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Old 7th April 2007, 08:57 AM   #111
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Quote:
Originally posted by agent.5


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?
Good point. Op-amp and digital crossovers are more acceptable below 120 Hz as far as I can tell - it's in the midrange and HF where the sonically noxious qualities of mid-fi electronics become audible. The dipole part of the system is where you want the pristine amplification - but for the sub, eh, I'm doubtful that tubes are the way to go. Both transformers and RC-coupled vacuum-tube circuits are at their worst in the deep-bass region.

I certainly would avoid passing the mid/high part of the signal through an ultra lowfi device like the Behringer DEQX, which uses cheap 741-type opamps and even cheaper electrolytic caps in the signal path. The digital part of the DEQX is decent, but the analog portion is truly terrible, using the same quality parts as a Wal-Mart boom-box.

Nothing wrong with separating the signals following the volume control, passing the mids and highs to the high-quality part of the system, and the low-bass to the subwoofer crossover, amp, and subwoofer drivers. Gary Pimm tried the DEQX for his subs and warned me that the electrolytic caps (in the DEQX input circuit) are such low quality they contaminated the mid-highs just by connecting the DEQX in parallel following the volume control. He had to replace the caps with good-quality polypropylenes in order to prevent the mid-highs from being contaminated whenever the DEQX was connected to the system.

As for field coil drivers, ooh la la, they sound so nice. I heard the Cogent horns at the RMAF and was deeply impressed at the subtlety and shimmer of the tone colors. While the signature of Alnico are bright and vivid tone colors, field-coil speakers have a sound all their own, with an almost luminous quality to the tonality. It's hard to describe unless you've heard it for yourself.

I don't know why it's even controversial that magnets sound different. The magnetic system is why the voice coil has significant inductance in the first place - the free-air value of the voice-coil inductance would be almost nothing. And of course, the magnetic system is NOT a perfectly linear inductor - what iron-core inductor is? Going further, chokes wound with ferrite and different magnetic materials sound different - and the (nonlinear) voice-coil inductance is ALWAYS in series with the signal going to the voice-coil, no matter what kind of amplifier you use.
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Old 7th April 2007, 09:50 AM   #112
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by Scottmoose
................................

Nothing fancy at all. Cheap Eminence Beta 15 bass driver mounted fairly close to the floor, XO to a Fostex FE167E at ~180Hz. Series XO, utilising the natural roll-off of the drivers. The baffle is relatively narrow, roughly 16in, triangular side-pieces forming a semi U frame, roughly 12in deep at the bass, tapering to nothing at the top. Sonics were downright staggering. For the megre price, I've heard little that gets close. Easy 40Hz, seemless transition. I went hunting for the XO, and I flat couldn't find it. Design was by JamesD, who frequents the World Designs forum, and has much fancier & most expensive acrylic versions -effectively a very highly modified version of the original Quasar using a 12in Supravox bass unit and AER MK1 up top. Bass isn't really dipole in radiation pattern in this layout, but it certainly sounds like it, and the things can be used close up to a wall or in corners will little degredation. Don't ask me how or why -it's beyond me! .............................

question for Scottmoose........and others:

what are easy predicted consequences (or differences) of using Eminence Alpha15 (97db/41Hz/Qts 1,26) vs. Eminence Beta15 (98,2db/35Hz/ Qts 0,58) in JamesD lookalike OB ?

which one is better if I need to place it as closer to wall I can?

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Old 7th April 2007, 10:06 AM   #113
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Default More Thoughts ...

Not to tease you guys too much, but I've had some good ideas in the last few hours about the bass unit for the dipole - I'll share what I have in the absence of firing up Canvas and making proper drawings - so bear with me in these descriptions.

One of the key design points of the bass unit is the woofers are right next to floor level. This has the desirable quality of creating a mirror-image "virtual" driver just below the physical driver, thus doubling the effective cone area. You lose just a bit of efficiency if there's carpet right there on the floor in front of the speaker, but the loss is pretty small.

I've been thinking all along of only one or two bass drivers, but that might not be the most interesting approach. Let's stretch the envelope a bit and think of FOUR 12 or 15-inch drivers.

I'm visualizing a pair of drivers side-by-side in the front, another driver on the left side of bass unit, and another driver on the right side of the bass unit. Maybe a pair of 12's for the front - Alnico Tone Tubby's, maybe? - and a pair of 15's on the sides. All four drivers are in a quasi-box that is filled with recycled cotton home-insulation material, like ground-up blue jeans.

The box is open on the rear, the sides are no more than 16 inches deep, and there are partitions on the inside, extending from the front to the back, between the front pair of drivers, as well as a partition going from the magnet of each front driver and extending to the rear of the box. (Three internal partitions in all, with uneven distances between them.)

This is a variation of Gary Pimm's cardioid speakers, which are open-ended boxes fllled with recycled cotton insulation, about 12 to 16 inches deep. Think of them as very short transmission lines, if you will.

Now let's take the next step and look at the crossover for these four drivers, which are connected in series-parallel. The front drivers are in series (with themselves), and the side drivers are in series (with themselves). The front drivers handle the range from 160 to 220 Hz downward, and the side drivers handle the range from 80 to 100 Hz downward. All drivers are in-phase and work together at the lowest frequencies.

The 160 to 220 Hz lowpass filter is 2nd-order, and connected to the series-pair of front drivers. There is a second branch to the circuit that jumps off from the front drivers and goes to another inductor that forms a 80 to 100 Hz lowpass filter, which then feeds the series-pair of side drivers. (I don't claim credit for the 2.5 way crossover, I saw it elsewhere on the loudspeaker forum, and it'll certainly work well in this application.)

This sequential crossover ensures the low-bass drivers share the same 2nd-order lowpass filter as the midbass drivers, and all drivers share favorable phase relationships at the lowest frequencies, where they work together. The 2nd-order filter also keeps the front set of midbass drivers sounding clean, avoiding the usual 1 kHz bumps-and-dips, and keeps energy out of the frequency range where the boxes go into box modes (although these are minimized by the asymmetric dimensions and asymmetric-spaced interior panels).

At first blush it seems outrageous to have FOUR 12 and 15-inch drivers per channel - well, it does nicely compensate for the 1/f dipole rolloff of 6 dB per octave, the crossovers can be precisely tuned for just the right degree of driver overlap and room compensation, and there's no reason the low-bass pair of drivers have to be super-audiophile-quality. Gary Pimm is using modestly priced MCM or Parts Express 15-inchers for his high-Q subwoofers.

For that matter, there no necessity for four drivers at all - you could get by with a single 16-ohm front driver and two 8-ohm side drivers, depending on how the efficiencies work out. The principle is the same whether 3 or 4 drivers are used, the crossover is pretty much the same, and 12 and 15-inch drivers can be mixed and matched, since they are doing somewhat different things - deep bass and midbass.
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Old 7th April 2007, 10:40 AM   #114
peufeu is offline peufeu  France
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Won't you get a strong pipe resonance with that type of "box", on your woofers ?

I tried something like that : picture a cube, 50 cm wide, two ends open, 15" woofer on a board in the middle of the cube (H-baffle). It worked pretty well and sounded good, too.

Of course there was large pipe resonance. It's strong enough to be completely immune to passive crossover equalization...

Anyway, please consider active filtering : low watt high quality tubes for the midrange and up ; solid state muscle for those woofers, which permits using active EQ... since dipoles need EQ anyway.
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Old 7th April 2007, 11:25 AM   #115
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Default To Box or Not To Box, That Is The Question

Quote:
Originally posted by peufeu
Won't you get a strong pipe resonance with that type of "box", on your woofers ?

I tried something like that : picture a cube, 50 cm wide, two ends open, 15" woofer on a board in the middle of the cube (H-baffle). It worked pretty well and sounded good, too.

Of course there was large pipe resonance. It's strong enough to be completely immune to passive crossover equalization...

Anyway, please consider active filtering : low watt high quality tubes for the midrange and up ; solid state muscle for those woofers, which permits using active EQ... since dipoles need EQ anyway.
Yes, with the Pimm-style semi-box, active EQ w/solid-state amplification for the bass units might be a necessity. Once the drivers go into short tunnels, there's the box mode(s) to consider.

In Pimm's setup, the semi-boxes are pretty much filled, with about 2 to 3 inches of free space between the back of the driver and all that filling. All you see when when you look at the back of the box is a wall of gray fuzzy stuff - that's why they remind me of short transmission lines, or maybe resistance-loaded boxes. The filling he's using was way denser than anything I've used in a TL - there's so much it's mostly absorbing the backwave, which is what gives the cardioid response pattern. Another hint of the backwave is being heavily absorbed is the flatness of the impedance curves, which have a very TL-like appearance.

The alternative to the semi-box is a pair of 12 or 15-inchers at the base of a true dipole, and conventional subs coming in around 120 Hz or so. I'm guessing the semi-box would probably go down to 40 or 50 Hz, depending on the degree of equalization.

Since there are two quite different ways to go, that's another reason to separate the HF dipole (acrylic) baffle from the bass unit. Dipoles and short-box cardioids are somewhat different animals, and have different needs for care and feeding (crossovers and equalization).
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Old 7th April 2007, 12:54 PM   #116
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I think peufeu is right. Even with a true dipole, when the depth is about 16" you will have to cope with a pretty strong resonance around 200 Hz (The 'pipe' is virtually longer than the depth of the dipole suggests). You could equalize that but even with active equalization I would stay away from such effects and use such a dipole only up to about 100 Hz.

I myself use 1 pair of 12" inch drivers per side (Peerless SLS-12), mounted on a baffle of 3.8" thick. This gives perfect dipole behaviour up to 400 Hz. They are equalized 6dB/oct with a shelving low pass set at 28Hz and 560 Hz corner frequencies. They provide me with uncoloured sound when I low pass filter them 2nd order at 200 Hz . Ofcourse, excursion limited SPL at the low end. For that, you already have a good solution in mind .
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Old 7th April 2007, 03:07 PM   #117
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There is no reason why the arrangement of 4 drivers couldn't be in the same kind of baffles envisioned earlier, the side panels would just need to be sized to accomodate them.

Designs with 2 or 4 bass drivers -- all on the sides push-push -- are already in the field. Given the low XO point, this arrangement could have significant advantages as far as newtonian energy loading/transmitting to the material of the baffle.

As far as 4 drivers seeming excessive -- an earlier sketch i posted had 8 x 12".

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Old 7th April 2007, 03:19 PM   #118
Paul W is offline Paul W  United States
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Lynn,
One of the things you can use to treat the pipe resonance is wool felt. I have a pair of BMS 18N850's in 15" u-baffles. A single 1/4" layer of F7 wool felt completely damps an otherwise huge spike at 200Hz. For me, the felt works better than active EQ.
Paul
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Old 7th April 2007, 10:56 PM   #119
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Default Meta Post - Waveguides for the Backwave

Well, once again I've led myself into a thicket with one of my bright ideas. Maybe stepping back a few steps will clarify things.

All of the typical acoustic radiators are intrinsically dipoles, with reverse polarity for the backwave. Note: Phase in only meaningful if the frequency is specified, while polarity exists independent of frequency. For example, it is trivial to invert the polarity of a signal with a bandwidth of 5 Hz to 5 MHz, while shifting the phase of such a signal through 90 degrees over the complete bandwidth requires an extremely complex all-pass filter. Do such wideband signals exist? Think of television, which is very sensitive to time distortion.

Returning to dipole radiators, what to do with the equal-intensity reverse-polarity backwave? One approach is a true infinite baffle, which in physical terms means cutting a hole in the living-room wall, and letting the backwave emit into the great outdoors. This is not a practical solution for most of us.

We can use a finite baffle of different shapes: let's look at four shapes.

1) A circular disk. This is the most resonant solution for an "open-baffle" loudspeaker.

2) A 90-degree circular horn, or megaphone. This is the most resonant solution for the horn.

3) An open-ended pipe with a circular cross section. This is the most resonant solution for the pipe.

4) A closed-ended pipe with a circular cross section. The other shapes can be closed off as well, creating a box.

All four shapes are actually transforms of each other, with performance that can be modified by selecting some degree of asymmetry, which decreases the Q of the standing wave, or some degree of absorbent filling, which decreases the Q and magnitude of the standing wave. In principle, 100% absorbent filling should absorb all of the backwave, and also completely remove the standing wave created by the structure. In practice, filling materials exhibit frequency selectivity as well as reflectivity at some frequencies.

Note again that shapes 1 through 3 are simply transforms, with an infinite range of intermediate shapes. All exhibit standing waves due to their finite size and the reflection created at the outer boundary of the shape. Asymmetry in the shape blurs the reflection across time, but does not decrease its total energy content. This smoothing looks good in the frequency domain, but the time display still shows a blurred reflection.

What about edge termination? Ah, now that's different. One solution not thoroughly investigated is replacing the hard material with an open-pore mesh. Perforated metal mesh, such as the pattern of ventilation holes on top of an amplifier, is typically 50% open. This kind of stands the JBL Acoustic Lens on its head - instead of forcing the forward wave to disperse through the mesh, they would have been better off terminating the horn itself with the mesh, thus softening the edge of the horn.

How does this apply to the short boxes of the dipole bass modules? Well, imagine the back portion of the box not made of wood, but perforated metal or pegboard. The back of the box is still open as before, but the last 1/3 of the sides are perforated metal or pegboard, which partially reflects and partially transmits sound. This gives a much softer edge to the end of the box, since energy leaks out progressively as the pulse travels back through the box. Internal pressures are low, since this is not a sealed or vented box in the usual sense, but more of a resistance-loaded box - or very short transmisssion line.

The mesh is also a good way to terminate the edge of the dipole on the top - although I'd be the first to admit a pattern of small holes at the outer edges of the acrylic panel would look a little odd. It would work, though.

As odd as it seems, dipoles, transmission lines, horns, and closed boxes are all topologically similar in the way they create standing waves. As this continuum of shapes approaches a pipelike shape, the energy storage increases, and as it approaches a flat disk, the energy storage decreases - although it does not drop to zero.

The usual solution of modest edge treatment (rolled corners) and/or filling materials is only modestly effective, as evidenced by high levels of remaining coloration and not-so-good time measurements. The extremely wide dynamic range of the ear is why we can hear even small reflections so efficiently, and why a 60 dB decay threshold is really only the beginning. Our hearing is optimized to detect sizes, shapes, and the acoustic properties of materials, and is extremely sensitive to resonances and correlated reflections.

Edge treatments in the broader sense of EnABL, Mamboni, and open-mesh techniques have not been investigated as thoroughly as should be, since the results will show in impedance curves and improvements in impulse response.
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Old 8th April 2007, 12:40 AM   #120
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Hi Lynn.

I wonder if the enclosures you mention could be terminated with some modification of the Mamboni pattern, perhaps extended into three dimensional shapes of some size and made from a material that eats energy by vibrating small kinked bits of fiber?

There is a material of merit, the replacement carpet underlayment used by auto interior restoration folks. About an inch thick, with a tightly interlaced body of various length and diameter, kinked, moderately stiff fibers and small pieces of a rag like material. Has a shorter, more densly packed face, on one side, with the other face more open and with larger kinked fibers. Easily the most linear damping material I have found and good from infrasonic to about 2 k or so. Very linear in this range too.

Perhaps a Mamboni ring applied just before and entering into the peg board mesh area?

Bud
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