DIY Stereo Subs

My father and I have recently started to look into purchasing a sub for our stereo/HT system. We enjoy deep, plentiful bass for movies, but several larger or ported subs simply cannot reproduce the tight, punchy bass of subs such as the newer RELs. But as always, our budget is limited, and the newer RELs can't be had for less than $1200 dollars each. We heard positive reviews about Mirage's BPS 400, but the SAF factor will most definitely prohibit the use of this dual 12" monster. So now, we are strongly considering a pair of DIY subs.

We have a total of about $800 to spend on materials. Constructing the cabinets ourselves is no problem. However, several questions arise. Can we design a sub that extends below 25Hz in a sealed enclosure, that isn't verging on huge? We know that a ported system may be better for home theater, but we do not want to sacrifice the tight, punchy bass for music. We also must choose what drivers would be suitable, and what plate amps would be suitable also. We have a large room (18 x 22 with a high, triangular ceiling), so we need a fair amount of power, but we can also count on a fair amount of room gain. Any suggestions or designs would be greatly appreciated.

(Just ran some sealed enclosure response curves in WinISD, and the NHT 1259 seemed to outperform the Dayton Titanic, Shiva, and Audio Concepts SV12, with a -3db output at about 28hz)

-Bryan

[Edited by Super on 08-12-2001 at 07:23 PM]
 
Bryan,
First things first...SAF is a local phenomenon, and varies widely. That you'll have to judge according to local conditions. Around here, I count myself very fortunate in that department, as even the subs I have (seven feet tall and fifteen inches wide) only elicited comments on the order of,"Golly, they sure are big!" with no further repercussions.
Until I played them, that is...
Now, she's pretty enthusiastic about them, and even makes occasional comments about how the HT system subwoofer (a not-small unit, itself) could use a little beefing up.
Which brings us to the present situation.
First off, let's stick a wooden stake in the heart of this whole ported vs. sealed thing. Neither is superior, sound quality-wise. The ported enclosure will, all factors being equal, be a bit more efficient. That's about the extent of it. To hear some people tell it, all ported enclosures sound boomy or loose, and/or all sealed enclosures sound constipated. It's like the stereotypical (ahem) 'sound' of solid state vs. tubes, i.e. analytical vs overly warm--only the bad ones sound that way.
A properly designed enclosure will give properly damped behavior out of your woofer...period.
What's properly damped?
Oh, now *there's* a can of worms!
Technically, a properly damped driver has a Q of .707. That means that it's maximally flat down until it rolls off. A lower Q, say .5, will begin a gentle rolloff at a higher frequency, but for that you gain some low frequency extension. A higher Q, say 1, will give a hump before rolling off somewhat earlier than you would expect. There are *many* commercial examples of speakers with overdamped (Q>.707) bass. Why? Because psychoacoustically, the ear interprets that hump (essentially just a narrow band where it's louder) as being lower. For that matter, one of the interesting quirks of the human ear is that it interprets *any* frequency as lower if it's louder. It's all part of the voicing process.
On to particulars.
I'd suggest one of the current class of 12" drivers such as the Dayton Titanic. There are equivalent drivers from other manufacturers, such as the Shiva. However, as I've said before, I'd recommend against any driver, regardless of price or claims made, that has a foam surround. Other than that, they're all going to give roughly the same performance. They'll all take huge amounts of power. They'll all get down into the mid-20 Hz region. They all cost about $125-150 each.
As a starting point, just for giggles, think perhaps of one per channel (I use six per channel, but that's because I'm a sick and deviant individual). That pulls in your driver cost at about $300. A couple of sheets of MDF will run you another $50-75. This leaves plenty of budget left over for finishing touches (veneer, lacquer, whatever), not to mention binding posts and wire. At any rate, you're not likely to get shot for getting that kind of performance for half your suggested budget.
Just itching to drop the whole $800? Go for two drivers per side. Four 12" drivers will move a lot of air. Further (and to me, more important), the less each driver has to work, the less non-linear its behavior...in other words, lower distortion.
Back to where we started--SAF. These drivers will all clock in at around 3cf or thereabouts. Consider what proportions you might use on such a box, and see if you can find a cardboard box of about those dimensions. Show it to The Boss. Gauge reactions accordingly:
1) She smiles and says something innocuous like,"That's cute." You're cleared for takeoff.
2) She raises an eyebrow and says,"That big?" Check ticket prices for a random spot on the opposite coast...but don't make the flight reservations.
3) She screams and runs outside...into the rain...without an umbrella. (True story--it happened to a friend of mine when he brought home his new sub.) Take the first flight anywhere. Don't bother to pack a bag, as it might delay you long enough that The Boss might come back and find you still in the house. I take no responsibility for medical bills resulting from lengthy stays in the Intensive Care Unit of your local hospital.
Good luck.

Grey
 
Ok, here's what I'm thinking. Pops has changed his mind, and is now insisting on a single cabinet, with no more than 2 drivers. If I were to use the NHT drivers in a vented enclosure, the thing would be about 54 inches high. If I made this thing, my mother would kill me, and then bury me inside of it. The Shiva's on the other hand, in a vented enclosure, have a -3db slope around 23hz, with a much more reasonable cabinet size. I also noticed that Parts Express has the 250 watt subwoofer amplifier for under $200 dollars, which is right up our alley price-wise. Now I have 2 questions. The Shiva's are each 4 ohms. The sub amp is supposed to go down to 4 ohms, but is that per channel? If not, how could I compensate for the load? Also, I'd have to get rid of that ridiculous +6 db bass boost. Is this simple to disconnect? Do you think that the two Shivas in a single enclosure would be suitable for stereo use and home theater? I really don't want my father to have to go spend $1200+ on a 10 inch REL if its possible for me to make a good sub that's at least comparable for less than half the price. (I need that extra money to build a rack and buy some good IC's now ;))

-Bryan
 
Bryan,

From the posts it appears that you already possess the NHT drivers among others. If not, I suggest that you try out the sub-woofer designed by Jeff McCaulay and published in Electronics World. It uses Four 8" drivers in a bi-polar configuration. The box is ported from the top (rectangular port) and its footprint is just about one square foot. The dedicated filter uses a multi-feedback arrangement, has a variable frequency control (40 to 120Hz) and it -3db frequency is 20Hz. I have made a couple of these sub-woofers and to say the least, they are among the best. Probably they are unbeatable for music (considering the price) and may do just as well for Home Theatre if used in conjunction with a compressor/limiter or if the lower frequency limit is raised to about 28-30Hz.

Photostat copies of the original articles are available with Electronics World's official agent based in the UK or someone in this forum might be able to help.
 
Bryan: A few comments on what it appears you are trying to accomplish.

1) The NHT driver, while a very good driver indeed, is not recommended for a vented enclosure. Its ideal home is a sealed enclosure, mounted vertically as it is used in the NHT 3.3 system. Using it in a vented enclosure may present problems dealing with excessive excursion.

2) The parts express plate amp has a 6db boost that you should probably defeat. PE can typically tell you how to do this, it will involve a little PCB surgery, and will likely void the warrantee. To avoid this, have a look at the amps offered by <a href="http://www.adireaudio.com/cd/amplifie.htm">Adire Audio</a>. They are virtually identical and don't have the 6db boost.

3) If you are going to place more than one driver in a single enclosure, drive it with a mono signal, not a stereo signal. There are very, very few instances of stereo material <80Hz. Frequencies below this level are non-directional anyhow, so even if you did build stereo subs, you're not very likely to be able to tell the difference between that and a single mono sub.

4) Typically, the deeper you want the response of the sub (25Hz, 20Hz, etc) the larger the cabinet you are going to need. Pesky physics! The way to offset this is to look for drivers that have a very low Vas measurement. If you haven't already purchased drivers, have a look at the ones offered by <a href="http://www.audiomobileinc.com/">AudioMobileInc.com</a>. They are more expensive than the average subwoofer, but are vastly superior in construction (rubber surrounds) and performance. They are intended for car audio (excellent response from small sealed enclosures), but work extremely well for home use as well. Another alternative to a large box might be to use cardboard concrete moldings for making pillars that is called "Sonotube". Its very sturdy, and a taller column might be more visually pleasing than a large box. If you do this, be sure that the driver is designed to be mounted horizontally, otherwise gravity will take its toll on the driver.

5) Also, be careful about using WinISD, particularly as you approach the <30Hz high power region. WinISD is designed to model small signals only (<1watt), and may not accurately predict performace at 25Hz with 300 watts of power. There are other box simulations available that are better than WinISD for this purpose. This is not to say that WinISD is not a good product, just be aware of its limitations.

6) The titanic drivers are pretty good, but lots of people report being able to make the driver "bottom out" with very little effort. I haven't verified this myself, but it may be worth looking into a little more (again, if you haven't already purchased drivers). A solution to this is to keep the driver in a sealed box (the Titanic is basially a copy of the NHT driver). See #1 above.

7) When designing your enclosure, design for flat response as deep as you can. Room gain will factor into overall performance, but try not to count on it to help you out. For example, don't design your sub so that it rolls of in the low 30's and then assume that room gain will make up the difference. It might, then again, it might not. In a room as large as yours, room gain is likely to be small. So, make the best design you can, and figure room gain as a bonus at the end...

For $800, you can build an awesome subwoofer. If you go with the AudioMobileInc driver for $300, an amp for $200, you still have $200 left over for the enclosure. Should be plenty...
 
First, in regards to the 8 inch drivers - the sub will be used equally as much for HT as it will for music, so I am going to have to sacrifice the quickness/tightness of the smaller drivers, mainly because I need to go deeper.

I had looked at Adire's site earlier, and perhaps missed their AVA250 amplifier. This looks very well suited to this project, and seems superior to that from Parts Express. If this project goes through, that is more than likely the amp that I'll be using.

I did some more experimentation last night with various types of enclosures and drivers. No matter what configuration I used for the Daytons, they rolled off quite early - in the mid to high 50hz region if I recall. After putting the Shiva's in a sealed box with a relatively small footprint, (19x30 approx), I got a smooth response, with a -3db rolloff in the mid 30's. Putting the NHTs in a sealed enclosure also gives a smooth response, can go down to about 29hz before the -3db rolloff. The box would be a bit larger, but not ridiculously large.

So what I am likely to go with are 2 NHT's in a single, closed box, with a 250 watt adire audio amp.

Now for some more questions. If using the NHTs, how should I configure them in the cabinet? Should I place one over the other? Or should I have each driver on the opposite wall? Would doing this have some form of cancellation? Also, could I even out the dimensions of the box, perhaps making it shorter and wider, depending on MAF (mother approval factor)? I shouldn't have a problem as long as the internal volume is the same, correct? I'd just like to thank everyone for their help once again.

-Oh yea, one more problem. The NHTs have a single, 4 ohm voice coil. Is there a way that I can wire the NHTs to provide a stable load for the amplifier? The amp can handle a 4 ohm load. (With the Shivas, I could just wire one of the 8 ohm coils on each driver, though 4 would again be preferable)

-Bryan

[Edited by Super on 08-13-2001 at 12:56 PM]
 
The exact configuration of your box should not make *too* much of a difference - just make sure it is heavily braced if you go with a standard box to prevent resonance of the box itself.

As for wiring, if each NHT driver is 4 ohms, you have two options. Use a stereo amp (feed it only a mono signal) and wire each driver to its own channel. This will give you a 4 ohm load on each channel, which most amps can handle without too much trouble. The other option is to wire the two drivers in series so that the combined drivers have an 8 ohm load. Drive this with one channel of your amp or with two channels bridged (only if your amp can handle this!).
 
If the amp can't handle the load as is, then I'll probably bridge the two channels.

I changed the box dimensions to approximately 22x37x22, and I think that I'll use the drivers in a vertical arrangement, much like the NHT 3.3. I will probably use 3/4 inch MDF, covered in 1/4 inch veneered ply, and brace it along all edges, with cross pieces between the panels.

Now I just have a few questions about how to dampen them. I am going to line the walls with acoustic foam, and maybe thick felt. Could I put sand in the base of the enclosure for dampening? Also, I plan to use acoustastuff. Approximately how much of this should I use? 100% fill? (I'm just looking for a ballpark figure, I'll tweak them before finishing.) Should I prevent the acoustastuff from touching the drivers themselves? Thanks again

-Bryan
 
I don't think that I would put sand in the enclosure. Too many moving parts for sand to cause trouble with. For your faceplace where you will be mounting the drivers themselves, you may want to use a double thickness of MDF so its about 1.5" thick. Just get a tube of "liquid nails" and glue two sheets of mdf together. Put a heavy weight on them for a few days - should be good enough. Remember, you're going to be weakening this part considerably by cutting two 12" holes in it!

As for stuffing, this starts to become a matter of personal taste. Typically, a vented enclosure is just lined and a sealed enclosure is stuffed somewhere between 50-100%. Remember, 100% stuffing is one pound of fill per cubic foot of enclosure space. You might want to start out with 50% and see how it sounds. Then experiment a little and maybe take some measurements with a test disk and an SPL meter. Play around a little. I don't think it will cause too much trouble if the stuffing comes into contact with the driver - but you may want to push it a little further back before mounting your drivers.
 
I forgot to mention the front baffle, but I was planning on using double layers of MDF already, but was unsure about how to combine the two together. I'll try the Liquid Nails. I'll do what you said and start with about 50% fill.

Any ideas as to how I should attach the drivers? Would wood or drywall screws and nuts on the inside suffice? I dont think that t-nuts grab well enough into MDF to use them. Also, does anyone know of any freeware CAD programs that I could use to do a diagram of all measurements and bracing?

Oh, one last thing. Does anyone know how to get clean edges where 1/4 inch veneered ply comes together? I'd like to round it off if possible. Thanks again

-Bryan
 
For mounting the drivers, t-nuts are the best because you can remove the driver to perform maintenance, then put it back in just as tightly as before without fear of stripping the hole. Wood screws into MDF will last once or twice, and if you over-tighten, you're done!

Some people put small pieces of plywood on the back of the baffle where the t-nuts go to give them something more solid to grab into.

Practically no amp is going to be happy with a 2 ohm load, so I would not suggest paralleling 4ohm drivers. Bridging two channels will definitely provide more power than a single channel (at a cost of higher distortion). However, an amp can only be bridged if it is designed to do so. Have a look through the manual for your amp. If it doesn't say anything about bridging, I wouldn't try it.
 
Ok. This sub is going to be connected to the subwoofer output of a receiver. There is only a single output jack. Would their be a way of maybe using two plate amplifiers? So that I could have one amp dedicated to each driver? One of the primary reasons I want to use plate amplifiers is because they have a built in active crossover. If there are any 250 watt 2 channel amps with a built in active crossover, that can support a 4 ohm load per channel for less than 3-400 dollars, please, by all means, let me know. I would wire it in series, but I really dont think that 180 watts will be enough to power these two NHT drivers. If not, could I possibly use two plate amplifiers from a single subwoofer out? Thanks for the help. I just dont want to have to sacrifice over 70 watts of power

-Bryan
 
Sure, there is nothing wrong with taking the line level sub out from your receiver and using a Y-cable to drive two plate amps. Just be sure to match the output level of the amps if there are any types of output controls present.

On the other hand, you might be surprised what 180 watts will do. A few suggestions here: When you build your box, get a 4 way binding post and wire each driver to its own set of binding posts. This way, you have maximum flexibility to drive it in parallel, series, whatever. Then, give your current amp a try, pushing each driver with one channel. Most amps can drive a 4 ohm load with no problem even if its not directly stated that it can. Just keep an eye on it to make sure it doesn't get too hot...

If this is not satisfactory performance for you, then get two plate amps and drive the sub that way. This way, you don't spend the money unless you really have to.
 
Just an idea

If I buy the plate amp, could I put it in its own separate enclosure? That way, I could just put binding posts/terminals on the subwoofer, and put the amp in a location where the airflow is better (The sub is likely to be place near a corner.) Also, I noticed that the subwoofer amplifier doesnt have a line in for the subwoofer output. Could I just use the left/right line level outs for the fronts and connect that to the subwoofer amp instead? Thanks

-Bryan
 
Bryan,
The Shiva, NHT, and Dayton Titanic should all give roughly equal performance, along with other drivers in their class. I'm concerned that you were getting rolloffs in the 50's for the Titanic and 30's for the Shiva. There's something fishy there.
Eric is correct, all drivers in that class are generally going to give optimum performance in a sealed enclosure. Yes, people use them in ported enclosures, but that's likely to lead to bottoming of any of the drivers, not to mention pretty loose bass, no matter how you tune the enclosure.
I disagree with Eric on the topic of mono vs. stereo subs, as there are enough recordings that are stereo all the way down to make it worthwhile (used to be true that recordings were mono in the bass, but that hasn't been the case for a while), not to mention directionality is evident much lower in frequency than most people realize. That said, if you're running a 5.1 system, you're trapped as far as the .1 signal goes.
It's not difficult to run the left and right line outs into a crossover and have separate plates pushing individual drivers.
Placing a sub in a corner is going to make your life interesting--the corner will act as a rough horn and you'll get unpredictable response. Be prepared to tinker.

Grey
 
I may have some space to tinker, but it may be limited position-wise. After discussion with my dad, the o-holy provider of funding, he wouldn't detest too much to having to purchase a second plate amp. However, two amps in a single box may cause some resonance/cabinet strength problems, so we were still wondering, can the plate amps be put in a separate cabinet? I don't see why not. So, I would be using a separate amp to power each channel. I figure this way, I could hook it up to the stereo outs, or put in a Y cable for the LFE channel. This way, I wouldn't have impedance issues with the amp, either. One more quickie about wiring. My receiver only has a single jack for the outputs of the left and right channel, i.e no red and white rca jacks. The sub amps however, have both red and white input jacks. How could I make this connection work? Same problem for the LFE channel. Only a single output jack, but the signal must be sent to two separate amps, which have no input jack for this type of connection. Thanks again

(BTW, I would make a pair of subs, rather than two in one enclosure, but a pair is out of the question in terms of what I'm allowed to make)

Oh, if possible, could someone use a boxplot program and tell me what results they get using two, NHT 1259's, in a normal configuration in a sealed enclosure? I get an internal volume of about 278.9 liters, and a -3db rolloff of about 29 hz. I appreciate it.

-Bryan

[Edited by Super on 08-14-2001 at 10:18 PM]
 
Re: Just an idea

Super said:
If I buy the plate amp, could I put it in its own separate enclosure? That way, I could just put binding posts/terminals on the subwoofer, and put the amp in a location where the airflow is better (The sub is likely to be place near a corner.) Also, I noticed that the subwoofer amplifier doesnt have a line in for the subwoofer output. Could I just use the left/right line level outs for the fronts and connect that to the subwoofer amp instead? Thanks

-Bryan

You don't really need an enclosure for the amp at all. If you are worried about SAF, perhaps a low-profile mounting of some sort is in order. I also wouldn't be too worried about ventalation, but if you are, don't seal the enclosure. More ventalation is better. Usually an amp won't produce THAT much heat, and it's over a reasonably wide surface area. Still if you must separate the two, just make sure your amp-to-subwoofer wire is of a high power rating and not very long. From the experiences I've had with car audio subwoofers, you can almost never have wire that is too thick.
 
Unfortunately, these plate amps should be in some sort of enclosure, since the face of them is all exposed wiring and components. Having them in an enclosure also gives me a foundation to attach the binding posts. One of the Madisound employees verified that my approximate dimensions were correct. With bracing I should come out with about 8.2 ft^3, and the recommended volume is between 7 and 10 feet^3. Now it seems that my only "problem" will be the wiring. Hopefully I'll be able to get the full 250 watts per channel, if not, I'll settle for the 180 per channel. Can anyone suggest some freeware that I can use to perhaps make a CAD drawing of the cabinet? Also, does anyone have suggestions as to how the cabinet should be braced? I'd like to keep the added volume to a minimum due to size restraints. I'll probably line the seams with 1/4 inch round strips, and 3 or 4 one inch crossbraces from front to back, and the same from left to right, since it will be a fairly tall unit. Thanks once again

-Bryan