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Author Topic: 21" subwoofers - opinions  (Read 1894 times)

Trevor Jalla

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21" subwoofers - opinions
« on: February 22, 2017, 04:29:49 am »

I've been contemplating a pair of double 18s to upgrade from the four PRX718XLFs I have.

I now have a line on 4x 21" units (dB Tech S1521N - powered, bass reflex). Does anyone here use 21s and in what situations would these present an advantage over 18s? Are 21s overkill or unsuited for rock/R&B bands?

Mostly, I operate and supply for 6-10pc bands for corporate events and weddings although I would absolutely be looking for wider applications - maybe EDM or outdoor cinema clients, council festivals etc. I'm wondering if 21s might be too specialised.

Mains are 4x dB Tech D12HP, and they outrun the JBLs. Thanks for any input.
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Ivan Beaver

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Re: 21" subwoofers - opinions
« Reply #1 on: February 22, 2017, 07:26:13 am »

I've been contemplating a pair of double 18s to upgrade from the four PRX718XLFs I have.

I now have a line on 4x 21" units (dB Tech S1521N - powered, bass reflex). Does anyone here use 21s and in what situations would these present an advantage over 18s? Are 21s overkill or unsuited for rock/R&B bands?

Mostly, I operate and supply for 6-10pc bands for corporate events and weddings although I would absolutely be looking for wider applications - maybe EDM or outdoor cinema clients, council festivals etc. I'm wondering if 21s might be too specialised.

Mains are 4x dB Tech D12HP, and they outrun the JBLs. Thanks for any input.
I would look closely at the response graphs and specs and make sure they will do what you need them to do.

I have no opinion about the specific products mentioned, but there are a good number of 21" cabinets that don't actually go as low as people would like to think.

People often "assume" that a larger driver will give more "bass".

That can be true, and it ALSO can be false.

It all depends on how the particular driver is used in a particular cabinet.

There are advantages and disadvantages to larger drivers.

I honestly feel a number of manufacturers simply make cabinets with 21" drivers because they assume people will buy them because they are "better" than smaller drivers.

You MUST dig into the specs (for any product) to see if it is suitable for YOUR applications.

Think of it this way-if bigger was better-why are so many companies making "mini" line arrays.

BTW-it goes the other way also.  Smaller is not always better.
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Ivan Beaver
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Don T. Williams

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Re: 21" subwoofers - opinions
« Reply #2 on: February 22, 2017, 01:10:44 pm »

+1 for Ivan again!
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lindsay Dean

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Re: 21" subwoofers - opinions
« Reply #3 on: February 22, 2017, 02:39:32 pm »

"specs" can be " embellished by the marketing depts. to make the product seem more competitive, in theory.
   maybe see if you can rent or audition ?
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Luke Geis

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Re: 21" subwoofers - opinions
« Reply #4 on: February 22, 2017, 04:15:48 pm »

From a theory standpoint, the 21" should be able to produce more low end at a higher level, but how much more is the question. The next issue is box design. People like the idea of the larger drivers, but it also makes for a non standard box size. So did the company try and repackage a 21" driver into a box more standard to an 18" driver? The proof is in the pudding. The last thing That I would consider is quality of sound. 21" drivers are pretty big in comparison to 18" ones and there is that much more mass to move. So it will react slower to the electrical input from the amp. Many 18" boxes these days will get right around 35hz these days and some even breach the 30hz zone in double 18" form. Shoot the JBL SRX-728's were rated down to 27hz. My question would be more how loud do I really need it to go and will 5hz lower make or break the show? 
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Ivan Beaver

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Re: 21" subwoofers - opinions
« Reply #5 on: February 22, 2017, 05:23:57 pm »

From a theory standpoint, the 21" should be able to produce more low end at a higher level, but how much more is the question. The next issue is box design. People like the idea of the larger drivers, but it also makes for a non standard box size. So did the company try and repackage a 21" driver into a box more standard to an 18" driver? The proof is in the pudding. The last thing That I would consider is quality of sound. 21" drivers are pretty big in comparison to 18" ones and there is that much more mass to move. So it will react slower to the electrical input from the amp.
There are a number of double 21" boxes that barely get to 40Hz.  SO that kinda blows the whole "going lower" theory.

It is a myth that larger drivers move "slower" than smaller drivers due to the mass.

ANY driver will move back and forth at the same speed as the freq applied.

For example-they will move back and forth at a 50Hz rate-if 50Hz is applied.

If it moved "slower", then the freq would change.  That would be VERY noticeable.

As the freq goes lower the "speed" will be slower and as the freq goes up the "speed" will go up.

But the speed is solely dependent on the freq applied.
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Ivan Beaver
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Scott Holtzman

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Re: 21" subwoofers - opinions
« Reply #6 on: February 22, 2017, 06:50:51 pm »

There are a number of double 21" boxes that barely get to 40Hz.  SO that kinda blows the whole "going lower" theory.

It is a myth that larger drivers move "slower" than smaller drivers due to the mass.

ANY driver will move back and forth at the same speed as the freq applied.

For example-they will move back and forth at a 50Hz rate-if 50Hz is applied.

If it moved "slower", then the freq would change.  That would be VERY noticeable.

As the freq goes lower the "speed" will be slower and as the freq goes up the "speed" will go up.

But the speed is solely dependent on the freq applied.


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?


How would one measure such a thing?


I am sure like anything that better materials are making this a non issue. 



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Marjan Milosevic

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Re: 21" subwoofers - opinions
« Reply #7 on: February 22, 2017, 07:06:51 pm »

Eeerm, 21 inch have about 50% more cone area, so they need to move less to shift same amount of air. :-)

Ivan Beaver

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Re: 21" subwoofers - opinions
« Reply #8 on: February 22, 2017, 07:23:59 pm »


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?


How would one measure such a thing?


I am sure like anything that better materials are making this a non issue.
Very often people confuse "speed" with freq response.

They "assume" that the loss of HF is due to "lack of speed".

But in reality it is a combination of the self inductance of the voice coil (that acts like a low pass filter-knocking out the higher freq) and the mass of the cone (not being able to vibrate as easily).

Years ago-whizzer cones were popular.  They were very light and stiff.  They moved along with the cones-but were able to vibrate easier than the main cone body-so it added some more highs.

As usual-it is a combination of several factors, and not a single one.

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Ivan Beaver
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Ivan Beaver

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Re: 21" subwoofers - opinions
« Reply #9 on: February 22, 2017, 07:28:00 pm »

Eeerm, 21 inch have about 50% more cone area, so they need to move less to shift same amount of air. :-)
The old EV 30W (a 30" woofer) did not have very much excursion, yet could go quite low.

They were limited in max output.

Something else that is often forgotten is the fact that the more a woofer moves-the more distortion it produces.

 Large drivers moving a small amount or woofers that are properly horn loaded produce less distortion than woofers that are "flopping around".

Of course some people associate the distortion of woofers as "normal"-because they are not used to clean accurate sound.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!
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