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Old 27th June 2007, 01:49 AM   #11
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The surest way I've found to get rid of the rear wave is to reflect it into oblivion and absorb the small remainder. Do this by constructing a tallish pyramid or cone and mount the driver in what would otherwise be the base. Add a little stuffing, especially as the tip, and you've got a very dead box that turns the rear wave into heat.
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Old 27th June 2007, 02:17 AM   #12
terry j is offline terry j  Australia
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thanks lech and john. seems you are both saying the exact same thing, except lech has the idea of using quite a few inverted pyramids. My initial thoughts of using simply an absorbing 'mat' seems to have a few problems eh? I can see conceptually the concentrating via the pyramid funnel to a very high pressure and then absorbing it as a a valid method.

The above approach sort of has a different feel than the conceptual one I had, namely that there is still pressure behind the cone, whereas in a type of OB (with rear absorption) that back pressure is removed.Hmmm

John, seein' as how you're the OB man (wasn't he on star wars?? Ob'man Kenobi or something), do you know if I can automatically use my mid in an OB type arrangement??

Leaving aside the sound escaping thru the cone bit, I would suppose that the mere fact of pressure behind the cone must distort the sound somewhat, in addition to the escaping soundwave.

Like you Ceramicman, I must do some experiments myself. The worst thread of all is a talking whatif? thread which ultimately goes nowhere!

The first thing for me to check out is the comparison of how the driver sounds on a large baffle (around 750 by 1200 mm, as that will ultimately be the size of the baffle on the mains) with a sealed box behind one, no box behind the other.

That will be a more valid test of 'sealed vs OB' than any previous listening to OB I've done.

JohninCR, there is there any reason for the driver to be less detailed in OB than sealed is there?? If not, then the lack of detail I heard on the Orions I can put down to driver differences rather than alignment differences I would assume.

Also John, what is your gut feeling about the disadvantages (if any) about the interaction with the wall behind the speakers in the normal dipole arrangement. That, after all, is kinda the starting point on whether or not there is any advantage in removing that by this method of trapping the back wave.

Ceramicman, are you into the car races?? Come next Bathurst, a whole bunch of audio (and car) nuts will be coming for an alcohol fuelled (not talkin bout the cars here), music soaked exhaust fumes and fun GTG. Praps you could make it down?
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Old 27th June 2007, 03:17 AM   #13
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I have drawn up a speaker plan after months of reading and modelling that has a damped open back. This would probably satisfy your original requirement, i.e. no back waves bounced back through the speaker cone while the back waves are significantly attenuated from the upper bass up. With this hybrid approach the power response should be more even than the typical dipole speaker.

The idea is to have the "front panel" as narrow as possible so as to minimize diffraction <10ms. A 2"+ round over to smooth out diffraction, and angled side wings extending the effective baffle width such that the dipole eq monopole frequency is brought down to just above 100Hz. Some felt may be applied to the panel to further reduce diffraction. That back of the speaker would allow acoustic stuffing material of up to 120mm thick and is adjustable.

With my limited knowledge, I guess it has the best of both worlds and through extensive modeling, I have not found any faults with it so far. Things are looking good.

The only trouble is that I have not found anyone to do the wood work for me.

Regards,
Bill
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Old 27th June 2007, 03:46 AM   #14
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HiFiNut is on the right track. I prefer to attenuate the HF of rear wave too, and considering that potenial, open alignments give you the ability to tune your speaker to your tastes, room, and placement with changes taking only a few seconds. No box affords that flexibility.
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Old 27th June 2007, 04:33 AM   #15
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The kind of TL people are talking about are aperiodic TLs... the idea being to completely (or close to it) absorb the back wave (in the vernacular, "suck the back wave down the pipe") This can be done with a 1/4 wave line or with a half-wave line (this is what the B&W nautilus uses)

Click the image to open in full size.

This one is for a bass driver (their lit has lots of good idea provoking pics), for a mid it doesn't have to be all that long (~1/2 the w/l of the lowest frequency of interest) You want to avoid having surfaces that reflect back at the cone,,, probably the biggest contributor is the driver itself. One has to be even careful with absorbtion material behind the driver as it is reflective at certain frequncies -- there should be none near the driver and get progressively denser as you approach the terminus...

Easiest implementations are a big PVC pipe firing out the back othe box or a simple folded & tapered TL with the terminus just by the driver.

dave
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Old 27th June 2007, 12:27 PM   #16
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Quote:
Originally posted by terry j
Ceramicman, are you into the car races?? Come next Bathurst, a whole bunch of audio (and car) nuts will be coming for an alcohol fuelled (not talkin bout the cars here), music soaked exhaust fumes and fun GTG. Praps you could make it down?
Sounds like a plan!
I'm quite new to Oz so I definitely need to see Sydney at some point

Quote:
Originally posted by planet10
One has to be even careful with absorbtion material behind the driver as it is reflective at certain frequncies -- there should be none near the driver and get progressively denser as you approach the terminus... [/B]
What about putting damping material only on the convex edge of the line so that the sound waves 'curl' inwards?
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Old 27th June 2007, 03:04 PM   #17
terry j is offline terry j  Australia
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hi ceramicman

try this link http://www.stereo.net.au/forum/index...howtopic=2528, you may have to register, but if you do then welcome aboard!!

Dave, re your pic, does the TL pictured load the driver in anyway?? still trying to get my head around whether or not removing the back wave w/out pressure on the cone (a la OB or IB) will sound different than removing it with pressure (eg very heavily damped box). Time for my
experiments I would assume!

BTW, when you say terminus does that mean the exit of the TL? If the soundwave exits the 'tube', where in the picture does it exit (after the other skin is added).
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Old 27th June 2007, 04:52 PM   #18
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Quote:
Originally posted by terry j
Dave, re your pic, does the TL pictured load the driver in anyway?? still trying to get my head around whether or not removing the back wave w/out pressure on the cone (a la OB or IB) will sound different than removing it with pressure (eg very heavily damped box). Time for my
experiments I would assume!

BTW, when you say terminus does that mean the exit of the TL? If the soundwave exits the 'tube', where in the picture does it exit (after the other skin is added).
This box could be considered sealed as far as loading goes, but i expect B&W designed a driver where they got very little pressure from the back-side.

This TL is half-wave so doesn't have an opening to the outside world, so terminus would refer to the very end of the spiral.

Personally i'd adopt a 1/4 wave line damped until it was aperiodic.

dave
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Old 29th June 2007, 10:49 PM   #19
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Quote:
Originally posted by terry j
[B]cool Drew, will check it out.

You obviously have some experience with both, and bear in mind I haven't checked out what you've just said, but the pluto is not a true dipole as the orions are?ie more along the lines of 'removing' the back wave etc.
The Pluto is a sealed speaker which approximates a true monopole up to 3KHz.

http://www.linkwitzlab.com/Pluto/intro.html

I speculate that they sound similar because polar response is relatively uniform w.r.t. frequency and neither has a cabinet storing energy which is re-radiated through the drivers.

Quote:
It may simply be a matter of terminology, but I always think of a transmission line as a means of tuning and deepening bass response, does it have that effect in the midrange as well or is it used in a different type of sense?
A transmission line directs transmission of energy (electromagnetic, acoustic) from one point to another. The Pluto uses a terminated transmission line like the B&W Nautilus where sound waves start at the driver, bounce off the end of the transmission line, and loose 99% of their energy by the time they make it back to the cone. It's just used to control the backwave and not provide bass output.

Quote:

Does anyone have any data on whether or not you can nilly-willy throw any old midrange driver into an OB alignment?? or indeed a no-pole ie could it lead to excursion problems etc etc etc? (ie is it only suitable for some mids and not others).
Read Siegfriend's site http://www.linkwitzlab.com. Since dipoles roll-off at 6dB/octave, maintaining constant output requires boosting displacement by 2X for each octave and SL uses 8" midrange drivers with beefy tweeters and 1440Hz LR4 cross-overs.
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Old 30th June 2007, 09:01 AM   #20
terry j is offline terry j  Australia
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aarrgh

had just talked myself into believing that there is no real danger in trying my mids in OB type baffle, you go and put the fear of the 'god that makes smoke come out of drivers' into me!

I put up a post on what parameters etc are suitable for a mid in OB over on the audio circle open baffle forum, received only one reply which is a bit of a bugger. So I had decided to just do it.

I don't mind doing a bit of research, but the trouble with Seigfreids site is it is so HUGE!

Is there any reason that using WinISD would not work for midrange drivers?? I only ever see it being applied to bass drivers, quite naturally I suppose, but if I plugged in (say) 1000 l for a sealed box it should at least show it's IB behaviour, and that way I might be able to get a feel for the excursion vs extension of the driver, which is my main concern and which is the potential problem that was echoed by Drew.

If from my fumbling attempts at modelling look 'safe' for the driver, I will mock up a quick OB for the mids next week and have a quick listen.

thanks for replies so far, much appreciated.

I don't want to damage the driver, I only got a replacement yesterday would you believe, I've been music-less for the last three weeks, so to damage it as a result of my own stupidity would be even more annoying.
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