Hi, if nothing else helps, follow VituixCAD REW measurement manual very closely and you are fine. In short
- measure each driver 0-axis maintaining mic distance from baffle plane (any common plane, baffle makes most sense) using timing reference
- process all data with same windowing
- leave z = 0 in vituixCAD drivers. Use y coordinate to move driver up / down where it is in reality, relative to intended listening axis (height).
An impulse is dependent on phase. Phase will not lead you to a clean impulse with an arbitrary response.
The varied baffle arrangement won't be without it's own effect..
This is how I know it, even for the simpler method for far-field measurements, in which all drivers are measured from one axis - e.g. that of the tweeter. All the time information that VituixCAD or similar programs needs for simulation is contained in the measurements if a time reference (electrical or acoustic) is used. Changes to the Z values are then even harmful.
To be honest, I don't really understand the intention of the question. Should the acoustic offset of the drivers be corrected mechanically? I would prefer a pure phase adjustment, because steps and edges on the baffle are not exactly advantageous from an acoustic point of view.
Or you could develop an active crossover and then adjust the offset using delay.
Oops, I left possibility for misunderstanding. Just in case:
Yeah the z is seldom used when measured like that, becaue it's included in the data. If somebody want's to physically align the drivers it's an option yeah, but perhaps not necessary most of the time, especially with DSP as you say. It's important to understand how the coordinate system works so one can work with it as required, so that all the measurements represent reality no matter how was measured.
If tweeter was bit closer to observer, say 1cm, than a mid woofer, one could flip the box around put mid above tweeter and listen at mid height, or put tweeter above listening height, or just don't mind about it because it's "just" the crossover region that is affected and depending on c-c distance and acoustic slopes it might matter some or not, but it's all visible in the simulator so one can see it and work with it as appropriate.
- when each driver is measured mic at each driver 0-axis, then one should use x and y-coordinate.
- when every driver is measured without moving the mic or dut, then the coordinates are left to zero.
Yeah the z is seldom used when measured like that, becaue it's included in the data. If somebody want's to physically align the drivers it's an option yeah, but perhaps not necessary most of the time, especially with DSP as you say. It's important to understand how the coordinate system works so one can work with it as required, so that all the measurements represent reality no matter how was measured.
If tweeter was bit closer to observer, say 1cm, than a mid woofer, one could flip the box around put mid above tweeter and listen at mid height, or put tweeter above listening height, or just don't mind about it because it's "just" the crossover region that is affected and depending on c-c distance and acoustic slopes it might matter some or not, but it's all visible in the simulator so one can see it and work with it as appropriate.
-(Slightly off-topic)
It then gets complicated when you want to reverse engineer the measurement scheme to take advantage of the vituixcad placement options, as you have to include the Z axis to make it right.
I sometimes do this anyway, although I still use the simple far-field measurement method, but only to get an impression of the vertical radiation behaviour. Of course, I have to reset this for the actual simulations.
I would like to measure each driver on its own axis. But the whole thing presents me with mechanical challenges: it would be nice if my measuring turret could be height-adjustable, but I haven't thought of a suitable mechanism yet. Changing the height of the microphone stand is difficult because it is so wobbly that there is no guarantee that the X and Z coordinates can be maintained.
Z adjustment in Vituix is there for the case of tilted baffle, stepped baffle, etc. Measurement is done with straight baffle (I use wedges). Physical Z is calculated from geometrical relationships of the driver and the tilt angle.
This is great feature, I usually measure the drivers in prototype cabinets with straight baffle. Than I model the crossover and try to find suitable baffle tilt (by manipulation of Z) that will suit my crossover targets and slopes, and spinorama overall.
This is great feature, I usually measure the drivers in prototype cabinets with straight baffle. Than I model the crossover and try to find suitable baffle tilt (by manipulation of Z) that will suit my crossover targets and slopes, and spinorama overall.
Yeah these are real practical issues that affect system quality. Imagine building big system, but have no reliable way to measure it, so the end result is within the error margin one happened to have. Making "error free" measurements can be quite some work, easy to make mistakes and resolution ends up quite bad <1kHz with indoor measurments etc. I always double and triple check and even then catch some errors every now and then.
Previous big system I built I purposely designed it to have a stand adapter on the bottom so I can spin it at high height outside to get some resolution down below 1kHz, and mic far enough to be far field. New error source was introduced at the same minute, my "floor" wasn't level anymore so the spin tilts some... well, just practical issues. First big system I built I had to fabricate special jig and use ratchet straps and all kinds of stuff, took whole day only to notice there was serious DC error in the data and had to do it all again 😀 fun time 😀
Classic error that happened to me multiple times this year was that my sound card ( RME ) settings had different clock rate than REW, which shifts everything in frequency. This error happens because my DAW (music recording and mixing) changes the RME clock, while REW does not and it's somethign I haven't remembered to check until noticing the responses look weird.
It's just very easy to make mistakes as there is several steps involved with the procedure before data is in VituixCAD.
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Fortunately, my largest loudspeaker to date was not that big:
But that doesn't mean it will stay that way.... 🤔
Only my livingroom subwoofers are bigger (here at the finish):
I've thought about this. A jig with a pair of linear rods would ensure the only axis you are changing is the Y axis. Eventually I will build one.
On the rotateably measuring turret that holds the speaker or on the microphone stand? Do you already have a concrete idea that you could sketch out?
I don't see this mentioned yet, but I could have missed it.
Two instances you would want to know z-offset:
1) You are going to slant the baffle to align acoustic centers, so you want to know the acoustic offset BEFORE building the cabinets so you can determine the specific slope for specific drivers.
2) You want to design a waveguide for a specific tweeter/midrange combination such that the acoustic offset is aligned.
Here is an older paper using the 3-measurement method. Note that PCD and VituixCad use different notation for the plus / minus directions. I'm quite sure VituixCad has a built in auxiliary tool for this but I'm not at my computer at the moment.
Two instances you would want to know z-offset:
1) You are going to slant the baffle to align acoustic centers, so you want to know the acoustic offset BEFORE building the cabinets so you can determine the specific slope for specific drivers.
2) You want to design a waveguide for a specific tweeter/midrange combination such that the acoustic offset is aligned.
Here is an older paper using the 3-measurement method. Note that PCD and VituixCad use different notation for the plus / minus directions. I'm quite sure VituixCad has a built in auxiliary tool for this but I'm not at my computer at the moment.
It would be linked to the turntable. I have it designed in my head. Just needs to get into CAD format I guess. Hardest parts are deciding how tall the rods need to be. Then they need to be secured. Linear rods at this length do start to bow. A linear rail, however, would be dead on accurate within the lengths you could fit inside of a building. Much more expensive though.
You also might get reflections from the jig. I had this happen on a jig I made before. I have started to keep the mic forward in an arm. So that the mic point is at least 2 feet forward of the rest of the microphone holder. Now I'm just thinking out loud. Again, haven't hashed it all out but haven't given it a ton of thought or engineering time tbh.