Hey all, I was gonna post this in the car audio section, but I figured it might be a little more appropriate here since the gist of the whole thing is just the box itself.
So I drew up a design for a vented enclosure for a 12" Polk subwoofer in my car, and before building it I wanted to run it through people with experience to see if anything immediately stands out as a problem. This is my first build so bear with me
The outside dimensions are 28" x 15" x 14", using .75" MDF. The net volume is 1.54 cu. ft., and the port is slotted at 12.5" x 2", tuned to 27.4Hz at 54.75" long (probably gonna end up extending the port slightly to put it at an even 27Hz, but that's also gonna affect the net volume a little so I haven't bothered yet). The sub is a Polk Audio MM1240DVC, and the suggested ported box volume is 1.58 cu. ft.. I put in the values to Speaker Box for android and came up with a few graphs.
Ignore the green line, the red is the ported box I designed and the blue is a 0.8 cu. ft. sealed enclosure, which is what I used to have the sub in.
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I'm not really familiar with phase response, does this all look good? My goal was to get a smoother (less punchy), deeper sound from my sub. The sealed enclosure I have sounds tight and clean, but I wasn't satisfied with how early it rolled off. I wanted to try and create a vented enclosure that was much deeper but still relatively responsive, and clean sound is also important. Another goal was to keep the time delay as small and/or as low (out of music range) possible. The lowest tone I have in like any of my music is 30Hz, and according to the graph the delay would only be 20ms. I wouldn't think that's even really mentionable since they'll basically just be sine waves in hip-hop, and any fast drum or beat would be further up the spectrum at like 10ms delay. On paper, I don't think a 10ms delay would be very noticeable, but I've never listened for it before. Is that a good place to be?
Also here's a rough sketch of the design itself, not all the measurements are written down (internal box height is 12.5"). On the final design i planned on putting angles in the corners of the vent to allow the air to travel more smoothly.
Imgur: The most awesome images on the Internet
So I drew up a design for a vented enclosure for a 12" Polk subwoofer in my car, and before building it I wanted to run it through people with experience to see if anything immediately stands out as a problem. This is my first build so bear with me
The outside dimensions are 28" x 15" x 14", using .75" MDF. The net volume is 1.54 cu. ft., and the port is slotted at 12.5" x 2", tuned to 27.4Hz at 54.75" long (probably gonna end up extending the port slightly to put it at an even 27Hz, but that's also gonna affect the net volume a little so I haven't bothered yet). The sub is a Polk Audio MM1240DVC, and the suggested ported box volume is 1.58 cu. ft.. I put in the values to Speaker Box for android and came up with a few graphs.
Ignore the green line, the red is the ported box I designed and the blue is a 0.8 cu. ft. sealed enclosure, which is what I used to have the sub in.
Imgur: The most awesome images on the Internet
Imgur: The most awesome images on the Internet
Imgur: The most awesome images on the Internet
I'm not really familiar with phase response, does this all look good? My goal was to get a smoother (less punchy), deeper sound from my sub. The sealed enclosure I have sounds tight and clean, but I wasn't satisfied with how early it rolled off. I wanted to try and create a vented enclosure that was much deeper but still relatively responsive, and clean sound is also important. Another goal was to keep the time delay as small and/or as low (out of music range) possible. The lowest tone I have in like any of my music is 30Hz, and according to the graph the delay would only be 20ms. I wouldn't think that's even really mentionable since they'll basically just be sine waves in hip-hop, and any fast drum or beat would be further up the spectrum at like 10ms delay. On paper, I don't think a 10ms delay would be very noticeable, but I've never listened for it before. Is that a good place to be?
Also here's a rough sketch of the design itself, not all the measurements are written down (internal box height is 12.5"). On the final design i planned on putting angles in the corners of the vent to allow the air to travel more smoothly.
Imgur: The most awesome images on the Internet
The design looks Ok, if a bit complex. Consider what you may have to do for example if the resonance frequency turns out to be lower than expected when you build the box. How would you adjust the length of the vent? I also suggest flaring the vent at both ends to reduce turbulence, which may be more noticeable at higher SPL levels in a car audio environment.
Edit - just read that you intend on putting angles in the corners. Flaring the vent properly will require a bit more than that.
Edit - just read that you intend on putting angles in the corners. Flaring the vent properly will require a bit more than that.
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Here's a little 3d model I drew up really quick, without the top or front pieces, to maybe give a better picture of it
Thanks for the reply! So my simplest thought to smoothing the airflow was to round off the inner corners with a router or sander, and put round trim in the outer corners at an equivalent angle. It probably won't be as efficient as using a larger piece of wood, but do you think it'd be worth it? Main reason I ask is because it's easier to build and I'm trying to sacrifice as little airspace as possible.
Flaring the ends, there's a good thought. For that, would I pretty much just need to use a router on the outside and on the end inside the box, or is there more to it than that?
Hm, I never thought about needing to adjust the port length... I could try and think of a way to make the top board removable (but sealed enough for tests), at least until I get it tuned to where I want it
Instead of folded very long port, use Passive Radiator(s). Less overall enclosure volume and easy to build and tune.
I was thinking of a little more than that 🙂. e.g. a 3" section at each end of the vent that's about 45 degrees to the vent's centerline might do it. If the cross-section of the vent is more than 1/3rd of the driver's rated Sd, you might be able to get away without flaring though.
You could make the last few inches of the first internal bend in the vent removable. Temporarily screw it into place for testing, and then when you've hit the target Fb, glue it in permanently.
Problems with car movement and PR "floppiness" are overstated, expect in the case of off-roading.
For best car sub sound quality I recommend closed box design, with EQ if needed.
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When you say the cross-section, do you mean just where the vent bends? Since it's 2 inches wide and 12.5 inches tall, at the first corner that'd be 2" x 2" x 12.5", which is 50 cu. in.? I've got a pretty strong feeling I'm misunderstanding though
The sub's rated Sd is 80.6 cu. in. Even if I can get away without flaring, I'm still willing to do so if it'll help the performance!
Ah, I like this even more. 🙂 thank you!
My only thing against sealed enclosures is this specific application. Space is pretty limited, and the sub is designed for a .88 cu. ft. I had two in .8 cu. ft. chambers (and it almost completely filled my trunk), but like I said I wasn't satisfied at all with how early it rolled off.
BUT the main thing with that is, the only way to go up in size is to sacrifice one of the two subs I have, and I want to get the most output out of it while still maintaining good sound quality, AND I'm not really comfortable putting a sub in a sealed enclosure that's twice (or almost) the size of its recommendation -- I don't want it to bottom out, especially since I'm feeding it a lot of power. The PR is something to think about, though, I may design one and put my leftover sub in it. Whichever one isn't quite as good as the other, I'll just keep in my home, hooked up to an amp I have inside 😎
Am I missing something? Your space permit only two 0.8 cubic ft. boxes (total of 1.6 cu. ft.), and you are building one vented box with internal volume (including the port) of about 2.5 cubic ft. (external 3.4 cu. ft.)?!
Why not one 2.5 cu. ft sealed box with two woofers? Or two sealed boxes with 1.25 cu ft. (or less)? Two woofers have 6 dB more output than one woofer, so two closed box with two woofers will have higher/same output as one vented box (with one woofer) for all frequencies above 30 Hz, without EQ!
Edit: the woofer is not designed for a 0.88 cu. ft. sealed box, it is only a recommendation, you can use any volume from 0.8 cu. ft. up!
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The recommendation is .88 cu. ft., which IS what is what it's designed for. Going too big runs the risk of the sub bottoming out and causing damage, no? Also I thought if you went too far above the sealed recommendation, you muddy up the sound. The vented enclosure I have designed now is right at the recommendation, which is where I'm more comfortable being.
But I somewhat miss-worded my statement. My ultimate goal is depth, and at .8 cubes the roll off begins all the way up at like 50hz. Sure, I can go lower by making a larger sealed enclosure, but it won't go to the lengths of the equivalent-sized ported box. And you're right, two typically are louder than just one, but since it doesn't reach as far, the upper frequencies will increase in volume even more than the lower ones I'm looking for (also aren't two sealed somewhat comparable to a single vented?).
I guess the main thing is I simply don't want to bother with a sealed box, at least for this particular sub in my car. I also read somewhere that 1lb of polyfil per cu. ft. increases the effective box size by 25%. I have done this with the two chambers I had, which would in theory cause the sub to react as if it were in a 1 cu. ft. box, and the response I was looking for simply wasn't there. I don't imagine adding a measly .2 extra cubes will get me the rest of the way there.
Not saying you're wrong or anything -- I mean i might be a complete idiot here -- just that I've already made up my mind in that area
"The recommendation is .88 cu. ft., which IS what is what it's designed for."
No, it IS NOT.
"Going too big runs the risk of the sub bottoming out and causing damage, no?"
Yes and no, depends on amplifier power.
" Also I thought if you went too far above the sealed recommendation, you muddy up the sound."
No, on the contrary!
"Sure, I can go lower by making a larger sealed enclosure, but it won't go to the lengths of the equivalent-sized ported box. And you're right, two typically are louder than just one, but since it doesn't reach as far, the upper frequencies will increase in volume even more than the lower ones I'm looking for"
Perfect for EQ and saving the amp power at higher bass frequencies for lower ones.
" I also read somewhere that 1lb of polyfil per cu. ft. increases the effective box size by 25%. I have done this with the two chambers I had, which would in theory cause the sub to react as if it were in a 1 cu. ft. box, and the response I was looking for simply wasn't there. I don't imagine adding a measly .2 extra cubes will get me the rest of the way there."
1.25 cu. ft. is 56% increase over 0.8 cu. ft.
1.25 cu. ft. increased by 25% is 1.56 cu. ft. - not measly increase, but almost double volume increase from 0.8 cu. ft. empty sealed box.
"I've already made up my mind in that area"
OK, I tried...
Good luck with your DIY project.
No, it IS NOT.
"Going too big runs the risk of the sub bottoming out and causing damage, no?"
Yes and no, depends on amplifier power.
" Also I thought if you went too far above the sealed recommendation, you muddy up the sound."
No, on the contrary!
"Sure, I can go lower by making a larger sealed enclosure, but it won't go to the lengths of the equivalent-sized ported box. And you're right, two typically are louder than just one, but since it doesn't reach as far, the upper frequencies will increase in volume even more than the lower ones I'm looking for"
Perfect for EQ and saving the amp power at higher bass frequencies for lower ones.
" I also read somewhere that 1lb of polyfil per cu. ft. increases the effective box size by 25%. I have done this with the two chambers I had, which would in theory cause the sub to react as if it were in a 1 cu. ft. box, and the response I was looking for simply wasn't there. I don't imagine adding a measly .2 extra cubes will get me the rest of the way there."
1.25 cu. ft. is 56% increase over 0.8 cu. ft.
1.25 cu. ft. increased by 25% is 1.56 cu. ft. - not measly increase, but almost double volume increase from 0.8 cu. ft. empty sealed box.
"I've already made up my mind in that area"
OK, I tried...
Good luck with your DIY project.
...no backup or reasoning? Just no?
Okay this contradicts your "no" statement. If it's possible to bottom out the sub by making the box too big, which you say it is, then it has to have a reference size. Hence, the recommended .88 cubes. It's designed around that size, I'm not trying to say it HAS to be as close to that size as possible, I simply don't want to go too much beyond that.
Also, for the record, the sub is getting quite a bit of extra power. Please ask questions and try to be understanding before jumping straight to arguing with every reason I give you for my actions.
Then why does every search result I've found suggest that making the box deliberately smaller tightens up the Sound but sacrifices low-end, and going too large makes it rumbly and inaccurate? Higher air pressure means more controlled, and a faster spring back to the sub's default state.
I'm not trying to argue here, just nearly everything you're saying goes against what I read elsewhere, and I don't understand because youre not explaining why. Youre just telling me no over and over.
Why would I build a box with louder upper frequencies just to turn them down, when I can build a vented enclosure where the lower notes are more prominent to begin with, being that depth is my main goal? Going with the former is totally inefficient. And turning down higher frequencies doesn't "save amp power" for lower notes, unless it's playing high bass and low bass at the same time -- id much rather have a flat, powerful response across the board naturally. If the box and sub allow for X dB at 35hz, then that's what it's capable of. Perhaps it's capable of more dB with more available power and taming frequencies may be of concern where there are limitations, but as I've said I have plenty of power and have no desire to aim for high frequencies to mute them.
You misunderstand me. My box WITH the polyfil is roughly the equivalent of 1 cubic foot. You suggested building a box at 1.25 cubes, which would be a small increase. And even at that, I wouldn't fill a box that big with polyfil because it's already nearly half a cubic foot over the suggested size. I've already said I don't want to go that route and would rather stay close to the recommendations, please understand this.
Thanks....... That's why I came here in the first place
Did some quick modeling cause I got curious after reading through the post.
I found that the sub seems to prefer a larger box than what was recommended. (Marketing won the coin toss when they decided to make a box recommendation, they are clearly targeting a car audio crowd who generally have smaller spaces to work with.)
The Mfg list the Thiel/Small parameters as having an Xmax of 25mm which seems high to me, but I can't prove it wrong. If it's accurate, then you really would have to put a large amount of power to this to push is beyond Xmax. (Over 300 watts with a large ported box will do it as shown below.)
The box you came up with will work, but the driver clearly will play well with a large and lower tuned port, the sealed box of .88 seems lackluster to me and I can see why you wanted to go ported.
Graph Legend
Red = Your design
Blue = Sealed with Mfg sq ft (0.88)
Orange = 5 sq ft with 18hz tuned port
Signal power was set to 300W (these drivers are rated for 270W RMS, so that more than covers what you can toss at them)
As you can see the Excursion barely exceeds Xmax with a bigger box, which is what generally causes damage with larger ported boxes. (Still not sure I buy the Xmax of 25mm)
Enjoy your project, don't see any reason why I'd say stop with what you've got planned. It looks like it will work.
I found that the sub seems to prefer a larger box than what was recommended. (Marketing won the coin toss when they decided to make a box recommendation, they are clearly targeting a car audio crowd who generally have smaller spaces to work with.)
The Mfg list the Thiel/Small parameters as having an Xmax of 25mm which seems high to me, but I can't prove it wrong. If it's accurate, then you really would have to put a large amount of power to this to push is beyond Xmax. (Over 300 watts with a large ported box will do it as shown below.)
The box you came up with will work, but the driver clearly will play well with a large and lower tuned port, the sealed box of .88 seems lackluster to me and I can see why you wanted to go ported.
Graph Legend
Red = Your design
Blue = Sealed with Mfg sq ft (0.88)
Orange = 5 sq ft with 18hz tuned port
Signal power was set to 300W (these drivers are rated for 270W RMS, so that more than covers what you can toss at them)
As you can see the Excursion barely exceeds Xmax with a bigger box, which is what generally causes damage with larger ported boxes. (Still not sure I buy the Xmax of 25mm)
Enjoy your project, don't see any reason why I'd say stop with what you've got planned. It looks like it will work.
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This is helpful, thank you! I really can't do much about airspace, there's a likely chance the current design is already too big, but I'm going to mock up a cardboard box just to see if it fits in the trunk.
After that I'll definitely play with the port length, try it a little longer and go from there.
One question though, where did you get the 270w figure? The Polk website shoes an RMS rating of 425w and a peak of 800w, is this incorrect? Mine are DVC 4Ohim coils, and my amp pushed the two at 400w rms each at 2ohms. And since it's not single voice coil, my only options are 2ohms and 8ohms but I don't think I'd want to send it 800 watts, unless the gain is turned super low.
If 270 is a more accurate figure though, I'll probably just stick with 8ohms and call it good
Hi Kukuraai,
One thing you may want to consider is using a single 4" precision port. I have used the 2", 3", and 4" on several projects. I have a two 10" sub with a 4" port, overall displacement is more than your single 12" and I haven't had any problems with port noise. IMHO your port area is overkill for that subwoofer and the port length is too long. The manufacturers xmax of 25mm is the peak to peak measurement, so real xmax is probably 12.5mm. The benefits of using a round port over a slot port, is easier to tune and build. Also using the 4" port the box will be smaller than your slot port design. What ever way you choose, post some pictures and your feed back of your build. Good luck.
Regards,
Matt
One thing you may want to consider is using a single 4" precision port. I have used the 2", 3", and 4" on several projects. I have a two 10" sub with a 4" port, overall displacement is more than your single 12" and I haven't had any problems with port noise. IMHO your port area is overkill for that subwoofer and the port length is too long. The manufacturers xmax of 25mm is the peak to peak measurement, so real xmax is probably 12.5mm. The benefits of using a round port over a slot port, is easier to tune and build. Also using the 4" port the box will be smaller than your slot port design. What ever way you choose, post some pictures and your feed back of your build. Good luck.
Regards,
Matt
Please provide some proof that it's not ~1"/25 mm:
Polk Audio AU | Products > Subwoofers > MM1240DVC 12” Subwoofer
This one is more specific: http://www.polkaudio.com.au/uploads/files/Files/MMsubwoofer_WebMN.pdf
GM
Polk Audio AU | Products > Subwoofers > MM1240DVC 12” Subwoofer
This one is more specific: http://www.polkaudio.com.au/uploads/files/Files/MMsubwoofer_WebMN.pdf
GM
Hi GM,
I don't have proof of my claims of xmax. I should have said that I believe the manufacturers xmax is peak to peak. My reasons for doubting Polk Audio's claimed xmax of 25mms is.
1. Price, the MM1240 has an internet price of $129. Displacement costs money and I'm not aware of another 12" subwoofer any ware near this price with 25mm of xmax.
2. Mounting depth, it has a mounting depth of 4 5/8". Most subs with 25mm of xmax have a mounting depth of 6" or more.
3. It has a single magnet motor with a small bumped back plate.
4. The surround doesn't look like it would handle 2" peak to peak excursion.
5. Most subs with big xmax are very power hungry 1000w rms plus.
Just to be clear, I like this subwoofer. I would buy it if I needed a shallow mount driver for a good price. I just don't believe the xmax is right.
Regards,
Matt
P.S. I attached some pictures of my fi audio 10" with 25mm of xmax.
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Greets!
Thanks, fair enough, I came to the same conclusion, but I used a lot of PA's drivers decades ago and they were so 'honest' + quality that I wanted a bit more proof that they've gone the way of their competitors. 🙁
Still, at least this one appears to be a good bang/buck woofer.
GM
Thanks, fair enough, I came to the same conclusion, but I used a lot of PA's drivers decades ago and they were so 'honest' + quality that I wanted a bit more proof that they've gone the way of their competitors. 🙁
Still, at least this one appears to be a good bang/buck woofer.
GM
I've been thinking about using a circular port since it's more efficient than a slot. I went with the size because of the standard (box-volume x 16 = port area in sq. in.) equation, which would be just a hint under the 25sq. in. I came up with. As for the length, that's simply to tune it as low as my goal
The 4" circular is just over 12.5 sq. in., but I'll take your word for it on not having size issues. Now with that, my net volume can be up to 2.2 cubes and to tune it to somewhere between 25 and 27Hz, it'll need to be 19-22 inches long, which is certainly better than the 54" I had drawn up for the rectanglular one 😀
One thing though, since the "ideal" port area for a box that's ~2.3 cubes is 35 sq. in., would it be a better idea to go with the 6" precision port? I don't really know the issues that come with getting a port that's too small/big
The formula that you are using to figure port area is popular in car audio for high output box designs, but are usually tuned higher 35 to 45hz. They tune it high and let the bass transfer function of the vehicle fill in the low stuff. The port length is usually under 25". The drivers used in these types of boxes are high power handling large excursion stiff suspension subwoofers, and don't rely on box loading alone to control excursion. Disclaimer, I'm not an expert with this type box. Now that I think of it, I'm not an expert at a single thing 🙁.
I'm a sound quality type of person, so I have built way more sealed enclosures than ported. If you are picky like I am, this ported box you are building may need a lot of tweaking to get it to sound right to you. Using round ports are way easier cut down than slot ports. My current project is a home theater subwoofer, and I have had to cut the ports six times to get it right.
I don't think that the 6" port will fit in the dimensions of your box, and I don't think they make a 90 for it. Hope this was helpful. I know I am kind of all over the place.
Regards,
Matt
I'm a sound quality type of person, so I have built way more sealed enclosures than ported. If you are picky like I am, this ported box you are building may need a lot of tweaking to get it to sound right to you. Using round ports are way easier cut down than slot ports. My current project is a home theater subwoofer, and I have had to cut the ports six times to get it right.
I don't think that the 6" port will fit in the dimensions of your box, and I don't think they make a 90 for it. Hope this was helpful. I know I am kind of all over the place.
Regards,
Matt
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