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Old 28th August 2007, 07:21 PM   #11
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If you do end up putting wool or other material around the tweeter, I'd recommend making the tweeter-facing side into a star shape so there are no edges concentric with the dome, which would give you another discontinuity, admittedly smaller because of the absorptive nature of the stuff, but still there.
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Old 28th August 2007, 07:45 PM   #12
Svante is offline Svante  Sweden
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Default Re: not convinced it's all baffle step

Quote:
Originally posted by ttruman
Hi Svante,
I am little confused by your interpretation of the simulation. Based on the graph I agree with the frequency but the dip amplitude appears to be about -2.5 db given a mean of the 6db on the chart. Apparently I am not interpreting that correctly. I also donít see the adjacent peak in my measurement which is the same amplitude as the dip. The tweeter exhibits the classic smooth 12db roll off of a closed box system when not measured in the baffle so I would expect to see the effects of the peak as well. Yes I am confused. Additionally I mentioned extending the baffle with cardboard in both horizontal and vertical directions. The dip did not move or change amplitude.
Well, it is true that your dip appears larger, but you can actually have the peak at 1500 Hz, without noticing it. The driver, as i see it on measurements I find on the web, seems to start rolling off at 2000 Hz, so the peak is more or less compensated by the roll-off.

If I simulate the same thing in Basta!, which has the same diffraction engine as The Edge, this becomes reasonably visible. The response curve of the infinitely baffled tweeter matches the responses I found reasonably good, except from the 10k+ range.

The simulation then looks like this.

Click the image to open in full size.

The green curve is there to demonstrate that slight adjustments in the microphone position smoothes out the ripple at higher frequencies. Rounding the edges of the box can have similar effects.

When it is put on top of your measurement, it is reasonably clear that diffraction contributes to that dip. I trust the diffraction simulation, I have seen it match so many measurements now that I can say with confidence that diffraction is at least part of the issue here.

Click the image to open in full size.

PS, it seems less likely to me that room reflections is the cause, you seem to have that under control.

PS2, the harsh advice here would be to re-design the baffle and place the tweeter based on diffraction simulations. Sorry about that, it is always hard to say this when a lot of work is invested in fine woodwork.

You could, just to test it, you could put the tweeter in a piece of MDF 220 mm wide, but slightly to the side and slightly lower:

Click the image to open in full size.

PS3, about the cardboard extension, I am surprised that it did not change the response. In my experience, cardboard is fine for quick and dirty diffraction tests. However, it is very important that the attachment between the box and the cardboard is tight, otherwise the diffraction will be there anyway.

What happens if you put the (free) driver in a piece of cardboard of the same geometry and size as the baffle? (You dont need the lower half metre I guess. a piece of 220x500 mm would do.)
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Old 28th August 2007, 08:44 PM   #13
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Default Re: Re: not convinced it's all baffle step

Quote:
Originally posted by Svante
[...]
PS2, the harsh advice here would be to re-design the baffle and place the tweeter based on diffraction simulations. Sorry about that, it is always hard to say this when a lot of work is invested in fine woodwork.
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.
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Old 29th August 2007, 12:15 AM   #14
owdi is offline owdi  United States
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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
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Old 29th August 2007, 01:12 AM   #15
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Quote:
Originally posted by owdi

...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

You said; "The 2.5khz dip is gone" because you measured at "15 degrees off axis"?
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Old 29th August 2007, 07:35 AM   #16
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Quote:
Originally posted by Inductor

You said; "The 2.5khz dip is gone" because you measured at "15 degrees off axis"?
Sure. That makes perfect sense because an edge presents a discontinuity to the plane wave coming from the driver. The plane wave will diffract from the edge, possibly causing constructive or destructive interference, which could be what you're seeing here.
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Old 29th August 2007, 09:24 AM   #17
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Quote:
Originally posted by DSP_Geek


Sure. That makes perfect sense because an edge presents a discontinuity to the plane wave coming from the driver. The plane wave will diffract from the edge, possibly causing constructive or destructive interference, which could be what you're seeing here.

There's still the question of the extent to which the baffle 'loads' the speaker driver.

The baffle step is an extreme case, but the secondary dip could also be present at a variety of angles, leading to an overall unevenness in the speaker's power response.
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Old 29th August 2007, 09:31 AM   #18
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Quote:
Originally posted by ttruman

Did you experience a similar issue with your two way Accuton configuration? What doesnít make sense to me is that if I change the baffle dimensions using cardboard, the anomaly does not change. The measurements taken with the extended baffle combinations do show a slight influence in the frequency response like those in BSD simulations.

At the time, I didn't really pay much attention to the baffle step, otherwise I would've made the baffle wider and with a much bigger chamfer. Plus I didn't have any measurement setup, other than an oscilloscope.

Maybe the cardboard is just too soft to have much effect at those frequencies?

Cheers,
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Old 29th August 2007, 05:42 PM   #19
ttruman is offline ttruman  United States
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Default Re: Re: not convinced it's all baffle step

Quote:
Originally posted by Svante
When it is put on top of your measurement, it is reasonably clear that diffraction contributes to that dip. I trust the diffraction simulation, I have seen it match so many measurements now that I can say with confidence that diffraction is at least part of the issue here.
Never an argument there but the pictures do show the truth of it. The question now is why it is so deep. As Inductor pointed out the OEM frequency sweep does show a dip that could explain it.
So maybe my out of baffle measurement is flawed. The Accuton site has not been updated in some years so I am still unsure if that is useable as a reference.


Quote:
Originally posted by Svante
You could, just to test it, you could put the tweeter in a piece of MDF 220 mm wide, but slightly to the side and slightly lower:
Click the image to open in full size.
I do believe I have several large pieces of 1Ē MDF around here somewhere. I will attempt the test this weekend.


Quote:
Originally posted by Svante
PS2, the harsh advice here would be to re-design the baffle and place the tweeter based on diffraction simulations. Sorry about that, it is always hard to say this when a lot of work is invested in fine woodwork.
Well, learning doesnít always take you down a linear path. It is my first speaker project and part of learning is doing. Did I learn this lesson it the hard way? Yup, sure did. Itís not the first time and certainly wonít be the last. I learn a great deal from my mistakes, so truly, I am glad I make so many of them.


Quote:
Originally posted by Svante
PS3, about the cardboard extension, I am surprised that it did not change the response. In my experience, cardboard is fine for quick and dirty diffraction tests. However, it is very important that the attachment between the box and the cardboard is tight, otherwise the diffraction will be there anyway.

What happens if you put the (free) driver in a piece of cardboard of the same geometry and size as the baffle? (You dont need the lower half metre I guess. a piece of 220x500 mm would do.)
[/B]
I did tape the seams (partly) on the side extension but not the top. I will try this again. Looks like I will have a busy weekend!
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Old 29th August 2007, 09:13 PM   #20
ttruman is offline ttruman  United States
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Default A thought

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.
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