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[Bill Hornibrook]

Thanks for any input.

I'm just a guy in the field and not a pro like many in this forum, so with a bit of reluctance here goes...

As a DJ I own and use a couple of 21" subs in boxes very similar in size to the ones you are considering. I built them primarily for contemporary hip-hop - which brutally and relentlessly dips into the low 30s on a continual basis. They work great for that.

I've used them a few times for bands. The kick is definitely there, but with tonal characteristics that are a bit different than 18s. They sound cleaner but not as warm (for lack of a better word).

That said, I don't think my 21" subs are unsuitable for bands, and don't imagine those dB Tech subs would be either. I also think most bands would be very happy to see them replace your PRXs. And as far as DJs are concerned (if you end up doing some festivals) they are always impressed with 21" subs... just because they are bigger. And I'm not going to explain any further than that...

I looked these up online. If you are spending over $12,000 on subs there are many excellent choices out there. I'd suggest that you don't zero in on one product in particular until you've done some research. You mentioned that you "have a line" on 4, so maybe for considerably less?

[Mac Kerr]

However a 21" driver will require a much more powerful motor and amplifier to accelerate the greater mass.  Is there a practical point where the driver can't articulate fast enough and small transient and detail are lost?

What transient program would make it past the crossover that sends all higher frequencies to a different speaker? Any transient that does make it through doesn't belong there so isn't a concern.

Mac

[Trevor Jalla]

Thanks all for your input. More contentious that I had imagined, and these are not my sole consideration for upgrading from the PRX subs. Yes I concede it comes down to box design, but I can barely find any info on the DVA range of subs (except the S30, which is popular in the field). Also these are an out-of-state buy with no means to demo. I'll ask around rental houses to see if anyone has stock to rent - but I'm doubtful! I'll also look around for response graphs. The on paper specs put +/- 3db at 30-105hz, fwiw.

[Scott Holtzman]

What transient program would make it past the crossover that sends all higher frequencies to a different speaker? Any transient that does make it through doesn't belong there so isn't a concern.

Mac

The transient could be in the pass band of the filter.  What I am trying to say is you have a pounding kick with a fundamental of say 45hz and it each hit is pushing the driver to say 75% of Xmax.  Then the bass player plays a more nuanced note with a fundamental of around 80hz.  Just for giggles let's say that that note is 9db down from the kick signal.  A smaller cone could more easily be accelerated and managed by the motor to create that complex waveform.  The larger driver has so much more compliance volume, shear weight and resistance for the motor to work against. 


Just like a small car can more easily navigate a winding road than a tractor trailer. 

[Mac Kerr]

The transient could be in the pass band of the filter.  What I am trying to say is you have a pounding kick with a fundamental of say 45hz and it each hit is pushing the driver to say 75% of Xmax.  Then the bass player plays a more nuanced note with a fundamental of around 80hz.  Just for giggles let's say that that note is 9db down from the kick signal.  A smaller cone could more easily be accelerated and managed by the motor to create that complex waveform.  The larger driver has so much more compliance volume, shear weight and resistance for the motor to work against. 


Just like a small car can more easily navigate a winding road than a tractor trailer.

The small car doesn't have the advantage of a filter that straightens the road for it. Frequencies in the sub bass range are by definition slow moving. actual impulses go to a different driver.

Mac

[Craig Hauber]

The small car doesn't have the advantage of a filter that straightens the road for it. Frequencies in the sub bass range are by definition slow moving. actual impulses go to a different driver.

Mac

which is exactly why that really slamming kick-drum hit also requires output from the mids and the highs too -all in proper alignment.

[Tim McCulloch]

The transient could be in the pass band of the filter.  What I am trying to say is you have a pounding kick with a fundamental of say 45hz and it each hit is pushing the driver to say 75% of Xmax.  Then the bass player plays a more nuanced note with a fundamental of around 80hz.  Just for giggles let's say that that note is 9db down from the kick signal.  A smaller cone could more easily be accelerated and managed by the motor to create that complex waveform.  The larger driver has so much more compliance volume, shear weight and resistance for the motor to work against. 


Just like a small car can more easily navigate a winding road than a tractor trailer.

No, the cone is already moving, whether at 45Hz or 80Hz.  What you get is intermodulation.  You're mixing wave forms not creating discrete events.

[Tom Bourke]

The on paper specs put +/- 3db at 30-105hz, fwiw.

at what output?  With how much distortion?  Is that distortion "good sounding?"  How big of truck do I need to move it?

Once you get out side of the engineering department the size of the cone is just a marketing thing.  The frequency range stated is easy, depending on output needs.  I have a set of computer speakers that have a 6.5" sub that can hit that.  It just can't cover much more than a small office or bedroom doing it.  The 15" subs I designed years ago can also hit that, and cover a good size room doing it, provided I  add some filtering and EQ.  Speaking of processing, ever hear of BagEnd?  They had a processed system that could get 8 Hz out of very small drivers and boxes.  They had single 18" and dual 12" boxes that were not much bigger than the drivers alone.  I use to run a rig that had 9 18" drivers PER SIDE that would make you feel like the earth it self was opening up right under your feet.  The trade off was the distortion was high enough to fill the next octave above them.

What's the need of your business?  21" drivers in huge boxes because your clients are running EDM shows and like big?  Or like my former biz where renting out a bunch of smaller systems paid the bills?  Or my current situation where one week I have a designer that did not plan for speakers existing  yet the CEO wants to feel the stage shake when he walks on and the next week the client saw there favorite band and HAS TO HAVE!!! the same brand speaker system?

Any sub can make you money, it just has to fit your clients needs, real or imagined.

[John Ferreira]

About size, there are subwoofers that are less than one inch in diameter, but of course with low wattage capacity.

For sub frequencies it is more important the inherent resonant frequency of the woofer, than its size.



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[Chris Grimshaw]

The transient could be in the pass band of the filter.  What I am trying to say is you have a pounding kick with a fundamental of say 45hz and it each hit is pushing the driver to say 75% of Xmax.  Then the bass player plays a more nuanced note with a fundamental of around 80hz.  Just for giggles let's say that that note is 9db down from the kick signal.  A smaller cone could more easily be accelerated and managed by the motor to create that complex waveform.  The larger driver has so much more compliance volume, shear weight and resistance for the motor to work against. 


Just like a small car can more easily navigate a winding road than a tractor trailer.

Cones being fast/slow is a myth and needs to be sorted IMO.
The motor in any modern driver is more than capable of keeping control over the cone and accelerating it as fast as needed.

Here are two drivers that use pretty much the same motor, one 18" and one 21":
http://bcspeakers.com/products/lf-driver/21-0/4/21sw115?impedence=4
http://bcspeakers.com/products/lf-driver/18-0/4/18sw115-4?impedence=4

The difference we are talking about is more like this:
One has a moving mass of 304g, and the other has a moving mass of 335g. That's about 10% difference.
Subwoofers are like having a car being asked to accelerate to 30mph in about 30 seconds, and then smoothly decelerate. That car will not care if it has a couple of passengers that add 10% to the total weight. It's still well within the capability of the engine.
Low frequencies do not require a fast-moving cone. If the cone is moving quickly, it's producing treble and your crossover is set incorrectly.

High-frequency drivers need to be small and light. Here's an example.
A 21" driver moving 10mm one-way at 50Hz has a peak velocity (as it passes through the zero position) of just over 3m/s. The acceleration is around 1000m/s/s, so the voicecoil is exerting just over 300N of force (about 30kg, 66lb). That will result in an SPL of a smidge over 121dB at 1m, groundplane. That's around 1KW power input.
That same cone producing 121dB at 5kHz would be moving 0.01mm, with peak accelerations of 15000m/s/s, requiring 4500N of force (992lb). To produce that force, you'll need to put around 60KW in there and hope it survives.

A 4.5" driver producing 121dB at 5kHz needs to move 0.07mm one-way, requiring a larger acceleration of 77,000m/s/s. The moving mass is about 3g, though, so you only need 231N of force to get it there. That particular driver would need around 5KW input power to manage that, which it won't survive. Dropping the moving mass further (thin titanium diaphragms) and bolting a horn on the front would improve efficiency and reduce the power requirements, though.


Edit - couple of people have posted while I typed that.
Free-air resonance is not more important than the size of the driver. Resonance (Fs), among a bunch of other parameters, will help determine the shape of the response of the driver in a given cabinet. The size and linear excursion, and thermal power handling of the driver will tell you how loud it can go.

Chris

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