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Generating driver FRD setup questions for passive xovers
Generating driver FRD setup questions for passive xovers
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Old 26th June 2018, 03:10 PM   #1
keithostertag is offline keithostertag
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Default Generating driver FRD setup questions for passive xovers

Starting out, and getting confused by the many different setups to generating FRD files for drivers (for passive crossover design). Probably a two way (MTM) at first.

I'd like to generate my own FRD files for individual drivers, probably with REW, then use something like Xsim. Because my house is small and not empty, seems like I will need to use near-field for low end and some other technique for above that- still reading about that.

It's not clear to me when a driver should be in a baffle or a box or in neither for generating the FRD files. In audiojudgement's useful articles the driver is always shown in a box, regardless of measurement method, for instance. While Dickason says use a 2x2ft baffle (if I remember correctly). I see others using a baffle approx to the size they think the finished box will have.

So..? do I understand correctly that most people follow this procedure:

1) Obtain drivers based on what you want to accomplish, budget, etc.
2) Obtain T/S parameter measurements to establish size and type of enclosure.
3) Build box, install drivers.
4) Generate FRD files with drivers inside box, one driver at a time (I will use a UMIK-1).
5) Design passive crossover from the above FRD file.
6) Use REW (or similar) to test room response to tweak xovers and placement, etc.

Since I'm new to this, I'd really like someone to just confirm that is the general procedure, and that it's best to make the FRD measurements with the driver mounted inside the final box.

Thanks if you can help clarify,
Keith
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Old 26th June 2018, 04:14 PM   #2
CharlieLaub is offline CharlieLaub  United States
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I suggest you....

1. Build cabinet (complete, not just front baffle) and mount drivers

2. Perform measurements:
2A. Choose a far-field measurement position: on or off axis within the horizontal plane of the T in the far field (e.g. 1m away from T)
2B. First measure each driver's response at this position, one at a time
2C. Then measure the response of all drivers, all operating together with no filters used, at the same position. DO NOT MOVE THE MIC to a different position for measurements 2B and 2C
2D. Go back and measure a woofer nearfield (e.g. 1cm away from dustcap). This is just one measurement, done at the end.

3. Process your measurements: use the program FRDBlender (I am a co-author) to merge the nearfield and far-field responses for the woofer, and to generate minimum phase responses for woofer and tweeter

4. Crossover Modeling: use the program PCD (by Jeff Bagby) to:
4A. First you use the individual driver measurements PLUS the multiple driver measurement at a given position to determine the offset of the acoustic centers for each driver WRT each other. This gives you a model for the loudspeaker as seen by the mic at this position. Do it for the on-axis position first. You should repeat this for each off axis position as well, since the relationship between acoustic centers will change slightly.
4B. Model the response at the on-axis position. Use PCD to design your passive crossover.
4C. Use the crossover you have designed in 5B but now use the FRD data and acoustic offset info for your off axis position(s). This is a check to see how the system response looks off axis (it will be different). Continue to tweak crossover until both on and off axis responses look good.

The procedure above can also be used when designing a DSP crossover. Just use my ACD (A=active) program instead of PCD (P=passive) to do the crossover design. The general procedure remains the same. In ACD you can view multiple on and off axis responses simultaneously.
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Last edited by CharlieLaub; 26th June 2018 at 04:22 PM.
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Old 26th June 2018, 04:33 PM   #3
keithostertag is offline keithostertag
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Thank you CharlieLaub. Under 2C, how does one "measure the response of all drivers, all operating together with no filters used,"?

Do you mean connect woofer and tweeter together with no xover?

Also, how would your suggested procedure differ for an MTM configuration?
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Old 26th June 2018, 04:45 PM   #4
CharlieLaub is offline CharlieLaub  United States
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Quote:
Originally Posted by keithostertag View Post
Thank you CharlieLaub. Under 2C, how does one "measure the response of all drivers, all operating together with no filters used,"?
You feed the same signal to all drivers. You can do this by connecting them in parallel, or connecting each to a separate amplifier channel and providing the same input to the amp inputs. You can do all drivers together, or pairs of drivers. Since there is symmetry about the T of an MTM if you keep the mic at the same elevation as the tweeter you can measure just the T and one of the M's. The other M will be mirrored about the T in terms of its position, and the distance from each M to the mic will be the same. For a beginner, just measure driver pairs (T and upper M, T and lower M).

Quote:
Originally Posted by keithostertag View Post
Do you mean connect woofer and tweeter together with no xover?
Yes. You are using the measurements to probe the response of the drivers in the cabinet without the effect of the crossover. This is the information that will be captured in the FRD file of each driver. The crossover design program adds the effect of the crossover to each driver. The result is a model of the system response (drivers+crossover) at the microphone position and without any room effects.

Quote:
Originally Posted by keithostertag View Post
Also, how would your suggested procedure differ for an MTM configuration?
It doesnt!
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Old 26th June 2018, 05:18 PM   #5
keithostertag is offline keithostertag
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Thanks again, this is really helpful. I have a MTM box and drivers that I want to test, so that's a situation I'd like to be more clear about.

This has me confused: "For a beginner, just measure driver pairs (T and upper M, T and lower M)", because I'm not seeing how to integrate that with steps 2C and 2D. Can you clarify?

Do I hook all three drivers together in parallel (that could be quite a load!) in step 2C?

For 2D I still need only measure one of the two woofers?

Or are you saying for an MTM there are actually two different steps, one for "T and upper M" and a second for "T and lower M" for 2C?

Sorry, it's probably easier done then said...
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Old 26th June 2018, 05:55 PM   #6
CharlieLaub is offline CharlieLaub  United States
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I will try to claify. I think the confusion is with 2C: measure the response of all drivers, all operating together with no filters used...

Here's more info about the measurements. Now that measurements software and calibrated mics are available to the DIYer it is possible to make accurate measurements in your home. The problem is that the space in your home has a finite size, that is there are room boundaries (floor, ceiling, and walls) acoustically nearby. When you make a measurements (typically an impulse measurement) the driver will emit the impulse. It travels through space in all directions. The shortest path is directly between the driver and the mic. Slightly longer paths are from driver-->floor--> mic (the floor reflection) or reflections off of the other surfaces. These will come later, a few milliseconds later in fact, and add to the signal of the "direct" sound. They essentially "contaminate" the measurement and if/when you include the reflected sounds the measurement will start to have large peaks and dips from them. This will in general be different depending on where in the room everything is located: speaker, mic, other stuff in the room, etc. What you want to know is the frequency response of the speaker itself. What can be done is that the time record of the microphone's signal can be inspected. It's typically relatively easy to see where the reflected sounds kick in. You just throw out all the data starting from that point on. What you are left with is a few milliseconds of the initial time record. Then this data is converted from a time record into a frequency response. The problem is that in order to get accurate low frequency information you need a long time record, but because you had to truncate your data at a few milliseconds you don't have that for frequencies less than, e.g. 200Hz.

But you also made a nearfield measurement. With a nearfield measurement, the "loudness" of the sound from the driver is much greater than that from the room reflections of that sound, such that the room reflections become so much less in magnitude that they can be ignored - you can use the full time record. This means that you can also get very accurate low frequency information. But the nearfield measurement cannot capture effects like the baffle response and other frequency response changes that are seen farther away from the speaker itself.

The far field frequency response valid down to 200Hz captures these plus most of the baffle step. What you can do is then take the nearfield response, adjust it to account for the baffle step using a model (it's semi-accurate) and then combine the far-field and near field responses together. This used to be done by splicing them at a single point, however, I can up with a way to "blend" them together over a range of frequencies (thus the name of the program FRDBlender). The FRD Blender allows the DIYer to properly combine responses, account for the baffle step, and then will generate the "minimum phase" response for the combined curve. You can think of the minimum phase response as the frequency response of the driver at the microphone position, but without the additional phase delay caused by the time it takes the sound to travel thru the air. The process, as I have explained it up to this point, is repeated for each driver to yield that driver's wideband (e.g. 20Hz-20kHz) minimum phase response. For the tweeter you do not need to make a nearfield response measurement, only for woofers and other drivers that have <200Hz sound within their passband.

Strictly speaking, you should do separate measurements on each driver. In some systems like an MTM when there are symmetries about the tweeter (including the cabinet) each of the M's will have the same minimum phase frequency response. If your MTM cabinet is e.g. a floorstander so that one M is near an edge and the other is kind of more central in the cabinet, then you should measure each M separately.

Now that you have processed your measurements into one minphase response per driver, the next step is to create the model of the loudspeaker in the crossover design program. The missing information is the relative location of the acoustic centers of each driver in space. Remember the minimum phase measurement doesn't have the phase delay resulting from the time it takes the driver's output to travel between the acoustic center (where the sound "starts") to the microphone. But there is a clever way to determine it, or make it possible to figure it out: the multiple driver measurement. When two or more drivers are operating together the sound at the microphone is a mixture of all of them. Because of the different pathlengths from each driver to mic, the phase lag will be different and each driver will have its own frequency response due to cabinet diffraction and the off-axis angle. You import e.g. the minphase FRD data for two drivers AND the data taken when both were operating together into the crossover design program. Then you vary the parameters (e.g. X,Y,Z location) of one of the drivers until the crossover design program's frequency response prediction matches the MEASUREMENT of the two drivers operating together. At that point you have correctly determined the relative XYZ location between driver 1 and driver 2. Then you move on to the third driver, replacing one of the FRD datasets (e.g. driver 2) with that of driver 3 and importing the two-driver measurement (e.g. of driver1+driver3). AGain you vary the XYZ coordinates for driver 3 until the prediction by the crossover program matches the driver1+driver3 measurement. At that point you have enough information to know the relative positions of drivers 1,2, and 3, which means you have a good model of the output of the loudspeaker at that microphone position.

Once you have an accurate model of the wideband output from the drivers and their relative positions in space you are ready to move on to designing the crossover.
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Old 26th June 2018, 06:12 PM   #7
keithostertag is offline keithostertag
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Thanks for the clarification. I'm going to spend some time studying your pages, and your software documentation. I appreciate you taking the time to help me (and other beginners) with these concepts.
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Old 26th June 2018, 07:33 PM   #8
ernperkins is offline ernperkins  United States
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Here' some information from Jeff Bagby that should help. If you don't have Excel for PCD you can use WinPCD by David Ralph. He basically duplicated the PCD functions with a few enhancements. Since it's a Widows app you don't need Excel.
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Old 26th June 2018, 07:41 PM   #9
CharlieLaub is offline CharlieLaub  United States
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Also, here is a link to a tutorial I wrote on HOW to do a gated (truncated) impulse response derived frequency response measurement using ARTA:
FR measurement using ARTA.pdf
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Old 26th June 2018, 07:55 PM   #10
keithostertag is offline keithostertag
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Thanks!
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