Sub w Passive Radiator(s)

[baddow]

I have a pile of older sub and passive radiators sitting around.
They were from my previous attempt to build my "Ultimate" speakers.......These ended up way too big for my wife and got scraped.

But the drivers all still around.

there are four pairs of Scanspeak scan-speak 23w/4557t and matching radiators.

I've tried selling them but only got stupidly low bids and zero interesting in the radiators.

So my next plan is to build a pair of small subs
- aprox/ 1 Cubic Foot volume - as recommended by Scanspeak
- 1 sub / side
- 1 or 2 radiators / side

I modeled these in WinISP, but could not get any good numbers for box size.

Question:
when using passive radiators, does box size matter much at all to tune the base?
OK - I'll be in the ballpark, but does exact volume matter?

Ex, could I go smaller and just cram in the extra radiator?

Thanks

Olaf

 

[planet10]

Scanspeak 23w/4557T

Looks like each driver needs to be in 30-85 litre sealed. It does not look at all suitable for a vented (ie PR) box. I am not a big fan of PRs, just give them away.

With 4 pair that would look like 4 push-push 60 litre (at least) sealed boxes. With some EQ to reduce a rising response at the bototm (and the worst of the room modes) a smaller box could be used.

These things are really inefficient, but faced with a simialr 2 x 4Ω drivers per box i got an inexpensive 4x100W 4Ω Sure aplifier boards, and used all 4 channels to drive the 4 woofers in the 2 subs — since i now have 2 or these amps i could drive 4 boxes like what you could build.

A sub will maximal invoke room gain, it is imortant to have a slow steady roll-off to counter.

dave

 

[planet10]

Undamped response of 30 & 85 litre sealed. Adding volume fill effectively increases the box size and reduces the Q of the box.

Box responses are F10 less than 20 Hz, which means in-room of well below 20 Hz.

If you build 4 much of th eroom anomalies can be worked out acoustically, EQ might be needed to reduce some peaks and possiblut too much box gain.

Do you have any EQ? Amplifiers you will use?

dave

 

[baddow]

I understand your arguement and have gone down this route before.
BUT - the requirement here is to minimize size, not optimise sound.

So I'm looking at a 1 cubic foot volume aprox. using passive rads
The question was,

1 - do I need to optimise volume?

or
2 - does it not matter +/- 50% or so.

Thanks

 

[GM]

Think of the PR as a pipe and as you add mass it makes it longer, ergo tuned lower, so its volume, etc., accuracy will mirror the pipe vented box's.

 

[baddow]

OK thanks

I played with WinISD and optimum size was 28L - about whats recommended by Scanspeak.

Once I get it built, I can play with different weights.

 

[GM]

😀

 

[Milus]

Lower volume will mean less output at tuning frequency and more weight on PR, though how much smaller one can go has to be modelled per driver, it depends a lot on Qts.

 

[Milus]

By the way - I am building a PR subwoofer myself, basen on Peerless XLS and matching Scan PR. Since the box is pretty slim the only way I can position the drivers is to put PR on the wall opposing to the one with active driver. I would have about 1 inch clearance between magnet and PR, inside the box. Are there any downsides to this configuration? Would the output from PR sum correctly with the driver above tuning frequency? Or should I try to move the PR down a little bit to avoid having it directly behind the active driver?

 

[wolf_teeth]

Use a thicker baffle to space the drivers.
Wolf

 

[Milus]

I know what can be done, I just don't know why such close spacing would be a problem, given that PR system is pressure driven, it shouldn'y matter, or could You elaborate why it does matter?

 

[Milus]

I researched the subject and putting the passive directly behind the active driver is correct, it may even improve coupling of passive through air spring. The only problem that came to my mind is that woofer magnets are very strong, and weight discs on PRs are steel, so in extreme excursion the disc could be torn from PR by woofer's magnet if they are too close.

 

[GM]

Lower volume will mean less output at tuning frequency and more weight on PR, though how much smaller one can go has to be modelled per driver, it depends a lot on Qts.

Right, and really surprised that with just a 1/2 ohm for wiring it calcs a 0.53 Qts', quite a bit higher than I figured would work with today's generally [much] less efficient drivers.

Seems like the _{bass would sound obviously 'loose' unless the room's Q is presumed to offset it, which considering the weather in the higher latitudes seems reasonable, but where I live, pretty much only below grade basements are rigid/massive enough.}

 

[GM]

I researched the subject and putting the passive directly behind the active driver is correct, it may even improve coupling of passive through air spring. The only problem that came to my mind is that woofer magnets are very strong, and weight discs on PRs are steel, so in extreme excursion the disc could be torn from PR by woofer's magnet if they are too close.

True enough and on the sides for dual.

Aren't they held on with threaded hardware?

Regardless, it gets too close and the magnet can potentially cause the PR to excurse/extend inward [rarefaction] more than the signal says to, ruining its timing, so with PRs already tending to be a tad 'slow', don't want to make it worse.

FWIW, I know from experience WRT placing a 10 ga galvanized steel sheet between a CRT TV's gun and some of the most powerful prosound magnets up through the '80s [15" with [~0.16-0.2 Qts] was sufficient to negate any visible distortion to then youthful eyes, so maybe an option worth experimenting with.

 

[tomtt]

I have a pile of older sub and passive radiators sitting around.
They were from my previous attempt to build my "Ultimate" speakers.......These ended up way too big for my wife and got scraped.

But the drivers all still around.

there are four pairs of Scanspeak scan-speak 23w/4557t and matching radiators.

I've tried selling them but only got stupidly low bids and zero interesting in the radiators.

So my next plan is to build a pair of small subs
- aprox/ 1 Cubic Foot volume - as recommended by Scanspeak
- 1 sub / side
- 1 or 2 radiators / side

I modeled these in WinISP, but could not get any good numbers for box size.

Question:
when using passive radiators, does box size matter much at all to tune the base?
OK - I'll be in the ballpark, but does exact volume matter?

Ex, could I go smaller and just cram in the extra radiator?

Thanks

Olaf

it's my understanding, that with pr's,

one has the option, to closed box systems,

of,

1/3 reduction of box size,

1/3 octave lower response,

1/3 less distortion.

or combinations thereof.

from the David Weems book(s).

usually, any thing Planet 10 says,

i'll agree with. bc he knows.

however,

since you already have tho$e

pa$$ive radiators,

that are Not Cheap,

you might definitely keep those, to experiment with.

-------------

and what GM says about grounding.

definitely worthwhile.

some people run ground wires to,

a ground, like the ground, or at least to the pluming of the house.

from the shielding.

also the rear of the magnets pole piece.

-----------

with those woofers, an amplifier, with high damping factor,

is a necessity.

 

[baddow]

Lower volume will mean less output at tuning frequency and more weight on PR, though how much smaller one can go has to be modelled per driver, it depends a lot on Qts.

Not an issue for me. I will have 1 sub per channel, for a moderately sized room, as enhancement. I'm not using this for a night club, so volume levels will be lower (than in my bachlor days.)

🙂

Thanks for all the feedback.
It will be roughly a 1 cu ft enclosure.
Roughly cube shaped, but the left and right walls will be curved - to match the speakers.

the sub in the front.

Still undecided if I put 1 PR in the back, or
2 PRs top and bottom.

Its still all in the planning phase. I just moved in and still need to get my workshop up and running fully. Its still a mess and for some reason toys are a low priority, while we complete renos.....

thanks for all the feedback!

 

[Baldin]

Hi daddow

If you have 2 PRs per sub you should if at all possible have these on opposite sides of the box.

I do not really understand why PRs and Vented systems have this bad reputation. In most cases with booming sound, it is the room, not the speaker which is the problem. The reason why this is not as much a problem with a closed box, is because these normally do not go as low in freq.

The only problem with PRs is the weight of the passive, which if only one is mounted will rock the enclosure ... can even make the box "walk around" on the floor 😉
... and not on top or bottom ... they will sag over time ... remember you have quite some moving mass

My friend has just one of the subs you are proposing, and is very satisfied.

Haven't sim'ed it but you might as with all subs and rooms need / want a bit of EQ'ing.
These are super high end units ... will be stellar

KR Baldin

 

[baddow]

Thanks for the feedback.

I'll likely mount 2 PRs per side
(and use them all up - it seems I can't sell them)

Having a spare pair of sub drivers around, isnt a bad thing. 🙂


The subs will mimic the shape of the existing speakers.
Wider front baffle, narrow at the back.
So the PR's will be top and bottom.
Sub driver at the front

And the "satilites" will sit on top of the subs.

The sub amp is a Crown K2 - damping is about 4000 in that frequency range.
Mid and tweeter amps: Crown D-75
Crossover - Behringer UltraDrove Pro