12 replies to this topic

[Blackrazor]

Evening.

Just comparing my two options for my system i'm building : either Sealed, or a strange little invention of my own similar in most regards to a Passive Radiator box. The green dotted lines at 20Hz and 60Hz respectively indicate the range i'm using it to play, there will be a 36db/o LP at 63Hz and a 24db/o HP at 20Hz, maybe 23Hz if overexcursion on the PR's becomes a problem.

Heres the response curves of the two, overlaid. Qtc 0.707 sealed in Yellow, Passive Radiator box in Cyan.



I've tried to minimalise the problems associated with passive radiator boxes as much as i can.

• The PR obviously has a MASSIVE advantage with output at the low end of events... i want that, i can always EQ it out if i want a flatter response but sometimes blatting out 25Hz is good to get a few funny looks.

• The Group Delay is almost identical till about 32Hz, so transient response shouldnt be an issue on most notes. It really does take a trip to see Jesus about 25Hz, but i'm figuring there isnt much demand for ultra accurately timed <25Hz tones Could be an issue tho.

• Phase Response : I'm quite proud of this aspect of the PR... its phase response is actually BETTER than the sealed box until about 22Hz. The main phase shift occurs below 20Hz on the PR, so i'm happy with that

• The Cone Displacement is fine and dandy for the woofers in both boxes, but the PR displacement gets huge (about 26mm) with only 500w input, if i give it the full power it goes to about 50mm and will be way into overexcursion, so i think i might have to raise the subsonic filter point to get a nice rolloff and minimise cone and pr excursion at this point

What are your thoughts? Any suggestions?

[>WAYCON<]

wow. My first thought is where are the schematics to this box?

second thought is about the feeling that would pass through ones inards as a super loud 25 hz tone gets played.

hmmmm, how big is the PR to acheive this?

[Blackrazor]

The PR box is 96L using twin 15" passive radiators with an Fs of 4.2Hz. Yep, as in Four Point Two :hehe:

[>WAYCON<]

:shock:
Dare I ask how much they set you back?
I assume that your arvus dude made them to spec for you?

[Blackrazor]

I havent got them yet as i havent finalised if i will go PR yet, but if i get them they will set me back about $250 NZ each including a mass loading kit. And no, they wont be Arvus ones, although i would get them if i could, Arvus do 5", 6", 8", 10" and 12" passive radiators and the 27" behemoth passive radiator from the Bladder Buster home subs (which i did enquire into, and it was only $650, which is ruddy cheap considering the size and build of it But trying to fit a big 27" cone into an S13 Nissan Silvia was going to be... interesting to say the least)

[Mr_Bob]

hmmm the brahma strikes again!
the PR box needs to be effectively tuned lower....
that group delay spike is still too high it'll make your low notes sound notchy and give them a rough texture.
i think the best ported enclosure for a 12" brahma is ~70L tuned to 23Hz
bass box pro also models teh brahma differently to WinISD, so try plugging the specs in tehre too for some added confusion.

[Blackrazor]

Mr_Bob said:

i think the best ported enclosure for a 12" brahma is ~70L tuned to 23Hz

Just tried that out, the group delay is similar but shifted up about 4Hz, and there is a quite pronounced chuffing noise at high output levels even when using aero ports

Do you really think the group delay being high but only in the mid-20's or less will have that much difference? How? :?:

[Mr_Bob]

are you using 15" version? it'll be different.

my group delay was similar to yours, mostly in the 30's or under, IMO it needed to be tuned lower to force the group delay peak into subsonic frequencies, not in the 22-28Hz region.

aeroports? pffft, i used a 6X8X44" slot port.

[Blackrazor]

Mr_Bob said:

are you using 15" version? it'll be different.

Nope, if i go the PR route it will be a single B12mkII with a coupled pair of PR-15 Passive Radiators (32mm excursion each way)

I can tune it lower if i like but the cone mass for each PR is already up to 1.3kg on that plot, and the maximum recommended is 1.5kg per... i could get the rise to start around 22Hz but that would tune to 16Hz and require 2.8kg loading per PR and i dont think the PR will stay stable if i try that

In reality, how much of an issue is group delay if its below normal operational threshold for all but the lowest 4Hz or so?

BTW i'm a slot port fan to, but to tune to 20Hz with a port and still not have too much compliance for the Brahma, the port would have to be 800mm long and make a noise like a vacuum hose Hence the PR's, all the benefits of porting but none of the box displacement or chuffing noises for low tuning

[Mr_Bob]

as i said before, my output gra[h was similar to that in my ported box and the low notes were "rough" instead of "silky" less control over the cone, but awesome output,
Infact, i ended up using a 36Hz 24db/oct subsonic filter to tame it for SQ comps, so the box's benefits became useless

[Bassaholic]

I'm mostly with Mr Bob on this one..

How much power are you going to be using?

The PRs have the same problem as the ports - although it comes in the form of overexcursion, rather than chuffing. EQing the response (inc using a subsonic filter) will have the same effect on the port, as it would on the PR - so you wouldn't need quite as much port area.

Why 20hz?

Why go for such a large peak when all you are going to do is EQ it out the repsonse?

Why not try it out with a test ported enclosure (you could probably get away with about 20 sq inches, ignore port noise if it happens, depending on power) before going to all of the expense of buying the PRs?

Do you want SQ, or just a lot of SPL at 20hz?

Or you could go for a pair of 15" brahmas sealed...

[Blackrazor]

Bassaholic said:

How much power are you going to be using?

Using? I have no idea, pends on the music. The amp i have for them is rated at 2000wRMS (1000w each) @ 2 ohm for daily driving, 4500wRMS (2250w each) if i wire it to 0.5 ohm (the amp is stable at that) for the odd SPL showoff attempt.

Bassaholic said:

Why 20hz?

Because noone else ever goes that low, and i'd like to try something different

Bassaholic said:

Why go for such a large peak when all you are going to do is EQ it out the repsonse?

Because i figure its better to have the potential and EQ it out, rather than try and create it via EQ when you have a shortage

Bassaholic said:

Why not try it out with a test ported enclosure (you could probably get away with about 20 sq inches, ignore port noise if it happens, depending on power) before going to all of the expense of buying the PRs?

I probably will

Bassaholic said:

Do you want SQ, or just a lot of SPL at 20hz?

SQ... i'm still thinking seriously on Sealed, but like i said, i'd like to also try something difference, hence this post, to try and figure out if the PR design has any major SQ shortcomings

Bassaholic said:

Or you could go for a pair of 15" brahmas sealed...

LOL i could... considering that the subs are where the backseat was, and will be firing almost directly at the back of each front seat, it might be a little... overpowering for SQ usage

[Bassaholic]

All right, try it (ported at first) and find out.

You could always build a sealed box afterwards. :wink: