If I could impart a little something which I now have little reason to doubt after playing with hifi and PA drivers for over forty years - "Move a Lot of Air - GENTLY". At a given spl and frequency, a small driver flapping around at Xmax is most unlikely to sound as good as a large one barely moving, hence the 4 x 21" and 6 x 18" subs in my system to cover up to 120Hz, each with their own individual amplifier.
My multiple B&C 18" sealed subs are 50l and are the most accurate sounding I have ever heard. They require absurd amounts of power and massive EQ, but try it - you might like it!
I agree, and have seen papers stating that as long as the compression ratio Vd/Vas is less than 5% then linearity and distortion is inconsequential. In reality it is quite difficult to exceed this percentage in a practicable enclosure; 2.5l displacement volume in a 50l enclosure for example.
I realize this is not ideal but I have a 21 Lavoce driver I am considering putting into an 80l sealed cabinet and will be used for movies/music 50/50 up to 100hz. I know this driver is not intended for that use but I have it now and resale would not recoup a great deal. Selling probably leaves enough for a 12 or maybe a 15 Dayton. Neither of which have the output of the lavoce whilst also being pushed to xmax and max rated rms.
Modeled in winisd given 1000rms it actually hits the output I was aiming for even if you take a few db off the predictions. It is not really pushed and at 20hz is only using half its xmax and it is rated at 1700rms. I would be using eq to flatten out the output and the room has a lot of cabin gain so it doesn't worry me too much. Subs are always inadequate in the 50-100 range in my room.
Is it as simple as that or am I missing something (heat buildup, modeling inaccuracies with high bl drivers etc). Any advice is appreciated.
Modeled in winisd given 1000rms it actually hits the output I was aiming for even if you take a few db off the predictions. It is not really pushed and at 20hz is only using half its xmax and it is rated at 1700rms. I would be using eq to flatten out the output and the room has a lot of cabin gain so it doesn't worry me too much. Subs are always inadequate in the 50-100 range in my room.
Is it as simple as that or am I missing something (heat buildup, modeling inaccuracies with high bl drivers etc). Any advice is appreciated.
Hmm........
a = 226 L/80 L = 2.825
Qtc = [2.825+1]0.5 * ~0.28 Qts' = 0.5476 Qtc
Fc = [0.5476*30]/0.28 = ~59 Hz
============
0.707 Qtc max flat = ~42.05 L
Fc = ~76 Hz
With such high tunings [Fc], how much LF boost you use could cause some 'choking' due to VC heat rise with such a high inductance [Le].
I doubt I will need much boost to be honest with the room gain I get from sealed subs in the current room, but I appreciate what you are saying.
So VC heat rise will still be an issue even though its only ever getting 1000rms and it's rated at 1700rms?
My simplistic approach was, whilst not ideal, a high displacement high power handling driver in a small enclosure would be better than a smaller driver suited to the box but with 50% less displacement capability/power handling.
My originals plans changed so I wouldn't buy this driver again but I still have to decide whether it can be used or can it be outperformed using the same 80 liters and amp by a driver that is much cheaper (I guess I'll lose about 50% selling and shipping)
So VC heat rise will still be an issue even though its only ever getting 1000rms and it's rated at 1700rms?
My simplistic approach was, whilst not ideal, a high displacement high power handling driver in a small enclosure would be better than a smaller driver suited to the box but with 50% less displacement capability/power handling.
My originals plans changed so I wouldn't buy this driver again but I still have to decide whether it can be used or can it be outperformed using the same 80 liters and amp by a driver that is much cheaper (I guess I'll lose about 50% selling and shipping)
I can practically guarantee that you will not be short of bass in any sane domestic environment with a sealed 21" - anything around 100l - but treat yourself to as powerful an amp as you can afford, preferably not digital. Any big old hulking transformer amp will do the task admirably, and if you strain your back lifting it, it's probably perfect for the job! You are most likely going to blown away by its performance, and a BIG performance trick is to run the mid/tops through a high pass filter to take the LF load and excursion off them. An old dbx DriveRack is perfect. This is all budget stuff - don't get hung up of the best of everything. Build. Enjoy.
I have run a pair of sealed B&C 18s in a pub PA setting, driven by an iNuke '3000W' amp and aluminium cables (don't laugh - it was a dare) with zero complaints, with one musician saying it was the cleanest bass they had ever experienced, and what on earth had he been playing through all these years!
So with heat rise already causing problems, high Le won't audibly aggravate it?
Here's another 2 cents worth about your prospective project:
If with 80 liters you get Fb=60Hz and Qt~0.5 then you can make it work very nicely using an LT or even a second order shelving filter and at least around 100W (~500W is better). The LT or shelving filter should be used to create an EQ that is cuts power at 12dB/oct starting at 30Hz and ending (flattening back out) at 60Hz (Q=0.7 in each case). The result will be Fb=30Hz, Q=0.5, which will be awesome for a large sealed sub combined with some room gain. It's almost perfect. You will want to be able to adjust the overall gain of the amp channel to match it to the rest of your system.
Well, unless there's some aspect of the design that can sufficiently wick heat away from the VC, then the higher the rating, the greater the coating required to keep the windings intact, which means an increasing insulator with increasing power, so 1000rms is still a lot power to 'sink' and not suffer significant heat rise and inductance's impact on it.
That said, my involvement with high power prosound apps ended back when 500 W transients was huge, so while materials/manufacturing has improved by leaps and bounds, find it hard to believe there won't be significant distortion, though of course would love to be proved wrong.
Last edited:
Yes, it depends on circumstances, needs and signal. But I certainly made my 21DS115 power limited in my 120l box.
But it was ported, so different circumstances.
Now I don't see how air adds linearity if one halfperiod it is pushing, and second one it is pulling inside the box. Onle when you count with this in the suspension system.
If you don't run your sub hot for long, it is okay but with continuous power, I would bake my 21"s at around 1000W. Restricted cone movement doesn't help. You burn a lot of heat to move the cone against small box volume.
Your thought process is about right. If there is enough motor strength, larger driver is better choice. The problem comes with displacement. If that one is restricted, you have long coil hanging out of the pole piece, not getting cooled down. And again not cooled down in small closed box...
If you just have 80l, go for it. 140 would be way nicer.
But it was ported, so different circumstances.
Now I don't see how air adds linearity if one halfperiod it is pushing, and second one it is pulling inside the box. Onle when you count with this in the suspension system.
If you don't run your sub hot for long, it is okay but with continuous power, I would bake my 21"s at around 1000W. Restricted cone movement doesn't help. You burn a lot of heat to move the cone against small box volume.
Your thought process is about right. If there is enough motor strength, larger driver is better choice. The problem comes with displacement. If that one is restricted, you have long coil hanging out of the pole piece, not getting cooled down. And again not cooled down in small closed box...
If you just have 80l, go for it. 140 would be way nicer.
I have never noticed it - by the time it becomes even remotely problematic it is many tens of dB below the main output on decent PA drivers playing ordinary music. I dare say it would be audible if trying to reproduce subsonic frequencies, but you wouldn't generally try that with a sealed enclosure.