My Box Is Assembled!

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mrmsudawgs

Copper - Posts: 145

Joined: February 22, 2005

Location: United States

Posted: March 13, 2005 at 12:04 AM / IP Logged

I have posted numerous questions to this group regarding enclosure design for my 2000 F150 extended cab truck. I started with no knowledge of port tuning or understanding of T/S parameters. I learned from this forum about using WinISD which I have become comfortable using. I now know how to use T/S parameters in subwoofer selection. I have learned much about ports - square, round, dual ports, etc. With that knowledge in hand I set forth to build the ultimate, port-tuned enclosure for my truck. A man on a mission My Box Is Assembled! - Last Post -- posted image.

My Box Is Assembled! - Last Post -- posted image.

I carefully designed my box using WinISD and a 3D modeling software called Autodesk Inventor. I laid out full scale templates in my truck to make sure everything fit. I modeled the port to scale. All was good. Then I built "Rev A" of my box. I installed it in my truck and learned that the fit was just too tight. The seat fell flush onto the box (I then posted several posts about jacking up the seat) which was not good at all. I had to reduce the height by over an inch.

The result was that the volume of the box changed from about .85 cubic feet to 0.67 cubic feet. That, in turn, changed my tuning frequency so I had to change my port length in order to maintain my 32 Hz desired frequency. Well, the reduced volume caused the port length to grow so long that it hit the subwoofer. My Box Is Assembled! - Last Post -- posted image.

And, because my box height had shrunk and because one end of my box had an offset in it to clear the hump, my 3" dia port was now too large for the end of the box. A 2.5" diameter port was now the max.

WinISD showed that this reduction in pipe diameter at my tuning volume would require an even longer port. I could make it shorter but I would loose my tuning frequency but then my stinking air velocity through the port was too high. Blaa Blaa blaa. You know the story.

Now, my box is a sealed enclosure. My Box Is Assembled! - Last Post -- posted image.

The tech support guys at Polk promised me that I will not be dissapointed in my MM2104 in a sealed enclosure. I am sure that I will love it just the same but it just kills me that I went to all that trouble to learn about port tuning then ended up not being able to use it. My Box Is Assembled! - Last Post -- posted image.

Dratts!!!

Anyway - thanks to all that have helped me. I'll post some pictures when I get the system installed completly.

Mike

xb2002

Member - Posts: 6

Joined: February 18, 2005

Location: United States

Posted: March 13, 2005 at 12:30 AM / IP Logged

Why didnt you just take the back seat out? It's not like you have to set back there anyway! When I finally get me an extended cab truck I'm gonna build a monster downfire ported box!!!

mrmsudawgs

Copper - Posts: 145

Joined: February 22, 2005

Location: United States

Posted: March 13, 2005 at 12:23 PM / IP Logged

Well - the main reason I did not take my back seat out is that I have three children. Two of which I take to school every day in my truck (along with a neighbor's kid which makes three). My back seat is used every day so taking it out was not an option.

stevdart

Platinum - Posts: 5,816

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Joined: January 24, 2004

Location: Pennsylvania, United States

Posted: March 13, 2005 at 7:04 PM / IP Logged

mrmsudawgs wrote:

WinISD showed that this reduction in pipe diameter at my tuning volume would require an even longer port.

Are you sure about that? Reducing the diameter yields a shorter length tube with the same tuning frequency, but the drawback is possible port noise at high SPLs.

mrmsudawgs

Copper - Posts: 145

Joined: February 22, 2005

Location: United States

Posted: March 13, 2005 at 11:40 PM / IP Logged

My mistake. I check WinISD in that indeed was my problem. My air velocity was way over 80 ft/sec. To get it down to about 50 ft/sec I had to make my port's cross-sectional area larger which, in turn, makes the port longer in order to keep the same "loading" on the driver. The result was that at 32 HZ my port was longer than my box by about 15 inches.

In retrospect - I would like to ask that you check something for me stevdart. Referring back to your post under the topic of Port Length Question - 8" VS 10" Subs you presented a very nice graph showing how a port can smooth out the wrinkles at high impedance. When I was tuning my port I was merely looking at the frequency range where I thought I would like to hear more SPL - mainly the low base notes around 32 HZ. After I looked at your graph again I realized that my approach may be incorrect.

Would you be so kind to take my specs and analyze them for me. Compare the same Polk MM2104 sub in a sealed and ported enclosure whose volume is 0.67 cubic feet. The maximum port size I can physically have (based on the size of my box) is 2.5" tall x 5.5" wide x 20" long. For a round tube port 2.5" diameter is as large as I can have and it also must not exceed 20" (otherwise it will hit the sub).

I entered my maximum port sizes into WinISD and let it calculate the tuning frequency. It came up with 50 HZ. I'm afraid that this will sound "boomy" and I have no idea if this corresponds with any impedance curves as you discussed in your graph. The curve that WinISD calculates shows that the ported box has a lower drop off point and it has 2 to 6 dB more output between 43 and 100 HZ. All this tells me to go with a ported box but my gut feeling says that it will sound boomy or hollow.

Can you help me?

Mike

stevdart

Platinum - Posts: 5,816

Joined: January 24, 2004

Location: Pennsylvania, United States

Posted: March 14, 2005 at 12:09 AM / IP Logged

Instead of looking up the driver specs and going through comparisons I can tell you this: consider that if you were to add a port to the already small box you have, the volume of the port structure would take away from the net volume needed for the subwoofer. I can't imagine a vented enclosure for any sub being that small.

The graph you referred to showed a small driver I'm using in my center channel project that I'm working on, but the principle remains the same as it does when you are working with subs. The graph showed an application where the enclosure was tuned to the resonant frequency of the driver. Your Polk's resonant frequency (Fs) is probably in the 20's, low 30's tops. To tune an enclosure to get the best SQ you would set your goal to get enough enclosure volume and the proper port volume to tune the box somewhere close to that resonant freq. Tuning high, like around 50, is only done in systems where SPL is the goal...considering that we're talking SUB woofer here. (In my home setup, I have midbass drivers in enclosures tuned to 50Hz, but there is also a subwoofer box tuned in the 20's.) The key ingredient is enough available space to work with...which you don't have in your truck.

Other comments: if you were to find the area of opening of a 2.5" round port (use pi R^2) and compare that to the area of a slotted port 2.5" X 5.5" you would quickly see which would yield the greater area...but the longer length port needed. (It's a difference of something like 5^2" compared to 13.75^2".) The smaller opening is easiest to work with because of the shorter length required, but you could end up with port noise. The larger area opening takes care of the port noise problem but requires more room inside the box for added length...and slotted ports are often built with the same thick MDF as used on the walls which further infringes on the air space available for the driver.

...and, when you're shooting for minimum port noise with the smallest possible port diameter in car audio...look at about 100 ft/sec as the maximum. Reason: Mach is the speed of sound in air (about 1130 ft/sec) and the general rule regarding port air speed is to keep it less than 10% of the speed of sound.

If that small sealed box just won't cut it using the Polk, you should look into trading it out for a sub that will be at home with that amount of air volume. You'll only really know after you have everything up and running.

mrmsudawgs wrote:

I am sure that I will love it just the same but it just kills me that I went to all that trouble to learn about port tuning then ended up not being able to use it.

That's not all so bad to know, is it?

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

mrmsudawgs

Copper - Posts: 145

Joined: February 22, 2005

Location: United States

Posted: March 14, 2005 at 9:50 AM / IP Logged

Your comments are almost identical to those in the Polk user's forum where I posted the same question. All of those who responded agreed that a sealed box is the way to go based on the volume I have to work with. And that is what I have decided to do. Sealed all the way. My Box Is Assembled! - Last Post -- posted image.

I can save the port tuning for a another project.

Thanks again!

Mike

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My Box Is Assembled!

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