Isobaric Bandpass

Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=89397
Printed Date: July 03, 2025 at 6:13 AM

Topic: Isobaric Bandpass

Posted By: Sharp21
Subject: Isobaric Bandpass
Date Posted: January 22, 2007 at 10:00 PM

Ive got a Land Rover Discovery that I built a regular ported box for with a 12" sub. But it takes so much space & the sub & amp are unprotected, so now im looking at building an 6th order isobaric bandpass box but have a few questions.
Does the size of the ambient pressure chamber matter?
I have 2 bridgable 2 channel amps. I was planning on using one per sub. Or should I use one amp for the 2 subs, wired in parallel?
S.

Replies:

Posted By: Sharp21
Date Posted: January 22, 2007 at 10:03 PM

Here is a pic of my current setup. It shakes pretty good, but takes too much space...
posted_image

Posted By: haemphyst
Date Posted: January 22, 2007 at 11:11 PM

ABSOLUTELY. And, if both chambers are not sized correctly, you can do MANY bad things, from poor frequency response to complete and UTTER DESTRUCTION of your woofer(s). Bandpass enclosures are not something you can "just put together", and generally speaking, they will be significantly larger than your present vented enclosure, for a given, EQUAL frequency response. Their tuning is MUCH more critical than a vented enclosure. Are you happy with the way it sounds right now? Then my suggestion would be put a grill over the woofer, and call it a day. Bandpass enclosures typically exhibit MUCH higher distortion figures as well. The only thing you will gain with a bandpass box is protection for your woofer, and sheer output. You will lose linearity, transient response, and deep bass extension. If you LIKE that kind of bass, then go for it, but have somebody design the enclosure for you, FOR the woofers, don't by-guess-and-by-golly it.

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Posted By: Sharp21
Date Posted: January 23, 2007 at 3:08 AM

Obviously I would pick out some speakers & start from there. Ive also built a few boxes so I feel im up to the task.
Something else I didnt mention is that I would also like to build a cargo storage box from the sub box right to the rear door. Dont get very good sound with a ton of gear covering the speaker & port! With this box I can have it porting straight up.
Its all going to be one large box, with the subs taking up the forward portion.
Also, while the sub hits pretty hard given the right track, there are a lot of flat spots. If possible I would like to design it to cover a broader frequency range.
Once I get the drivers im going to use ill calculate the box out exactly. Right now Im just trying to get an overview of the influencing factors
S.

Posted By: haemphyst
Date Posted: January 23, 2007 at 8:34 AM

Sharp21 wrote:

Obviously I would pick out some speakers & start from there. Ive also built a few boxes so I feel im up to the task.

As long as you have the mathematical abilities, and the formulae into which you plug your chosen speaker's T/S parameters... Go for it. I, personally would never try it. (The fact that I dislike bandpass enclosures tremendously, notwithstanding.) I have built some very esoteric enclosures myself, and I still wouldn't do a bandpass. I figured out what the (minimalist) capabilities and (grandiose) shortcomings of them are long ago...

Sharp21 wrote:

Also, while the sub hits pretty hard given the right track, there are a lot of flat spots. If possible I would like to design it to cover a broader frequency range.

That's not going to happen, with a FLAT response. The BEST bandwidth a bandpass enclosure can get, without a (somewhere around) 3dB hole in the middle is ONE OCTAVE. i.e. 25 to 50Hz, or 40 to 80Hz. That's what they are designed to do, and it's what they do well... They are VERY loud for a given input, in their bandpass but NOT very good for any type of bandwidth.

Sharp21 wrote:

Once I get the drivers im going to use ill calculate the box out exactly. Right now Im just trying to get an overview of the influencing factors
S.

Also, many drivers CAN'T work well in a bandpass enclosure. It's not something you can do by "choosing a woofer", then "building the box"... It is a hand-in-hand process, the bandpass. You need to first figure several enclosure sizes, based on the amount of space you want to give up in your car, THEN build (on paper) perhaps as many as 10 or 12 designs, using parameters provided by the manufacturer of SEVERAL different drivers you might like. To do a bandpass RIGHT takes a LOOOOONNG time, my friend. There are some MORE influencing factors for you. posted_image

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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

Posted By: Sharp21
Date Posted: January 23, 2007 at 7:54 PM

I can dig it. I never thought it would be a cakewalk. But as I am also going to take the time to design & build a well thought out cargo arrangement I will at the same time build a well thought out box. Im leaning towards the bandpass as I said for speaker protection.

My original idea was to mount the speakers facing down, having the whole box on a 2-3" riser. Do you think this would be a better solution? My concern with that is limited amount of airspace in that space for the subs... I would then still port up towards the roof.

Posted By: stevdart
Date Posted: January 23, 2007 at 9:25 PM

]Im le wrote:

ning towards the bandpass as I said for speaker protection.

There are certainly other ways to protect the woofer from external damage than giving up fidelity for this reason.

]My or wrote:

ginal idea was to mount the speakers facing down, having the whole box on a 2-3" riser. Do you think this would be a better solution? My concern with that is limited amount of airspace in that space for the subs... I would then still port up towards the roof.

Check the Gloss in WinISD Pro, or use the sag calculator to make sure the driver of choice is suitable for downfiring. Sag factor below 5%. You should port to the same side as the driver is facing. Here, again, is a design where it is a good idea to protect the driver with a grill. A good heavy duty grill like shown here will protect the woofer from bumps and bruises (found at Parts Express and some of your local sellers).

posted_image

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Posted By: coppellstereo
Date Posted: January 23, 2007 at 9:40 PM

as far as the amps are concerned, you will have to have some sorta bridging module or pass-through to keep the woofers in phase. or they will likely xplod!

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Posted By: Sharp21
Date Posted: January 23, 2007 at 11:17 PM

coppellstereo wrote:

as far as the amps are concerned, you will have to have some sorta bridging module or pass-through to keep the woofers in phase. or they will likely xplod!

I was under the impression that one speaker is run from the amp like regular & the speaker from the other amp is run with reverse polarity?

Ive never heard about having the ports fire the same way as the subs...

I suppose I could build with the subs facing up, then have them countersunk in the box, covering that with a mesh grill...

Posted By: Sharp21
Date Posted: January 26, 2007 at 11:04 PM

What about building midbass subs into the same box, but in a seperate airspace? Would having the ports from them & the sub firing the same way cancel any of the sound?

Posted By: stevdart
Date Posted: January 27, 2007 at 12:58 PM

Sharp21 wrote:

What about building midbass subs into the same box, but in a seperate airspace?

Midbass goes up front. Keep the subwoofer separate.

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Posted By: Sharp21
Date Posted: January 27, 2007 at 7:11 PM

Roger that.
S.