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Old 6th April 2007, 07:41 AM   #101
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by planet10


If RCAs, Olson designed them...

dave

I know that....................
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Old 6th April 2007, 08:01 AM   #102
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In post #88 Floric wrote of the line source;-

In normal listening environments this (comb) effect is smeared by the room with the consequence of an inaccurancy in the time domain. But you will never hear the sound of the outermost speaker with a delay.

In post#97 Lynn Olson wrote;-

the power does not drop off according to the square law, thanks to the very narrow vertical dispersion. This means the acoustical joining to a radiator with spherical dispersion, such as a subwoofer or supertweeter, requires a listening-distance compensation.


Last year I tried line arrangements, and was able to attain enjoyable listening, but, the clarity of reproduction improved with distance away for the line, thus it was NOT best for use within a room at normal room listening distances. The idea was shelved.

As an additional note - I found that any form of straight side (rear back pressure reflective) baffling of the line source (including rear folded/tapered longitudinal 1/4 wave loading) had a deleterious effect upon reproduction, maybe as the graduated back-pressures caused non-coherently additive driver reactions.

The best reproduction arose by having the drivers narrowly mounted as a strip between two vertical soft long/fine pile carpet rolls with these pulled together behind the drivers to leave a rear gap equal to front cone area. There was no additional reflective rear pressure induced blurring tonality with this rounded damping baffle.

Whether the same clean sounding reproduction could be achieved by using a (well lined) curved perspex (or other) open baffle where the curve is rolled right back to a slot behind a single driver is more than I could ever farbricate or test.

Through my experiments I concluded that whizzer cones simply cannot remain time coherent at hf and yet, excepting low bass, drivers needed to be single point radiating for normal within-room listening.

So Lynn, I have been watching your thread with interest, though I have opted for cheap/cheerful 6x9 multi-way car drivers with expected aperiodic 12" bass augmentation, when (if) I get the chance.

Gee, can we ever be satisfied ?
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Old 6th April 2007, 08:24 AM   #103
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even if subject of combing or non-combing isn't really issue-long time ago recognized in pro and PA fields,ther is just one example how pro boyz'n'grlz choose to solve it........
http://www.meyersound.com/products/i...alseries/sb-2/

hehe,I know problem of lousy intelligibility is......I have few church systems behind me.........

so- mentioning line arrays in this thread is really a side step- good for some another thread where cheapskating is first condition
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Old 6th April 2007, 11:45 AM   #104
Floric is offline Floric  Europe
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You are right Lynn,

I was thinking of the problem from the wrong direction. I thought of an array where the distance of the sources is small compared to the wavelength - which is an error if you think of audible frequencies. In addition I was thinking of sines and not of Impulses.

My first thought was that e.g. an optical grid produces a relatively sharp 0th maximum which is coherent - if it wasn't it would not exist.

But that is not valid for audible frequencies and real world drivers, there is always a part of the spectrum where the wavelength is in the same order of size than the distance of the speakers or even below that.

The picture Dave posted shows that effect very good: in the white areas you get coherent sound. In the "grey" areas you don't hear anything. If you choose the right distance for a given line length everything is o.k.

On the other hand, I did not hear the incorrectness in the time domain when I listened to line arrays right now. I was only surprised by the dynamics, the really low distorsion and the effect of a large sweet spot.

But for the correct reprodution of an impulse response which consits of a superposition of an infinite number of sine waves of a continuous spectrum they are definitely the wrong tool.

I wish you a nice day.

Best regards

Floric
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Old 6th April 2007, 05:29 PM   #105
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Default back to back...

Lynn,

What are your thoughts regarding the value of mounting drivers back to back in a kind of warped open baffle isobarik loading magnet to magnet, with the rear facing driver wired out of phase to the front facing driver, so the two move as one unit?

Here are my own assumptions why this may be worth the added expense to some folks (and I am in way over my head here, so everyone go easy on me!):

1) Even though the "box" has no top, bottom, or sides.... the non-linearities of any driver will be somewhat improved by having a pair of them closely coupled in this arrangement, lowering distortion and other artifacts by having the "pull" driver creating a wave directly behind the "push" driver. The "push" driver could even be delayed ever so slightly, possibly introducing other advantages (depending on your take on Tierry Budge's work).

2) A little stuffing along the open sides and bottom should make the turbulence and/or any roaming non-cancelled sound waves that manage to survive in that sandwich a non factor from the listening chair. Open top still allows maximum heat escape to help keep drivers as close to optimum operating temp as possible.

3) Rear, out of phase signal is now closer in profile to front signal providing (I assume that due to all the mechanics behind a driver that the sound from the back is somewhat different than the sound from the front) so the dipole environment is now a bit closer to theoretical perfection.

4) if coaxial drivers, then you have the rear tweeter to use depending on your taste and you could eq just the rear facing tweeter for different preferences regarding this effect at the listening seat (following the linkwitz crowd hear on the potential value of the rear facing tweeter depending on room, speaker placement, and listening preferences).

5) Could also wire the two drivers in serial fashion for a nice 16 ohm load, OTL biamping mode with passive line level crossover..... By the way, what are your thoughts regarding passive line level crossovers? It seems to me that this would be a better way to optimize a system if you were building the amps as well, coupling them directly to the drivers and shaping the signal behind the preamp instead of behind the power amp.

Awesome thread by the way, and glad to see you exploring the dipole coax approach. Will be watching with great interest, and participating when possible (both in discussion and in potential build/experiment opportunities, though I don't have any measuring equipment yet...)
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Old 6th April 2007, 05:37 PM   #106
Variac is offline Variac  United States
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On his Orion, a very highly regarded speaker, Sigfreid Linkwitz has had dipole, open baffle, bass and mids from the beginning.

He recently added a back tweeter only and says it's the missing Link or similar:

http://www.linkwitzlab.com/orion++.htm

I believe it is a dipole also, like the rest of the drivers. So now all three are dipoles- when the front pushes, the rear pulls, so to speak..



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Old 6th April 2007, 07:26 PM   #107
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Default Re: back to back...

Quote:
Originally posted by Greggo
What are your thoughts regarding the value of mounting drivers back to back in a kind of warped open baffle isobarik loading magnet to magnet, with the rear facing driver wired out of phase to the front facing driver, so the two move as one unit?
Given the depth of the drivers -- especially if coax -- this arrangement would limit the extension into the midrange of the main driver.

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Old 6th April 2007, 11:22 PM   #108
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Default Iso-Whatsis

Quote:
Originally posted by Greggo
Lynn,

What are your thoughts regarding the value of mounting drivers back to back in a kind of warped open baffle isobarik loading magnet to magnet, with the rear facing driver wired out of phase to the front facing driver, so the two move as one unit?

Here are my own assumptions why this may be worth the added expense to some folks (and I am in way over my head here, so everyone go easy on me!):

1) Even though the "box" has no top, bottom, or sides.... the non-linearities of any driver will be somewhat improved by having a pair of them closely coupled in this arrangement, lowering distortion and other artifacts by having the "pull" driver creating a wave directly behind the "push" driver. The "push" driver could even be delayed ever so slightly, possibly introducing other advantages (depending on your take on Tierry Budge's work).

2) A little stuffing along the open sides and bottom should make the turbulence and/or any roaming non-cancelled sound waves that manage to survive in that sandwich a non factor from the listening chair. Open top still allows maximum heat escape to help keep drivers as close to optimum operating temp as possible.

3) Rear, out of phase signal is now closer in profile to front signal providing (I assume that due to all the mechanics behind a driver that the sound from the back is somewhat different than the sound from the front) so the dipole environment is now a bit closer to theoretical perfection.

4) if coaxial drivers, then you have the rear tweeter to use depending on your taste and you could eq just the rear facing tweeter for different preferences regarding this effect at the listening seat (following the linkwitz crowd hear on the potential value of the rear facing tweeter depending on room, speaker placement, and listening preferences).

5) Could also wire the two drivers in serial fashion for a nice 16 ohm load, OTL biamping mode with passive line level crossover..... By the way, what are your thoughts regarding passive line level crossovers? It seems to me that this would be a better way to optimize a system if you were building the amps as well, coupling them directly to the drivers and shaping the signal behind the preamp instead of behind the power amp.

Awesome thread by the way, and glad to see you exploring the dipole coax approach. Will be watching with great interest, and participating when possible (both in discussion and in potential build/experiment opportunities, though I don't have any measuring equipment yet...)
Once again it is useful to step out of the frequency domain, which conceals phase and time information, and look at impulses instead. Everything is travelling at the speed of sound (345 m/Sec), and for small sources, emission is hemispherical, in one polarity for the front wave, in the opposite polarity for the rear wave. We can choose to absorb the back wave - with only partial success in conventional enclosures - or let it travel freely and reflect off the walls of the room.

Putting one driver in front of another introduces a delayed image for both front and back waves, with the delay being rather short, about half-millisecond or so (a millisecond is about 14 inches long). Since the time window of 0 to 1 milliseconds is used for localization by the ear/brain/mind system, this delayed image will inevitably degrade image quality. All the fancy propaganda by well-financed hifi companies located in Scotland doesn't change these facts of physics, acoustics, and psychoacoustics.

The floor and wall reflections in the 2.5 to 20 millisecond region, by contrast, are benign, since they fall outside the localization window, and do not interfere at all. If anything, localization is improved, since reverberent content in the recording is assigned to these early reflections.

This is not new information, by the way: research into stereophonic sound in the Fifties and Sixties established the critical time intervals used for localization and perception of spatial impressions. Unfortunately, the hifi industry, with the exception of the BBC, KEF, and B&W, choose to ignore this until the early Eighties, so many speakers and surround systems are fundamentally mis-designed.

Designers who refuse to leave the frequency domain, or discuss measurement artifacts like comb filtering, are missing the forest for trees. The perception of sound involves both time and frequency domains, and ignoring one in favor of the other is a mistake. Reflections appear as what they are in the time domain: you see picket-fencing on the impulse response. If you only look at the frequency domain, it looks like comb-filtering, a series of narrow nulls in the frequency response. But it should never be forgotten that phase information is discarded in frequency-domain measurements, and time artifacts like an all-pass function (phase rotation over 180 or 360 degrees) do not appear at all.

It was probably working at the Tektronix Spectrum Analyzer division, with both a scope and an analyzer on the same bench, that underlined this. As a technical writer, it was my task to design sample measurement protocols, photograph the display, explain to the user what they were seeing, and explain the limitations of the measurement protocol. TV signals, for example, look quite different in the time (scope) and frequency (spectrum analyzer) domains. Twiddling a few controls on the SA can alter the appearance of that kind of spectrum quite a bit.

Returning to the Iso-whatsis, it's one way to make a push-pull woofer, which cancels most (but not all) of the 2nd harmonic distortion(at low frequencies). There's no change in efficiency from a single driver, but the required enclosure size is reduced in half, which is an advantage for building little-bitty speakers. The distance between the front and rear cones, though, is very troublesome, grossly degrading impulse response, and introducing our old friend comb-filtering. The usual crude fix seen in commercial speakers is lining the tunnel with felt, and not using the woofers into the midrange, hoping the lowpass crossover will roll off the midrange artifacts. (Hollow-sounding - gee, do you think a tunnel might sound that way? D'ya think?)

The whole concept has no benefits I can see, and serious and completely unnecessary drawbacks. If you want push-pull operation, fine, use two drivers, but side-by-side, with one facing in, and one facing out. Of course, the esthetics aren't as pretty, with an unfinished magnet and basket facing the customer, so I suspect the Iso-whatsis was invented as a way around the esthetic objection.

As we all know, looks win out over sound every time in the hifi biz, especially if you can get the Marketing Department to cook up some ridiculous story about making the cabinet disappear "by magic". If magazine reviewers buy it, so will the dealers and the customers, so everybody is happy.

Oh, it doesn't sound as good? That's because you're not using an Audiophile-Approved (tm) CD to listen to your system. The first time I saw my audio-pals using a StereoPile CD to assess their hifi, I knew the business had come full circle - people had so completely forgotten what music sounded like they had to buy a "HiFi For Dummies" CD with special sound tracks and liner notes to tell them what to listen for. Me, I'll take the "Rain, Steam, and Speed" soundtrack every time - nothing like a 80 mph fully-loaded freight train in a driving thunderstorm to really hear what your hifi can do!

OK, all snark aside, there are some good questions you've asked. Although line-level crossovers sidestep passive networks (good), they introduce the (much worse) problem of combining a top-quality linestage with an active crossover. This is a non-trivial problem, unless you're a member of the all-electronics-sound-the-same school, in which case you throw op-amps or digital EQ at it.

For better or worse, I'm one of those guys that can easily hear single parts changes, much less an entire linestage, and I consider designing a sonically good-sounding line-level crossover a huge and very difficult challenge, every bit as difficult as designing a new power amp or speaker system. There are very few good-sounding linestages, and the commercially available active crossovers are quite a bit worse, regardless of the high prices and fancy names on the box.
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Old 7th April 2007, 12:09 AM   #109
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Default Field Coils anyone?

Lynn, All,

there seems to be little discussion of field coils so far. I had the thought that the ability to tune the Q of a field coil speaker by altering the current density in the coil might allow using two very similar speakers on the OB and tuning them to fill their required roles.

I don't know if there are any field coil hemp cones available, I emailed the guys at Hemp Acoustics but got no response, which I believe is typical.
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Old 7th April 2007, 12:36 AM   #110
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Default Re: Iso-Whatsis

Quote:
I consider designing a sonically good-sounding line-level crossover a huge and very difficult challenge, every bit as difficult as designing a new power amp or speaker system.
At the beginning of this thread, you indicated that you will use stereo sub crossed at 80Hz, don't you need a line-level crossover to do that anyway?
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