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Old 17th April 2004, 04:20 AM   #1
Saurav is offline Saurav
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Hi,

I'm playing around with Speaker Workshop, trying to model crossovers and learn how the software works. I have a few questions that I would appreciate help with. I'm trying to design a passive crossover between an Audax PR170M0 and a Fostex FT17H. These will be mounted to an open baffle (that's the plan so far).

First, I used offset.xls (don't remember where I found that software) to try and find crossover frequencies/slopes to give me a smooth frequency response. I assumed that the acoustic center of the horn tweeter will be at the diaphragm/voice coil, and for the midrange driver I took the midpoint of the magnet. Feeding this into the spreadsheet and playing with values, this is what I got:

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I cropped that too small - those are 2nd order Butterworth filters, 2900Hz lowpass and 5400Hz highpass, and a 3cm offset. That was the smoothest response I could get while staying within the limits of the drivers and avoiding shallow but broad dips.

I used SPL Trace to create FRD and ZMA files for my drivers. I used the network optimizer tool in Speaker Workshop, as well as PCDC.xls (Passive Crossover Design Calculator, most of this software can be found on FRD Consortium website) to pick L and C values to get to this target response. I put an L-pad on the tweeter to bring its level down closer to the midrange. The final network consists of one L and one C on each driver, and two Rs on the tweeter for an L-pad.

The best result I could manage:

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It looks pretty horrible until you see that the scale on the left is 2dB. If I add in the individual driver responses, that makes the scale become something more sensible, and the result looks like this:

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I then played with the component values manually, trying to flatten the transition. This is one of my results:

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Looks better at this resolution, but when I zoom in by removing the individual driver curves:

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Which looks worse than the previous crossover, though this is at 1dB resolution so maybe it isn't all that worse.

So, my basic question - which resolution level makes more sense, and correlates better with what I'll hear in-room? Should I be trying to flatten 1-2dB peaks and dips, or is it more important to get the broad shapes correct?

Also, if there are any big holes in my approach, please point them out. I know I haven't looked at off-axis response and phase and impedance. I'll try those later once I figure out the first step, on-axis response (even though I know they all go hand-in-hand). I'll also measure my drivers once I've built the baffle and mounted them, and re-do all of this with that frequency response data. Which means I need to learn how to correctly measure them too.

Thanks for looking,

Saurav

Edit: The tweeter crossover ended up with just a series capacitor (the optimizer suggested 1000H for the inductor, so I took it out).
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Old 17th April 2004, 03:33 PM   #2
sreten is offline sreten  United Kingdom
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Its more important to get the broad shapes correct.

Peaks and dips are only an issue if they exist on all
your off axis traces, a single on axis trace cannot
be taken too literally as the actual units response.

sreten.
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Old 17th April 2004, 03:36 PM   #3
Saurav is offline Saurav
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Thanks.
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Old 18th April 2004, 04:20 AM   #4
GM is offline GM  United States
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We are amplitude oriented, so if the peaks/dips are high Q, then we tend to only 'hear' the peaks, so if they present a ~flat FR you're OK. As for resolution, +/- 2dB is exceptional. I used +/-3dB and all were pleased.

WRT horn acoustic centers, as you go down in frequency, it moves towards the mouth, so at 5900Hz it will be somewhat in front of the VC.

GM
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Old 18th April 2004, 08:32 AM   #5
Saurav is offline Saurav
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Oh I don't think I have +/-2dB or even +/-3dB across the board. I was trying to figure out when a peak/dip would be considered bad enough that I'd have to treat it individually. If I understood your point about peak/dip Q right, then for the sharp/narrow peaks I should try and line those up with the flatter smoother regions at other frequencies. That makes sense.

Thanks,
Saurav
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Old 20th April 2004, 04:57 AM   #6
GM is offline GM  United States
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Correct, like sreten said, it's the shape of the FR on and off-axis that counts.

GM
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Old 20th April 2004, 06:08 AM   #7
Saurav is offline Saurav
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Thanks. This is what I have so far. This is purely modelling based on printed frequency response and impedance curves, I haven't mounted and measured any drivers yet. I'm sure some (or a lot) of this will change when the drivers actually go onto the baffle. Anyway:

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I started out with component values calculated by the software, and then I changed them (at least the Ls and Cs) to values that I know I can get, or I already have. I'll try and bring that one peak down some more. It's in the on-axis FR plot of the Audax driver, though some people have said that those peaks aren't all that audible, and someone said he's seen an FR plot where those peaks aren't even present. We'll see.

Does this look OK so far? I went for target 2nd order Butterworth acoustic slopes, at 2900 and 5400 (got those values from playing around with offset.xls). The schematic includes my estimated driver offset. Should I be aiming for steeper acoustic slopes? I thought those frequencies looked good, because the Audax shouldn't start beaming too much at 2900. The resistor across the midrange is the zobel, Speaker Workshop calculated a 0F capacitor in series so I took that out. I don't think the phase plot can be trusted, because my FRD and ZMA files were generated using SPL trace so they say 0 degrees at every frequency.

The low pass will be an active XO at 500Hz, probably 4th order L-R. The baffle is 14" wide, which puts the BSC corner at 325Hz, I can shift it a little lower by adding small wings to the sides, so I think that end of it should be OK.

Can I get Speaker Workshop to show me what the off-axis response will look like? Or do I just use the 30 and 60 degree FRD files with the same crossover network and that's how I get my off-axis response?

Thanks in advance,

Saurav
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Old 20th April 2004, 06:35 AM   #8
Saurav is offline Saurav
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And if my assumption about modelling off-axis FR is correct, this is what that should look like after 1/3 octave smoothing at 0, 30 and 60 degrees (the graph in the previous post is unsmoothed):

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Old 20th April 2004, 03:04 PM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Saurav
. The resistor across the midrange is the zobel, Speaker Workshop calculated a 0F capacitor in series so I took that out.
Saurav
Well in that case remove the resistor as well,
it serves no useful purpose and wastes power.

sreten.
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Old 20th April 2004, 03:15 PM   #10
Saurav is offline Saurav
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It completely changes the frequency response if I remove it though, adding a big bump at around 2kHz. I'll see if I can adjust the tweeter to match it. Thanks for the help.
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