BMS 18N862 8 or 4 Ohms?

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You're welcome!

To the first approximation, no, since the driver wants to 'feel' a certain amount of compliance to help effectively drive the large vents of the day [0.75-1.0x Sd] since the driver's effective motor strength [Qts'] was fairly low and max practical acoustic efficiency was the goal to offset the high output impedance, low power, amps required.

Qts' = Qts + any added series resistance: mh-audio.nl - Home

With today's high power/Xmax drivers, vents ideally need to be quite large to keep them from 'chuffing'/'closing up' in overload, often too large to keep them short, so limiters are used and the vents sized for this lower power at tuning [Fb]. FWIW, here's the math to calculate the vent area [Av] based solely on driver specs, then you can find its length with one of the speaker programs or on-line calculators: https://www.trueaudio.com/st_ventq.htm

T/S filter theory cab design came along after amp output impedance had become a moot point in most apps and the driver's motor strength [Qts] was tailored to dominate until Vas becomes quite high, which in turn allowed much more design flexibility, though not necessarily better performance since the higher power required increases materials, manufacturing, etc., costs and higher thermal power distortion. In short, I view T/S cab design as a diet plan as it pares off all the built in 'fat' [acoustic efficiency] of the earlier cab design routines.

Yes, normally there's not much difference, so for whatever reason BMS chose to make each optimized for either different apps or my SWAG that it's an effort to keep the thermal limited peak SPL response ~same between 4-8 ohm.

GM
 
Great info GM.
I had to read it a couple of times and even study a bit to grasp. It all makes sense.

I see double 18 bass bins of different brands have not changed that much in size in the last 10-15 years but the tuning has shifted downwards quite a bit.
It still amazes me how they manage output so much LF -even considering new drivers with 2-3 times the Xmax of a high power driver 10-15 years ago? Is it clever port design?

The 'problem' using BMS as DIY is that it's really difficult to get in dialogue with them or getting additional technical specs.
 
Hi

Did you ever make those subwoofers?

If you did. How are they performing?

Best regards

Uwe
Hi Uwe.
I only tried them in some old sub cabinets approx. 150l tuned to 30Hz and they do very well. I was hoping to find some way of calculating a curved port put I had no luck so far. Maybe I will end up with a standard cab with ports in the corners.
Remember to make good space in front of driver for the extra excursion :0)

Cheers, Andy
 
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