb155-pyles.html
 

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I was thinking of ports or a slot port in the center towards the floor. the deck however does vibrate some at the rear. The front covered with ply does not. Interestingly enough it seems to have a little more bottom with a window open than all closed, just enough to notice. the small ports would not be active at 20Hz or better so they would not affect anything. this is a great setup considering.

Seems like this would be the same thing as building a bandpass with ports tuned at two frequencies....would it work normally above the tuning of the lower tuned port?

Oops forgot the link
http://www.diymobileaudio.com/forum...5s-ib-baffle-into-seat-yes-plwb155-pyles.html
any vibration of your enclosure equals passive radiation/cancellation. when you wind the window down and relieve the pressure mode of the car that almost always is helping a standing wave/cancellation again. the small ports aren't "active" but they still would be holes in the box and sound is going to come through it just like "air or water and sound" goes everywhere you allow it to. well the premise of bandpasses are to tune specific chambers seperately, not try to "double tune" the same enclosure-rear wave in your case- and with different port sizes too although you are on the path of a series tuned bandpass, in that case you will be stuck with an windows up and windows down type of bass response for your car and you need to know EXACTLY how big your cabin is in regards to this especially when you sit in it with the windows up so get out that tuxedo measuring tape measure and get your physical dimensions:laugh: try to put say 3 pieces of 2 x 4 vertically under the rear shelf for standing support and see if that cures the response any as it is now, how hot is it in the cabin with not 1 but 2 non defeatable electric powered 'cabin heaters" in the car? :D
 

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Yep that's why I would shy away from the 1 inchers, cause then they would become "active" for sure then
 
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