Dipolar or box sub for Visaton B200's

[bvan]

Hi. I'm hoping you can help a complete beginner here.

I have some Visaton B200's in open baffle and a "Critical Q" sub that I made. They were my first ever diy projects.

The Critical Q sub I found here: http://www.customanalogue.com/sub_index.htm
It promised to be fast and accurate and I cant say I am disappointed. It blends in seemlesly with the B200's with the help of a Behringer BFD1124.

But I have heard it mentioned that dipolar subs are theoretically better. I've seen some used with the B200 in a sort of H-shaped open baffle and I'm wondering if this might be worth trying.

The sub used a Peerless 12 inch XLS - Model 830500 driver, (Fs: 18.1 Hz * SPL: 90.6 dB 2.83V/1m * Vas: 4.92 cu. ft. * Qms: 3.7 * Qes: .21 * Qts: .20 * Xmax: 12.5mm)

Which design do you think would be better? Is Q=0.5 ideal? I would be willing to give up 10hz of extention if required.

many thanks for any help

bevan

[Calvin]

Hi,

the parameters of this Peerless are not very suitable for a dipolar sub in an ´folded´baffle design. I´d rather use it in cabinets it is intended for.

jauuu
Calvin

[bvan]

Thanks Calvin.

Could you say which parameters?

Linkwitz uses this driver in his dipoles so I thought it might be o.k. (I was thinking of a single driver in an H-frame)

cheers

bevan

[Calvin]

Hi,

Fs is too low for my taste, same with Qts. moving mass on the other side is rather high.
Folded Baffles -regardless of their shape- lower fs even further. But thats contraproductive when Fs (freeair) is already in the subsonic-range. For a 12" driver I prefer a fs around 25-30HZ.
The very low Qts makes massive equalization a must. Since transient response is best with Qts between 0.5-0.7 I prefer drivers with higher Qts values. My rule of experience says the bigger the driver, the lower its Qts can be (for dipole usage), but I wouldn´t choose a 12"er lower than ~0.35.
Have a look at the set of parameters of the Peerless SDS-Series. To my taste the 10" and 12" of this series has nearly perfect set of parameters for folded baffle dipoles..and they have proven to work outstandingly good in such an application. Since they are much cheaper than the XLS or XXLS series, You might think of using 2 SDSes instead f one XXLS with superior results (You might not even need the bassboost in many cases!) ;-)

jauu
Calvin

[bvan]

Thanks very much Calvin.

A question for anybody, what are the subjective qualities of subs with Q less than 0.5? i.e how do they sound (I know EQ will be needed but if I have an equalizer and can live with less extention what are the remaining drawbacks of using a Q=0.2 driver?)

And, if a driver has published Qts of say 0.5, will it still be 0.5 in an open baffle frame? (i.e is it only boxed enclosures that change the Q of a driver/)

Thanks for helping a newby out.

Bevan

[Calvin]

Hi,

when built into an open baffle the Fs sinks slightly and the Qts rises slightly. This effect becomes stronger with folded baffles and reducing the physical dimensions of the folded baffle. In extreme up to 10Hz reduction is possible. Adding passive filtering (i.e. if there´s a ohmic value in series with the driver) will risen the Qt further (normally less than 0.1). But the change in parameter values is of course much less than with any other cabinet.
For a good transient response Qts are normally chosen between 0.5 and 0.9, though it should be kept in mind that the room response spoils even the best transient response of the driver itself.
A Qt of 0.7 is a good choice, because it is a good compromise between output level, good transient response and box volume.
Qts below 0.5 as same as Qts above ~0.8 lead to a longer decay than the before mentioned values and sooner or later this becomes audible.
If You equalize than You add an electrical Filter with Q>1. The product of driver´s Qt and the Filter´s Q forms the resultant Q. In theory it is easy to find a filter Q that gives the system an overall Qt in the right range, but in praxi does this mean to push the driver with more power. I prefer to have a driver doing it´s task rather by itself instead of forcing with brute amplifier-force it to do something it wouldn´t do ´naturally´.

jauuu
Calvin

[bvan]

That helps, many thanks

Bevan