EBS with high qts ceiling speaker?

dre7

Member
2004-07-15 3:37 pm
U.S.
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
 
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
 
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
 
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
 

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dre7

Member
2004-07-15 3:37 pm
U.S.
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
 
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

Member
2004-07-15 3:37 pm
U.S.
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? :xeye:

Andrew
 
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
 
Hi,

As a follow up, and using your driver's spec, a sealed enclosure design with a Qtc of 1 would yield a box volume of 1.105 cu ft. with an f3 of 50 Hz. That is without the HR partition.

Then you run a sim for the driver in the calculated volume to get the resulting resonant frequency (the peak in the resulting impedance curve). That is the target frequency for the HR to damp. Plan for a partition placement, then calculate Veff. Veff is used to determine the HR port length, tuned to the resultant frequency above.

When the HR port is tuned to the box + driver frequency, you will notice a single impedance hump centered at the target frequency, but with a lower Q. Q can be further lowered by damping the port openings. The impedance plot can then be used to estimate the resulting Qtc, which would be by then lowered from the value of 1 that was without the HR.


Thanks,

Dexter
 
Hi All,

Sorry for the monologue. If it's OK with you, I will upload my actual enclosure impedance plots (before and after tuning) tomorrow.

Please note also that this is not the same as aperiodic design, as aperiodic blindly targets damping (plus it vents outside the enclosure) while the internal HR selectively targets the box resonant frequency.

Thanks,

Dexter
 

dre7

Member
2004-07-15 3:37 pm
U.S.
Thanks, DCRs are something I know little about. Most of my BR experience comes from poor sounding consumer grade stuff.

I also have a pair of SP8Bs rescued from the dumpster, and IIRC that what Augspurger used in his original design. Wish I could find a link to his article. That way I can judge on the merit of the design and not have to blame my math skills :rolleyes:

Andrew
 
dre7 said:

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

Andrew

Hi,

The short is No, it is not an EBS and No, 7cuft boxes would be pointless.
Open baffle with another driver or subwoofer is likely your best bet.

:)/sreten.
 
dre7 said:

So you're saying a (roughly) 25x11x19 enclosure moved back towards the wall would do me?

Or would a terminus close to the floor (hardwood with area rug in front) be preferred?

You're welcome!

??? You're putting a ~full range driver into a box height better suited for a sub? I designed the TL to put the driver at ~seated ear height. Regardless, TLs benefit from floor loading, though with such a short height I doubt it matters much where you put the terminus short of down-firing.

GM
 

dre7

Member
2004-07-15 3:37 pm
U.S.
GM

You're absolutely right, that's probably carrying WAF a bit too far.

Actually, what you're proposing is a larger scale version of some 1/4 wave pipes I made for the PE 269-469 which I have on either side of my desk as computer monitors. And yes, considering the small footprint and placement close to the rear wall, they won't be nearly as intrusive as the old speakers.

Plus it can be done with one sheet of ply. Right on! I'll give it a go.

Andrew