As I mentioned in this thread (
https://www.audiosciencereview.com/forum/index.php?threads/dual-opposed-subwoofer-theory.19186/), I'm considering to build Amphion type DOS tower that also could work as speaker stand. I found out there is no complicated theory in DOS so I feel free in choosing driver. But driver needs to fulfills some conditions.
1. Under 400$ per driver - I have to use 4 driver so I have to cut price...
2. Depth under 12cm - my speaker is 7" MTM build, sealed Qtc 0.707 so has 24cm width. Double driver need to be shallower than 24cm.
3. 10" or 12"
4. Purpose well in sealed design. - I want to match to my main speaker Qtc .707
5. My subwoofer poweramp is 4ohm, RMS 300W for each channel. Power output compatible...
Any recommends?
View attachment 105585
View attachment 105585
I'm not clear on whether what you intend to build will have two woofers arranged vertically as shown in the picture, or whether you intend to use two woofers directly opposing each other. Since you emphasized "DOS", my inference is that you intend to build two stands/subwoofers each with two woofers directly opposing each other (such that the reaction forces mutually cancel), which is not like what is shown in the picture. But this isn't consistent with your statement that the depth of driver needs to be less than 24 cm. If they are arranged in a true opposing configuration, the depth of each will need to be no greater than 12 cm. Perhaps what you intend is to offset the woofers slightly as needed, so that each woofer can use the entire 24 cm if need be. But even if you do this, what you are going to find is that any pair of 10" or 12" woofers that are suitable for use as sealed enclosure subwoofers are going to individually need at least 40 liters of enclosure volume. If I am correct in estimating that your enclosure be in the ballpark of 50 liters, there will need to be an adjustment of your design goals.
By the way, a good place to find drivers is by using loudspeakerdatabase.com.
There are lots of subwoofer drivers that are optimized for ported enclosures, which unfortunately makes them less than ideal for sealed subwoofer use. For a subwoofer, I would want the F3 point to be below 40 Hz, ideally down around 35 Hz. Of course it would be better if it were lower still, down around 25 Hz, but this is practically impossible with a sealed subwoofer. Of course it will be helpful if the amplifier you use has built-in equalization, but even if so, it is still desirable for the speaker to require only minimal EQ, and possibly get by without it.
A good formula that I've found for estimating the theoretical F3 point for a sealed subwoofer is .73 * Fs/Qts. The multiplier .73 is good for Qtc values in the customary range, which you indicate will apply. There are drivers with Fs adequately low, down around 20 Hz, and even after multiplying by .73, it is when dividing by Qts that you discover the difficulty. As an easy hypothetical example, if Fs is 20 Hz but Qts is .4, you end up with a theoretical F3 of about 36 Hz. It is easy to see that Qts needs to be no less than .4, but it will most likely need to be bigger than this, because a driver that is suitable in other respects will probably have Fs a good bit higher than 20 Hz.
And with a high Qts value, you run into the problem of the required volume being too great, unless Vas is unusually small.
Following is a list of drivers that I had come up with when doing a paper exercise several months back:
Brand/model Size Fs/Qts F3 Vas SPL Cms Bl Mms
(inch) (Hz) (Hz) (liter) (dB) (mm/N) (T-m) (gm)
Scan-Speak
25W/8565-00 10 20/.41 35 225 88 1.5 8.2 43
Dayton Audio
RSS265HF-8 10.5 25/.49 36 52.5 84 .31 15 131
SB Acoustics
SW26DBAC76-4 11.4 22/.46 34 54 86 .39 10.8 134
Scan-Speak
30W/4558T00 12 17/.32 38 197 89 .65 11 135
Peerless XXLS-
300F50AL01-04 12 22/.41 39 113 85 .37 11 138
Dayton Audio
RSS315HFA-8 12.4 22/.45 35 97.2 86 .26 18 194
Scan-Speak
32W/4878T05 12.6 17/.32 38 180 89 .52 13 165
SB Acoustics
SB34NRX75-6 13.6 19/.40 34 260 90 .71 11 99
If you really want to arrange the two drivers directly opposing each other, the SB SW26DBAC76 would fit. Each of them will need about 40 liters. This is probably the lowest required enclosure volume for any drivers on this list, but when you put two of them together, you'll need about 80 liters. Since I'm more accustomed to thinking in traditional English units, I convert this to a number a little less than 3 cubic feet. If the interior dimensions are 2/3 foot in width and one foot in depth, the enclosure would need to be about 4.5' in height. When you put the small speaker on top, they will be higher than you'll want them to be. If you were willing to make the enclosures wider or deeper front-to-back, you just might be able to pull it off using this driver.
If you use a pair of smaller 8" woofers, this will resolve the problem with enclosure volume, however the smaller woofers are not going to have the same low Fs value, and will not give you the same deep bass extension. The end result would likely be less desirable than going with a single 10" or 12" driver, since F3 will be lower with the single 10" or 12" driver, due to the lower Fs.
If it were me and I was set on this being a sealed enclosure, I would be content with a single woofer in each enclosure. Given that you'd still have a pair of them, it does not seem likely that you'd find the bass lacking. There is possibly another way to solve the conundrum, which is to use EQ to suppress the big peak in the mid-bass that will be associated with the high Q and due to the enclosure being too small for a pair of larger woofers. I suppose you could do this, but I would carefully calculate the theoretical Q and compare it to the EQ capability that will be provided by the amplifier. And even if the amplifier's EQ would be able to flatten the big peak and allow you to use a pair of 10" or 12" woofers, either from this list or something similar you find elsewhere, I'm not sure whether this is what I would do.