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Old 27th September 2006, 03:28 PM   #11
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Hi MBK

> floor boundary gain should come in roughly when the distance to the floor is less than 1/2 wavelengths

. . thatís centre of driver to floor?

If eg 1/2 wavelength is at 40 Hz, is the gain:
∑ only at that Hz
∑ from that Hz down
∑ that Hz +/ - ?

Was that in SLís material?

Your baffles - nice finish ~
What is your dipole bass driver, EQ & Hz range?

Which software do you like?


Cheers
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Old 27th September 2006, 03:34 PM   #12
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Default U-frame baffles

Hi

You chose this shape based on JohnKís work? . . . is there a model using TS for this?

Cheers
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Old 27th September 2006, 04:08 PM   #13
MBK is offline MBK  Singapore
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Hi Rick,

the frequency is a rough rule of thumb. SL actually mentions 200 to 100 Hz as the transition zone for the 6 dB gain based on empirical results. In any case it matters little whether it's driver center or boundary, given the wavelengths involved. At 200 Hz a 1/2 wavelength is ca. 86 cm or ca. 3 ft... so it's really plus minus a foot or two.

My baffle shape came about from various considerations - going for width, not depth, as shallow as possible to avoid cavity resonances but deep enough for a complete driver depth, including the woofers in the same baffle, and ease of build. The tweeter is conventional, the mid is in a 7" deep U frame, the woofers are in the same depth H-frame, very shallow indeed.

The foot houses the x-o and amps, so each speaker is a complete active setup that only needs an AC power cord and the signal cable. Mounted on rubber wheels of furniture supply origin...

Basically the shape is a mix of practical considerations and design intentions. The needed baffle dimensions were estimated using Linkwitz's formulas, and the intent was to run the mid with no EQ by having the dipole rolloff start at ca. 200 Hz where the floor gain kicks in - leaving the mid at no EQ until 100 Hz, in theory. The low Q mid (0.22) should have its first pole around 130 to 140 Hz, and I intended to use both dipole rolloff and low Q rolloff as 2 of the 4 poles of an eventual acoustic LR4. In practice a 125 Hz 3 pole HP seems to achieve the closest to an LR4, but I need to do better outdoors measurements. I first built a prototype, which turned out to measure OK for my intentions, then I built the final baffle. Asd I still haven't got around to confirm LF performance in detail by good measurements, but the dimensions are very close to the prototype.

The bass is courtesy two Vifa 10" in push pull configuration, Q=0.5 and Fs=23 Hz, EQ is currently from 110 Hz dwn or so (I keep modifying it by +- 10 Hz or so, a matter of taste really). I used to EQ down to Fs or even lower, but remarkable with music it made strictly no difference if I cut it off at 40 Hz. So, to save on excursion needs, I now use 40 Hz. I am toying with the idea of an 18" pro woofer, it would fit perfectly and give more volume displacement and possibly sonic benefits.

Software: I mostly use Audiotester, and have also started to play around with ARTA now. But my main problem is really to find a suitable measurement location , my outdoors is not quite large enough (small yard) and way to boomy in the lower octaves (city hum) , say in the 50 to 300 Hz region where you can't do any meaningful windowing, I get too much background noise outdoors and due to the large baffle, and the dipole, I can't just do nearfield measurements (at least not for optimizing the X-O).

I used almost exclusively SL's materials to learn about dipoles. I learned about JohnK's approach at a later date, and also about The Edge. Honestly I think SL's material ought to be absorbed first, then you might add alternate visions and ifs and buts.

In the end much of it boils down to personal tastes and trade offs anyway, AND, hard to stress this enough, to know what and how to measure!! That's actually the hardest part. Software and mics are not enough. You must know enough theory to know why you are getting what you are getting, in measurements. And simulations are just not enough, the deviations from reality are too large.
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Old 28th September 2006, 02:40 PM   #14
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Thanks your suggestions.

I certainly agree that SL's materials are first principles, but as I have struggled to understand some of it, Martin Kingís worksheet is vrtainly the quickest way to trail the 6 odd drivers Iím considering

Iíll digest your suggestions, esp ďto know what and how to measureĒ

Have you posted at the (almost inert) Yahoo Group for dipoles?

Cheers
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Old 28th September 2006, 04:05 PM   #15
MBK is offline MBK  Singapore
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Hi Rick,

I've seen Martin King's site but I can't find the actual worksheets there... Anyway, yes, SL's site can be hard to digest, not so much because of the difficulty of the material in itself, but because he presents so many different angles of view, all to be considered together for a coherent system. I built my first version of a dipole speaker in '99 when there was only the Phoenix... and I am still learning and my opinions keep evolving.

I still think the main problem for the home builder is measurements. This is what makes worksheets and design applets so attractive - they suggest that comprehensive measurements may not be necessary. But this approach can only get you so far - simulations for an approximation of what to expect, and the underlying theory for explanations of why you get what you are getting. But *what you are actually getting* will remain unknown unless you measure it.

Don't get me wrong, I say all this precisely because good measurements are my personal final frontier, my area of greatest need and lack. I have no problem inputting data in a worksheet and optimizing a simulated curve to maximum prettiness. But I have severe doubts as to how realistic my measured results really are, which problems are real, and which are artefacts. Maybe the best would be to build a simple speaker, box or dipole, and just measure it into the ground, under all possible conditions. Just to get a feeling for the process, before moving on to more difficult devices... mumble, mumble...

I did visit the yahoo group at some point but I haven't been there in the longest time. DIYAudio is more diverse and there's always a tidbit of two of interest here...
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Old 28th September 2006, 06:38 PM   #16
v-bro is offline v-bro  Netherlands
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Quote:
Originally posted by bear
The link appears to point to basic "crossover filters."

You'll need to take into acount the actual acoustic response in the design of any real-world crossovers.

_-_-bear

I guess you're right, first a good functioning enclosure/baffle, than filtering...

But not many people (hobbyists) use this very basic and easy to experiment with (due to much lower values) type of filtering.
Most hobbyists use passive filtering on the power-amp's output, where the current is much higher and the components have to be able to handle this(thus are very big and expensive....).

This filtering method is between the line-level signal on the amp's input.....

For those who didn't know, hope I've been of some help....

look at:http://www.marchandelec.com/xm46.html
When good components are used, the soundquality can be phenomenal (due to it's simplicity)...
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Old 30th September 2006, 02:53 PM   #17
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I am familliar with Marchand.

I would not use electronic xovers other than for Sub Woofer LP functions myself.

One could build very high quality line level crossovers, but it is non-trivial to do so. The distortion inherent in 99% of the opamps used for such things, the lack of headroom with opamps in filter sections (they require gain you know, and line level being a nominal 1v leaves you with nil headroom, unless you pad between stages... etc.) means that regular passive components are cleaner.

Imho, the ultimate way to go for "active" is to use buffered passive components!

For passive xovers:

The cost of 100v or 200v polypropylene caps is not so high - just don't buy the fancy "audio" ones! The ones used by switching supply mfrs are quite good enough for xover use. Imho the results are superior to opamp based electronic xovers.

Coils are the cost of wire and a bobbin to wind on... cheap.
Wind ur own to exact spec.

Of course much depends on what ultimate quality level you are aiming for with your system. No doubt that adjustable electronic xovers are simple and fast to implement.

Personally, I prefer to stay away from designs that require convoluted "EQ" in the passband (meaning you can't implement simply in passive no matter what) and assorted narrow filters in the xover regions or notch/boosts elsewhere...

_-_-bear
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Old 30th September 2006, 03:20 PM   #18
v-bro is offline v-bro  Netherlands
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True about the op-amp issue, though the xm46 from marchand is a PASSIVE line level filter using NO op-amp circuitry....

The quality is really high (look at the s/n ratio!!).
A kit for one filter costs 65 USD, and probably even cheaper when you build it yourself.....
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Old 2nd October 2006, 03:25 PM   #19
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Oh... lemme see... yeah! Ok.

But you better have one heck of a buffer to drive these suckers!!
Not a bad price... of course ur locked in to a specific frequency since he is going to supply the inductors wound to fit... and the filter shape will be pretty much what he supplies, although you can dork the cap values yrself and get some other filter types within reason...

So, that gets back to the cost, since imho you need a very clean and relatively low Z buffer... a straight 600ohm output impedance is probably not low enough - so that leaves out ur chips generally speaking.

Consider some iron?

Once you build a buffer, you've added some expense, but things could be worse!

Also, don't forget about the phase thing if these are used as 2nd order filters - if you use them like that...

_-_-bear
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Old 6th October 2006, 12:25 PM   #20
v-bro is offline v-bro  Netherlands
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For those who're interested:
http://www.marchandelec.com/ftp/xm46man.pdf
The schematic and component list!
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