EBS with high qts ceiling speaker?

[dre7]

Hi:

I'm getting the urge to cut some plywood again. I have a pair of Pioneer BOFU and AMK CX802. I've been listening to the AMKs for a while but I'm not entirely satisfied with the enclosure they're presently in. I'd like to try them in a bass reflex. The Qts is less than ideal (.7) and when I model them in WinISD beta, it suggests a very large enclosure (7 cu ft.) tuned well below the driver Fs. The plot shows a slight "hump" at the frequency the port's tuned to. Is this an EBS design and would it be worth building? Here's the info on the driver

http://tinyurl.com/2etg46

The other option would probably be a coaxial conversion of the BOFU in a reflex box.

Any input would be greatly appreciated.

Andrew

[Badeck]

Hi,

Since you you have not yet cut the plys for the enclosure, maybe you can design for a higher system Q, say 0.8 to 1.0. This will give you a smaller enclosure, then fit in a horizontal partition to divide the enclosure into two, and connect them with a vent.

This effectively places an internal Helmholtz resonator inside your enclosure that selectively damps and lower the Q of the offending frequency range (that occurs about the enclosure + driver fs) that results from the high Q driver + enclosure system.

If you plan to go along this solution, what’s left to calculate is the port dimensions only, since you can arbitrarily divide the enclosure to about two volumes. Criteria for division is so that the port (length) can fit. Then the two volumes will have an effective volume of V1*V2/(V1+V2). Then you can calculate the port length. Formula used would be the vented box port formula where you plug in enclosure volume using effective volume above and port diameter, to get port length.

Then you run an impedance plot to see that the hump at fs being lowered. System Q would be adjusted by stuffing port openings, leaving the volume inside the port free from damping materials to maintain its mass. The goal of damping the port is just to create/adjust turbulence at the port openings, maintaining the internal volume to maintain the tuning frequency.


Thanks,


Dexter

[dre7]

You mean something like the Weems Double Chamber Reflex minus the upper chamber's room firing port? Neat.

Just a thought: could I make the lower chamber a slot and fire into that, or would that be another design altogether?

Thanks

Andrew

[Badeck]

Hi,

Yes, just like Weems, but it could be with or without the outside port. Also we think of it as Helmholtz resonator really, and as such we can make calculations and little cut and try for HR tuning.

With a vent (tube) is much easier as it can be temporarily attached during tuning (cut shorter / replace with longer).

You will notice two humps when the HR is out of tune. Magic happens when the two high humps in impedance join to form/become a smaller hump as a result of port tuning and damping.

First task is to tune, goal is for the two humps to become one. Then after that comes damping the port by covering with polyester pillow material.

Thanks,

Dexter

[GM]

Greets!

No, while an EBS is an over-sized cab tuned low, it requires a < 0.55 Qts cab tuned no < 0.707*Fs for best performance and won't have any audible peaking at Fb. Bottom line, just because you can load this one into a huge cab tuned well below Fs, it's not a good plan, especially with so little Xmax.

The specs are ideal for a max flat impedance TL in a modest size tower though:

L = 48"
WxD = ~91.5"^2
zdriver = 10"
zport = 91.5"^2 cutout at the bottom

All dims i.d. and 1.0 lb/ft^3 polyfil stuffing density simmed. With an easy load for a tube amp and a nice pumped up mid-bass, then if positioned close enough to a wall or corner to help boost the extreme low end a bit it should be a pretty decent performer.

GM

[GM]

FYI guys, you can sim DBRs with MJK's MathCad worksheet.

GM

[dre7]

GM

Interesting you should mention TLs. Presently , these speakers are being used in what I'd call an aperiodic rear loaded horn as I had to stuff the compression chamber a bit to tame some shoutiness. The wife's been pretty patient with me over the last 5 years these 4 ft behemoths have been in our living room. To add insult to injury, I purposefully made the tops slanted so they couldn't be used as plant stands!

So you're saying a (roughly) 25x11x19 enclosure moved back towards the wall would do me? Sounds mighty tempting. The speakers would sit on either side of a fireplace with heavy drapes covering part of the back wall. The enclosure I'm thinking of would have the line terminate in the front under the driver. Or would a terminus close to the floor (hardwood with area rug in front) be preferred?

Thanks for the help.

Andrew

[Badeck]

Hi All,

A consequence of tall/long enclosures are pipe resonances that are then tamed by stuffing/filling which normally and unselectively damp out vibrations, with a result of reduced output SPL.

With the damped internal HR, you will not need stuffing the enclosure when the HR is tuned to the hump in FR/impedance, as it only selectively damps the problem frequency that arises due to smaller-than-optimal enclosures.

Thanks,

Dexter

[dre7]

Dexter:

I'm a little confused by your suggestion. When you say

maybe you can design for a higher system Q, say 0.8 to 1.0. This will give you a smaller enclosure

I assume you mean to calculate as if it were a sealed enclosure? I'm not familiar with Qtc as applied to a bass reflex enclosure.

Also, frequency should the ports be tuned to?

Andrew

[Badeck]

Hi,

Yes, closed box, I admit. I have tested the HR solution for a closed box design, but a vented enclosure with a capped port is also a closed box.

The port should be tuned to the box + driver resonant frequency. It can be also be measured with the partition with the hole in place, less the tube. The HR resonance would be noticed above/beyond fs, as the tube is not yet in place.

Port length is calculated using vented box port length formula, but using Veff = V1||V2 = V1*V2/(V1+V2), where V1 and V2 are the individual partition volumes.


Thanks,

Dexter