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

[morpheusX]

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):

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]

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.

 

[Valhalla]

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]

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.

 

[morpheusX]

How important are the Vertical measurements for XO design?
I'm struggling to find a way to do them in a secure way

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

 

[witwald]

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.

 

[fluid]

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

There is another option to try in the attached document

 

[kimmosto]

How important are the Vertical measurements for XO design?

It depends on radiator types. Significance of separate vertical data is very small with circular drivers in conventional box, but measuring vertical is mandatory with rectangular planars, ribbons , elliptical horns etc. Measurement instructions linked on the main page help but final decision is yours.

 

[tvrgeek]

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.

 

[thewas]

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.

Above transition frequency preferred loudspeakers have a neutral on-axis/listening window response and smooth directivity, so those anechoic polar responses tell much more than any non-windowed measurement at a listening position and that is the reason why all experienced loudspeaker engineers use those for more than 5 decades now. Below transition frequency it is also a better basis to tune a loudspeaker for a flat direct sound or sound power than just by measurements at the LP as otherwise the contribution of the loudspeaker and the room cannot be clearly separated which need different compensations, plus the flexibility for different rooms and placements will be lost.

 

[Bghead8che]

Hey guys. I am new to REW and speaker measurements in general and I have been following the guide. For kicks, the first speaker I am going to measure is the following:

Ascend Acoustic Sierra Towers w/ RAAL tweeter (http://ascendacoustics.com/pages/products/speakers/SRT/srt.html)

The Sierra has one tweeter, one mid-range 5.25" driver, and two 5.25" bass drivers. I have some questions about filling in the "baffle edge diffraction" simulator."

Since I am measuring a tower speaker instead of a bookshelf, I have a few questions:

1. I assume the "observation point, vertical axial, and horizontal axial" settings can all be left at their default for our purposes?

2. When measuring a tower speaker with multiple bass drivers, I assume you fill in the location of the driver you are measuring, correct? In other words, I put in the x and y location of the bottom woofer.

3. Lastly, the "baffle edge radius" setting is the width of the inset around the front baffle, correct? The Sierra's have a very small 1/4 inch rounded edge around the front baffle sides. Is this what I enter for the radius setting?

4. Does my diffraction response curve look as expected in the simulator?

Thanks in advance guys for helping me with my beginner questions.

 

[Holmz]

Converting near field to far field is not entirely trivial.

 

[napilopez]

Hey guys. I am new to REW and speaker measurements in general and I have been following the guide. For kicks, the first speaker I am going to measure is the following:

Ascend Acoustic Sierra Towers w/ RAAL tweeter (http://ascendacoustics.com/pages/products/speakers/SRT/srt.html)

The Sierra has one tweeter, one mid-range 5.25" driver, and two 5.25" bass drivers. I have some questions about filling in the "baffle edge diffraction" simulator."

View attachment 192972View attachment 192973

Since I am measuring a tower speaker instead of a bookshelf, I have a few questions:

1. I assume the "observation point, vertical axial, and horizontal axial" settings can all be left at their default for our purposes?

2. When measuring a tower speaker with multiple bass drivers, I assume you fill in the location of the driver you are measuring, correct? In other words, I put in the x and y location of the bottom woofer.

3. Lastly, the "baffle edge radius" setting is the width of the inset around the front baffle, correct? The Sierra's have a very small 1/4 inch rounded edge around the front baffle sides. Is this what I enter for the radius setting?

4. Does my diffraction response curve look as expected in the simulator?

Thanks in advance guys for helping me with my beginner questions.

1) Observation point should be the distance you're measuring the speaker's far-field gated response. It won't make a significant difference in the frequency range we're concerned with, but it's technically a little more accurate.

2) Yes

3) Yup!

4) seems pretty normal to me!

I will say you should try to learn use VituixCADs Diffraction tool if you have the time(the VituixCAD manual is quite detailed), as it allows you to add multiple woofers and is just the more powerful tool on the whole.

I've been meaning to update this guide with VituixCAD's tool as an alternative, especially for towers, but haven't had much of a chance to use it "in the field" yet as I don't measure speakers too often these days.

 

[Bghead8che]

Thank you for the response. I measured the port first and then one of the front woofers. My charts look similar to yours, however, after aligning and tracing the graphs seem wrong. The large dip at 55hz in the final frequency response definitely does not look correct. I am not sure what I did wrong. I've attached the REW file. Would someone mind looking at files and see where I made my error? Also, since there are two bass drivers, I assume I add +6 db to the entire response. How do I do this in REW to account for the second woofer?

Thanks in advance for any help. I really appreciate it!

Update: Looks like I can't upload REW files on here. Not sure how to post the REW file for others to download?

 

[Holmz]

If the room is in the measurement and not gated out, then one could do the testing outside.

 

[fluid]

Update: Looks like I can't upload REW files on here. Not sure how to post the REW file for others to download?

zip it if it is small enough to attach or post a link to it on google drive or something similar if you have it.

 

[napilopez]

Zip file should work for uploading! happy to take a look

 

[Bghead8che]

Thanks guys. I attached the zipped REW files. Below are the numbers I used for the diffraction step. I'm curious what I did wrong. I also need to know how to account for the two 5.25" bass drivers and one port. Greatly appreciate the help!

I also don't mind paying someone a fee to help coach me through the steps of measuring tower speakers. If someone is available for consult, please PM me.

 

[napilopez]

When summing two identical woofers, you just add 6dB to its SPL. If you're using the method of aligning the low-end response tails, it doesn't really matter. I will note that if you haven't, you should also measure the midrange driver too. I don't know what the crossovers are for the speaker

So I'm not sure of the top of my head why this happened so dramatically @Bghead8che , but its basically the phase of the two measurements are not time aligned correctly. This is good for anyone following this thread, so for reference, this is the way the port and woofer are summing in @Bghead8che's measurements:

Whenever you see various dips like that in succession ("comb filtering") when summing measurements it's a sure fire sign that there's a timing issue.

If you go to the 'Impulse tab' and look at the two measurements, you'll see that the references timing is very different. Close to 0ms for the woofer, but -27ms for the port:

Woofer:

Port:

Whenever I see comb filtering like that, you can try using REW's "alignment tool" to sum the graphs and have control over timing as well.
(All SPL> Controls> Alignment tool). Here you can control both the gain (dB) of the response and the timing, and see what would happen if you changed the timing.

That showed that modifying the timing of the port by adding ~26-27 ms gets us a respones more like what we'd expect. A good way to do this is to tap on the little slider and hold the left right up or down arrow on your keyboard and see the response change live.

So I'm not sure why that difference in timing happened. You can see from the alignment tool that delaying the port by 26.75ms is equivalent to moving the port 30 feet/ 9m away lol. But that gets us closer to what we want. You can then tap on "aligned sum" to save the response.

Alternatively, you could make a copy of your responses, go to the impulse tabs, click on controls, and set t=0 at impulse start. That can cause issues during far field measurements but should be fine for these nearfield ones.

EDIT: On closer look, the measurements have a note saying you used an acoustic timing reference. Was that on purpose?

For some reason that seems to have screwed things up.

Also these measurements are using the 90 degree calibration file. Not a big deal for these frequencies, but FYI it should be the zero degree one.

 

[Bghead8che]

Thank you for the generous, detailed response. Before I respond to your post, I'd like to switch gears a bit. I tested down to 200hz using your guide. Unless I am mistaken the results seemed pretty close from 200hz up, which is exciting!

Official Ascend Acoustics chart

My chart

I tested the speaker at 1m with the mic aimed directly the tweeter and used a 5ms gate based on the guide. The speaker was on a stand which put the tweeter halfway between the ceiling and floor. I've attached the REW file. Would you mind looking at the file and making sure the impulse looks correct? My impulse response was not as clean as your official example. Also, do you think the final response matches the official chart enough to be considered accurate or am I off? Thanks in advance!