I'd like some pointer on an idea for a 12" sub - Morel or ScanSpeak - diyAudio
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Old 25th June 2014, 01:16 PM   #1
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Default I'd like some pointer on an idea for a 12" sub - Morel or ScanSpeak

Hi all,

First of all: let me start by telling you all that I have little to no experience in loudspeaker design and building. I know a few things about the theory and I have a pair of good ears, but no practical experience. I have a 10" subwoofer, but I'd like something bigger and beefier. What I think I want is very expensive when bought in a store, so I thought I'd check the DIY route. I have a plan (several, actually), but I'm not sure which will work best. I'd like some insights from experienced builders.

My current Adam Sub10 mkII does fine with music, although I'd like some more extension. I get 25Hz, but only just. I had to seal the sub to get it to intergrate well, but it's efficiency in the range where I need it most (sub 40Hz), is really low because of that. It manages 75dB at ca 25Hz before distorting like mad. So movies a bit louder than usual, is a problem.

I'd like a sub that works best with music: quick, deep extension, low distortion, high headroom. HT-use is of equal importance, but I prefer accuracy over grunt. Since my current sub sounds okay but really can't keep up with my Dynaudio Special 25's with movies, something with more power, headroom and extension is very welcome.

I'm looking at two drivers, both have good specs, as far as I can tell. They are the Morel UW1258 and the ScanSpeak 32W4878T00. Both suited for subwoofer duty, I believe. I've run a few simulations in WinISD, but I'm not sure how to relate them to real-world results.

With different alignments, I get different results, obviously. On one hand there is low group delay at around 30Hz (higher at 20Hz), which I think is good for speed (preferable for music). But these alignments lack deep extension. -3dB at ca 40Hz. I think 12" and 13" drivers should be able to do better. I can get lower bass from these simulations, but that results in enourmous boxes, of up to 250 liters!

On the other hand there are alignments with extension to 20Hz or so, but these have high group delay, of more than 30ms at around 25Hz. Less suited for music, but poweful and deep bass. Boxes of 140 - 150 liters.

According to SPL simulation, some 200Watts should be enough to get 105+ dB at is lowest useable frequency. But this 200 Watts wil cause the Morel driver to exceed it's Xmax in the deep bass alignment. 100W will prevent this, but I wonder if with 100W I'll still have headroom and dynamics.

Bottom line: how should I interpret these numbers? Will a box with low group delay and -3dB at ca40Hz deliver deep bass to ca 20Hz in a typical (Dutch) living room of ca 35m^2 using room gain? Or should I really aim for 20Hz f3? How should I interpret the SPL data? Is that comparable with reality? E.g. if WinISD says a box can manage 105dB at 20Hz, will it really do 105dB at 20Hz when it's built and up and running?

These questions are the first that came up. Ultimately I'd like to build a box that gets me quick, deep and powerful bass. But on the other hand: there are some space restrictions...

I hope somebody guide me in the right direction with this.

Thanks!

James.

PS: perhaps totally useless to mention, but boominess is a definite "no go". How, based on the simulations, can one predict if a subwoofer wil sound boomy? Has that to do with resonances in the port(s)?

Last edited by Jamesblond; 25th June 2014 at 01:53 PM.
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Old 25th June 2014, 04:00 PM   #2
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Do you have a crossover/processor, and amplification?

PS - I had good results using an under-sized ported box and applying EQ to boost around port tuning to get a flat response. Beware power issues, though - impedance and cone excursion are at a minimum, so the drivers will have to have good thermal management.

Last edited by chris661; 25th June 2014 at 04:06 PM.
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Old 25th June 2014, 05:23 PM   #3
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The other way around (no EQ) you can use the ScanSpeak 32W4878T00 but not in a sealed enclosure. F-3dB 27Hz more like not 40Hz as you mention. I wouldn't consider that ""lack deep extension"".
Voice Coil specs
Madisound alignments link
You should use passive radiators better for best performance. Very long ports is not desirable. 2 passives for each 13" driver is an option.
ScanSpeak 30W/0-00 12" Passive
Passive Radiators

Quote:
When should I use a passive radiator? With what woofer?
Use a passive radiator when you would like to have the extra output of a vented system, but are not able to fit an adequate port into the enclosure. The tuning of a PR is determined by the amount of mass on the diaphram, where the tuning of a port is determined by length. Often if you want to get low tuning in a small enclosure, an adequate port will just not fit. You either need to sacrifice output by going with a smaller diameter port, or sacrifice low end extension by going with a higher tuned port. For example, we use a single 15" woofer in an enclosure tuned to 20Hz with a pair of 18" PR's. If you wanted to get the same tuning with an adequate port, it would need to be 6" diameter and nearly 60" long and you would STILL have problems with vent noise.

You typically only want to use a passive radiator along with a woofer that is designed for a vented enclosure. A passive radiator enclosure is NOT a sealed box enclosure. You will typically want to use a lower Q woofer with a PR system. This usually means a woofer with Qts of .4 or lower.
link
You can plug the reflex in the mains and get better summation with sub at crossover frequency (100/150Hz).
Dynaudio Special Twenty-Five loudspeaker Measurements | Stereophile.com

You can always opt for a different woofer (or longer 1M ports) if you don't like the passives alternative.

You can use the Morel Ultimate UW1258 working as above (EQ or not) it has plenty of excursion but in reality is 5 dB's (sensitivity) less than the SS.
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Old 25th June 2014, 06:52 PM   #4
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Go with a sealed box for the best sound quality, and add a custom, diy Linkwitz transform EQ to make flat response.

How: Make a box as big as your wife will allow, to minimize the amount of power you'll need for bass boost EQ. If big is bad, then make it small, no problem, the EQ can make it behave and sound like a big box, but it will take more amp power and a higher driver power rating. So pick a driver that has high sensitivity and high power rating and strong cone construction, like a quality pro-audio bass guitar speaker. Eminence Alpha need not apply. The hifi speakers you mentioned will work fine. Build the box, install the speaker and use woofertester to measure the speaker/box parameters to get the values you need to calculate the transform filter parts values. Use the calculator, info and PCBs here to build your custom EQ for $50-75. You could also use a commercial EQ box like Behringer DEQ2496 or miniDSP with a SPL meter to create the FR you want but they are expensive for this simple job and you want to diy it anyway, right?

If you don't want no stinkin electronic EQ, then you will need a big box that can naturally make Qts=.57 with a big enough driver cone that works with the bass boosting reflections of the room to yield a net flat bass response. Rolloff of sealed box is -12dB/oct, while bass boost of room is +12dB/oct, so if you get the right rolloff slope (Qts) and big enough cone to start the rolloff at the right freq, then you end up with flat FR - in room. Smaller cone for smaller rooms. Use a free speaker box designer software like "WinISD Beta" to determine the desired Qts for a given box volume and driver parameters.

A low Qts will boost the bass at LF for more sensitivity, but it also improves the transient response, making the bass sound drier, faster and clearer, at cost of size, weight and more wood. Qts=.57 is an ideal value for sound quality if you can tolerate the big box. The lowest Q is .5, most people think that sounds too dry and the box is too big. Qts = .7 would be the smallest box for high sound quality, but with a linkwitz transform, up to Q=1 is OK.

Remember, the box volume (Vb) does not include the internal volume displaced by the driver protruding into the box, nor the internal box bracing, but Vb does include the volume occupied by soft stuffing.

The larger the box, the more difficult it is to prevent it from flexing and resonating from the pressure and vibration of the driver. If it flexes a lot, like it will with no bracing, you will hear it and you will hate it. The braces should connect opposite and adjacent faces, not just beams glued to the inside panels. That's another benefit of Linkwitz transform: it allows the same sound from a smaller box which is easier to prevent flex and vibration.

A 250W plate amp should do the job if you use Linkwitz Transform bass boost, with listening peaks of bass around 100dB in a medium size room. That's very loud for bass. Good luck!
Rich
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Old 26th June 2014, 07:08 AM   #5
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@ Chris661: I have a Rotel RSX-1067 and intend to use it's bass management to drive the subwoofer. I have my current sub hooked up to one of it's subwoofer outputs and that works okay. Only draw back is that the cross-over point can't be set lower than 40Hz on the AVR, no lower than 50 if I use the sub's LPF. I prefer the subwoofer to cover as small a frequency range as possible. At least: based on my current configuration.

@ Inductor: I've nor simulated boxes with PR's. I'm not familiar with that technology, and I'm guessing it will become too large and too expensive. For now, I'm focussing on ported, or sealed and possibly EQ'ed.

@ Richidoo: do I understand correctly that the Linkwitz Transform EQ only works with sealed boxes? How does the added gain affect X-max with regards to the extra amplifier power needed? I think I can't simulate that in WinISD. I'll try to post some pictures of the simulations I have in mind. If I'm completely off with them, please say so. As mentioned before, I'm ony just losing my virginity on this subject...

Last edited by Jamesblond; 26th June 2014 at 07:12 AM.
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Old 26th June 2014, 09:13 AM   #6
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Some pictures of the plans I have. Your comments, please.

Also please note the X-max curves (simulated at 200W input). How do I relate that to real-world situations? E.G. I put a 500W amp in them, EQ-them for 20Hz extension and play a bass-heavy movie loudly? Will the drivers be in danger of mechanical damage of overload? Other possibe issues that may arise?
Attached Images
File Type: png Morel UW1258 transfer function.png (46.4 KB, 96 views)
File Type: png Morel UW1258 SPL 200W.png (39.5 KB, 95 views)
File Type: png Morel UW1258 ConeX 200W.png (45.2 KB, 95 views)
File Type: png ScanSpeak 21W4878T00 transfer function.png (48.8 KB, 95 views)
File Type: png ScanSpeak 32W4878T00 SPL 200W.png (44.9 KB, 95 views)
File Type: png ScanSpeak 32W4878T00 ConeX 200W.png (45.9 KB, 7 views)
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Old 26th June 2014, 04:50 PM   #7
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Quote:
Originally Posted by Jamesblond View Post
Also please note the X-max curves (simulated at 200W input). ...Will the drivers be in danger of mechanical damage of overload?
Lets' say you have both drivers in a sealed 110L enclosure, to make things easy.
You apply high pass filters to both drivers to protect 200W at max excursion Xmax. In the first case for the MOREL UW1258 using a Butterworth 6 dB/octave (-3 dB)@27 Hz and in the second driver SCANSPEAK 32W/4878T00 a Butterworth 6 dB/octave (-3 dB)@19 Hz.
Spl is 108.3 dB in the first case and 110.1 dB in the second, for one driver/speaker/sub (1M), less ~2.8 dB for thermal att. (TA).
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Old 26th June 2014, 05:45 PM   #8
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Quote:
Originally Posted by Jamesblond View Post
@ Chris661: I have a Rotel RSX-1067 and intend to use it's bass management to drive the subwoofer. I have my current sub hooked up to one of it's subwoofer outputs and that works okay. Only draw back is that the cross-over point can't be set lower than 40Hz on the AVR, no lower than 50 if I use the sub's LPF. I prefer the subwoofer to cover as small a frequency range as possible. At least: based on my current configuration.

@ Inductor: I've nor simulated boxes with PR's. I'm not familiar with that technology, and I'm guessing it will become too large and too expensive. For now, I'm focussing on ported, or sealed and possibly EQ'ed.

@ Richidoo: do I understand correctly that the Linkwitz Transform EQ only works with sealed boxes? How does the added gain affect X-max with regards to the extra amplifier power needed? I think I can't simulate that in WinISD. I'll try to post some pictures of the simulations I have in mind. If I'm completely off with them, please say so. As mentioned before, I'm ony just losing my virginity on this subject...
Linkwitz transform only works with a sealed box.

Xmax is not related to power or gain. It's just a factor in determining max SPL at given freq. This calculator can help you focus on the drivers' capabilities. Piston Excursion Calculator

WinISD is most helpful for determining the Vb for desired Qts and a given driver specs.

Your preference for accuracy over grunt, and your desire for high SPL at low freq indicates that you would prefer a larger diameter cone. Going big has lots of benefits. 15-18"

Last edited by Richidoo; 26th June 2014 at 05:51 PM.
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Old 26th June 2014, 06:03 PM   #9
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My recommendation would be to go with the Scanspeak. This isn't to say that the Morel is going to be worse in spec, but Scan have a reputation for their drivers being very good. This is mainly because Klippel tests, at least with respect to linear excursion, are generally bang on spec for Scan drivers, whereas others seem to fall a little behind.

As we're comparing driver performance based on swept volume displacement, a driver actually being able to work up to its xmax spec, whilst being linear, is going to be pretty important and I would trust the Scan to do this more than the Morel.

It would be nice however to see a full set of measurements on some of Morels newer designs, just to see if they've improved somewhat. One things for certain though, Scan never needed to improve, they've always been top notch.
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Old 27th June 2014, 07:53 AM   #10
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@ Richidoo: That's a nice link, thanks! X-max is not all that important in the bigger picture, but it is a limiting factor in achieving SPL at a given frequency. It seems that with the idea I have in mind the ScanSpeak should be able to achieve 90+ dB at 20Hz. That's more than I probably need. So it seems this could be a very nice subwoofer with pleny of headroom, if I manage to finish the project. But let's first focus on designing the thing...
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