4th Order Bandpass Help - diyAudio
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Old 16th January 2007, 06:06 PM   #1
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Default 4th Order Bandpass Help

Okay here I go, I am a long time reader first time poster as now I need some help. It has been a long time since I designed a 4th order bandpass enclosure so I am rusty. I did some reading to brush myself back up on the theory and even used WinISD to help me model it. Some of the main things I worry about are how I did some of my own tweaking to provide a gain boost in the low end and most importantly having enough port area.

The single 12 sub will be used for car audio application and being powered with a true 750 watts. I am looking for it to be overall decent sounding also while still providing a lot of low end response through 40s to 60s hz range.


Speaker parameters:

Qts: 0.47
Qes: 0.52
Vas: 2.07 ft^3
Fs: 27 hz
Pe: 800w
Xmax: 21 mm
Spl: 85.3dB 1w/1m


Enclosure:

Fb: 45.21 hz
Vf: 0.94 ft^3
Vr: 1.14 ft^3
Av: 40.5 in^2
Lv: 51.5 in


All the enclosure specs are tweaked by using formulas they are not what WinISD suggests. The net volumes (sub, bracing and port displacements factored in), port length, port area and tuning are what they actually are after measuring out on graph paper. In my past I have always adjusted any enclosure with using formulas after modeling them in a program. I prefer pencil, paper and a calculator.

After my adjusting it to get a +2dB gain in the 40s to 50 hz range I figured the Q should be around .80ish and providing a decent transient response with a passband of 40 to 82 hz.

Now I am a little worried about the port area I am trying to avoid excessive port noise if at all possible. Going by the calculator on CarStereo to find my minimum vent diameter it says I need 99.74 in^2 but if I go by minimum port area from diyaudiocorner they say I only need a 6 inch circular port or 28.26 in^2 for a single 12 inch subwoofer. Can someone please tell me if my 40.5 in^2 will be adequate enough? Using WinISD it shows it as a 0.02% match. Lastly will the 51.5 inches long labyrinth port length (corners will all be beveled) affect anything?

Thank you all for taking the time to read this. I am looking forward to some helpful replies and of course any suggestions are also welcomed.



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Old 17th January 2007, 07:35 AM   #2
Volenti is offline Volenti  Australia
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Ok, you've gone for the classic flat top response which unfortunatly results in a pretty bloated response in a car after cabin gain, if you tune the front chamber to ~70hz you will will get a raw response that looks ugly on "paper" but will work very well in the car, combining with cabin gain to give you a large boost from 80hz all the way down to below 30hz (compared to sealed)

This plot has a modest 6db cabin gain added to show what I'm talking about.

Pink is sealed in 1cuft, green is your design, yellow is your design tuned to 70hz.
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Old 17th January 2007, 07:38 AM   #3
Volenti is offline Volenti  Australia
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Raw response for comparason.
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Old 17th January 2007, 02:32 PM   #4
infinia is offline infinia  United States
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Hi
Port area determines max air speed. It's a function of how much port noise you can stand.
I suggest if you have access to MS Excel; DL the Unibox program. Unibox is a little steeper learning curve than WinISD but tells you so much more.
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Old 18th January 2007, 10:14 AM   #5
djQUAN is offline djQUAN  Philippines
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following what volenti said in the first post, I gave it a try with the kicker compVR's that I'm thinking of building a bandpass box with........

the blue line is the bandpass with 1.5cuft sealed chamber, 1cuft ported chamber tuned to 73Hz. the volume the ports will take up is already taken into consideration. I would like a little more lower end extension but this has been the biggest box that I could fit in the space I'm willing to give up. the port will be rectangular 3" by 14"

the green line is the box I'm currently using at 1.5cuft tuned to 32Hz.

any comments?
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Old 18th January 2007, 07:34 PM   #6
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Thank you Volenti. I like how you shown that comparison in BBP. Please email me with your email address as I have a few follow up questions that I would like to discuss personally. I cannot email you through this board as I am a newer member with a small post count.


Thank you as well infinia, I have fiddled around in the past with UniBox. I am looking for a more all around guideline like on how ported enclosures I go by 15 square inches per cubic foot of net and I have never had any problems doing so. I am looking for a more definitive answer and or any tips and tricks regarding minimum port area for a bandpass enclosure.



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Old 19th January 2007, 12:04 AM   #7
infinia is offline infinia  United States
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Quote:
Originally posted by berserker

Thank you as well infinia, I have fiddled around in the past with UniBox. I am looking for a more all around guideline like on how ported enclosures I go by 15 square inches per cubic foot of net and I have never had any problems doing so. I am looking for a more definitive answer and or any tips and tricks regarding minimum port area for a bandpass enclosure.

- Gus -
Port area mainly goes by Freq. vs excursion so cannot use a fixed rule of thumb unless you use the same size drivers, Xmax, with similar tunings.
Guidelines/limits on port area are there in Unibox... take a look.
(ie Typically 5% of speed of sound or 8% when using flares.)
I would go as big as practical since all the sound you are after goes through it.
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Old 19th January 2007, 05:25 AM   #8
Volenti is offline Volenti  Australia
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Quote:
Originally posted by djQUAN
following what volenti said in the first post, I gave it a try with the kicker compVR's that I'm thinking of building a bandpass box with........


any comments?
That plot looks a little peakier than I'd expect but that program may not be calculating for box losses, it's not a big issue.

The beauty with this "alignment" is that for the most part all you are changing is the port length to tune it, and the usually high tuning frequency let's you use very large ports which keep vent air volecity down.



berserker, email me at agraham5(at)bigpond(dot)net(dot)au , put diy audio in the title or it may get lost in the spam.
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Old 19th January 2007, 09:46 AM   #9
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Quote:
Originally posted by infinia
Guidelines/limits on port area are there in Unibox... take a look.
(ie Typically 5% of speed of sound or 8% when using flares.)
I would go as big as practical since all the sound you are after goes through it.
Thank you Infinia for the added input.


Quote:
Originally posted by Volenti
berserker, email me at agraham5(at)bigpond(dot)net(dot)au , put diy audio in the title or it may get lost in the spam.
Thanks and will do.



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Old 19th January 2007, 03:56 PM   #10
djQUAN is offline djQUAN  Philippines
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ah. I'm using winisd.
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