Help with vented box alignment?

[chris661]

Originally Posted by iand

If the 1.25ft3 reflex box is tuned to ~30Hz then it'll need about 7dB of bass boost to flatten the response, then a fast rolloff (HPF) below this to prevent high excursion when unloaded -- you can probably do both with an underdamped 2nd order HPF, but to get a really flat response you will probably need more complex parametric equaliser (as well as a HPF).

The Xmax-limited maximum output of a reflex box is about 8dB higher than an Xmax-limited closed box (or 2/3 octave more extension for the same SPL), and the efficiency near cutoff is usually higher as well.

There's also an excursion peak 1/2 octave above port tuning. That would cause problems (45Hz is within the kick drum and bass guitar range).

You can't go saying that you get 8dB more out because, when tuned to 50Hz, a sealed box will have loads more output at 10Hz, using less excursion.

Chris

PS - putting a second order LP filter on it set to 20Hz would get it to go low (keeping it sealed), but efficiency is through the floor.

[iand]

Originally Posted by chris661

There's also an excursion peak 1/2 octave above port tuning. That would cause problems (45Hz is within the kick drum and bass guitar range).

You can't go saying that you get 8dB more out because, when tuned to 50Hz, a sealed box will have loads more output at 10Hz, using less excursion.

Chris

PS - putting a second order LP filter on it set to 20Hz would get it to go low (keeping it sealed), but efficiency is through the floor.

OK, I'll be more precise -- at the box tuning frequency or above, a reflex has 8dB more Xmax-limited output than a closed box with the same driver.

Yes there is an excursion peak 1/2 octave above the port tuning, but the excursion here is the same as a closed box with a 3dB cutoff 2/3 octave higher than the reflex.

[chris661]

I'm struggling to see where this 8dB more output has come from. Using a generic 8" driver, in a 19L sealed box, Q=0.707, f3=70Hz and vented 34L tuned to 37Hz gives an f3 of 40Hz. Using the max.SPL tab on winISD, the ported one, at tuning frequency, can put out over 20dB more. Below 25Hz, the sealed one has a higher maxSPL. Above tuning, the SPL increase is very slight (6dB, but the difference narrows quickly as the port becomes inactive). I fail to see where 8dB comes in to it (an average, perhaps?).

Anyway, the conclusion seems to be go for sealed here, as vented boxes need to be much bigger than that cabinet.

Chris

[iand]

Originally Posted by chris661

I'm struggling to see where this 8dB more output has come from. Using a generic 8" driver, in a 19L sealed box, Q=0.707, f3=70Hz and vented 34L tuned to 37Hz gives an f3 of 40Hz. Using the max.SPL tab on winISD, the ported one, at tuning frequency, can put out over 20dB more. Below 25Hz, the sealed one has a higher maxSPL. Above tuning, the SPL increase is very slight (6dB, but the difference narrows quickly as the port becomes inactive). I fail to see where 8dB comes in to it (an average, perhaps?).

Anyway, the conclusion seems to be go for sealed here, as vented boxes need to be much bigger than that cabinet.

Chris

If you ignore the frequency response and just apply as much power as necessary to reach Xmax -- this may be more or less than the thermal rating of the driver -- then above the box tuning you can typically get about 8dB more output (at the maximum excursion frequency) from a reflex than a closed box, this could be a bit more or a bit less depending on the driver and box used.

Since each extra octave needs 4x the excursion, 8dB means 2/3 of an octave extension without being Xmax limited (if the example you chose had 6dB this would give 1/2 an octave). To pick an example, the 10" driver I use in our band's PA (Celestion BN10-300X) will handle 300W down to about 125Hz (-3dB point) in a closed box or 80Hz (-3dB point) in a reflex before Xmax is exceeded. You don't want to be using high-Q drivers in this case.

You can build a reflex in a small box which works down to <40Hz, but you need a suitable driver (e.g. Ciare 12.00SW) and long ports with enough area to prevent port noise -- this can mean about 500mm long, but you can use drainpipe bends to get more length in. Of course there are limits, once the ports start occupying a significant part of the box volume you're onto a long slippery slope...

For low tuning in a small box passive radiators have advantages, but you have to find them and then build them into the box.

And of course if the driver has lots of Xmax and not so much power handling (which could be the case for this one), you won't run out of Xmax in a closed box anyway :-)

[bjorno]

Originally Posted by clm811

Need some help.

..generic 12" woofer.. need help with a vented alignment..

The parameters are:
Fs: 23.55 Hz
SPL: 89.89 dB 2.83 V/1m, 90.5 dB 1W/1m
Vas: 6.551 cu. ft.
Cms:0.0901 in/lb
Mms: 3.132 oz
Rms: 7.358 lb/s
Re: 9.207 ohms
Le: 2.836 mH
Z=11.05 ohms
BL: 4.231 lb/A
Pe: 150 Watts
No: 0684%
Qms: 3.936
Qes: 0.341
Qts: 0.314
Xmax: 0.42 in (11mm)
Xmech: 0.63 in (16mm)
P-Dia: 9.972 in
Sd: 78.1 sq.in
P-Vd: 0.019 cu. ft

.. so the net(actual) volume is is probably closer to 1.25 cu.ft...

..Anybody here to give a recommendation for box frequencies and vent length ..

Fyi, Referring to whats been said in post#2 and 8 : See the picture.

b

[clm811]

CharlieLaub wrote:

I recommend that you use it sealed...In that case, one of those plate amps that has a 30Hz boost of +6dB would be a great match for this speaker + sealed box combo. You could then get pretty flat response down to around 30Hz, where the room boost will kick in.

Since I'm using this to extend the low end of my small monitors, primarily for music (not as a LFE sub), I took your advice and used it sealed.

I've fine-tuned the plate amp with bass boost almost an octave below fb cutoff. The end result is very nice, tight bass in my room without over-excursion (at the levels I prefer).

Thanks again to everyone here for all your help!

-Chas

p.s. I was originally advised to use it in a vented box, but the box I got turned out to be much smaller than was required for this; Consequently, an equalized sealed box was the answer.

[chris661]

Originally Posted by iand

If you ignore the frequency response and just apply as much power as necessary to reach Xmax -- this may be more or less than the thermal rating of the driver -- then above the box tuning you can typically get about 8dB more output (at the maximum excursion frequency) from a reflex than a closed box, this could be a bit more or a bit less depending on the driver and box used.

winISD has the capability to model the Xmax-limited SPL.

See below - the first is using a generic 8" subwoofer, with recommended cabinet volumes etc, here's the maximum SPL. At 130Hz and below, it's Xmax limited, at above that, it's power limited.

The second one is with an Eminence Delta 12LF.

Edit - nice sub above. Wish mine looked like that.

[clm811]

I finally got around to testing anotherof the plate amps, and upon powering this one up, the 4A line fuse blew after a second or so. I touched the two "Jun Fu" 6800uf electrolytic caps and they felt unusually warm for such a short power-up.

To be sure it wasn't just a turn-on surge, I slowly powered it up through a variac. The fuse didn't blow this time, but the caps felt
very hot !

Looks like I'll have to replace them anyway, after all.

Anyone else have this experience with these caps?

Recommendations?

-Chas

p.s. I tried to contact Steve at Apex Jr., regarding the Jun Fu caps(he sells these), but he's away on vacation for another week.

[chris661]

Sounds faulty. Perhaps check the wiring, as it's a potential capacitor explosion if something's wrong...

[clm811]

I removed the suspect caps and upon shaking them they rattle as if there is something loose inside. Other electrolytics on hand do not exhibit this.

Ill assume they have "dried-out" and that's why they are bad(?).

I'm now shopping for some good quality 80V snap-ins...

-Chas