multi-driver sub question

taylor

Member
2001-10-04 9:10 pm
Hi, I have decided to build a narrow footprint sub, either a BR or TL to replace my large cube subs. I am going to use either two 8" or 10" drivers and would like to know what the best placement for them would be. I can either mount both on the front baffle(my last choice), opposite each other (front/back or left/right), or at 90 degrees (either one front/side or one front/bottom firing). This last option is the one I would prefer. Both drivers will be identical so I should be able to wire them in series and just adjust the VAS (2x)while using all the other T/S parameters for just a single driver right? Any help will be much appreciated. Im an industrial designer and not a very good engineer. Thanks Taylor
 
Not sure about your other questions, but remember that when wiring in series, you're going to have to drive them harder than you probably did with your old subs, since you're going to have an 8-16 ohm load. Can you wire it in parallel? And yes, all that you should have to worry about if driving them in mono is the vas (to the best of my knowledge). I would suggest going with the 10 inch drivers (at least), and not mounting them to the same baffle. You'd be losing a lot of strength, and you'd be increasing the footprint. If your main concern is size, I have some other ideas that you can try (if your not fixated on a TL or BR sub). Subwoofers don't have to be cubes, they can be taller and narrow, or as I prefer, SonoSubs ;)
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
If you are going to wire them in parallel, they should be 8 ohm woofers. If each woofer is 4 ohms, you'd better have a very fine amplifier capable of driving 2 ohm loads.

When you wire in parallel, you divide the impedance in half. When you wire in series, you double the impedance. If you have a fairly inexpensive amp, then I would go with hooking them up in series. It is better to have impedance slightly too high rather than too low.

If you plan on having one woofer firing in front, having a second one above it will not increase the footprint any. If the speaker is wide enough to fit one, it will be wide enough to fit the other.

I agree with Bryan about encouraging you to go with the 10" woofers. Reason? There are a lot of very good 10" subwoofers around. There are not so many 8" subwoofers to choose from that can go really low.

One more thing. There are some 10" subwoofers, such as the Dayton Titanic, which have a very very long excursion. The whole idea of a subwoofer is to move air, and two woofers can move more than one. But they take twice the volume. The Dayton Titanic, (as well as others), can move two or three times as much air as most other woofers-very long excursion. So in a sense it is like having two or three woofers in one.

If you keep the volume the same as for two woofers, but install just one, then that woofer will give you half an octave extra bass-a significant amount.

But before we go any further, would you care to tell us

a) how large you would like to build your cabinet,

b) what amp do you have driving the subwoofer?
 

taylor

Member
2001-10-04 9:10 pm
Thanks for the info gentlemen. To elaborate further, I suppose I should have given you more info. I would like to use dual subs as pedestals for my satalites (braun L200s or dyi scan speak mtms) or in the corners of my living room. I would like a footprint of 12" square or less. With 1" thick cabinets I felt that two 8" or maybe a 10" would supply similar or better LF than my old M & Ks and be far easier to "decorate" with (recently married) I am driving them with bridged Hafler DH200s which I am going to replace. I thought that series wiring would yield a higher impedence that would be easier to drive for a single amp. My days of high spl are over and I'm more concerned with tight controlled bass. I would like to use a single amp to drive the sats and the subs. Simple is now becoming better. As far as the height goes, if they support my sats, 26" is about it but if I use them in the corners, anything up to 9' will fit. What do you think? Thanks Taylor
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Taylor:

Thanks for the info. Things are much more clearly in focus now.

If you use the subs for stands at 26" high, with a footprint of 12" square, and using 1" thick material, then you have an internal volume to work with of 1.38 cu. ft.per channel.

If you go with one amp for both the satellites and the sub, you will not be able to use an active crossover. Passive crossovers that cross over at 200 Hz or so get expensive, but you can still do it.

I have never built a transmission line. But you mentioned that you might decide to put the subwoofers in the corner, and that they can be 9' high if you do. Assuming that the internal height will be 8', (you want to leave some space between the top of the sub and the ceiling), then a single internal barrier will give you a line that is 16' long. That is approximately one quarter wavelength of a 17 Hz tone.

Sounds exciting. Very few people in the world, I imagine, can boast of a transmission line that long, or which goes that deep. But I will leave transmission line advice to those who have experience with them.

[Edited by kelticwizard on 10-05-2001 at 09:26 AM]
 

taylor

Member
2001-10-04 9:10 pm
Kelticwizard, I checked out the Dayton Titanic 10" and it looks pretty impressive. At 90db and an Fs of 25.8, I think a single driver will be sufficient. I will have to relocate the site but I found a plan for the 12" version that was used in a 12 x 12 x 48 inch cabinet to be hidden behind a couch. Very nice design and performance. I'm going to abandon the speaker stand idea unless a driver does exist that can perform at 25 hz in < 1.5 cf. So corner columns are now my focus. If I can design it right, a TL should be able to drop the f3 to under 20hz and be close to a 12 x12 footprint although it will be quite tall. Im also now looking at combining my mtm w/ the sub. This might make an excellent full range speaker. Thanks again for the help. Taylor
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Taylor:

I found at least one woofer to fit your speaker stand enclosure perfectly. It is the Audiomobile EVO 2210 S4. It will go into a 1.4 cubic foot box and be 3 db down at 24 Hz or so-extraordinary. Frankly, I thought you would have trouble finding a subwoofer to go down as far as 30 Hz in such a compact box.

It is $169 plus shipping-just a tad more than the Dayton Titanic. It is rated at 86 db sensitivity, but I would wager your satellites are probably about 88 db or so. You will never hear the sensitivity difference, and the bass extension is outstanding.

Here are the specs: http://www.audiomobileinc.com/main.htm
Here is the price list: http://www.audiomobileinc.com/main.htm

Another possibilty was the Cupric 10 by Ultimate Sound, which still remains a possibility. However, it was a dual voice coil, and I wanted to check how they rated the sensitivity of the subwoofer-different dual voice coil hookups give different sensitivities. http://www.ultimate-sound.com/ProFrame.htm

Overall, the Audiomobile looks the best overall choice right now.

There were other subwoofers that would fit into your 1.4 cu. ft. box and go down to 25 Hz or so, but they were all over $200. Usually well over $200.

A 3" diameter port of 21.5" should tune the box to 24 Hz, which will be your cutoff. This will require an elbow in the port-easy to do, and the price you pay for getting a woofer that goes so low in a small box. You will get a slight dip of 1.5 db at 30 hz or so, but I have never found such a dip to be audible. Don't split hairs-the speaker fits the box!

PS: In no way do I intend this post to discourage you from the transmission line corner subwoofer, (which I still think sounds like a great idea!), or the behind-the-couch enclosure. I just posted to let you know that IF you decide to do the speaker stand idea, you have a woofer to do it with.

PS: Just checked the efficiency of the MTM's. It is 86.5 db @ 2.83V compared to the Audiomobile's 86. Nice match.

[Edited by kelticwizard on 10-07-2001 at 10:07 PM]
 
Cyclotronguy: Yes, this is generally correct. However, some of the higher priced drivers for car audio are of very high quality and can be used for a home environment. Audiomobile is a manufacturer that has been found to produce suitable drivers.

(Another, for example, would be Focal's 11 and 13 inch kevlar reinforced units, which they call "autosound" drivers)
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Cyclotronguy:

I am going by the Thiele-Small parameters. Assuming they are honestly stated, they cannot be adjusted for room or cabin gain.

I think the specs to which you refer are for enclosure recommendations for subwoofers. As in: "Put this woofer in a 1 cu. ft box and you will be 3 db down at 20 Hz." There, yes, cabin or room gain is frequently factored in.

And of course, there is the specs given with the woofer without specifying a box, such as "200 Watts, 56 oz. magnet, 25-400Hz." We all learned to ignore specs like this long ago.

But your advice is well taken, Cyclotronguy, for automotive specs as regards enclosure recommendations.
 
Kelticwizard:

Cyclotronguy has a point. There are some manufacturers out there who don't give anechoic measurements. They publish their specs as "goes down to 20 hz", when its actually a 10 inch driver with an f3 of 40 hz. Some of them do factor in cabin gain, and this must be watched for, but is usually found among lower quality manufacturers.

Although I have no personal experience with the drivers themselves, I've heard positive things about them, and I'm <b>not</b> into car audio by any means. What I've heard about them was evaluated on in home use.

Often, the materials used in car drivers are what make them useful for in home purposes. The cones are usually very stiff and light, often made of aluminum, paper/kevlar or paper/carbon fiber. Also, they can generally go deeper than some standard drivers, because they have a great xmax. Their thick surrounds give them higher power handling, and it doesn't limit their ability to displace air in comparison to some other drivers. (Excursions can be compared subs such as Carver and a few other makes/models)

What I would do when searching for one to use is verify that the measurements are anechoic, make sure you know what the materials are, see what the impedance is, and depending on your system, make sure the power handling is in your range. If you have an amp that puts out 350 watts into 4 ohms, don't go in search of a 10 inch driver with 1000 watts RMS...
 

taylor

Member
2001-10-04 9:10 pm
All things being considered, I think I'm going to try and stay with a driver that is intended for home/monitor use. I would guess that speakers designed for a harse autosound environment must give up alittle "finesse in exchange for robustness". Design compromise in manufacturing is an unfortunate reality unless dealing with an unlimited budget on a benchmark product with little regard for price points. I may be wrong but since my goal is quick, articulate and accurate bass (hence my obsession w/ the TL design) I think that I'll try and stay w/ a proven manufacturer/product used indoors. I do appreciate the research on an appropriate driver for the 1.4 cf cabinet, but the design constraints have dropped it from consideration. By the way, my MTMs are 87db! Good call. So far, the Titan looks pretty good although, as mentioned, Focal drivers seem to have a big following (especially in TLs). I may also now have approval for something w/ a larger footprint as long as they go into the corners. If 2'X 1' will pass muster, I am thinking of doing something like splitting the "Perfectionist" sub in two and turning them upright. Check out the site or an updated version (coffin subwoofer) on the TL Links site.
http://www.hogheaven.com/diyaudio/subwoofers/PATL/patl.html
http://www.t-linespeakers.org/linx/linx.html

Its a monster but if halved and uprighted, it could really be a performer (provided the stated specs are accurate)
Comments? Thank again for all the info, people.

Taylor
 
I'm not sure how you envision this, but it doesn't appear as though there's any way to split the cabinet in two, without having to completely redesign it.

Just wondering, but do you still need a pair of subs? My current suggestions right now would be either a Shiva or a Titanic driver. They rival any driver on the market, cost about $150 (Shiva is about 12), and both are amazingly deep, tight and quick. And not to mention, they can sound amazing in a simple 1cu foot enclosure. Give it some consideration. I'd also suggest going with a plate amplifier, such as the Adire Audio AVA-250, because placing a sub in a corner may cause the need for adjusting the phase and crossover levels: an option not found on most stereo amps. And it costs less than $200. Let us know what your situation is.
 
Taylor,
Splitting the coffin will work. (Bryan, there are two independent channels in there, side-by-side, one firing left, the other right. The diagrams don't make this as clear as they might, but it's a classic TL design.)
But...
The problem is this: That TL is designed for the KEF B-139. If you're thinking of using a different driver, you're not necessarily going to get an optimal match from that particular cabinet. A different driver will probably have a different Fs, hence a require a different line length. It may also have a different piston area. This isn't so hard to overcome as it might be, as the B-139 was roughly equivalent to a standard 10" driver.
Your two choices are to try to lay hands on some KEF B-139s and use the cabinetry as-is. (I've got some, but am not willing to let them go at this time. If you look around, you'll probably be able to turn some up.) Or, to try to match the piston area and resonant frequency with something that is currently available.
Elseways, you might as well start planning a cabinet from scratch. It's not as hard as it might seem at first, I've done it many a time.

Grey
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
I looked up the specs at http://www.speakerspecs.com. The following specs for the KEF B.139-an 8" X 12" woofer-are as follows:

Resonance Frequency [Fs]
25.00 Hz

Mechanical Q Factor [Qms]
5.50

Volume Equivalent of Air at Cas [Vas]
164.0 liters

Volume Equivalent of Air at Cas [Vas]
5.79 ft-3

Suspension Compliance [Cms]
0.00 mN-1

Moving Mass [Mms]
43.50 kg

Mechanical Resistance [Rms]
0.00 kg s-1

X Linear [xmax]
3.18

Cone Area [Sd]
354.00



Electrical Q Factor [Qes]
0.40

DC Resistance [Re]
6.20

Voice Coil Inductance [Le]
0.56

Nominal Impedance [Z]
8

Force Factor [BL]
12.3

RMS Power [Pe]
100



Total Q Factor [Qts]
0.37

Efficiency

0.00

Sensitivity [SPL 1W/1m]

84.0 dB

SPL 2.83V

0.00 dB



[Edited by kelticwizard on 10-10-2001 at 09:32 AM]
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Slight Discrepancy

Taylor:

I just want to point out that "The Perfectionist" seems to claim a sensitivity of 95 using drivers rated at 84 dB @ 1 W/m. Notice The Perfectionist specs never say "95 Db @ 1 W at 1 meter".

My guess is that they parallel the channels, giving out higher Db at any given voltage level, but drawing more power. If you split this in half, and run the halves on two different channels, I think you will find it puts out 84 Db @ 1 W/m. Your MTM's are rated at 87.

Some would like a more efficient sub-but it is not too far away from your satellites' sensitivity. Any substitution driver you might use with similar Thiele-Small specs is likely to be more sensitive, but of course will not be exactly the driver the project calls for. The choice is yours.

[Edited by kelticwizard on 10-10-2001 at 09:30 AM]