Multi Driver .5

[Einric]

I am looking to use two RS150-4's as the .5 driver in a 3way slim tower.
I was wondering about Zobels.
Would it be better to put a Zobel on each woofer or could I calculate the equivalent Zobel for a driver of twice the inductance and resistance since they will be wired in series?
It would definitely be cheaper to have one Zobel on two woofers.

[PeteMcK]

a zobel is unnecessary... its effect on the .5 way is neglegible, its effect on the main driver can be replicated by tweaking existing xcrossover parts. If you must have one, do one for both drivers.

[Einric]

At the frequency that I am intending on bringing the .5 in (100hz) each driver has an impedance of 6ohm.
I will have a series impedance of 12 ohms which makes for a large inductor?
Correct?
19-20mh or so?

[AllenB]

With the drivers in series, your zobel (if you use one) would use twice the resistance and half of the capacitance compared to a single driver. The inductance would double.

For the pair in parallel, the resistance would halve, the capacitance double, and the inductance would halve compared to a single driver.

[PeteMcK]

either you have a very wide baffle, or you're trying to bring the .5 woofer in too low.
It should be at the baffle step freq = baffle width(metres)/115. A bonus is that you'll also get more sensible size inductors. Go for parallel if possible.

[Einric]

The baffle frequency is about 600hz.
That is the frequency of the low pass in the crossover.
I was thinking that the .5 would just reinforce the frequencies 1.5db below the F0 frequency and lower.

[Einric]

Any suggestions?
I could bring the .5's in at 300 or so.
The RS150-8 main woofer will run sealed in a separate chamber from the pair of RS150-4's which will be ported for low extension.

[AllenB]

It is common to use a second identical driver that is crossed to cut in as the baffle transitions. It is normally done with a first order filter. This would require impedance compensation or equivalent. You'd normally make sure it is rolled off before the upper crossover.

In your case you'll want to ensure you are getting the correct sound volume out of the other drivers.

[Einric]

My calculations show that the two .5 drivers would be 3db higher than the main woofer.
That is why I thought it would be better to cross in lower.
I had figured on implementing zobels for all the woofers.

[AllenB]

Ok, the eight has a sensitivity of 87dB, that's per Watt. If we make the sweeping assumption that the fours' impedance is half of the eights', two in series will dissipate 1W at the same drive level as the eights will. At this point the eight will be at 87, and the fours, which have a sensitivity of 92 will be at 89 due to them only dissipating half a watt at that point. However, since they're paired up they'll be running together at 92 at low frequencies.