TQWT for SB12PFC25-4

[grahamgraham]

Oi oi, everyone.

I'm modelling a woofer for a compact TQWT project I am working on. It will, in turn, become a 2 way system within a yet to be decided tweeter. My aim is for a short and compact pair of floor standers for medium to near field listening.

The mid-woofer is the SB Acoustics SB12PFC25-4.

4" SB12PFC25-4 :: SB Acoustics

4-5mm XMAX and low FS so I feel I should be able to get politely low on hertz.

154cm line (not including the mass loaded vent)

Can Anyone give me feedback on this please?







Am I pushing this driver too far?

Cheers!

 

[grahamgraham]

Straight MLTL

Here's a straight MLTL as an alternative:

109cm line length (not including port)
75cm cross section
9cm x 3.5cm port
Fundamental frequency is 48.2hz

Thoughts anyone?

Cheers!

 

[PeteMcK]

FWIW My BR model has it reaching an F3 of ~36 Hz in 12L tuned to 40Hz. IIRC, these PFC drivers all have a distortion plateau at ~ 1Khz, so it might be better used as bass support for a full ranger such as Dayton PS75 or Peerless TG9... in which case I'd use the 5" version...

 

[PeteMcK]

the 'Alignment Tables Spreadsheet' at the link below is probably the best way to check your results:
Transmission Line Theory

 

[grahamgraham]

FWIW My BR model has it reaching an F3 of ~36 Hz in 12L tuned to 40Hz. IIRC, these PFC drivers all have a distortion plateau at ~ 1Khz, so it might be better used as bass support for a full ranger such as Dayton PS75 or Peerless TG9... in which case I'd use the 5" version...

Thanks, are there any tweeters out there that would go super low to avoid using a wide band driver?

Peerless by Tymphany TPY25T04S0062 - 1" Tweeter , perhaps?

 

[Mayuri]

SB26ADC is the one of the best options for low crossover frequency. I've used it with 1,5kHz BW3 crossover.

 

[PeteMcK]

Re:'avoid using a wide band driver" - why?
wide range drivers are ~ the same size as a tweeter, considerably cheaper, and will give much lower distortion, easier to cross over (I'd cross over at the baffle step freq)

 

[grahamgraham]

SB26ADC is the one of the best options for low crossover frequency. I've used it with 1,5kHz BW3 crossover.

Thanks, wow that frequency plot is astounding! Pricey tho in comparison to my woofers. Maybe if I can pick up a s/h pair. I was hoping to keep this project at a modest budget. Very appealing!

Re:'avoid using a wide band driver" - why?
wide range drivers are ~ the same size as a tweeter, considerably cheaper, and will give much lower distortion, easier to cross over (I'd cross over at the baffle step freq)

A few reasons but I am aware of the benefits of a WAW system as I have Alpair 7MS supported by tang band subs. A great way of eeking a little more dynamic range!

My reasons against are:

A low crossover point as you mention means expensive crossover components.

I have already given myself a challenge to flush mount the woofer and most frames for 2" drivers are square (also muddling aesthetics).

Introducing another box in to the transmission line that might pinch or disrupt the CSA is something I am anxious about as I haven't built my own tl design from scratch yet.

I'm totally up for suggestions, tho, thank you!

Obviously the SB65WBAC25-4 is a contender but too big for my liking. I have also been looking at Peerless by Tymphany PLS-P830983 & inverted 830970.

All food for thought!

 

[GM]

Here's a straight MLTL as an alternative:

Am I pushing this driver too far?

For whatever reason your link doesn't work for me: 4" SB12PFC25-4 :: SB Acoustics

Based on published specs, this is what I'd recommend. Lowest practical reflex tuning is a 1/2 octave below Fs = ~0.707*58 = ~41 Hz, so you're fine.

GM

 

[GM]

I have already given myself a challenge to flush mount the woofer and most frames for 2" drivers are square (also muddling aesthetics).

Introducing another box in to the transmission line that might pinch or disrupt the CSA is something I am anxious about as I haven't built my own tl design from scratch yet.

FWIW, having no router during my speaker building 'career', I always cheated by adding a cosmetic 'skin' to the baffle to mimic recessed drivers.

Better to mount the 2" in a damped tube with it exiting at the back, which will have virtually no impact on the MLTL's pipe action, though if you want to be anal about it, increase net Vb to offset the added displacement same as for the driver, vent and if you line top, walls with damping rather than loose stuffing, then cover the pipe with some damping as even round objects generate eigenmodes with any parallel surface.

GM

 

[grahamgraham]

For whatever reason your link doesn't work for me: 4" SB12PFC25-4 :: SB Acoustics

Based on published specs, this is what I'd recommend. Lowest practical reflex tuning is a 1/2 octave below Fs = ~0.707*58 = ~41 Hz, so you're fine.

GM

Thanks, Greg! I always gleen some wisdom from you! But, how to push the driver lower? It doesn't behave too well, plot-wise, below 47/48hz. I will continue to tinker.

 

[grahamgraham]

FWIW, having no router during my speaker building 'career', I always cheated by adding a cosmetic 'skin' to the baffle to mimic recessed drivers.

GM

Ingenious ideas come from sticky situations! I am fortunate enough to have a router tho, but only a small one.

I did start to wonder about having a template 3D printed to router around> Hmmmmm.

Better to mount the 2" in a damped tube with it exiting at the back, which will have virtually no impact on the MLTL's pipe action, though if you want to be anal about it, increase net Vb to offset the added displacement same as for the driver, vent and if you line top, walls with damping rather than loose stuffing, then cover the pipe with some damping as even round objects generate eigenmodes with any parallel surface.

GM

A damped open tube, like a short TL? Or closed?


This is all academic at the moment as the drivers have not arrived yet and I haven't heard this distortion nor been able to find any info on it. Perhaps Pete can help me out here, please?

It may be that a normal crossover configuration works just fine for this budget build but we will have to see once they get here!

 

[PeteMcK]

I think Klang & Ton did tests on all the PFC drivers when they came out, Google & you're sure to find them. I have them, but not sure which computer they're on... I'll have a look
edit: K&T June/July 2017 p46

 

[GM]

A damped open tube, like a short TL? Or closed?

Some folks have used closed, but I've only done well damped open to get as wide a roll off BW as practical, which most times allowed me to just cap them off.

GM

 

[grahamgraham]

Evening all. Here is the measurement of the single folded prototype. 80g of stuffing, I think I will revert back to my initial 60g.

But what is really weird is the measurement I got from the port on its own. There is a massive spike between 600-1800hz. It doesn't appear to effect overall combined output but what the hell is it?

I shifted the port up to 15cm from the bottom but I expected to see less ripple not more?

Measurements summed and adjusted by -6.6db in xsim.

 

[Qts]

Pipe resonance of the port tube? Is port length half of the wave length of the peak?

 

[grahamgraham]

Hmmm, port length is 11cm so that would be 1500hz odd.

 

[grahamgraham]

I had a moment this evening to test a couple of things.

First of all changed the driver for the second one I have for the pair. No dice.

Second of all I thought I'd see what happened if I added some length to the port to see how the FR reacted. I added 2 inches of pipe and the peak has clearly moved. I am now thinking that I have just picked a really awful spot for the port to be placed (15cm from the bottom of the line).

I have expoxied two more ports on my previous prototype to see if in the unfolded version the port placement gives the same results in the unfolded version. The second port I glued was the same diameter but at the bottom of the line. Let's see..

Green is correct tuning and purple is with 2 inch extension.

 

[GM]

Thanks, Greg! I always gleen some wisdom from you! But, how to push the driver lower? It doesn't behave too well, plot-wise, below 47/48hz. I will continue to tinker.

You're welcome!

When tuning lower than T/S suggests, the cab must be larger than predicted, enough to use damping or digital EQ to shape its response.

GM