Open backed boxes ? Anyone ?

[norman bates]

I loved a flat open baffle with a foam box around rear of driver.

Super clean mids, no boxy sound, no front to back nonlinearities in pressure.

Now I need higher waf.

I'm not looking for intoxicating dipole front wall bounce ambience.

What will a no backed box sound like with stuffing ?

I think a box 8" wide and 21.5" deep would look like an open baffle 47.5" wide.
47.5" wide would give a 6-10db dip at 285hz (front and back 180 degrees out of phase) then near 6db peak at 142hz (front and back 90 degrees out of phase thusly adding) and rolling off to oblivion.

I figure a driver will roll off at its mass corner (Fs / qts) regardless of how big the baffle is.
Then add the baffle rolloff to that.

Long ago I sawed off the back to a small yamaha 8" 3way. Mid and tweet were closed back.

Good sound, voice was better, but I heard a few resonances (bare walls inside), so I removed the sides.
Well, that was a mistake, probably rolling everything off below 1khz.............

Norman

[Godzilla]

Someone at work brought in a pair of very cheaply made speakers and asked me to upgrade the parts for him. They were old no names and sounded very boxy on first listen. Inside was a small super tweeter and a full range driver. I believe the cap on the super tweeter was 1uf and the main driver played full. Since there was no way to get to the drivers without cutting into the box I cut out the back to have a look inside. Then i played them without the back and they did not sound boxy at all. In fact, i compared them with the Pioneer B20 speakers i had on hand and the cheaper speaker without the back of the box sounded better overall! I was shocked as was my friend Tom. So we took off the backs of both and nothing but good sound came from them always. Secretly i wanted those drivers for myself but there was no way to dispute how good they sounded. He wanted them back without any changes. I helped him screw a fancy metal grill on the back and he brought them home, bought a sonic impact amp and has it all connected to his computer, tv, nintendo. He tells me all the time how much he enjoys his stereo and listening/watching sports. He says his daughter is always in there with her friends. What i am trying to say is that an open back speaker could sound really great and i think this type of cabinet is very well suited for many of the speakers we use on this forum.

Godzilla

[Nelson Pass]

Originally posted by norman bates
I'm not looking for intoxicating dipole front wall bounce ambience.

What will a no backed box sound like with stuffing ?

Besides the aforementioned intoxicating dipole front wall bounce ambience,
open baffles also eliminate the sound bouncing around inside the box.

An open back stuffed box does not have as much ambience, but it does
do wonders for the sound bouncing around inside.

[Godzilla]

>>> An open back stuffed box does not have as much ambience, but it does do wonders for the sound bouncing around inside.

I wonder if a BIB takes advantage of this as well, among other things???

[Nuuk]

This is similar to what you are looking for but I'm WAF is just about zero!

[Nardis]

I've always been intrigued by the slatted-back, tunable design from this French company:

http://www.phy-hp.com/English/Commun...ication_E.html

Has anyone tried the idea with more affordable drivers?

[nosmok]

Let me be the first guy to mention the Hartley "boffle" concept here. Hartley (he of the white coned drivers, including the legendary 220 full ranger) was for years a champion of what he called a boffle-- a large backless box with several curtain-like layers of cotton or foam to attenuate the rear wave. Never heard one myself, but there have been periodic mentions of them over at AudioAsylum, and the old timers who remember seem to like them.

--nosmok

[GM]

FWIW, I've periodically mentioned that I 'voice' drivers in a foam box to get a feel for what they can do and not being a dipole fan, I've used them in conjunction with woofers to good effect, so pick a baffle size, add a slender frame for the sides, back (I use dowels) and wrap it with whatever works best for you. For a finished product you can cover it with an expanded knit or actual grill cloth.

GM

[norman bates]

yea, I'm all for absorbing the backwave.

Seems it may be tricky to balance box depth with how much, what and where the stuffing.

Like the old consoles, only fill with stuffing (and no amp inside).

To me a benefit you be golden ratio off set.
I'd been wrapping my mind around a 4' wide x 6' tall open baffle.
If the driver center is 18 1/3" from one side, it'd be 29 2/3" from the other, then place it 48" from the top.
That give a 1 : 1.618 ratio.
Problem is you'd want the driver in the middle so you'd have 24" min distance to cabinet edge.

here seems to be a golden ratio offset open baffle.
http://pic4.picturetrail.com/VOL706/...0/97442407.jpg

But what if you could put a box right next to your tv?
The speaker driver would be next to the middle of the tv.
The open baffle could be L-shaped (the drive at the edge).
It could be 29" wide and a wall going straight back (18 1/3").

There was an open baffle test showing nulls and such but I can't find it now.


"There are surprisingly little knowledge in the issue, I think. Harry Olsens "elements of acoustical engineering" , D. Van Nostrand Co; 2d ed edition (1947) is most the most recent reading I have found in the issue. He for example discusses of open-back boxes as a loud speaker type. Basically, adding wings behind the baffle causes similar effect, only in smaller scale. You have a peak in the response, whichs "fS" is baffles width. The deeper the wings, the higher the peak. rectangular base gives highest amplification in "fS". Twisting -or folding- another- or both- of the wings reduces effect. Look at the Auditorium23:s site for some really interesting baffle and folded baffle ideas, meant for PHY:s, but will of course work with any OB driver as well. Folding the baffle does anyway not affect to the baffles lower limit, compared to the same width of straight baffle. the only effect is that peak(s) in response in upper frequencies. The major advantage is higher WAF, I think. Or, one can have advantage of that peaking effect in compensating defects of the baffle and driver.

http://auditorium23.de/PHY/Plaene.html

then, "weitere bauworschläge". Only auf deutch. BTW; some really, really nice TT's also there."

http://members.myactv.net/~je205d/monob.htm


Boffle, neat stuff

http://homepage.mac.com/planet10/TLS...eybrochure.pdf

"As I remember, the original Boffle was an open-back cabinet with curtains of felt hanging inside. Later, the cabinet was stuffed with a layer of felt and fiberglass that were rolled up in the manner of a jellyroll..............H A Hartley’s intent was to minimize the audibility of the back wave, and to mimic, as far as possible, a baffle of infinite size. He was quite opposed to ports, horns, etc."

Norman

[Scottmoose]

FWIW, it should be able to ~model an open-back cabinet design using Martin King's H-frame MathCAD worksheet. Just reduce the length of the front cavity to 0.001in. It'll be a bit of a fudge, as it still assumes the entire front of the box is the forward radiating area, but better than nothing & probably more accurate than the U-frame sheet which doesn't account for the top.