Displacement (Xmax VS Diameter, which to choose?)

[Hybrid fourdoor]

In doing some simple math comparing, I came up with a question.

Consider the following:

18" PartsExpress PA 18" woofer with Xmax of 6mm.

12" Peerless XLS 12" Woofer with Xmax of 12mm.

Now with some volume calculations it seems that the 18" will displace 2x what the XLS will even though it has half the Xmax. Plus it has a 4" voice coil VS a 2" on the XLS. So could the 18" driver be just as viable? It seems the load would be better spread out due to the larger VC, plus even though it has a much less Xmax, its displacing so much volume it takes less physical movement to make the same amount of output. They are also at a very close price point.

Other than an enourmous box (but were talking home audio application), would the 18" driver be a good (not asking better ) choice?


Thanks for any help even if this turns into a smaller drivers have faster bass debate.

[kelticwizard]

Well, I don't know which Parts Express driver you are talking about. Their Dayton line does not have an 18 incher, so I suppose you are talking about another brand that Parts Express happens to carry.

The Peerless XLS has a cone area of 466 sq cm. The Selenium 18 incher carried by Parts Express has an Sd of 1190 sq cm. So the 18 incher moves 27% more air, or about 3 dB higher SPL.

The big thing is, though, that most 18 inchers are PA type speakers, not intended for deep bass. They are intended to give big SPL in the bass range of 50 Hz on up. If you can find an 18 incher with the Thiele-Small parameters that can go deep in the size box you don't mind building, then go for it.

Most of the time, though, if you want deep bass, you will probably end up with a 12 incher or, at most, a 15 incher.

[Hybrid fourdoor]

18" Dayton Profesional Driver

Fs: 25Hz
Vas: 16.12 Cu Ft
Qms: 6.74
Qes: 0.36
Qts: 0.34

Seems good, Fs is similar to most good 12" drivers, but still very low.

[454Casull]

You can always use it in an IB if you're afraid of the large box size.

[sreten]

Some classic errors here.

SPL does not depend on total swept volume, so you
cannot use this to compare different sized drivers.

An 18" has ~ 2.25 the cone area of a 12", in the case
of the XLS this is nearer 2.5, and for the same swept
volume will produce 4dB more output due to more efficient
air loading.

(Consider 1 driver versus 4 in series parrallel, 4 drivers
produce 6dB more output with the same input power as 1,
though the swept volume is the same in both cases.)

The difference between the drivers is probably around 7dB.

sreten.

[454Casull]

But _maximum_ SPL is determined by total displacement.

[Svante]

Originally posted by sreten
Some classic errors here.

SPL does not depend on total swept volume, so you
cannot use this to compare different sized drivers.

An 18" has ~ 2.25 the cone area of a 12", in the case
of the XLS this is nearer 2.5, and for the same swept
volume will produce 4dB more output due to more efficient
air loading.

(Consider 1 driver versus 4 in series parrallel, 4 drivers
produce 6dB more output with the same input power as 1,
though the swept volume is the same in both cases.)

The difference between the drivers is probably around 7dB.

sreten.

No, Sreten. Max SPL is determined by the maximum volume flow for a given (low) frequency. So if a driver has twice the Sd and half Xmax, the max SPL will be the same.

Remember, the free field sound pressure from a point source is jwU*rho0/(4*pi*r), where U is the volume flow, r is the distance and rho0=1.2 kg/m3.

It is true that if you double the area the sound pressure will double and the power will quadruple, and the efficiency will double. But that is a different story.

[Bill F.]

David,

If the 18's Qts is appropriate for your desired alignment and you aren't worried about box size, I would certainly go for the 18 over the 12 (assuming motor linearity is good, distortion is low, etc.). The positive effects of the higher efficiency would not be subtle, IMO, paying off in reduced dynamic compression and less distortion in your bass amp's output.

Big cones rock. I listen to a pair of 21-inchers.

[kelticwizard]

I think what Sretan figured I was talking about sensitivity, which is given as SPL @ 1W/1M, which indeed is not determined by swept volume. However, I was talking about max output.

[kelticwizard]

Hybrid:

Sorry I told you that Parts Express did not have 18 inchers in their own brand.

I went to the Parts Express home page, selected "Dayton Loudspeaker", and when the screen came up, I clicked 18" under "Woofers". I got a screen that said my search had returned no results.

Well, you have something here that will give you an F3 of 27 Hz in a ported box of 14 cu ft or so, which is pretty good for a 18" woofer. If you have the place to put the box, go for it.

One question. If you are willing to build a huge box, have you considered using two Shivas? They cost only a little more money, the box size should be 10 cu ft, the sensitivity should be 90 dB, the F3 should be much lower and the displacement for the two Shivas should be much more than for the Dayton.

Just something to think about.