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Boomer1950

69 yr-old guy needs advice with 1st DIY sub

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After further consideration I've made another adjustment to my plan. If I leave the box @7ft^3 & change the tuning to 20Hz, the port length drops to 22.25" - removing the need to curve the ports.  Plus, blocking one port should take it into the teens for HT, if I've understood properly.

If I get this type of performance from my first DIY sub, I'll be thrilled. 

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I get an "Internal Server Error 500" when trying to load the link you posted.  The link may only work for you.

That design should work decently, though depending on the type of ports used (like flare characteristics if any), the length you gave might tune a bit lower than 20 Hz.  Also keep in mind that the vent area limits how much air can be moved which limits how much output you can get at the bottom before the vent overloads.  And it's home theater content that's more likely to push the sub to its limits.

One thing I'd consider changing is to add a 3rd 4" vent, increasing the lengths and cabinet depth accordingly.  I'm not sure you can do this with the Precision Ports I posted a link to, earlier in the post, and if you can, it'd probably be a (maybe ugly) asymmetrical arrangement.  Another option is to do a slot vent, which can be built right into the cabinet.  It should be braced every ~8" or so, which can also be used to the vent into individual chambers that can be plugged.  See for example the Skhorn and Skram designs posted here.

Do you think you'd be able to fit a second one of these subs somewhere?  Subs tend to sound better with at least two in different parts of the room.

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Thanks for your comments, @SME.  Because the problem you mentioned with arranging 3 Precision Ports on the front of my box is virtually unsolvable, I'm going to go with a slot vent.  This construction is a little more complicated than using pre-made ports, but I don't think it will be beyond my capabilities.  I've settled on a 7ft^3 box with a 70 sq.in. port that is about 40" long which I hope gives me a tuning of around 24Hz.  I will be dividing the port into 3 pieces for added support & to give me the possibility of closing one or two of the ports.  I assume dividing the 70 sq.in. port into 3 ports will lower the tuning, but that seems fine to me.

I plan to start working on the box in the next few weeks.  I'll keep you posted as to my progress.  If there are additional comments or suggestions, I'd love to see them.  Thanks again to everyone's help.

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About the suggestion of having two subs - I can't do it right now.  I understand the advantages, but it is unaffordable at the present time.  This sub will be such an improvement over what I've been listening to for the last 13 years that two subs are not very appealing to me.

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7 hours ago, Boomer1950 said:

Thanks for your comments, @SME.  Because the problem you mentioned with arranging 3 Precision Ports on the front of my box is virtually unsolvable, I'm going to go with a slot vent.  This construction is a little more complicated than using pre-made ports, but I don't think it will be beyond my capabilities.  I've settled on a 7ft^3 box with a 70 sq.in. port that is about 40" long which I hope gives me a tuning of around 24Hz.  I will be dividing the port into 3 pieces for added support & to give me the possibility of closing one or two of the ports.  I assume dividing the 70 sq.in. port into 3 ports will lower the tuning, but that seems fine to me.

I plan to start working on the box in the next few weeks.  I'll keep you posted as to my progress.  If there are additional comments or suggestions, I'd love to see them.  Thanks again to everyone's help.

Just to double check some things.  The 7 cuft should be the *internal* volume left over after subtracting stuff like the internal bracing and *especially* the port.  Also the driver takes up some space, though probably not more than 0.25 cuft or so.

The port dividers reduce the cross-sectional area of the port slightly, so you are right that this will drop the tuning a bit more than without them.  It shouldn't be hard to calculate how much area you lose ... For 2 dividers of 3/4" thickness in a 3" tall slot port you lose (2 * 3/4 * 3) = 4.5 in^2.  It's that simple.

There is one more important consideration worth mentioning.  Vents have a "pipe resonance" (like an organ pipe) at a certain frequency depending on its length.  Your 40" pipe is likely to resonate around 150 Hz.  That's a bit lower than we'd like but is "do-able", especially if you are crossing over at 80 Hz and not higher.  Certainly we wouldn't want a vent that's any longer.

As you can surmise when trying to make a low-tuned cabinet as small as possible, there is an inherent trade-off between getting vent area large enough to not overload at high levels while keeping the length short enough to fit into the cabinet and to avoid making the pipe resonance too low.  This is a trade-off that vented system designers have to routinely make, and it gets worse when either tuning lower or trying to use a driver with more output.

Your final design looks pretty good as far as balancing trade-offs.  For the length, area, and volume you specified, Hornresp indicates a tune of around 22.5 Hz.  A 3rd order high pass filter at 18 Hz appears to control excursion well and gives you a response that's roughly -6 dB at 20 Hz and -10 dB at 18 Hz.  Depending on what your room is doing, you may get useful output quite a bit lower than that.  If you plug a vent and use the same HPF, you lower the tune to 18.5 Hz, which gets you -6 dB at 17.5 Hz and -10 dB at 15 Hz.  Again, your room may help things get a bit lower.  In both cases, vent velocities are very reasonable, so you're not likely to see much compression.  While plugging two vents will technically get you a 13 Hz tune, you don't really gain any usable output or extension this way, so I wouldn't recommend using it this way.

Anyway, good luck with the build!  We look forward to hearing of your progress.

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23 hours ago, Boomer1950 said:

 I've settled on a 7ft^3 box with a 70 sq.in. port that is about 40" long which I hope gives me a tuning of around 24Hz.  I will be dividing the port into 3 pieces for added support & to give me the possibility of closing one or two of the ports.  I assume dividing the 70 sq.in. port into 3 ports will lower the tuning, but that seems fine to me.

Are you sticking with the box dimensions posted earlier of 20x26x29.35"? It sounds like you would be placing the driver and ports on the 20x26" face? 

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