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Old 29th December 2008, 11:43 PM   #1
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Lightbulb Low-distortion OB

Having done few OB prototypes I have few understanding how the drivers behave on them and really want to do one now that is full-range and most importantly: Low distortion.

The obvious answer is to build an Orion but it's so expensive. Even then the W22EX is being strained to 1500Hz according to SL himself due to him needing them to cover 150Hz zone, perhaps the H-frame woofer can only cover up to 150Hz due to cavity resonances.

So I have on hand now:

- 4x Eminence alpha 15A (2 are broken in)
- 2x Vifa P13WH midbass
- 2x Vifa TC20TD tweeters (cheapish)

The objective is to operate the drivers at their "pistonic" and non-beaming properties. I have experimented using Eminence up to 1kHz as CS2 but convinced that this is way beyond its ideal operating range and although it sounded "good" and "Live" perhaps distortion is at play here. I also have played with P13WH on OB and it did not sound good at the time when run at its well-known flat response 250-5kHz, so it has to be revisited.

According to SL the rule of thumb is to operate them at 1/2 wavelength of the cone diameter. This gives:

20 - 300Hz: use 4x eminence alpha
300 - 1500: use 2x P13WH
1500 - 20kHz use 2x TC20TD.

The tweeter is bad as it has Fs of 1400 so it will need to be replaced with better one. P13WH had been extensively measured and reviewed by SL and Lynn, and I have done plenty distortion measurement on the alphas.

All will be crossed using active 4th order filters and tri-amped. Baffle width will be 45cm x 120cm (or shorter). Flat.

Is this sane thinking and what are the pitfalls here?
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Old 30th December 2008, 01:42 AM   #2
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The P13WH midrange selection. It is interesting that the 1/2 wavelength rule coincide with that bit of dip (breakup?) at 1500Hz. Perhaps ideally the upper freq. selection should be at 1kHz but then there will be no tweeters able to cover this (?)

The other option is perhaps lower at 1300-1400Hz. Orion ASP shows that the tweeters are crossed at 1440Hz. It seems this is as low as it would go (and they're excellent tweeters).

Click the image to open in full size.


Being small this midrange came up excellent when SL tested it. His complain is it is audibly strained when tested, maybe because of the low frequency requirement. Hopefully by running only from 300Hz up, this defficiency would not show.

"The Vifa drivers 10 (PL14WJ) and 11 (P13WH) exhibited very good burst behavior, something one might have expected for the small cones. Unfortunately, they audibly strain even at the modest volume displacements of this test series." -SL

I hope the good burst behaviour will give the speakers good clarity in the midrange. Lynn seems to emphasize them although I read as well that the Vifa changed over the years.

"I first heard the 5.5" Vifa in the Lineaum LFX, and was impressed with the wonderfully transparent midrange. I was not so impressed with the thin bass. " -LO

Again, the bass is removed from the equation by running them 300Hz up. Hopefully this is logical thinking.

Click the image to open in full size.
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Old 30th December 2008, 02:54 AM   #3
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You might want to consider that if you're using 2 drivers the directivity of the pair will be different than that of a single driver. Thus if you use SL's rule of thumb for 2 tightly spaced 15s you have to (roughly) half the upper cutoff frequency to get a maximum of 150 Hz. In the long dimension the situation is actually worse than a single 30" driver though because more cone area is away from center, so if you want wide directivity you need to lower the cutoff even more.
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Old 30th December 2008, 03:06 AM   #4
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That is very interesting. I have no experience with multiple drivers on same frequency space. I did read that SL is against 2x P13 instead of one W22 albeit the strain -- on the ground of polar response problem exhibited by multiple drivers.

Is this what's called "Lobing" ?

Halving the upper cutoff frequency to 150Hz will be problematic with the mid driver and quickly will need SL's compromise of using 8" drivers.

The other option is just use one 15" alpha per channel and compromise the extreme lows.
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Old 30th December 2008, 03:14 AM   #5
terry j is offline terry j  Australia
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Quote:
Originally posted by Rybaudio
You might want to consider that if you're using 2 drivers the directivity of the pair will be different than that of a single driver. Thus if you use SL's rule of thumb for 2 tightly spaced 15s you have to (roughly) half the upper cutoff frequency to get a maximum of 150 Hz. In the long dimension the situation is actually worse than a single 30" driver though because more cone area is away from center, so if you want wide directivity you need to lower the cutoff even more.

interesting, can you expand a bit on this?
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Old 30th December 2008, 03:39 AM   #6
Andy G is offline Andy G  Australia
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TC20 , 2 of ???? and at 1500.. gulp !!!!

(I assume one front and one at the rear ??)

You are right about them needing replacement

Try the DX25, its got plenty and you can probably cross it anywhere from 1k up..... with a steep enough filter from a digital x-o.

And if you are concerned at running the 15" drivers one above the other, you could always run one front and one back, but still open baffle. Any PA 15" is more than comfortable reaching way above 300, in fact, most are often used as vocal rather than bass drivers, particularly ones like the Eminence alpha and beta range.


ps, and you are right about the P13WH, much better to cross it 2.5k or below than to use its full range.
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Old 30th December 2008, 04:31 AM   #7
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Thanks for the tips on DX25 Andy. Price is very reasonable, the only gripes is they only stock the 4 ohms ones at speakerbits.com

The woofer:

This should be the simpler part of the design. I have sucessfully used the alpha 15s up to 1kHz with appropriate notches. They are at 2kHz and 400+ Hz. It's really curious that the 1/2 wavelength rule coincide in this driver's frequency range as well. Hopefully crossing at 300Hz 4th order will need no more notch at 400Hz. Definitely not at 2kHz.

Shelving lowpass will be needed to compensate dipole losses and with 45cm baffle it starts around 200Hz down to high 30s. A second SLP can also be implemented to raise the frequency even more but that's pushing it. Furthermore linkwitz transform can be investigated to exploit the high Qts of the woofer.

Click the image to open in full size.


Alpha's distortion is acceptable down to 30Hz. At 20Hz it's not. If a 20Hz tone is fed to them, they give out loud 60Hz tone. There is alot of debate about the audibility of these harmonics though and there is no harm to try to equalise them to low 20s.

30Hz:
Click the image to open in full size.

20Hz:
Click the image to open in full size.
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Old 30th December 2008, 04:34 AM   #8
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gainphile and terry j,

There is really just one extremely general idea here. Suppose you have two points that are each emanating a wave (of any kind). If you are the same distance from the two they add constructively to form a wave with twice the amplitude. If you are a different distance from the two sources, they won't add perfectly because they aren't in phase- there is a phase offset from the difference in path lengths to the two sources. You might have a look at the wikipedia article:

http://en.wikipedia.org/wiki/Interference

In the case of a speaker, as you go off-axis you are closer to one side of the cone and further away from the other. At low frequencies this doesn't matter because the path length difference this creates is small compared to a wavelength and hence doesn't create much phase shift. At high frequencies this makes the speaker become more directive because the phase shift from the path length difference makes the sound coming from the near and far parts of the cone add destructively. "Lobes" occur when you get high enough in frequency where at some angle(s) off-axis you get near perfect cancelation. The drawing below illustrates the directivity of a piston (decent approximation to a speaker) at various frequencies.

Click the image to open in full size.

Note a wider piston will become directional at lower frequencies because going the same angle off-axis creates a larger path length difference. This is the same reason why two drivers mounted on the same baffle will have greater directivity than 1. If they are arrayed vertically, the horizontal directivity will be the same, but the vertical directivity will be higher. The reason why it won't be simply the same as that of a piston of twice the diameter in the vertical dimension is that more of the radiating surface area of the pair of drivers is off-center than in the case of a piston, so going off-axis creates more cancelation.

Does that clear things up?
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Old 30th December 2008, 04:44 AM   #9
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Thanks Ryb. That clears a lot of things. I think I'm sold to using 1 woofer per channel first and only if needed, try the second one later. I got massive bass already with 1 woofer anyway. The other issue with using 2 alphas is the tweeter axis will not be at ear height.

But, considering that horizontal directivity is less affected and normally people don't change height when listening, in practice how acceptable are these vertical lobing?
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Old 30th December 2008, 06:50 AM   #10
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The issue isn't the direct sound. The issues are that (1) you have reflections (for example ceiling and floor) arriving with holes in the response and (2) that the power response (reverberent field) will be uneven. These effect the overall tonality of the system and aren't correctable with EQ upstream.

The hard part is determining the relative importance of these effects. I don't have any hard numbers for you but I think it's safe to say vertical directivity does matter but isn't as important as horizontal directivity.

Now that I think about it, I guess I never really consider the trade off you're talking about- the extra dynamics and max SPL of the 2 woofer setup for the cost of narrowed vertical directivity. I only consider setups with well-behaved directivity (in that frequency range at least) and design within that realm for the output I need. I think it's just not necessary to make that comprimise... you can use a more efficient driver and/or a different enclosure to boost the efficiency and power handling of the device. I guess I consider directivity to be non-negotiable in this case. That might not be a very useful reply but that's how I think of it.

In most speakers I do end up making a dynamics vs. directivity trade-off in the mid to high transition because at the crossover the center to center spacing of the mid and HF driver is big enough where there is a little vertical pinching if crossed where the tweeter is comfortable and a lot of stress on the tweeter if crossed low enough where the vertical directivity is smooth through the transition. The increased vertical directivity is mainly from the center to center spacing and only a little bit from the size of the mid's cone. A partial solution (what I do) to this problem is to minimize the frequency range this occurs over by using steep filters.


Enough rambling... I like the idea of starting with 1 15 and seeing if that will provide enough output. If that isn't sufficient, go from there.
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