Woofers in Series v Parallel, Pros and Cons?

GeoffMillar

Apologies if this has been covered before - I'm sure it has somewhere - but I'm curious about this.

My interest was sparked when I saw this open source three way project which had two four ohm woofers connected in series to handle the bass.

https://sbacoustics.com/product/tifa-8/

I'm not intending to build it but I just found the concept interesting: why have two four ohm woofers instead of one eight ohm woofer?

The stated dB of the four ohm is 92.5, compared with 90.5 for the eight ohm. The woofers are from the SB Acoustics budget range, SB20pfcr-8 and -4.

From my very basic understanding:

• theoretical dB from the two woofers wired in series would be 92.5
• the cabinet for one woofer could be half the size (VAS is the same for both drivers)
• the two woofer design would move more air, so would suit larger rooms
• two woofers would give you higher power handling
• the crossover would be pretty much the same, with some tweaking to deal with 2dB difference
• I assume that the designer didn't use two eight ohm woofers in parallel - for six more dB - as the mid range would not have sufficient output to match, or for the speaker to present a more benign impedance to the amplifier, as was the case with Curt's "Tritrix" MTM.

Are those assumptions correct, please?

Thank you for your indulgence with what's a pretty basic question

Geoff

Wolf

Sounds about right to me.

GeoffMillar

Thanks Wolf

Geoff

4thtry

Also be aware that using two 8 inch woofers, as shown in your linked example, instead of using just one 8 inch woofer, will alter the specific "floor bounce" effect AND the average "floor boundary reinforcement" effect. You can calculate the specific floor bounce frequency using VituixCAD's time window tool. When using two woofers, the specific floor bounce center frequency will be spread out over a slightly wider band. At a listening distance of about 2500mm, the floor bounce goes up in frequency and tends to even out. So you will probably be unable to hear a difference between a single verse double 8 inch woofer set up.

The average floor boundary reinforcement effect, however, will probably cause the low frequency SPL in the 80 to 300Hz region to be boosted by about 1dB when using two woofers instead of one. Which means that you will probably need to apply a little less baffle step compensation (bsc) in the two woofer situation. You can attempt to estimate the amount of floor boundary reinforcement change using Jeff Bagby's Baffle Diffraction and Boundary Simulator (BDBS).

ani_101

I have a design in mind using 2 4 ohm woofers. The reasons for selecting these are as follows:

• these would give me a 8 ohm nominal load - better for some amps
• Do not need high SPL due to the mid having lower efficency
• 2 8 ohm will move more air than one 4 ohm so better distortion at the same SPL or capable of handling higher SPL
• 2 8" woofers fill up a 40" floor stander baffle nicely to place the tweeter at 36" height
• spreads floor bounce
• It would be a sealed alignment - so having extra displacement of 2 woofers over one woofer is helpful to extend the low end with DSP
• The 4ohm woofer was on sale, the 8 ohm wasn't!

On the flip side, these were the reasons for selecting 4ohm woofers in my MAC-05 3 way build

• The mid is a 4 ohm MAC-05
• The woofer section is iso-baric, so it has 4 4 ohm woofers, which sums to 4 ohm and the same efficiency as the mid since there is no gain
• I gain surface area and lower extension due to the box design than using just one woofer. The box is bigger than using just one driver, but not 4x due to the isobaric alignment
• MAC-5 comes only in 4 ohm
• It's DIY so we can combine as we want to - no specific reason as long as it works!

DrewsBrews

@GeoffMillar said:

• the cabinet for one woofer could be half the size (VAS is the same for both drivers)

Qts is .33 and .37 . That will affect the enclosure size too.

GeoffMillar

Fascinating stuff, thank you all.

Another interesting aspect of that design is the cabinet's backwards tilt. Apart from making construction more difficult, it's supposed to provide better time alignment for the drivers.

It's nice that SB makes some of its designs available as open source; full kits with or without cabinets are sold here as well, but they feature SB's more expensive drivers.

Geoff

a4eaudio

@GeoffMillar said:
...Another interesting aspect of that design is the cabinet's backwards tilt. Apart from making construction more difficult, it's supposed to provide better time alignment for the drivers.

Yes, I was surprised to see the tilted cabinet, especially in a build using the budget drivers. My assumption (which could be wrong) is that someone building something with the budget drivers is also more likely to be newer to speaker building and possibly the woodworking. On the other hand, the tilt isn't nearly as complicated as some other things that could be done.

Steve_Lee

One could simply tilt the cabinet with adjustable shims and forgo the extra effort/complexity of building a tilted box . . .

dcibel

GeoffMillar

Of course, why didn't I think of that!

Geoff

DrewsBrews

Gosh making a plinth with the angle seems more difficult than just the cabinet.

6thplanet

If you have a router box for leveling pieces, it wouldn't be to bad. Pretty easy to build a jig to do it.

DrewsBrews

Yeah that was the only practical way I could think of. But I don't have a setup like that.

4thtry

You could use small wedges and then leave the center section of the base hollow. I had a pair of infinity speakers about 30 years ago that used just one small front wedge and one small back wedge to tip the entire rectangular tower back by about 7 degrees. The base was bolted to the bottom of the box with 4 long screws.