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Multi-Way Conventional loudspeakers with crossovers

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Old 11th April 2007, 09:00 AM   #171
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It is a question of trial and error to find out if all these damping tricks are as effective at dealing with the rearwave as a simple flat open baffle already does intrinsically.

Regards

Charles
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Old 11th April 2007, 09:11 AM   #172
rjb is offline rjb  New Zealand
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In 1951 W.A.Hartley of Hartly-Turner was singing the praises of his 215 full range loudspeaker in a Boffle, which basically was an 18 inch open backed cube, with nine internal divisions of heavy felt, spaced approximately 1 7/8 inches apart. Each division had a different sized hole, with the final one having no hole to form the back. He claimed the sound was improved because of the non-resonance design of this enclosure.
He produced two frequency response curves for the same speaker, one mounted in a large flat wall, the other in the boffle. The curves above 100 Hz were very similar indeed, but below that the frequency response of the boffle fell at double the rate of the wall, about 10db per octave from memory, as might be expected.
Many years later I built a boffle with a cheapish 12" unit that ideally required a five cub.ft BR cabinet, to compare the sound of the two cabinets, (hoping to get a box of more acceptable size). The sound from normal listening position was obviously cleaner than the BR, but also obviously had less bass output. Only a low level rumble could be heard with an ear placed close to the back.
The same unit in a large OB was quite unsatisfactory. I did not follow this line of thought, but moved to TL's via Voight QW designs.
It seems to me that Lynn's suggestion of a leaky boffle, with modern equalisation may well be worth pursuing.
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Old 11th April 2007, 09:50 AM   #173
rjb is offline rjb  New Zealand
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Sorry, I meant H.A.Hartley. The firm at that time was H.A.Hartley Co based in London. It produced only one design, a full range 4 ohm unit with an 8" cone using an alinco magnet. The cone was curvalinear, and divided by a compliance at about the three inch diameter giving a mechanical crossover, and the voice-coil isolated with a latex coating over an aluminium former. The surround was untreated cloth. A very interesting design.One recently sold on ebay for big money.
The firm later became Hartley-Turner, producing a range of sizes including 12" and I think 15" for sale in the States.
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Old 11th April 2007, 01:56 PM   #174
Paul W is offline Paul W  United States
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I just finished a boffle-like midbass baffle for testing this weekend.

The top and bottom are closed (to sit on a woofer module and support a tweeter module above). The sides and back are open, with wool felt in a semi-circle side to side. Felt density is adjustable by changing the number of layers and position around the semi-circle.

I'll post a picture if interested.
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Old 11th April 2007, 03:10 PM   #175
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This from a Radio-Electronics article (i could pdf if any interest).... the Boffle patent (sep 1935) is also downloadable 434,563

This concept was recently revisited in aXp with modern materials & data aquisition.

dave
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Old 11th April 2007, 04:08 PM   #176
Variac is offline Variac  United States
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Quote:
Folks who get cute and make a room-divider folding baffle with a piano hinge are sort of kidding themselves - small air leaks through the hinge reduce the effective size to not much larger than the hinge-to-baffle-edge distance.
Is it established that this is true for low frequencies?

You won't be "fooling yourself" if you use the "Clear Flat Hinges" here:

http://www.outwatercatalogs.com/2006...86&catalog=otm

They fit into a saw kerf in the edge of the panel.
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Old 11th April 2007, 04:23 PM   #177
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Quote:
Originally posted by planet10
This from a Radio-Electronics article (i could pdf if any interest).... the Boffle patent (sep 1935) is also downloadable 434,563

This concept was recently revisited in aXp with modern materials & data aquisition.

dave

handy : http://www.pat2pdf.org/

btw that 434,563 is combined latch and lock
useful- but for other type of boxes
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Old 11th April 2007, 04:53 PM   #178
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Quote:
Originally posted by Zen Mod
btw that 434,563 is combined latch and lock
I'll check the number when i get home... i was (trying to) typing it while looking at the patents cover page.

dave
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Old 11th April 2007, 05:03 PM   #179
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It's very difficult to get you guys out of box mode. Sure if the hinged wings are folded sharply back, pressure is contained in the cavity and some may leak through the hinges, but the effect is going to be far less than you think. The rear wave is going to take the path of least resistance for the most part. I've played woofers with about 1/2" of space between the driver and the baffle all the way around and it had surprizingly little effect compared to being well sealed.

I'm sure there are all kinds of imaginative ways we can come up with to absorb the rear wave and retain an open look to the thing, but doing so to obtain bass means a shift toward monopole and detracts from the biggest advantage of open alignments. Plus things like a "boffle" will have an effective range sloped in the wrong direction. We need more attenuation the lower the frequency, ideally at a 6db/oct slope.

Before I tried the dual pathway with a Helmholtz slot as a divider to send only LF to the longer pathway, I tried a series of low tuned helmholtz traps for the rear wave. I never could get that method to make more than a barely audible difference, and I had much more success with the slot arrangement feeding an expanding pathway.

Back then I was wanting to create a single driver speaker with an open alignment that didn't need the help of a sub, a true minimalist OB. It works well with a 15" coax, but they're too big for most. I also tried it with a B200, which didn't achieve the bass levels I wanted. I really need a 10"coax with a mid Q and 7mm of more of useable Xmax. Here's the B200 version, same concept as before, just a different layout:
Click the image to open in full size.Click the image to open in full size.
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Old 11th April 2007, 08:59 PM   #180
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Hi Variac,

I'm not sure where your 'hinges' quote comes from.

It has been my experience that gapping a solid baffle from the wings (as with hinges) does much to relieve the increased rear of cone pressure from deleteriously affecting the forward response.

I would not imagine the boffle design releasing the immediate rear pressure, though what Lynn has described could.

I can also imagine John's B200 OB-RLH design sounding better in the forward direction because the rear pressure has an 'escape' route, yet one which is then put to good use by augmenting the overall bass response.

Cheers .........Graham.
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