Need advice on building a ported box

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menaztricks

Member - Posts: 28

Joined: October 30, 2005

Posted: November 16, 2005 at 11:22 AM / IP Logged

So I have been trying to figure out how to build a good box for these subs, but I have found a lot of conflicting information everywhere I look.

Winisd tells me that if I want to use a 2^3ft box tuned to 30hz using a 4" round port, the port needs to be 15.81" but on the infinity spec sheets, it says the port length should be 12.25." So that 3"+ difference confused me.

I wanted to use a slotted vent anyway, because I had heard these in an MTX box with a port that was 1.75" x 14" and I liked the way they sounded, so I figured if I made a box that was 2^3ft with the port tuned to 30hz, like recommended in the spec sheet, that it would sound even better. But after reading the tutorials on the JL audio site, it says "The cross-sectional area (found by multiplying h and W) should be the same as that prescribed for a round port. For example, if our design calls for a 4" diameter port, our duct's cross-sectional area should be 12.57 square inches." So according to that, I should use a much smaller vent than the MTX box, so winisd tells me that using a vent of 1.75" x 7.25" would give me a cross area of 12.69 and length of 15.97".

To confuse me more, I enter the sub specs in one of the calculators at http://www.bcae1.com/spboxnew2.htm to see if this sub is better suited for a sealed or a ported box, and it tells me I am better off with a sealed box.

If you were me, who would you listen to? Should I make one with a rounded port the length in the spec sheet, or using the length winisd gives me? Or should I use a port thats 1.75"x14" at whatever length winisd tells me to use, and forget what JL audio has to say about the cross area? Or should I just make it a sealed box.

Any help would be appreciated.

Also, this is the link to the specs of the sub, if anybody wanted to double check anything for me http://www.bcae1.com/spboxnew2.htm

Thanks

sedate

Silver - Posts: 1,173

Joined: July 03, 2004

Location: Colorado, United States

Posted: November 16, 2005 at 1:03 PM / IP Logged

Yikes.

Nice job on the homework before the post.

Okay... uhh.. hmm. Lets get a wee-bit more specific .. what sub are you using? How many? Any you want to make it like an MTX box because you liked the way they sounded in that box eh?

Hmm. I'd be real suprised if we couldn't do a bit better.

First off, I'm not sure if you really need to use a slot-port at all.. thats kinda a style thing.. most ppl here would tell ya that for fidelity a round port would usually be the best choice tho in a car it hardly matters. More a "ease of construction" standpoint. I hate building slot-port boxes.

Anywho, as a verteran of struggling with JL's excellent but heady and not-very-applicable port-box tutorials, just ignore those for now. Lets concentrate on using WinISD and then we'll compare to the Infinity spec sheets, any, for fun, maybe the JL site too eh..?

Aight so you need to tell me

1) What sub and how many (really a link to the t/s parameters)

2) What amp

3) Rough overview of ur system wouldn't hurt either

And no, absolutely do not make it a sealed box.

Venting is fun.

"I'm finished!" - Daniel Plainview

menaztricks

Member - Posts: 28

Joined: October 30, 2005

Posted: November 16, 2005 at 2:02 PM / IP Logged

Oops, I thought I had posted the model number on subject line, and that last url was supposed to be the spec sheet, I guess my brain hasnt fully booted today.

It is one infinity 1242w http://manuals.harman.com/INF/CAR/Boxes%20and%20Parameters/1242W%20Tech%20Sheet.pdf

The amp is going to be either a bazooka ba1500 or a kenwood kac-7202, thats what I could get on my budget, though I hope to get an MTX TA3401 sometime soon. Most likely than not I will end up using the Kenwood, it is rated at 150w x 2 @ 4ohm, and was planning on running each channel to a coil. If bridged, it is not stable at 2ohm, so I guess I have to use separate channels. Though a local shop did tell me that if I wire the coils for 8ohm, that the amp sees the load as 4ohm, and that it would work perfect, I had never heard that before though.

I have a Pioneer DEH-P7300 HU, the factory 4x6 front speakers are going to be replaced by infinity 5012i 5.25"s and for about a month or so I'm going to stick with the factory 6x9s in the rear dash until I get the infinity 6x9's I want (cant remember the model number off the top of my head). The car is a 99 cavalier.

Yes, when I listened to them they were in a 1.75^3ft mtx box, hooked up to the Bazooka ba1500 and the MTX TA3401, the sound was a lot cleaner with the MTX, but I already have the bazooka and kenwood amps.

I dont care for style much, I want to get the best sound I can get out of this sub.

The sealed box option is no longer an option.

j_darling2007

Copper - Posts: 210

Joined: January 21, 2005

Location: United States

Posted: November 16, 2005 at 2:44 PM / IP Logged

That is the exact same sub that I have. i used a sealed box built to 1.25 cubes. I like the sound myself but it sounds as if you really want a ported enclosure. I had a ported box a long while back, and I used the port length calculator here, and it seemed to work fine. i entered the cubes of the box and the diameter of the port, and it tells you how long to make the port.

And yes, if you wire the voice coils in series, it will create an 8 ohm load, which you can bridge because each channel will see a 4 ohm load.

There are 3 kinds of people in the world, those who can count and those who can't

sedate

Silver - Posts: 1,173

Joined: July 03, 2004

Location: Colorado, United States

Posted: November 17, 2005 at 12:45 AM / IP Logged

menaztricks:

Yea man, the shop has the bead on the best way to run it... wire the sub in series and then bridge it to the amp. You really don't want to run each channel discretely... it just isn't ideal. By bridging the subwoofer you sum the output of the amp, which, strickly speaking, is better for the sub than running each voice coil discretely.... and you'd run the exact same amount of power anyway.

Anywho, that said, I've played around with ur sub in WinISD, and I mean.. u got like 200 different options all depending on your taste and comfort level with tough carpentry jobs. I really like the curve I get with 1.75cft tuned to 28hz... uhh.. the easiest port solution I came up with at that configuration was 2 round ports @ 2" in diameter @ 10" long. If you'd want to do a slot port, try 1.75x7.75@ 21" long...

Uhh... WinISD is an absolutely outstanding predictor of port noise/port tune.. so I always trust it. I'm super sleepy so i don't feel liek doing the math but I'd betcha *at least* the JL formulas for port tune will check out with WinISD and the calc's here...

Anywho... here's the graph of the enclosure I recommended... like I say... check it.. +-3db from 30-80hz... haha try getting that with a sealed enclosure!!!

Driver : Infinity Reference 1242

Project by : sedate

Project for : menaztricks

----------------

Number of drivers : 1

Box type : Vented

Box size : 1.800 ft^3

Tuning frequency : 28.00 Hz

Vent : 2 vent(s)

10.30 in length for each

2.00 in round

Relative

Freq Gain SPL

[Hz] [dB] [dB]

30.00 -3.32 114.46

35.00 -0.62 117.16

40.00 1.17 118.95

45.00 2.12 119.90

50.00 2.45 120.23

55.00 2.45 120.23

60.00 2.29 120.08

65.00 2.09 119.87

70.00 1.89 119.67

75.00 1.69 119.47

80.00 1.52 119.30

"I'm finished!" - Daniel Plainview

menaztricks

Member - Posts: 28

Joined: October 30, 2005

Posted: November 17, 2005 at 2:38 AM / IP Logged

j_darling2007 - ever since I got into speakers, I've always preferred the sound of ported boxes, there was one sub in a sealed box that I heard long ago, it produced really nice tight bass, but it only sounded like that with certain types of music. It may be just me, but it seems like ported boxes do a better job with a wider range of music, and I listen to just about everything. That's why I'm not too fond of bandpass boxes either, seems to me like the only thing that sounds right in them is rap, though all bandpass boxes I have ever heard have never been custom made for the subs. BTW, what amp are you running it with?

I don't mind doing some carpentry work, but I am having the walls cut for me at the place I buy the MDF, just saves me an awful lot of time and mess.

Using those port sizes you gave me, I get the exact same results in winisd. I'm going to go with the round ports, if the only advantage of a slotted port is the eye candy, I really dont need the extra work just because it will look more modern or whatever.

Just to better understand how to do this on my own in the future. How did you come up with the 1.75cft and 28hz? Also, I'm still trying to fully understand the graph in winisd, if I understand correctly, it shouldnt be too, "irregular" can't think of a better word at the moment, not looking like a rollercoaster, etc.. But aside from that, what am I shooting for?

Thanks for all the help, I cant wait to get started on this thing.

menaztricks

Member - Posts: 28

Joined: October 30, 2005

Posted: November 17, 2005 at 2:44 AM / IP Logged

Oh yeah, and that the frequencies below the -3db line wont sound very well in that box, but aside from that I'm not certain what to look for when I am playing with the graph.

j_darling2007

Copper - Posts: 210

Joined: January 21, 2005

Location: United States

Posted: November 17, 2005 at 2:26 PM / IP Logged

Correct me if I am wrong, but sealed boxes provide a smoother curve than a ported box. A ported box is louder at certain frequencies than others.

But, if the ported box is properly designed and constructed, it can have a relatively smooth curve, but will provide a higher SPL. But it sounds as if you want a ported box, which I am not trying to talk you out of. I had 1 reference in a ported box a while back, and I liked its volume, but I like the clean, tight, accurate bass a sealed box provides. Anyway, just my 2 cents. Good luck with your box.

There are 3 kinds of people in the world, those who can count and those who can't

sedate

Silver - Posts: 1,173

Joined: July 03, 2004

Location: Colorado, United States

Posted: November 17, 2005 at 5:00 PM / IP Logged

menaztricks wrote:

Oh yeah, and that the frequencies below the -3db line wont sound very well in that box, but aside from that I'm not certain what to look for when I am playing with the graph.

Not exactly. -3 dB is a fairly significant shift in terms of output, but it certainly isn't going to render the note inaudible by any strech of the imagination. The box I use now has a curve quite a bit steeper actually, winding down about -5dB at 30hz and up to about +2db @ 60... it sounds absolutely outstanding... very smooth and fluid. By comparison, much flatter (which is the word you're looking for, but 'like a rollercoaster' is pretty good to Need advice on building a ported box -- posted image.

Need advice on building a ported box -- posted image.

) than a sealed box which brings me too...

j_darling2007 wrote:

Correct me if I am wrong, but sealed boxes provide a smoother curve than a ported box. A ported box is louder at certain frequencies than others.

Okay you're wrong. Any given sealed enclosure will have a standard -6dB/octave slope built into the response curve. This means that at 35hz, it is 6dB quieter than at 70hz.

A *properly designed* port will not function to make certain frequencies very loud, but actually to provide significant low frequency extention.. make up for that -6dB slope built into an otherwise sealed enclosure. As you can imagine, the slope we are discussing is significantly flatter than any given sealed enclosure with that -6dB slope.... Now, if you sayy.. want to see how loud you can get your sub, building a box tuned for the fs of the speaker in a larger enclosure will certainly get you a peaky response..... I built a box for a 13w6 once that would hit 140dB at 50hz but would be down *twenty-five* dB either way @ as little as +-10hz... the box sounded horrible beyond all imagination, but, a 50 hz test tone would make you damn near deaf for the rest of the day.

Anywho, all graphed out for ya:

The green line is the ported box @ 1.75cft @ 28hz and the yellow is a sealed box... notice the steeper slope.

Notice too that these graphs *do* indeed have a simaler shape.. they won't sound all that different. Still, the +3dB down at 30hz easily would seal my decision on the matter.

"I'm finished!" - Daniel Plainview

j_darling2007

Copper - Posts: 210

Joined: January 21, 2005

Location: United States

Posted: November 17, 2005 at 8:46 PM / IP Logged

Ok. It makes sense now. It really helped seeing the graph. So the advantage of a sealed box is to provide a more flat even response without peaks across the entire range the subwoofer plays, right. However, it will sacrifice a small amout of SPL, correct.

There are 3 kinds of people in the world, those who can count and those who can't

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