ISO passive radiator reco's for Mid-Bass coupler design

[john61ct]

Noob here, what follows is from just reading, so please correct any misconceptions below, I realise I'm swimming deep way over my paygrade...

This is not a trueSub use case, the point is bass reinforcement, using a single PR in the front to increase midbass SPL bandpassed well above 60Hz, maybe up to 250Hz if possible, in an enclosure of quality plywood, interior well under a cubic foot.

Inspired by Wolf's project 'Overdrive10' aka Kilauea.

I want as large a PR as possible within the main constraint, a FIXED MAXIMUM baffle size of 12.5" square. This will be strong, at least 1-1/2" thick, the side walls at 3/4".

Does that limit choices to a 10" diameter? If so, can the side walls go thicker? I hope to end up having more than enough interior volume, can go up to 15" in depth...

The Sd should be as high as possible relative to that diameter constraint.

VERY low Mms is critically important to raising Fs, I want the tuning target Fb to get up well above Fs, so unlikely to add any weights.

High Xmax (relatively within the above context) is also important, hoping to make good use of 200+Wrms.

Despite my need for high SPL, I plan to limit my choice of active driver within those at half the PR's Vd.

I know the active driver needs to be stiff (low Cms) and the PR will be driven hard, so I need it to be mechanically strong - should the PR's Cms also be low??

Please advise, and are there other (maybe non-TSP) factors that I haven't considered? e.g. materials used in the PR, general reputable makes for build quality...

Yes, ofc I plan to model before building - @Wolf please advise on which software to use (or anyone else with experience of the bandpass part)

and to use REW extensively, test / tuning feedback looping.

I hope I've understood the basic concepts so far...

 

[Wolf]

I used Unibox because I could track the models together. Model the bandpass and then model the PR chamber to match. Most PRs available have a heavy mass, so low volume is required to increase the tuning.

To reach 250Hz is a TALL order, unless not playing low on the bottom end. To achieve bandwidth gain involves losing output. Remember getting a gain from a bandpass involves further limiting bandwidth. Tuning should be just below the upper rolloff.

A 12" PR means a 10" woofer maximum, and I don't see why you want high output and such in this case. A small Vas is required for small volume, unless not reaching low bass, but a 10" is not normally a midbass.

 

[john61ct]

Thanks for responding!

Most PRs available have a heavy mass, so low volume is required to increase the tuning.

Can you point out low-Mms exceptions for me to investigate?

Do you think Cms should be high, low?

And yes, the low chamber volume requirement is a Good Thing but can't go shallower than the driver ;-)

...

> Tuning should be just below the upper rolloff

Tuning via chamber size sets the latter, smaller the better, right? I thought no added weights would help, that's as high as possible wrt tuning the PR?

...

> 12" PR

I did not expect any that big to fit a baffle size of 12.5" square!

> A 12" PR means a 10" woofer maximum... 10" is not normally a midbass.

Yes the active may well be smaller diameter, its Vd will be limited to half that of the PR so that may mean a relatively low Xmax?

I hope to get a strong Bl and maybe 150Wrms power handling to help deliver SPL, if not high sensitivity.

...

> To reach 250Hz is a TALL order, unless not playing low on the bottom end.

Yes from 65Hz that approaches a full two octaves.

I may be able to stop at 180Hz, so if from 80Hz that means just under 1.2 octaves

and these will be stereo, so two units gets more SPL - is that more realistic?

If necessary I suppose I'd consider splitting the job across one stereo pair at one octave, and then a mono unit for a lower / narrower bandpass.

That would get me right down to blending well with the top range of an Overdrive10 or two

Multiple boxen should also smooth out room modes.

But all that's putting the cart before the horse, at this stage just looking for PRs that come closest to fitting the bill?

 

[Wolf]

Honestly, you'll have to model and measure to know most anything you asked. I was wanting bass, so had no concern for low Mms PRs.

Physical size must by accounted for, including under excursion. If the PR uses a bolt fastener, it cannot hit the driver behind it.

Smaller volume is higher tuning, and only way to increase tuning once added mass is depleted. You can't really remove inherent mass from a PR.

Overdrive10 is 13" square, but there may be lesser-framed PRs out there that may allow a 12" to be used.

I hope you use floor spikes, build heavy, or go dual-opposed for the subs, as they can shake themselves pretty hard.

Also of note, they changed the Ultimax to version 2. I know I posted a remodel with the newer driver (DIYaudio, I think), and you should look for that to see if it still meets your goals.

 

[voodooless]

If it’s about midbass, there are probably better ways to raise the efficiency of the system than a PR. How about a kick-bass cabinet? Something like this:

Bass Cabinet

Community oriented DIY loudspeaker design plans, general resources downloads, chat and forum.

www.freespeakerplans.com

 

[Wolf]

I would too agree a thwack horn would do more for his endeavors in that direction to increase midbass kick. It will also likely have less group delay.

However, it appears he is limited in living space, or is just a PR junkie. Either way, he has some learning to do.

Have fun!

 

[Wolf]

Yes, the model I posted for the U2 is in the thread over there at DIYaudio. The box is in theory a little larger than the original, but not much.

 

[john61ct]

If it’s about midbass, there are probably better ways to raise the efficiency of the system than a PR.

I am hoping that "pretty closely" extrapolating Wolf's Overdrive10 design will work for what I am after, but I would of course be very interested in any other suggestions to get there.

However they need to fit within the < 1 cu ft restriction, specifically the external 12.5" square HxW dimensions limitation I clearly stated in the OP. Depth can go to maybe ~15", some leeway there.

Given the very high air spring / chamber compression of bandpass boxen I would like to go thicker than 3/4" walls rather than thinner, but that reduces interior volume by a LOT.

 

[john61ct]

However, it appears he is limited in living space, or is just a PR junkie.

The acoustic bandpass idea REALLY appeals to me, trading off a **physically controlled** narrowing of frequency bandwidth, for higher SPL efficiency.

Bandpass design using PR seems to me (so far) the best option to get solid low-end reinforcement within the above dimensional restrictions?

I really like the physical construction simplicity, and all driver output exiting out the front only.

My understanding is, mainstream bass reflex designs would require complex "very folded" porting that sacrifices too much interior volume, or simply do not fit at all in < 1cuft total.

 

[john61ct]

Smaller volume is higher tuning, and only way to increase tuning once added mass is depleted. You can't really remove inherent mass from a PR.

Well by selecting one with very low Mms to start...

I just realised, going to a 10" rather than 12" actually leads to a lighter Mms pool to choose from, doh!

So I'm retracting my "want the largest diameter PR that can fit", what I want is

recommended SQ brands for high strength

large Vd to handle as powerful / high SQ+SPL as possible active MBM driver to compensate for lower efficiency

If that is 5-8" so be it - Tang Band style?

So, for lighter Mms thinking better to lower my sights to keep the choice pool within 10" PRs.

 

[john61ct]

The answer to my Cms query - so far - seems to be "lower (stiffer suspension) is better"

along with low Mms helps transient response?

to help control the cone at high SPL and keeping "the response tighter" that is also transient response?

Aligned with lower Vas?

Helps raise realistic Fb tuning target?

finally, low Cms often correlates with a higher mechanical Qms - a good thing in this case?

Again just from googling, so feedback please whether this all makes sense...

 

[Wolf]

The 3ways Andrew Jones designed with the PR on the midbass peak out at 200Hz and use either a 7" or 8" very light mass PR. ELAC?

 

[john61ct]

Adante series, AS-61, AF-61 ?

Apparently 8" PRs, active woofer is 6.5"

But those would not be publicly available components, would they?

 

[john61ct]

my Cms query

...

Aligned with lower Vas?

Actually the Vas must be besides the point for PRs ?

For the active driver sure, Vas is critical in this case

but for the PRs I think (?) I only need to focus on Vd, Cms and Mms / Fs ?

 

[john61ct]

Just came across this

> I would just slap an ex-speaker cone (membrane + basket + suspensions) but without the magnet, onto the box, a size bigger than the active.

Like if you found an old "for parts only" active on eBay?

my mind is boggled...

 

[Wolf]

For the PR; Fsp, Mmp, Qmp, Sdp and Vasp are important. The P is not always used for PR specs, then Mms, Fs, Qms, Sd, and Vas.

These are what you need to model. Cms is calculated from those parameters.

Adante, yes, that's it. Nope, not publicly attainable.

No, a cheap woofer with motor removed will not always work.

 

[john61ct]

For the PR; Fsp, Mmp, Qmp, Sdp and Vasp are important. The P is not always used for PR specs, then Mms, Fs, Qms, Sd, and Vas.

These are what you need to model. Cms is calculated from those parameters.

Besides what I need for modeling, just from a selection POV for this use case these factors I feel pretty certain about:

High Qmp

High Fs

Vdp being as high as possible, within the context of lowest possible Mmp. Since that puts a limit of Vd/2 on selecting a high SPL active driver.

...

While these I am not so sure, corrections and feedback please:

Vas being low is correlated to stiffer suspension anyway, since the latter is the important performance factor, select based on low Cmp. Box size is set by Fb tuning - 70-90Hz?

That's the front PR chamber, correct?

Were you saying the rear sealed box gets tuned higher, say 150-200Hz if possible?

Finally, would going to multiple small PRs make this easier?

Really appreciate your time helping me on this learning curve...

 

[Wolf]

The rear chamber acts as a highpass and the vented or PR chamber act as a lowpass, this is why they are called bandpass designs.

Then the front chamber is tuned just below where you want the rolloff to start.

 

[john61ct]

The rear chamber acts as a highpass and the vented or PR chamber act as a lowpass, this is why they are called bandpass designs.

I must be dense, because I understand the above as a general principle, but haven't yet fully grokked how that connects to brass tacks implementation. Sorry ;-(

So, looks like I had it backwards?

> "rear chamber acts as a highpass"

Say the active driver Fs is 60Hz, and I want the lower rolloff of the whole system F3 to be ~75Hz.

The (only?) way to tune for that HPF, is adjusting the rear sealed box size? Q1

> "Then the front chamber is tuned just below where you want the rolloff to start"

So, as that is LPF, you mean the UPPER -3dB cutoff frequency point, correct? Q2

So, assuming I am not using any tuning weights on the (hopefully low-MMS enough) PR, the only way to tune that LPF any higher (say ~185Hz if possible), is to reduce the rear sealed box size? Q3

Stiff PR compliance required because air spring force is so, high suspension has to work harder against this resistance.

Which if too extremely small, could results in PR damage or blowing out the box joints.

Fortunately the Vd / excursion required is a bit less up at midbass as opposed to trueSub.

Am I on track? Q4

 

[Wolf]

1,2, yup.
Front chamber only for lowpass. Rear and front together decide bandwidth.