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How to make quasi-anechoic speaker measurements/spinoramas with REW and VituixCAD

morpheusX

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Yes, but as noted above it usually isn't practical. It adds extra variables, especially if measuring indoors. The two main issues are that 1) if measuring indoors, the distance to walls changes as you move the mic, and 2) you would have to be very careful to not change the distance from the microphone to the speaker as you move around the room, as that will affect the SPL level and though the shape of each off axis angle is unlikely to change much, it could misrepresent directivity (matters more if you are measuring at 1m than 2m, but still not ideal).

That said, I have moved the micr instead of the speaker on a couple of occasions where it was inconvenient to turn the speaker on its side for vertical measurements (side-firing woofers, for instance).

Based on your feedback:

1) Will be measuring outdoors, in a yard of 7x7 m2, and should be able to place the speaker higher that the surrounding walls, which are only 1.6m high.

2) Please see the following picture (the scale and measures are not properly represented, but i'm hopping this can represent well enough what i'm trying to describe):

PlcuDCV.png


1) The axis for the rotation will be the tweeter,
2) I should be able to elevate the speaker at about 1.8 from floor.
3) Distance from Microfone to the axis (tweeter), will be 2m,
4) Microfone placed in a stand, with a 0º angle to the axis, microfone at 1.8m pointing in a straight line to the axis (tweeter),
5) Microfone placed in a stand, with a 90º angle to the axis, microfone at 1.8m pointing in a straight line to the axis (tweeter),

For measurements, the stand would be moved in the exact angles from 0º to 180º with the mic pointing in a straight line to the axis (following the correspondent angle). If i'm understanding this correctly, this will ensure the same angles/distance between the speaker and microfone, as if we were rotating the speaker.

If the the above is true, i'm thinking in printing the circle with the angles in an acrylic sheet in real size, and use it to control the exact Mic/Stand placement. This would ensure that i would be able to align the Mic straight to the tweeter, ensuring the angles are being followed correctly.

Is the above valid, or my assumptions are incorrect?

Thank you in advance for looking into this!
 

fluid

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For measurements, the stand would be moved in the exact angles from 0º to 180º with the mic pointing in a straight line to the axis (following the correspondent angle). If i'm understanding this correctly, this will ensure the same angles/distance between the speaker and microfone, as if we were rotating the speaker.

If the the above is true, i'm thinking in printing the circle with the angles in an acrylic sheet in real size, and use it to control the exact Mic/Stand placement. This would ensure that i would be able to align the Mic straight to the tweeter, ensuring the angles are being followed correctly.
Unless the mic is fixed to some kind of jig to let it move in a circle it will be very difficult to position the mic accurately when having to move it so many times. It is much easier to make the speaker rotate and control that movement.

Certainly it can be done the way you describe but making a repeatable measurement will be a battle.
 

Arash

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Does anyone have any idea how/where microphone should be placed when measuring a woofer near filed response at off-axis? At 90 degrees or 180 degrees? Should microphone be placed as close as possible to the enclosure? Or what?
 

fluid

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Does anyone have any idea how/where microphone should be placed when measuring a woofer near filed response at off-axis? At 90 degrees or 180 degrees? Should microphone be placed as close as possible to the enclosure? Or what?
Off axis nearfield measurements tend to be used for checking for cabinet resonances or other problems. Directivity can only be measured farfield unless using nearfield holography / sound field separation.

Joe D'Appolito
https://audioxpress.com/article/measuring-loudspeaker-low-frequency-response

"In the near-field technique, the microphone is placed very close to the driver diaphragm to swamp out baffle and room effects. At low frequencies where the driver diaphragm behaves like a rigid piston, the measured near-field response is directly proportional to the far-field response and independent of the environment into which the driver radiates. Keele describes this technique in his paper.[1] I will summarize the approach and its limitations here.

For the near-field technique to work properly, the microphone should be placed as near to the center of the diaphragm as possible. Keele shows that a microphone distance less than 0.11 times the diaphragm effective radius results in measurement errors of less than 1 dB. As an example, a 6.5" driver will typically have an effective cone diameter of 5" or an effective radius of 2.5". For this driver, the microphone should be placed within 0.275" of the driver dust cap"


The directivity at low frequencies can be estimated by simulation of piston size in much the same way as baffle diffraction can be simulated and applied to a nearfield measurement and then spliced to a farfield measurement if that is what you wanted to do.
 

witwald

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How important are the Vertical measurements for XO design?
I think that they are very important. Vertical measurements will serve to verify where the main lobe of the response is pointing. It's not a good idea to have it directed at the floor, and the measurements will help to quantify that it is pointing directly forwards. Of course, the chosen crossover topology and driver layout may result in a main lobe that is pointing in a direction other than parallel to the horizontal.
This discussion on DIY Audio seems to imply its possible to design a competent XO without them:
- 3way 22W/4851, MW16TX-8, T25B in WG
It does seem possible, as the mathematical models can now embody a lot of the inherent driver behavior to use in the simulations. It would still be ideal to verify the design by measurement, just to make entirely sure that all is working as expected. It's worth the extra trouble.

If you choose an acoustic Linkwitz-Riley in-phase crossover network design, then it will be fairly tolerant of errors/limitations in the models. It will also tend to produce a main lobe that is horizontally aligned, so that the on-axis response is the one with the greatest output through the crossover region.
 

kimmosto

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tvrgeek

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Many years ago, I was interested in measuring speakers outside of the environment. Pole in the back yard, on a baffle facing up in the ground. No MLS back then.

I don't know about you, but I don't listen to speakers on a pole outside. I listen in rooms with reflections, room gain, and all that stuff that makes charts ugly.

So, I now only do nearfield to double check resonance or missed crossovers, MLS ( different tool, same technology) @ 1M to generate my .frd files for simulation, but I do all my tuning in a prototype box in-situ. Polar plots in free space just don't tell me anything about how to tune the speaker or room. I also so static pink noise. It can tell you a lot about how the room is doing nasty things. I do pure tone sweeps to find thigs in the speaker or environment that resonate. You would not believe how effective an unpowered speaker works as a Helmholtz trap. I mean a sharp 70 Hz suckout! Makes you wonder about those showrooms full of speakers!

Outside was a disaster as I lived a mile away from I-50 and the LF constant LF noise was very high. As it was constant, you did not notice it, but the mic sure did. Anyway, no room gain so LF measurements outside were useless anyway.
 
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