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Old 29th August 2007, 10:39 PM   #21
ttruman is offline ttruman  United States
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Quote:
Originally posted by DSP_Geek


I've had good results on a tall baffle by placing the tweeter about 40% of the distance from one edge, ie on a 10 inch baffle placing it 4 inches from one side and 6 inches from the other.

I came to this by trying to disperse the diffraction modes evenly across frequency, so one distance should be 1.0 and the other 1.414 (square root of 2). The mode frequencies then occur at multiples of both distances: 1.0, 1.414, 2.0, 2.828, and so on. The total distance is 1.0 + 1.414 = 2.414, so the portions are 1.0/2.414 and 1.414/2.414, or about 41.4% and 58.6% of the baffle width. It's not perfect, but it's a good starting point.

With three edges (left, right, top) you can try for the cube root of 2, which gives you three possibilities, ranked in order of distance of the tweeter from the top edge (all percentages relative to baffle width):
a) distance from top: 35%
distance from side A: 44%
distance from side B: 56%
b) distance from top: 49%
distance from side A: 39%
distance from side B: 61%
c) distance from top: 70%
distance from side A: 44%
distance from side B: 56%

I just simulated a and b on TheEdge: both still have a dip around 3 kHz on your baffle width, but it's fairly narrow and only 1 dB instead of the previous 4 dB problem. A bit of roundover on the edges doesn't hurt, either. Even a 3/4" radius helps.
Hi DSP_Geek,
Thanks for tips a, b and c. I have to admit I donít completely understand everything that going on in your explanation. I hate to sound so green but math is not one of my strong points (long story). I learn quickly though. So I will brush up on square roots, cube roots at wikipedia. Obviously the square and cube of two are related to frequency/diffraction modes somehow Öjust need to make the connection.
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Old 29th August 2007, 11:10 PM   #22
ttruman is offline ttruman  United States
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Quote:
Originally posted by owdi
ttruman - I have a very similar problem, but at a slightly higher frequency. My baffle diffraction sim indicates I will have 2-3db of ripple, but I measure 5-6db.

Need advice on crossover design for Vifa XG18 + Seas 27TBFC/G (measurements included)

The problem is within an octave of my crossover frequency, so modeling is a pain. It's driving me nuts.

Off axis it does get better, but I still have a bump at 4khz.

Dan
Hi Dan,

This weekend I plan on doing a series of well documents tests to try all the very helpful suggestions mentioned so far. Hopefully the secondary culprit contributing to the dip will be revealed. I would be happy to post them up for all interested.

Tim
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Old 30th August 2007, 12:20 AM   #23
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Quote:
Originally posted by ttruman


Hi DSP_Geek,
Thanks for tips a, b and c. I have to admit I donít completely understand everything that going on in your explanation. I hate to sound so green but math is not one of my strong points (long story). I learn quickly though. So I will brush up on square roots, cube roots at wikipedia. Obviously the square and cube of two are related to frequency/diffraction modes somehow Öjust need to make the connection.
No probs, my explanation was a bit curt. I used the square roots and cube roots to generate numbers equally spaced in the logarithmic domain. All the hard work's already done and presented in the three alternatives, so you can use them as a cookbook for the dimensions you need.

Let's try a 10 inch baffle with alternative a: 35% of 10 inches is 3.5 inches, 44% is 4.4 inches, and 56% is 5.6 inches, so you'd place the tweeter center 3.5 inches from the top and 4.4 inches from one side.

For alternative b, 10 inch baffle: 4.9 inches from the top, 3.9 inches from one side.
For alternative c, 10 inch baffle: 7.0 inches from the top, 4.4 inches from one side.
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Old 30th August 2007, 12:44 AM   #24
ttruman is offline ttruman  United States
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Quote:
Originally posted by DSP_Geek

No probs, my explanation was a bit curt. I used the square roots and cube roots to generate numbers equally spaced in the logarithmic domain. All the hard work's already done ...
I should have made my self clear! I am still learning how to communicate effectively in forums so please bear with me. I understand how to convert percentages into working measurements. It's the "hard work" that's already done that I was referring too:

Quote:
I came to this by trying to disperse the diffraction modes evenly across frequency, so one distance should be 1.0 and the other 1.414 (square root of 2). The mode frequencies then occur at multiples of both distances: 1.0, 1.414, 2.0, 2.828, and so on. The total distance is 1.0 + 1.414 = 2.414, so the portions are 1.0/2.414 and 1.414/2.414, or about 41.4% and 58.6% of the baffle width. It's not perfect, but it's a good starting point.
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Old 30th August 2007, 04:13 AM   #25
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Quote:
Originally posted by ttruman


I should have made my self clear! I am still learning how to communicate effectively in forums so please bear with me. I understand how to convert percentages into working measurements. It's the "hard work" that's already done that I was referring too:

Cool. Let's look at what we want to do: we want to make sure that edge reflections don't reinforce each other on-axis. As others have noted, what creates a dip on-axis might very well flatten the response off-axis. Flat off-axis response is nice, of course, but I'd rather be flat on-axis than be flat off-axis and have the on-axis response look like crap.

To this end, we want to make sure the tweeter-to-edge frequencies are reasonably well-spaced. All the modes will coincide if the tweeter is at the same distance from all edges, with the unfortunate results you found.

So, how do we space the modes? Let's say we only have left and right sides to worry about, so we want the tweeter-to-left-edge mode (call it Fl) to be well away from the tweeter-to-right-edge (Fr) mode. We also know that modes occur at multiples of their fundamental frequency, so they also occur at 2*Fl, 2*Fr, 3*Fl, 3*Fr, and so on. Since a number of things, including the non-zero diameter of the tweeter, come into play, only the first few edge modes are important. Nonetheless, we want to place one set of frequencies neatly between the other set.

We want the left modes to fit between the right modes: that means we want them ordered as Fr, Fl, 2*Fr, 2*Fl, 3*Fr, 3*Fl, etc. By examination, we'd like Fl to be exactly between Fr and 2*Fr. A naive implementation would make Fl = 1.5 Fr, but that would be wrong since frequencies tend to work exponentially. You want something known as a geometric mean, which is the root of two numbers multiplied together, like so Gm = sqrt(a*b). If we apply this to Fl, we get Fl = sqrt(Fr*(2*Fr)), or Fl = Fr * sqrt(2). This means we want the distance from the tweeter to one edge to be 1.414 that of the tweeter to the other edge, so the ratios are 1:1.414. Summing the two numbers gets you a total distance of 2.414 which represents the sum of left and right distances, ie the baffle width. Creating width ratios in relation to the total baffle width: 1.414/2.414 = 58.6%, and 1.0/2.414 = 41.4%. Let's just double check: 58.6%/41.4% = 1.415, which is pretty darn close.

For three edges the concept is similar, with the mode progression going (Ft = Ftop): Fr, Fl, Ft, 2*Fr, 2*Fl, 2*Ft, 3*Fr, 3*Fl, 3*Ft, and so on. You end up getting a series of distances all separated by the cube root of 2, to wit 1, 1.26, 1.59. [1] I merely applied that to the various edges to generate the three tweeter placement alternatives. For example, alternative a has distance 1 for tweeter to top edge, 1.26 for tweeter to one side, and 1.59 for tweeter to other side. Note that the two side-to-side distances add up to 2.85, so the first side is 1.26/2.85 of the baffle width, or 44%. The distance to the top is 1/2.85, or 35% of the baffle width. The other alternatives are constructed similarly.

[1] actually 2*1.59 is 3.18, or slightly bigger than 3*1.0, but that's OK because the modes are still reasonably spaced.
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Old 30th August 2007, 05:57 AM   #26
Svante is offline Svante  Sweden
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Default Re: A thought

Quote:
Originally posted by ttruman
It occured to me is that diffraction effects are not necessarily your enemy. Used wisely they could help flatten out the response of a problem driver.
Yes, that is indeed the case. One should remember though that the diffracion effects come with a directivity, so it cannot completely compensate for an (omnidirectional) artifact in the driver.
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Old 30th August 2007, 06:49 AM   #27
terry j is offline terry j  Australia
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Quote:
Originally posted by DSP_Geek


Cool. Let's look at what we want to do: we want to make sure that edge reflections don't reinforce each other on-axis. As others have noted, what creates a dip on-axis might very well flatten the response off-axis. Flat off-axis response is nice, of course, but I'd rather be flat on-axis than be flat off-axis and have the on-axis response look like crap.

Etc etc

nice post, but it got me wondering about just how many commercial offerings even vaguely begin to apply any of that?

That leads on to asking 'just how audible is an anomaly' like the one the OP posted?? (given that it usually is simply not addressed in the industry).

Sure we can see it on a microphone response, but then again we see lots of things with a mic which at the end of the day might not make a lot of difference.

Curious what thoughts people might have on all that.
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Old 31st August 2007, 03:45 AM   #28
ttruman is offline ttruman  United States
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Quote:
Originally posted by DSP_Geek
Cool. Let's look at what we want to do: we want to make sure that edge reflections don't reinforce each other on-axis.
...
Ahh. I see the logic in it now. The math orders the diffraction modes evenly between each other, thus smoothing out the overall response. It's amazing to me just how much I keep learning from building a pair of speakers. Thanks for breaking that down Geek.
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Old 31st August 2007, 03:51 AM   #29
ttruman is offline ttruman  United States
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It's begun. This is the setup for the tests...

I took an unused tripod and mounted a swiveling platform on it. I used the cabinet as reference to set the height so I donít have to move the gate window when going back to measure with the cabinet.
Click the image to open in full size.

Underneath are indicators that will allow the platform to be set at 15 and 30 deg off axis.
The MDF and cardboard test baffles can then be butt up against the platform. When flush against the front edge they will be at the same angle.
Click the image to open in full size.

Here is ECM 8000 one meter away.
Click the image to open in full size.

The amps Ö
Click the image to open in full size.

The C23ís under test.
Click the image to open in full size.

Pulse response.
Click the image to open in full size.

The base line (free) on axis measurements using the rig.
Click the image to open in full size.

I think the setup is sound and should provide good results. If you guys see anything that could skew the testing sure would appreciate the feedback.

Tomorrow night I will build the test baffles and begin the tests.
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Old 31st August 2007, 06:28 AM   #30
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Quote:
Originally posted by ttruman

Ahh. I see the logic in it now. The math orders the diffraction modes evenly between each other, thus smoothing out the overall response. It's amazing to me just how much I keep learning from building a pair of speakers. Thanks for breaking that down Geek.
My pleasure; I hadn't actually worked out the 3 edge problem before you asked, so (as they say in Santa Cruz) it's all good.

And as far as learning stuff, I'm still picking up _lots_ from the group here. I'd already designed a couple of well-regarded Pro Audio systems, but the learning curve accelerated seriously when I got here. My current project stalled for a while because I found an egregious error - after I'd cut the lumber, of course - and it took a year to get back on track. A shout out to LinearTeam's WinISD and Svante's Edge for a couple of outstanding software tools.
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