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Old 28th May 2007, 11:51 AM   #891
reins is offline reins  Germany
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Quote:
Originally posted by Lynn Olson
Been thinking about a 4-driver version, with two 18Sound 12NDA520's and two 18Sound 15NMB420's.

Hi,

I wonder about the usability of the 18Sound 15NMB420 as dipole bass. The resonance frequency of 42 Hz seems a bit high. In dipole configuration there is a drop of 12 dB/oct beyond Fs if I remember correctly.
IMHO bass should reach down at least to 30Hz.

Regards
Stephan
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Old 28th May 2007, 04:41 PM   #892
MBK is offline MBK  Singapore
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Some thoughts.

I have found it quite a challenge to achieve EQ goals with purely acoustic means (driver Fs and Q, baffle dimensions, driver placement). In my ca. 6, 7 year OB building experience I always tried to get rid of some of the filters and EQ by choosing a compensating configuration. This will work to some extent, but if you want a really exact solution, a blank slate and suitable EQ is far easier. In the 50-200 Hz region, measurement alone will be a huge challenge, because the data will contain a mixture of several effects: dipole rolloff, resonance at the knee of the rolloff (Linkwitz describes this in detail and I have consistently found the same effect), beginning driver rolloff if low Q, and "baffle step" (floor transition), plus floor reflections in an essentially un-windowable frequency range if not measured at "ground plane". To correct all these lumped effects with any precision when they in fact measure in compound, will require long trials. I found it much easier to do what SL does: address each issue in isolation, measure it in isolation (!! important), fix it independently, and then cascade the elements (in fact the filters).

An example: for my current OB implementation, I chose a 24x40" baffle, 7" deep. Width and depth were chosen to get the dipole rolloff to start just below 200 Hz, just when the gradual floor transition adds 6 dB, with the aim to begin dipole EQ at now 100 Hz and not 200 Hz. (In addition to that I wanted diffraction from baffle edges to begin at >700 us to avoid timing smearing, so a wide baffle was indicated). The depth was chosen to completely hide the back of the woofers yet without creating a cavity resonance. Driver placement and baffle height were chosen to smooth out nulls and peaks of the dipole effects over a wide range.

The result works, sort of: raw response is fairly flat down to 100 Hz, and I *could* now get rid of SL's recommended 200-100 Hz 6 dB floor transition shelving highpass, start dipole EQ at 100 Hz, and with another shelving lowpass EQ the low Q (0.22) bass driver, all down to 20 Hz. By starting dipole EQ a bit lower than calculated, the dipole resonant peak around 200 Hz would not be too apparent on measurements.

But in reality things work only approximately that way. The 200 Hz resonance is still apparent that way ( on certain notes a muddying is noticeable, especially on piano), and starting dipole EQ lower to mask it only resulted in missing EQ in the lower bass. So the peak EQ went back in and the start of the dipole EQ back to where the calculations by theory had it put. Then I noticed that the intended dipole EQ down to 20 Hz just created too much excursion for optimal sound quality. By calculation it was all sufficient, a 7.5mm Xmax 15" woofer should be enough no? It is, but not at optimal performance. But if I stopped EQ say at 40 Hz, the intended EQ starting frequency at 100 Hz meant that then the corner frequencies weren't far enough apart, so I wouldn't get the full gain and slope. So the 100-200 Hz shelving highpass went back in, and the dipole shelving lowpass went back to 200-40 Hz even though it is partially cancelled by the shelving highpass ... Basically in the end I did what SL does: address issues separately and fix them separately, usually by EQ.

In addition to that, measurements of what is going on are not really interpretable if you can't take the various filter elements in or out while tailoring the measurement setup precisely to the effect you want to pin down. For instance a ground plane measurement will not show the floor transition, so you can't use the 100-200 Hz filter - which is OK if you planned on using one. But if you plan on building the baffle such that you won't need the 100-200 Hz filter, you have to tailor the response such that a ground plane measurement will in fact show the sub-100 Hz bass 6dB down, starting at 200 Hz. (I found non- ground plane measurements completely unreliable from 100-400 Hz, floor bounce is just too erratic as an influence)

Another thing to consider: pro woofers such as my 15ND930, or the ones considered here, are spec'd at insane power levels: 500W pink noise for 2 h before taking T/S parameters, in my woofer's case. Actual T/S in-baffle at home power levels are *dramatically* different, I suppose because the suspension never softens up enough to reach the spec'd low Fs and Q. So planning should allow for deviation from spec'd T/S data, and EQ will very likely be the only option below 200 Hz.
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Old 28th May 2007, 08:28 PM   #893
mige0 is offline mige0  Austria
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Default BASTA! versus SoundEasy

Hi

Lynn
Quote:
We're doing pretty well for linear Xmax. The 12NDA520 has 8mm total linear Xmax, and 22mm total peak-to-peak excursion. The 15NMB420 has 13mm total linear Xmax, and 36mm (!) total peak-to-peak excursion.
MBK
Quote:
By calculation it was all sufficient, a 7.5mm Xmax 15" woofer should be enough no? It is, but not at optimal performance.



Taking a rough guess from the intermodulation I measured in post 824

http://www.diyaudio.com/forums/showt...43#post1215943
"http://www.diyaudio.com/forums/showthread.php?postid=1215943#post1215943"

Click the image to open in full size.

I doubt, that a XO 2,5...N system will be sufficient for let's say 110db / 1m down to 40 Hz (or as in MBK's case the chosen single 15" for more moderate listening levels)

Does SoundEasy software allow to simulate max excursion / SPL with different multiple drivers on open baffles in 2.5 way arrangement?
If not, BASTA! software from

www.tolvan.com
"www.tolvan.com "

would be a cheap alternative I would consider to buy for myself.

If I understood right with a 2.5....N approach the 12" wide range speaker would run down to its natural resonant frequency splitting the load by half every time an additional driver comes in. First with the 12" low-mid and second with the two 15" woofers.


Greetings
Michael
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Old 28th May 2007, 09:32 PM   #894
agent.5 is offline agent.5  United States
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Default Re: BASTA! versus SoundEasy

Quote:
I doubt, that a XO 2,5...N system will be sufficient for let's say 110db / 1m down to 40 Hz (or as in MBK's case the chosen single 15" for more moderate listening levels)
I think Lynn's goal is to have his speakers sufficient for listening down to 80Hz and a separate pair of subwoofer will come it at 80Hz.

Lynn, what are you using for the subs? Rythmik?
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Old 28th May 2007, 09:56 PM   #895
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I'm in agreement with both MBK and mige0. The plan all along was to transition to stereo subwoofers between 70 and 100 Hz. All the 2.5 ... N does is merely take the speaker from the 1/f transition of the widerange driver down to the point where the subwoofers take over - basically, from 200 to 70 Hz, a one-and-a-half octave range.

Within this range I'll be using separate amplification and equalizing as necessary, measuring on a flat boundary surface (the rear wall of the room, which I used before to confirm room energy spectra). There will be three sets of amplifiers: my Class A1/A2 Karna triode amplifiers, powering the WR and HF drivers; a Class AB or D amplifier for the Midbass and Bass drivers; and another Class AB or D amplifier for the stereo subwoofers. I'll use a Rane or DBX parametric equalizer for the MB, B, and SW amplifiers, and use passive crossover EQ (if necessary) for the mids and highs.

Within the 70 Hz to 20 kHz range, I want to keep upward EQ (response boosting) to the mininum, or failing that, have the boosted range covered by multiple drivers, instead of one highly stressed driver. I'm sure you can see what I'm aiming for - dipole sound with near-Klipschorn dynamics, which isn't going to happen with heavy EQ and jamming 200 watts of Class AB power into 88 dB/metre audiophile drivers - which were never designed for that kind of abuse.

So the whole business of 2, 3, or 4 midbass and bass drivers (and the way they work together) is wide open, and to be determined by measurement and audition. I don't know if I want the slant the OB backward by 5 or 10 degrees; I'll have to build it so it can adjusted by set-screw (probably on the base of the vertical strut) and audition that as well.

Similarly, the compression driver + horn will need some kind of cat's-cradle that mechanically isolates, supports the 10-kilo weight of driver+horn, and provides adjustment for front-to-back alignment and left-to-right rotation. The mounting system should also accomodate ribbon tweeters, which will need the same location in space as the CD for the horn system.

This project is not a ready-to-build speaker. People who want that should visit the websites that offer a range of plans built around popular audiophile drivers; you're getting a known quantity that follows the template of nearly every commercial audiophile speaker.

The winding path this project is taking is similar to the way I wrote about the Ariel speaker in 1992, followed by the Amity, Aurora, and Karna amplifiers, sharing my thoughts as I go along. The Ariel was a modest departure from a minimonitor, using well-known drivers of the day, a MTM, and a twin-path transmission-line cabinet. Nothing too astonishing there.

The fully-balanced triode amps, though, were a jump into the wild blue yonder, using circuits that hadn't seen the light of day since the mid-Thirties. These amplifiers were very different animals than the Williamson variants that dominated tube-amps since 1947 - and as I found out, measured and sounded quite different as well. When I started in 1997, all of the SET builders ridiculed the idea of transformer coupling and all-balanced circuits - many still do, ten years later. And of course transistor amp guys think any kind of tube amp is just goofy, a waste of time, since transistor amps solved all the problems worth solving decades ago.

In terms of builder risk, this project is about halfway between the Ariel and Amity. There's no risk of electrocution, and you don't need to know the finer points of ground-system layout and magnetic-field induction. But owning instrumentation, a decent measurement mike, and software similar to SoundEasy is a must, just as a scope and a DVM are essential for building an amplifier.

That's why the feedback and experience of ScottG, MBK, mige0, and so many other is so important - they've built systems outside the commercial sphere, and I'm very curious what they've come up with, what works, and what doesn't.

Rather than present perfect, 100% debugged, ready-to-build systems to the public, I'm willing to make my mistakes in public - and anyone that reads my stuff know I make plenty of mistakes!

I also know audio designers, like pro photographers, make lots of mistakes all the time. They just don't show them, hiding behind the image of the audio-expert. (And I do that too.) Well ... unfortunately, we don't have many geniuses in audio. The days of Major Armstrong, Blumlein, Bell Labs, and D.E.L. Shorter of the BBC Labs are long gone. We're all just working on the margins, assisted by computer modelling and better instrumentation.
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Old 28th May 2007, 10:26 PM   #896
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Default Re: Re: BASTA! versus SoundEasy

Quote:
Originally posted by agent.5

I think Lynn's goal is to have his speakers sufficient for listening down to 80Hz and a separate pair of subwoofer will come it at 80Hz.

Lynn, what are you using for the subs? Rythmik?
Right now, I have one REL Strata II, which complements the sound of the Ariels. But REL has moved on, and the Strata III is a different design with a different LF rolloff slope from the Strata II, so buying another REL to match isn't an option. (Huge phase shifts between L and R channels, very undesirable.)

I'll just have to sell what I have now and build a pair of subwoofers. The easiest thing is probably copy Gary Pimm's W-baffle subwoofers, which if I recall right, are built around pairs of Parts Express 15-inch drivers.
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Old 28th May 2007, 10:56 PM   #897
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Would a pair of Rythmiks serve as a viable alternative?

Ray
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Old 28th May 2007, 11:07 PM   #898
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Agent.5 and Ray Collins, thanks for the pointer to Rythmik Audio. The issues they're discussing are real and significant, and the engineering looks solid to me. I'll be putting a pair of DS15's on my short-list.



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Old 28th May 2007, 11:29 PM   #899
ScottG is offline ScottG  United States
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Quote:
Originally posted by Lynn Olson
Agent.5 and Ray Collins, thanks for the pointer to www.rythmikaudio.com/memory.html]Rythmik Audio[/URL]. The issues they're discussing are real and significant, and the engineering looks solid to me. I'll be putting a pair of DS15's on my short-list.

Thats the only commercial DIY sub that will work. Not only does it lower distortion above driver/port resonance, it so well controls an otherwise "droning" driver architecture, that it can actually be used as a bass driver (i.e. freq.s *above* 31 Hz) with more transient capable loudspeakers. Note though that its still cr@p near driver/port resonance, AND its overall output is still compromised at lower freq.s. My suggestion would be to vent it around 12-13 Hz - keeping the lossy resonance (and distortion) out of the way while providing higher power handling and a more modest size cabinet. (..of course such is suggestion is counter intuitive - most will gravitate to a sealed design without fully understanding the variables). Note: as a result I would choose the 2 12's over the single 15.
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Old 29th May 2007, 12:01 AM   #900
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Using the adaptive EQ is interesting - since the driver is sensing its own output, I guess the vented version would have to have the vent frequency (and Q) programmed into the little feedback-control card in the power amp.

I was thinking of a sealed box, actually, since the feedback servo takes care of most of the distortion. The primary benefit of vented is substantially reduced excursion (and distortion) close to the vent frequency. With servo, that benefit mostly disappears, leaving increased SPL as the remaining asset - but with the drawbacks of vent coloration and a 4th-order highpass characteristic.

In a servo setup, what are the sealed-box DISadvantages? Curious to know.

-----

It is interesting that Rythmik is addressing the same concerns that I'm looking at, just in a different frequency band. It does make you think about custom drivers for the Bass array in the OB, although that's a project for another day.
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