New speaker type? OB-RLH - diyAudio
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Old 25th February 2006, 01:31 AM   #1
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Default New speaker type? OB-RLH

I'm using the Hawthorne Silver Iris Coax in the cab pictured below. Before all the naysayers jump all over me with "that's not a horn" or whatever. With just the U-baffle top I was already getting good response down into the 30's. FYI, I use no EQ, I prefer to address things mechanically instead of electrically. Although I can't locate my SPL meter, a tone sweep shows a gradual rise in response of 3-4db at my listening position from 28hz to 300hz with no noteable peaks and valleys in between. Even if it was 6-8db it would still be unheard of with OB and my ears are well trained enough to know the difference between 3 and 6db even over a 3.5 octave frequency spread. It may be closer to flat, because to me, any more bass would make it bottom heavy. Note that my results are with corner loading because out in the room the bottom end has some peaks and valleys in response. Also note that I can apply more power with the "horn" base than with just the U-baffle before the cone loses control, which leads me to believe that I'm getting some horn loading. There isn't a lot of output at the mouth, but I really wasn't looking for much, just to fill in the very bottom that was missing with just the U-Baffle.

I took my idea from Steve Deckert's Sub Only Jensen Imperial, whick also has an open air throat loaded via some type of helmholtz resonation. My reason for posting is to solicit help in optimizing the design. This was just with my initial test cabs. I don't fully understand how it works, but it seems too short to be a TL since it's filling in only in the the 28hz to 45hz range. It's not just a resonator because if I plug the mouth it turns into a real one note resonator, big and flabby.

Any suggestions would be greatly appreciated, because I want to try something similar with a Visaton B200 and I'll need more bandwidth and more efficiency in the horn, since the smaller driver won't be pushing as much air across the slot.

Here's a drawing fairly close to scale:
Click the image to open in full size.
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Old 25th February 2006, 03:46 PM   #2
konut is offline konut  United States
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Interesting design. Have you tried plugging the back of the pentangle? Or plugging the back of the pentangle and putting in an aperiodic vent or two? Or put an aperiodic vent in the mouth? Or a combination of the 2 ? That might be a way to tune to the specific frequencies you want to accentuate. Just some first blush thoughts.
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Old 25th February 2006, 04:28 PM   #3
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Any resistance I add in the back is going to attenuate the HF of the rear wave and detract from the wide open OB sound. Closing it up would likely sound terrible because the driver has a Qts of .91 and would require a much large box than the 1.5 or so cuft back there.

First and foremost I want to retain OB and I know how to stretch that limit with specially shaped U-Baffles. Until the wave escapes the confines of the baffle it is still a pressure wave, so I have some pressure to work with. In addition, I've got Helmholtz principles loading that slot, which I can tune with slot size.

Thanks for the input.
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Old 25th February 2006, 11:50 PM   #4
Corloc is offline Corloc  United States
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The idea kind of reminds me of blowing across the top of a bottle.

You would be able to tune the TL to cover any hole left in the OB. I have to say I'm a little suprised that you getting enough spl to make a differance. It's pretty intresting that its working.

Have you tried to make a baffle behind the driver, just at the mouth of the TL (maybe 7")? You might be able to channel more air into it?

Chris

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Old 26th February 2006, 01:01 AM   #5
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Quote:
Originally posted by Corloc
The idea kind of reminds me of blowing across the top of a bottle.

You would be able to tune the TL to cover any hole left in the OB. I have to say I'm a little suprised that you getting enough spl to make a differance. It's pretty intresting that its working.

Have you tried to make a baffle behind the driver, just at the mouth of the TL (maybe 7")? You might be able to channel more air into it?

Chris

Corloc,

At first I was worried that I was only getting a resonator effect, like blowing across a bottle. That's exactly what happens if I close off the mouth, an ugly very pronounced one noter. With the mouth open it behaves very different.

With just my U-baffles, I was already getting great bass extension for an OB, but I wanted to totally eliminate the need for a sub with music. I suspect that I'm getting a combination of effects. First, that I'm shunting off a portion of the LF content in the rear wave with a frequency range based on the dimensions of the slot and air volume beyond it (like a helmholtz absorber). Instead of being dissipated in a sealed chamber with damping, it travels an expanding pathway of approx 1.8m , plus with the exit at the rear for dipole cancellation purposes that portion of the rear wave must travel a total extra distance of 2.2m. That equates to an Fequal of 26hz. Second, I believe the expanding pathway is having at least some effect on the waves that enter it.

For a smaller driver, some type of double baffle is exactly what I was thinking. I at least had some help via Helmholtz formulae to determine a slot that seems to work as intended, however, a double baffle has the potential of redirecting a major portion of the LF content. That's where I'm left with kind of a blank stare, because I'm not much for trial and error. Sure I try stuff that's out on the fringes, but I have very specific reasons for every part of a design.

I'd want to come up with some idea for an optimal size and shape for the driver cutout on this secondary baffle. The idea would be able to determine a mechanical XO frequency. Frequencies above that pass rearward unfettered, and below at an increasing slope they are sent where we want. I can make the spacing adjustable, so that variable is covered, but I'd like an educated guess on a starting point for the false baffle.
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Old 26th February 2006, 07:46 AM   #6
westend is offline westend  United States
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I'm liking this design. If you placed another baffle rearward of the back plane of the speaker mounting baffle, say, three inches and captured that LF into an enlosure around the cone that might load a lot into that space and still permit adequate dipole material to push to the rear. I'm envisioning a truncated doughnut coupled to the horn part. I've never seen any info on loading part of the energy of a transducer. It is almost always treated from the drivers total force. I bet a guy like Ron C would be the guy to draw into this discussion.
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Old 26th February 2006, 01:20 PM   #7
grrr is offline grrr  Netherlands
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Default pipe resonator

I like to point out but maybe you already know that a pipe closed at 1 end resonates at differed frequencies than a pipe open at 2 ends. A pipe open at 2 ends resonates for a standing wave with 2 high speed nodes at the end 1/2 ,1 ,3/2.... waves.
a open and closed pipe resonates with 1 high pressure side and 1 high speed side
1/4,3/4,5/4 standing waves so comparing a open and closed pipe in this design is not changing only just 1 parameter of your experiment so it is difficult to conclude anything from that.
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Old 26th February 2006, 03:53 PM   #8
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grrr,
I don't think I'm dealing with pipe resonances. Yes, I did have to shape the terminus of my U-baffle in a way to eliminate them without using stuffing because the 1st order low pass on the woofer is at 3500hz, so that was my primary concern with the U-baffle. With the "horn" the frequencies are too low and pipe resonances are pretty obvious when they occur unwanted.


Westend,

I was thinking about a similar shape, but in an easy to construct form. Also, if sufficient LF loading is possible with just a flat secondary baffle, then something easy is possible. Here are a couple of ideas. The one on the left definitely isn't to scale. It's just an idea of that "chamber" layout.

Click the image to open in full size.
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Old 26th February 2006, 04:37 PM   #9
grrr is offline grrr  Netherlands
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i just was reacting to this
At first I was worried that I was only getting a resonator effect, like blowing across a bottle. That's exactly what happens if I close off the mouth, an ugly very pronounced one noter. With the mouth open it behaves very different.
you can just not make conclusions from that.
there are always resonances when you have pipes even a competely flat open baffle has resonances and there are multiple resonances for every pipe.
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Old 26th February 2006, 05:08 PM   #10
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Quote:
Originally posted by grrr
i just was reacting to this
At first I was worried that I was only getting a resonator effect, like blowing across a bottle. That's exactly what happens if I close off the mouth, an ugly very pronounced one noter. With the mouth open it behaves very different.
you can just not make conclusions from that.
there are always resonances when you have pipes even a competely flat open baffle has resonances and there are multiple resonances for every pipe.
I appreciate the input and when I closed the mouth up, I can't say definitively whether the result was a 1/4 wave resonance and/or a helmholtz resonator because either or both could have been applicable in the 40-50hz range where it occurred. I believe the wavelengths of the frequencies involved (50hz and below) are too long for 1/2 wave resonances, however that may need to be taken into account once I start trying to load a wider bandwidth with a smaller driver.
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