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28th August 2007, 01:52 AM  #1 
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Stuck in a dip
Hello everyone,
Been away for a while reading, learning. I think I feel a bit like Alice falling down the rabbit hole sometimes. As part of course I believe I am now getting reliable measurement data from Speaker Workshop as to utilize the cross over designer features and I have begun to carefully measure all the drivers to obtain a good modeling. In that regard the tweeter measurement has exposed some kind of destructive cancellation when the driver rests flush mounted in the cabinet. I have tried extending the baffle using cardboard both vertically and/or horizontally. I also tried covering the upper portion of the baffle around the tweeter with felt containing a small hole (30mm) exposing just the dome. The location and the severity of the dip remained unchanged! However pulling the tweeter out about 100 mm from the baffle changes the response dramatically. The original dip disappears but is replaced with two sharp dips located on opposite sides of where the dip previously occurred. If I remove the tweeter from the baffle and measure its response positioned resting on top of the cabinet, the dip disappears. I have read much about diffraction and although it appears to explain the location it does not explain the severity of almost 8 db. Below is a chart showing the anomaly. The red line is the tweeter flush mounted on the baffle. The black line is the tweeter removed from the baffle and measured sitting on top of the cab. My only theory at this point is that the design of the tweeter faceplate is creating a waveform that when encountering the baffle creates a very destructive cancellation that so happens to be centered around a baffle step diffraction dip. I am hoping the experience on this board can help me make some sense of this. I don't know what else to try short of removing the grill and applying felt directly to the tweeter faceplate. The process will damage the grill; there is risk of damaging the dome and the felt test I tried above doesn't strongly suggest it would help anyway. Measurements farfield 1M gated. The box is 220 x 1080 X 310 (WxHxD)mm The Accuton C23 is horizontally centered on the baffle 110mm from the top.
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28th August 2007, 07:13 AM  #2  
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Re: Stuck in a dip
Quote:
Another thing, which is a longshot; as far as I can see from the measurement, the resolution is ~100 Hz. That would mean that the gating window is ~10 ms (?), which in turn means that the pathway of the first reflection must be no less than 0,01*345=3,45 m. Is that so? Could it be that you also have a reflection from one of the room surfaces inside the gating window? 

28th August 2007, 07:53 AM  #3  
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Re: Re: Stuck in a dip
Quote:
...at least 0,01*345=3,45 m longer than the direct pathway... 

28th August 2007, 11:43 AM  #4 
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Considering the relatively long wavelengths in question (circa: 110mm), you may need a layer of damping material that's in the vicinity of 3~4cm thick.
How about covering the corners and box edges with some pieces of sheepskin rug? A 100mm x 660mm strip (with two cutouts for the overlapping corners) may be sufficient. Cheers, Lech
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28th August 2007, 04:48 PM  #5  
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not convinced it's all baffle step
Hi Svante,
Quote:
Quote:
Quote:
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28th August 2007, 05:57 PM  #6 
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Hi, 2,5dB, 2,5dB = ... (a lot)
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28th August 2007, 06:46 PM  #7  
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Hi Lech,
Quote:
Quote:
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28th August 2007, 06:59 PM  #8  
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Quote:
I think in order for that to be usefull, the particulars of the circumstances under which that measurement was taken must be known. Assuming that the measurement was taken without the unit being flush mounted in baffle as you suggest is it should resonable to expect that I would see the same dip in my my out of baffle measurement, which I dont. In all fairness it may very well be, as Svante suggested, my measurements may not be accurate. I will check my setup this evening and confirm my original results.
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28th August 2007, 07:42 PM  #9  
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Quote:
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28th August 2007, 07:47 PM  #10  
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Quote:
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28th August 2007, 08:21 PM  #11 
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If you do end up putting wool or other material around the tweeter, I'd recommend making the tweeterfacing 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.

28th August 2007, 08:45 PM  #12  
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Re: not convinced it's all baffle step
Quote:
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. 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. 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 redesign 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: 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.) 

28th August 2007, 09:44 PM  #13  
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Re: Re: not convinced it's all baffle step
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. 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. 

29th August 2007, 01:15 AM  #14 
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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 23db of ripple, but I measure 56db.
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 
29th August 2007, 02:12 AM  #15  
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Quote:
You said; "The 2.5khz dip is gone" because you measured at "15 degrees off axis"?
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29th August 2007, 08:35 AM  #16  
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Quote:


29th August 2007, 10:24 AM  #17  
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Quote:
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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29th August 2007, 10:31 AM  #18  
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Quote:
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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29th August 2007, 06:42 PM  #19  
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Re: Re: not convinced it's all baffle step
Quote:
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:
Quote:
Quote:
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29th August 2007, 10:13 PM  #20 
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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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29th August 2007, 11:39 PM  #21  
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Quote:
Thanks for tips a, b and c. I have to admit I don’t 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 …just need to make the connection.
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30th August 2007, 12:10 AM  #22  
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Quote:
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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30th August 2007, 01:20 AM  #23  
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Quote:
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. 

30th August 2007, 01:44 AM  #24  
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Quote:
Quote:
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30th August 2007, 05:13 AM  #25  
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Quote:
To this end, we want to make sure the tweetertoedge frequencies are reasonably wellspaced. 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 tweetertoleftedge mode (call it Fl) to be well away from the tweetertorightedge (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 nonzero 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 sidetoside 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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