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MultiWay Conventional loudspeakers with crossovers 

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29th August 2007, 11:39 PM  #21  
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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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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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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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30th August 2007, 05:13 AM  #25  
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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. 

30th August 2007, 06:57 AM  #26  
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Re: A thought
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30th August 2007, 07:49 AM  #27  
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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. 

31st August 2007, 04:45 AM  #28  
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31st August 2007, 04:51 AM  #29 
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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 don’t have to move the gate window when going back to measure with the cabinet. 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. Here is ECM 8000 one meter away. The amps … The C23’s under test. Pulse response. The base line (free) on axis measurements using the rig. 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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31st August 2007, 07:28 AM  #30  
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And as far as learning stuff, I'm still picking up _lots_ from the group here. I'd already designed a couple of wellregarded 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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