Edge Diffraction with this setup? - diyAudio
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Old 23rd September 2006, 07:25 PM   #1
bogey is offline bogey  United States
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Default Edge Diffraction with this setup?

I understand a little about edge diffraction, but not much. I understand that it is good to round the edges of the front baffle of your box to do away with this.

However, I want to use a veneer to cover my floorstanding cabs and most veneers only come 2 foot wide, so I don't believe I'll be able to do a rounded edge. If I do I'm sure the seems would come out in some ugly places.

Anyway, the drivers I'm wanting to use are: Seas L22, L15, and 27TDFC. I am wondering if I had a narrow baffle (only .5" to each side) for the L22 (similar to Troel's TJLs) and then positioned the L15/27TDFC off center would this limit edge diffraction? I guess one might say I have concerns about ED. Ok, yeah, bad joke. Any ideas?
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Old 23rd September 2006, 08:18 PM   #2
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As I understand it, these effects are all related to wavelength.

The panel the tweeter is mounted in presents an obstacle to the wave front as it launches from the diaphram. The driver is therefore radiating into half space, just the air in front of the box. Where the box folds round the wavefront istill travelling but now ceases to "see" the baffle is so radiating into full space: the air all around the box. This abrupt change from one state to another sets up a reflection that travels in the opposite direction and this intereferes constructively and destructively according to the dimensions involved and the frequency of the wave. It's also responsible for "baffle step" that rise in amplitude response you often see at 1-3KHz.

Rounding the cabinet edges off to smooth the transition from half space to full space only makes a significant difference to edge diffraction if the radius of the fillet is significantly larger than the wavelength of the frequency you're concerned with.

This assumes your drivers have perfect 180 degree directivity at all frequencies, which of course they dont.

If you do a quick bit of maths you'll see that putting a 1/2" router bit down the side of your box is close to useless, its only effective at frequencies where your tweeter is starting to beam and doesn't put any energy across the face of the baffle anyway!

Offseting the tweeter, less so the woofer, is worthwhile in my opinion. Use the golden mean 1:1.618 to set the distance from one side of the baffle to another. This will ensure that those inevitable reflections at least will be spread out as much as possible and not reinforce each other. Avoid mathmatically even relationships wherever possible.

Simon
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Old 24th September 2006, 02:08 AM   #3
Svante is offline Svante  Sweden
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One way of understanding edge diffraction is looking at the baffle step (which is a result of edge diffraction). Assuming a point source on the baffle for a while, it will radiate in half space only for wavelengths much shorter than the size of the baffle. In this case, the baffle acts as a mirror and there is a gain of 6 dB for these wavelengths. On the other hand, large wavelengths will find their way around the edges of the baffle, and so the speaker will radiate in full space and there is no gain from mirroring.

This results in the baffle step, ie a transition from 0 dB at low frequencies, to + 6 dB at high frequencies. The only way to get rid of it is to have a baffle smaller than the shortest wavelength of interest (20 kHz <=> 17 mm) or larger than the longest wavelength of interest (20 Hz <=> 17 m). This is typically not possible, so the baffle step is an ever present phenomenon in loudspeaker design.

So why round the edges? The transition between 0 an 6 dB can look very different for different baffle shapes, and rounded edges smoothes the transition somewhat. However, the placement of the drivers on the baffle is far more important.

You can experiment with different baffle shapes using The Edge in my signature below.
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Old 24th September 2006, 03:22 AM   #4
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You can veneer the flat surfaces, then route a notch in the corners to take a piece of solid wood that can then be rounded over.

The best looking veneers are generally unbacked, where you'll have to do something about the seams anyway. www.joewoodworker.com has a lot of nice looking veneers and a wealth of how-to information.

For wider paper backed veneers check out http://www.tapeease.com/ (just a happy customer) The 10 mil stuff will usually wrap around a 3/4" roundover without much trouble.
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Old 24th September 2006, 04:24 AM   #5
bogey is offline bogey  United States
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Great feedback! Thanks guys. Also, I appreciate the links. I love getting additional resources.

Svante, I was looking at the screen captures for The Edge program. There are a couple fields that I wasn't sure about. What are "Edge Sources" and "Speaker Source Density?"
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Old 24th September 2006, 07:00 AM   #6
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How does offsetting the driver, particularily the tweeter effect polar and off axis response? Is BD so objectionable, and the small offsetting so helpful, that it's a better trade off then no offset but with better polar response?
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Old 24th September 2006, 06:29 PM   #7
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My experience is that edge diffraction is much more significant directly on axis than off. I think its down to the wavelaunch off the baffle and the edge diffraction is therefore a stronger effect when you are perpendicular to the baffle.

Maybe that contributes to why some speakers sound smoother if you don't toe them in. Svante's analogy of the baffle behaving as a mirror is a good one.

I've never noted any audible compromises in the polar response by ofsetting the tweeter, theres too much other fudging going on in the crossover reigon, with ZDP and all, for it to affect the response very much. You aint going to get rid of it unless you use big radii so you have to work with it.

Imagine standing in a concrete room that is square. Stand in the middle and speak, the echoes all stack up and flutter like mad. Stand off to one side, you still have the echo but its much smoother.
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Old 24th September 2006, 08:24 PM   #8
Svante is offline Svante  Sweden
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Quote:
Originally posted by bogey
Great feedback! Thanks guys. Also, I appreciate the links. I love getting additional resources.

Svante, I was looking at the screen captures for The Edge program. There are a couple fields that I wasn't sure about. What are "Edge Sources" and "Speaker Source Density?"
The Edge models the driver as a number of point sources, and the higher the "Speaker source density" is, the greater number of point sources is used.

Also, The edge models edge diffraction using point sources placed at the edges of the baffle. For each of the driver point sources, a number of edge sources are used, and this number is set by "Edge sources".

...in other words, The Edge uses a lot of point sources...

Quote:
Originally posted by augerpro
How does offsetting the driver, particularily the tweeter effect polar and off axis response? Is BD so objectionable, and the small offsetting so helpful, that it's a better trade off then no offset but with better polar response?
Well, for sure, off axis response is also affected by offsetting the driver. However, it is not obvious to me that a baffle with the driver to the side would produce a worse off axis response. Is it necessary a trade-off between the two?

To some extent the response in off-axis directions can also be modelled in The Edge, but the accuracy is less than on-axis.
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Old 25th September 2006, 01:20 AM   #9
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I see. So none of you think you give up much , in terms of polar response or lobing, by going from a D'Appolito MTM to an MTM with at least the tweeter offset? A smoother FR seems worth it as long as the lobing or comb filtering doesn't get much worse. Interesting.

Also what are some good baffle dampening coverings? Felt? Something spray on? How about rough suede for better looks? In one of Vance Dickasons books I just read he seemed to feel this definately made a difference.
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Old 25th September 2006, 08:36 AM   #10
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Hi,

in another forum I found a link to this article http://www.positive-feedback.com/Iss...ndingwaves.htm.

They suggest a specific pattern to apply to cone- and baffle edges that should prevent diffraction. It seams kind of strange to me, but my english as well as my technical knowledge are not good enought to really judge if there is anything of substance inthis or not.

best,
LC
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