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

napilopez

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#1
This is a guide to taking simple quasi-anechoic measurements for the least hardware expense possible, using Room EQ Wizard (donate!) and VituixCAD (donate!). This way, I can also stop repeating myself every time someone asks me how I do it!

Warning: this guide will be wordy. My hope is that even a beginner can learn to do quasi-anechoic speaker measurements this way, so I apologize if I repeat myself or state some obvious things. I started measuring speakers with absolutely no engineering background and barely any knowledge of acoustics, and I wished there'd been a wordy guide for me starting out.

I'd like to start by acknowledging the late Jeff Bagby, whose white paper on quasi-anechoic measurements is basically how I got started; much of what's in this guide is essentially just a 'translation' for REW. Of course, Dr Toole's book was invaluable for the initial inspiration and teaching me how to interpret that data. To Amir for providing a platform to emphasize speaker measurements, as well as Stereophile, Soundstage Network, Erin/hardisj, and others who provide valuable sources of speaker measurements that I've often used to compare my data with.

This guide will be divided into 6 parts. How many you read depends on how thorough you want to be with your measurements:
  1. Introduction to quasi-anechoic measurements
  2. Setup and gear
  3. On-axis measurement (sans low bass)
  4. Nearfield bass measurements
  5. Off-axis measurements
  6. Create a full shebang spinorama
Each part will build on the previous ones, so you don't have to the full guide. If you're already familiar with the ideas behind quasi-anechoic measurements and just want to know how to do them in REW, you can skip the 'On-axis measurement' section.

Please give this a try! Even if you just perform a single on-axis measurement without bass, that's already a lot more useful than most of the speaker information available on the web.

Finally, please keep in mind this is just one way of doing things, lots of which I've learned through trial and error. I am open to feedback =]

Update 4/12/21: Fixed some typos, reworded some bits for clarity.
Update 5/11/21: Fixed some more typos, reworded more bits for clarity.
Update 5/20/21: Added reminder to make sure sample rates for input and output device match (should be 48 kHz with Umik-1)

1) Intro

The best thing you can do is send your speaker to Amir or Erin for testing with the Klippel NFS. But if you can't do that for whatever reason, creating quasi-anechoic measurements can help contribute to the pool of valuable speaker data. Even a single on-axis measurement can be extremely useful.

Quasi-anechoic measurements are basically a way to take a speaker measurement indoors or outdoors and ignore the influence of walls (including the ceiling and floor). The gist of it is to make a sine sweep in REW and truncate the impulse response such that REW only includes the data from right before the first major reflection 'hits' the microphone (it's much easier to do than it sounds!).

1616973791989.png


This is called 'gating' or 'time-windowing' the impulse response. In doing so, you lose some resolution, which is most apparent at low frequencies, and the data becomes completely invalid for the bass (usually below 100-200 Hz). The wider the gate, the higher the resolution of your data. A 5ms gate, such as is used by Stereophile, will give you a resolution and lowest valid frequency of 200Hz. My measurements are typically done at 6.5ms, which gives me a resolution of 154Hz. The resolution calculation is 1/[window in seconds], so 1/0.0065, though REW will let you know too.

To make up for the lack of resolution at lower frequencies, we can take super-nearfield measurements of the speaker's bass components (woofers, ports, and passive radiators), and simulate the far-field bass response from it. (Another common, even more reliable method for bass measurements is the ground-plane method, but that requires an ample amount of space, so I've never really used it).

With a bit of care and trial-and-error, you can get results that greatly approximate those made in an anechoic chamber or with the Klippel NFS. For some validation of the method, and an idea of what you can expect, here are some examples of my own measurements compared to anechoic sources.

JBL HDI-1600 (vs Amir's NFS):

1616973876576.png


1616973912421.png

D&D 8C (vs Erin/hardisj's NFS):

1616973987182.png


Focal Chora 806 vs Soundstage Network's at the NRC anechoic chamber:
1616974037403.png


The Spinorama/CTA-2034A standard says that a ±1.5dB measurement agreement for the same speaker is considered 'good'. You can see the above measurements are very close to that, despite measuring different test units.

Note that this does not mean the quasi-anechoic method is as accurate as an anechoic chamber or Klippel NFS. In particular, resolution in the low mids pales in comparison, which means narrow resonances may be obscured partially or entirely. But the data is still very useful for determining trends in tonality and can become effectively equivalent to anechoic ones by the upper mids.

2) Setup and gear

Here's what you'll need:
  • Room EQ Wizard. This guide was written with beta Version 5.20 RC6. As of writing this guide there are a several important features in the betas not available in the 'stable' release that is currently available on the REW website (V5.19). In my experience, the betas are extremely stable for the type of work we're doing
  • (If splicing nearfield bass) The Jeff Bagby Diffraction and Boundary Simulator, for adjusting nearfield bass measurements to match farfield results. This requires Excel; I've not tried it on Google Sheets or other spreadsheet software.
  • (If doing full spinoramas) VituixCAD (version 2.0.65.0 was used for this guide). It will automatically create a spinorama once provided with enough horizontal and vertical off axis measurements. It can also adjust nearfield bass measurements, but I prefer the simplicity of the Bagby spreadsheet.
  • A MiniDSP Umik-1 or other flat measurement microphone. If you don't already have one, I'd highly recommend getting a calibrated Umik-1 from Cross Spectrum Labs for extra accuracy. It only costs a few bucks more than ordering one directly from MiniDSP ($110+ shipping). It's not necessary, but increases accuracy in the upper treble and lower bass and adds peace of mind.
  • A microphone stand. It just needs to be thin so as to be minimally reflective. I use something like this, about 20 bucks.
  • A sturdy way to elevate speakers far off the ground, preferably 5+ feet, but as far from surfaces as you can manage. I've typically simply placed my speaker stand on top of a table. The sturdiness of the speaker stand is particularly important if you want to do vertical measurements. I'm currently using this.
  • (If doing off-axis measurements) You'll need some kind of turntable to place your stand on. I use this and label it with angles in 5 degree increments. For added security, especially for vertical measurements, I highly recommend getting a rachet or cam strap to secure the speaker while it's off-balance. I use one or two of these.
  • Open space. If measuring indoors at 1m — sufficient distance for most bookshelf speakers, in my experience — you'll want the closest wall (including the floor and ceiling) to be about 1.5+ m (5+ feet) to match the time window and resolution I've used in most of my measurements (6.5ms). You'll also want to move all furniture out of that 1.5 foot radius — or as far as possible — but small objects shouldn't cause much of a problem. If you have low ceilings and can't measure outdoors, you might have to settle for a smaller gate or measuring at less than 1m.
The open space is key. Again, the greater the time difference between when direct sound hits the microphone and the first big reflection hits the microphone, the more resolution your data will have, and the lower the frequency your measurement will be valid to.

When setting up your speaker on the stand, it should look something like this (taken from the CTA-2034A standard):
1616974151676.png


It is important to make the edge of the speaker stand as flush as possible with the speaker's baffle, as otherwise the setup can introduce minor reflections that might look like resonances. And again, I'm using a 1-meter distance, rather than the 2m the spinorama standard technically asks for, in order to increase the available time window.

Neither 1m nor 2m are magic numbers, by the way. For horizontal measurements of small speakers, simply being 2-3x the baffle width is usually enough. For a single on-axis measurement of a small speaker, you might get away with less than 2 feet. Experiment and see how the response changes at different distances, and find the best compromise for your space. Vertical polar measurements will be the most affected by short distances, so I would try to keep at least 1 meter for those for most speakers.

Don't sweat your setup too much. It doesn't need to be too fancy. This is what I used for the JBL HDI-1600 measurements above (set up for vertical measurements):

JBL HDI measure.jpg


The important thing is to simply minimize reflections enough to keep your data sufficiently clean to be useful, which you can readily assess from the resulting frequency and impulse response. If the impulse response looks messy or the frequency response looks unexpectedly 'squiggly', try to move stuff around to make it as clean as possible, then remeasure. It'll take some trial and error, but again, don't sweat it too much. Perfect is the enemy of good.

One more note: make sure that the sample rate for your input and output devices are the same (Thanks for the reminder @sweetchaos). The Umik-1 can only operate at 48kHz for example, so you'll want your audio output to be at 48kHz as well. Many devices will default to 44.1 kHz and using a different sample rate can have a slight effect on the highest frequencies in my experience. Using a higher sample rate won't improve accuracy, per REW documentation.

On Windows 10, you can do this by going to Sound Settings> Sound Control Panel, tapping on your playback device's properties, and then changing the sample rate in the 'advanced' tab.

Snag_77f7316.png

You should also make sure any spatial audio effects and the like are turned off.

3) The On-Axis measurement (sans bass)

The most basic quasi-anechoic measurement you can do is a simple on-axis sweep.

It's way easier and faster to perform, say, a single on-axis quasi-anechoic measurement (or even a few horizontal off-axis angles), than to do a full vertical and horizontal spinorama with nearfield bass spliced in. In fact, if you can position the speaker fast enough, it only takes a few minutes to do.

As noted earlier, creating open space around the speaker is key and your setup will likely take the most time in this whole process. Before even making a quasi-anechoic measurement, simply moving your speaker away from walls and measuring from closer — thereby minimizing the 'loudness' of reflections — cleans up the data a lot.

To illustrate this effect, here is an old measurement of the Buchardt A500. This is an on-axis measurement taken as a single sweep from my listening position 3m (~10ft) away:

1616974394216.png


This doesn't tell us much about the speaker's direct sound.

Now here is another measurement taken from just 1 meter, after repositioning the speaker such that it is 5+ feet from every wall, including the floor:

1616974480115.png


The highs are much cleaner now, and we have a better idea of the speaker's sound, but this is still not terribly useful. Next, I'll show the exact same measurement file you see above, except with a gate or time window applied. Note that this was not a separate sweep, I am simply modifying how REW interprets the same file:

1616974619152.png


That's more like it! Although we lost the bass response, we have now removed the 'noise' of the room and have something that tells us something much more useful about the "true" direct sound of the speaker.

Here's how you do it.

Again, position your speaker as far away from walls as possible. Make the speaker's baffle flush with the edge of its stand. Aim your microphone at the speaker's reference axis; check the manual, but if not stated, it's usually the tweeter or midway between the tweeter and woofer. If you're using a boom microphone, try to keep the arm extended such that the microphone is far from the 'stem' to minimize reflections near the microphone.

2021-03-28_19-41-16.jpg


(Ideally, the boom would be in line with the microphone, but I couldn't get it high enough in this case).

Then just take a regular sweep measurement in REW. I assume most of you know how to do this, but it can be done from the 'Measure' button on the upper left (shortcut: Ctrl+M). These are my usual settings:

1616975109747.png


(Ignore the output and input settings, as the microphone wasn't connected when I took this screenshot).

There is one important setting in the 'measure' window that you should keep in mind for doing off-axis measurements later. By default, REW sets t=0 at the IR peak, but this causes problems once you go more than 90 degrees off-axis (basically, the reflection off a wall might be louder/have a higher IR peak than the direct sound). So it's better to set it to t=0 at IR start. The resulting FR should be the same.

1616975179602.png


Now tap start (or press the spacebar), and once the sweep is complete you have all the data you need!

From here, we just need to change the way REW interprets the data to get our quasi-anechoic measurement.

Head over to 'Impulse' tab, and make sure you're in the percentage view on the upper left. The impulse response shows us the same FR data we just captured from a time perspective.

1616975217158.png


You should now see something like this:

1616975232826.png


See that blip right before 7ms, and how the data is all messy after that? That is where the first reflection hits the mic (each subsequent 'blip is another reflection). Were going to remove those blips from our data. You may need to finagle a bit with the zoom controls on the upper left and bottom right corners to get a good view of the blips:

1616975247683.png


If you're measuring outdoors, you might not see such pronounced reflections. That's fine; don't worry about it too much, as we can always adjust the gate later.

Now tap on 'IR Windows' at the top of the REW window. Set the 'right window' to a time just before your first reflection. In my case, I set it to 6.5ms. The left window is usually not very important, but if measuring outdoors, it may help to shorten it to about 2-5ms to prevent loud sounds from contaminating the data.

1616975265991.png


If you're doing off-axis measurements, it's good to leave yourself a little 'slack' between your window and the first reflection, as sometimes distances change a little bit as you're rotating the speaker. Hence me using 6.5ms even though I could stretch the window a little higher.

As noted earlier, the longer you have before the first reflection hits the microphone, the more resolution you have in your data, and the lower the frequency your data is accurate to.

REW will tell you what the frequency resolution of your measurement is, which will also be the lowest frequency the data is useful to. As shown above, 6.5 ms gives a resolution of 154 Hz. If possible, try to get at least a 5ms gate (this is what Stereophile uses, for reference, although they measure from a further distance), which has a resolution of 200Hz. Still, even a smaller gate can be useful, just know you'll have lower resolution.

And that's basically it. Once you tap on 'Apply Windows' you should now see a cleaner frequency response. You can also get a live view of the changes caused by changing the time window by dragging the green 'R' marker at the top of the Impulse tab.



Then return to the 'All SPL' tab and you should see your new gated measurement.

Some miscellaneous notes:
  • Make sure REW is set to a reasonable scaling to get a useful view of data. It's easy to obscure flaws in the frequency response with very tall scaling. If you tap on 'Limits' in the All SPL window, the SPL Top and Bottom should be a 50dB difference in most cases.
  • For a better way to ensure consistent scaling when sharing your frequency response, I recommend using REW's built-in 'Capture' button on the upper left. Under 'graph aspect ratio,' select 25 dB/decade. This is technically the aspect ratio defined by the spinorama/CTA-2034A standard too, although not even Harman uses it most of the time. The good thing about using this method is that even if you set different vertical limits than the usual 50dB, your frequency response will export at the same scaling.
  • 1/24 is my preferred smoothing.
  • You can make your frequency response dashed or dotted by tapping 'Controls' and then 'Trace Options.'
  • As we're not measuring sensitivity for this guide, SPL choice isn't terribly important. 85dB @1m is a reasonable SPL, but I used 75dB for a long time to not annoy neighbors. It matters most when using DSP speakers whose frequency response might change (compress) significantly with SPL level.
  • Sometimes there are objects that cause unexpected reflections. Others matter a lot less than you'd think (like your own body, sometimes!). You can usually tell if something is amiss by how 'messy' the impulse response looks, or if the frequency response looks unexpectedly wiggly. Again, trial and error. Mess around with positioning and settings until you find something that works consistently.
Phew. I know that was a lot, but you should see it's really not all that difficult. Hopefully, this will get you started!
 
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napilopez

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4) Nearfield Bass Measurements

Because gating the impulse response means we lose bass resolution, we need to make up for it by capturing the bass some other way. Some people use the ground-plane method, which is essentially the gold standard outside of a giant anechoic chamber or NFS, but it requires having like 30 feet of open space around the speaker, which most of us don't have.

Nearfield bass measurements can be done virtually anywhere and can get us pretty darn close to an anechoic or ground plane response. As we are measuring extremely close to the sound sources, it drowns out any reflections from the room. Here's the summary of what we're about to do.
  1. Measure the bass sound sources from really close up.
  2. Adjust SPL levels for the different radiating areas.
  3. Sum the measurements
  4. Modify the sum to be accurate at farfield distances (baffle step adjustment)
  5. (Optional) Splice to on-axis in REW. This can be done later in VituixCAD too when creating a full spinorama.
For this guide we are going to use the most common speaker type: a ported two-way speaker (I'm using the JBL-HDI-1600 for this example).

We start by capturing the individual frequency response of the bass sound sources. You should start with the smaller sound source, usually the port (more on this later). Then:
  1. Measure the diameter/area of the port. If it's a flanged or tapered port, you can use either the 'throat' or 'mouth' of the port, but I lean towards the former as it'll be louder and have less potential contamination.
  2. Set your microphone flush with the port's throat or mouth, whichever diameter you measured earlier.
  3. Run your sweep (adjust volume and retry if clipping occurs).
  4. Measure and write down your woofer's effective radiating diameter/area. You can set your ruler or measuring tape starting and ending at roughly halfway through the woofer's surround. Important: this is not the woofer's spec-sheet diameter. A 6.5" woofer typically only has an effective diameter of ~5".
  5. Set the microphone as close as possible to the center of the woofer, leaving just enough of a gap to clear the woofer's excursion. Technically, the microphone should be about 0.05x the woofer's effective diameter, about 0.25" for a 6.5" woofer with a 5" effective diameter. But you can usually just eyeball it.
  6. Without touching the volume control, run your sweep. The woofer should be measured at the same source volume as the port.
Just to give you an idea, this picture shows how close to the woofer the microphone was when I measured the JBL L82.
Nearfield.jpg

Now that we have our two measurements, we need to reduce the port's SPL to match the woofer's.

The 'proper' way, as noted in Jeff Bagby's whitepaper, is to follow this formula if both port and woofer and circular: 20 Log ([port diameter]/[woofer effective diameter]). If one of them is not circular, you can use this generalized formula by calculating the area: 10 Log ([port area]/woofer effective area]).

The 'easy' way, which works just as well 95% of the time, is to simply eyeball it. Just adjust the port's frequency response until it aligns with the woofer's low-frequency tail, usually the region below 20Hz. You don't even need to measure the area of the port and woofer this way, although I still recommend going through all the steps just in case. This method is also useful when the port or woofer isn't circular, when calculating the precise area becomes more tricky.

2021-04-03_16-23-42.jpg


To adjust the port's SPL, go Controls>Measurement actions and then select the port's measurement. Change the SPL offset until the port's low-end tail matches the woofer's. Note the SPL offset value, but I recommend you don't need to add the offset to the data just yet — this is just to figure out how much we need to adjust the port. We will add the two responses correctly next. If you want, you can write down the required SPL offset in the measurement notes on the left of the main REW window.

1618167122667.png


Whether you calculate the adjustment or eyeball it, next go to Controls>Alignment tool, select the port and woofer measurements, and change the port's gain to the correct SPL offset. You should see a preview of the summed response, and you can mess around with the gain and delay if you want to see how that affects things. Then tap on 'Aligned sum' to add them up.

2021-04-11_13-13-28.jpg


The resulting response, seen above in white, will have an exaggerated bass. This is because we need to correct our nearfield measurements for baffle step. There are many ways to do this, but we will use Jeff Bagby's Diffraction & Boundary Simulator, as IMO it is the easiest.

Enter the speaker and woofer's specs into the simulator (sorry non-Americans, the simulator uses imperial units). Don't worry too much about being perfectly precise, but for the 'speaker piston diameter,' remember this is the effective diameter. Again, a 6.5-inch woofer typically has a 5-inch pistonic region.

You will see a handy preview of the predicted correction for the nearfield response.

2021-04-11_13-37-44.jpg


Once complete, tap on 'Save Baffle Driffaction.' You can then import (Ctrl+I) or drag and drop the resulting file right into the REW window.

From here we just go to Controls > Trace Arithmetic and select our Aligned Sum and the correction we just made, and set the operation to A*B. Tap generate, and you'll get the corrected response.

1618167277728.png


The corrected response will have less exaggerated bass, usually down by 4-6 dB by 100Hz.

Bass correction.png


This new response can now be aligned with your on-axis response. If all is well, you should see decent agreement between the contour of your corrected nearfield summation and your on-axis measurement. Let's use the 'Measurement actions' tool again to bring down the summation to the SPL of the on-axis (always bring the corrected measurement down to match the on-axis, not the other way around).

Line them up as best you can between roughly 200-800 Hz. Some deviation is fine (remember, our on-axis has lower resolution in the bass), they should just generally match. This time, do tap on 'add offset to data.'

Snag_3202ebf9.png


Now, we can merge the corrected nearfield response with the on-axis response. This will be done later in VituixCAD if you plan on measuring a full spinorama, but I often like to do a preview first in REW anyway.

Go back to Trace Arithmetic, select your on-axis response under field A, and your corrected nearfield response in B. Then select Merge B to A as your operation and choose the frequency roughly where the curves best line up. I recommend selecting the 'blend' function on, this will smooth out the transition within 1/3 of an octave. It just looks a little nicer.

1618167491270.png


And voilà! We now have an on-axis response that should be pretty close to an anechoic result.

HDI-1600 On-Axis.png


As has been demonstrated a few times on this forum, getting perfectly consistent bass results among different measurement techniques, even with trusted methods like the Klippel NFS, anechoic chambers, and ground plane, can be difficult. So don't fret too much if your results don't line up 100%. Our goal is to get 'close enough,' as the bass response will be altered dramatically by the room anyway!

Some more notes you might find useful:
  • It is important that you measure the smallest sound source first because it will appear loudest to the microphone. If you measure the woofer before the port, you may find the port measurements clipping at your chosen volume.
  • Ideally, the port and woofer should be measured at the same 1-meter SPL as your on-axis, but you might find this is too loud when the microphone is so close to the sound sources. For passive speakers, the bass response will rarely vary dramatically at different SPLs, so just use the loudest SPL you can get away with.
  • But keep the above in mind if measuring a speaker with DSP, which may have a response that varies significantly at different SPLs. You could also try changing the gain on your Umik-1.
  • If a speaker has multiple identical woofers, ports, or radiators (and you're sure they cover the same frequencies) you can simply add 6dB for each additional unit rather than taking an additional measurement.
  • Some DSP speakers, like the D&D 8C and the Buchardt A500, have different timing between different drive units. For example, on the D&D 8C, the rear subwoofer units actually fire before the front woofer and tweeter, leading to summations that seem off. For this, you can mess around with the delay settings in the alignment tool until the measurement looks correct. Often times with such time alignment, the delay may simply be roughly equal to the difference in distance between the drivers.
  • If you have doubts about the accuracy of your summation, you can roughly confirm it by measuring the speaker from a few close distances, say 1 or 2 feet. At this point, your room will have an influence on the measurements, but you should see the general trends line up decently.
  • For 3+ way speakers, the 'eyeball it' method gets trickier, so you might just want to do the math.
  • On the other hand, sealed speakers are easy-peasy. All you need to do is measure the woofer and do baffle step correction.
  • The nearfield response can sometimes show low-frequency resonances, but they don't always show up quite the way they would in an anechoic or ground plane measurement. So keep that in mind that it still doesn't quite perfectly capture measurements in this region.
  • If the woofer has a phase plug, or other unusual design, just get as close to the center as you can. If the woofer has a permanent grille, I usually put the microphone flush with the grille, although you should see if the grille is removable without damaging the speaker first.
  • If you want to be as accurate as possible, you can try to apply baffle step correction to the woofers and ports individually and then add them up. In my experience, the difference is so tiny that it's usually not worth the extra hassle. On the other hand, it does allow you to see a more accurate preview when using the alignment tool, which is nice.
I know, I know, this is all very long, but it's a lot more tedious than it is difficult. Once you've done it a couple of times, manipulating the data doesn't take long at all; It's only the measurement setup that really takes that much time.
 
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napilopez

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5) Off-Axis measurements (no bass)

Okay, so we now know how to get a pretty darn accurate on-axis measurement without an anechoic chamber. The only problem is if we want to make a spinorama, we need a total of 70 measurements. Yeesh.

Luckily, this isn't quite as awful as it sounds. Although it can be a little tedious, you'll find you probably spend more time setting up your measurement rig than actually performing all the measurements. And though a spinorama requires data for 70 angles, we can you can choose to skip a big chunk of those on most speakers, which are horizontally symmetrical. Getting everything set up might take you an hour, but actually capturing the measurements only takes about 15-30 minutes, with the most difficult bit generally being the vertical measurements.

By the way, you also don't need to create a full spinorama. You can tell a lot about a speaker by simply capturing a few horizontal off-axis measurements. That alone would be much more than we usually get.

This section is basically the same as capturing the on-axis response multiple times. It'll mostly consist of setup advice and configuration stuff in REW.

Some additional setup notes:
  • I will assume you are using a turntable/lazy susan with labeled angles. Really, you can use any way to measure angles, but a turntable will probably make your life a lot easier and make this go much faster.
  • When labeling your turntable, don't just label the top; draw some marks on the side edge as well. This makes it easier to see angles that might be be obscured by your stand.
  • I recommend using a speaker stand with a flat base or removing the feet, as it can be trickier to balance the stand on the turntable otherwise.
  • If possible, use a very heavy stand or fill it with something heavy. You'll thank me when doing vertical measurements.
  • Again, using a cam or ratchet strap also makes vertical measurements a lot easier.
  • REW is limited to something like 30 measurement max by default, which is not nearly enough for our purposes. Go to Preferences>View tab and change the 'maximum measurement' to the actual maximum of 199.
  • When doing off-axis measurements, the speaker must be rotated around the front baffle, or close to it. According to the CTA-2034 standard, the axis of rotation should not be more than 5cm/2in from the front baffle. This will be a bit of a balancing act, especially for vertical measurements.
From the CTA-2034A:

1618175130721.png

I'm assuming you've already tested your on-axis measurement as detailed in section 3, and know what time gate works for you.

To save yourself some time, there are some settings to keep in mind with REW. First I'd suggest setting up REW to automatically apply a time gate to all measurements so you don't have to do this later, and so that you can see what the gated off-axis measurements look like as you're making them.

Go to Preferences > Analysis tab, and then change the impulse response window defaults to the settings discussed in the on-axis measurement section. As noted, I normally use a 6.5ms right window, and 3ms left window, and remember to leave yourself just a little bit of slack with your right window as the timing of the first reflection will change slightly as you capture various angles.

Snag_3279b6ec.png


You can always adjust these settings later if need be, but it makes it easier to have this done by default when making so many measurements.

Now let's start the horizontal measurements open up the measurement window (Ctrl+M). As noted earlier, make sure that under the 'timing' section, you have 'Set t=0 at IR start.'

1618175208160.png


This is important for off-axis measurements as when you go far off-axis, usually past 90 degrees+, the first reflection may actually be louder than the direct sound, so REW gets confused and your measurements get all screwed up. This can be fixed in the impulse response tab, but it's tedious when doing multiple measurements, so it's best to avoid it.

You can also save yourself a lot of time by getting REW to label angles automatically, especially if you plan to import your data into VituixCAD to create a spin.

We can do this by changing the measurement's name to something like 'Speaker H' or 'Speaker Hor' for horizontal measurements and 'Speaker V' for vertical measurements and then setting REW to add the angle numbers in increments. You do this by tapping 'add number,' setting 'next number' to 0, and then 'increment' to 10. The settings look like this.

1618175264647.png


This tells REW to add 10 to the name every time you make a new measurement. Note: Don't make the speaker name too long, i.e., 'Focal-JMLab Grand Utopia III H.' REW has a short character limit.

Then tap Start to begin your measurement. Turn the speaker 10 degrees, and repeat until you reach 180 degrees.

Pro-tip: rather than doing all that pointing and clicking, just press Ctrl+M and then spacebar for each subsequent measurement.

Once you've measured the 180-degree position, you can call it a day for the horizontal measurements if the speaker is horizontally symmetrical.

If you do want to measure the other horizontal half, you'll need to return to the measurement window and change the naming scheme agian. You can then either:
  • Set 'next number' to -170, and continue rotating the speaker as you measure the other side.
  • Return the speaker to the on-axis position, set the next number to 0, and set the increment to -10.
Personally, I like to always measure both the positive and negative angles at least within the listening window, as this helps make sure that I've aligned the microphone to be right at the center.

For the vertical measurements, we are basically repeating the above steps. You just need to keep a few things in mind:
  • The speaker should rotate about the same reference axis as you used for your horizontal measurements. Hence the suggestion of the cam strap to tie down the speaker tightly to a heavy stand, as it will usually be very off-balance otherwise. You can also figure out some other way to keep the speaker from falling off, of course, this is just what I do.
  • If your speaker is too much off-balance, it may help to reposition your stand on the turntable as well. For example, I normally turn my stand 90 degrees so that the 'long' side is facing the microphone, giving the speaker more of a surface to sit on. Then I shift the whole thing a little bit to the side to make sure the reference axis is aligned with the microphone without having the speaker be falling off the side of the stand so much.
  • You do not change the distance between the microphone and the speaker. As the speaker will usually be at a lower height for vertical measurements, you will have to move the microphone. If using a microphone stand with a boom arm, don't lower the microphone by changing the angle of the boom arm, as this will change the distance from the microphone to the speaker.
  • Verify that the on-axis vertical measurement matches the horizontal on-axis measurement very closely, especially in overall SPL. In practice, turning the speaker on its side will usually change the response a small amount, usually some bumps in the midrange. But since the spinorama involves so many averages, this normally is not too big of a deal, so don't fret too much.
  • Obviously use + angles for above the reference axis and - for below.

As shown earlier, your vertical setup will probably look something like this.

Then you just repeat the same steps in REW for naming the angles, except you start with V 0.

Once done, you have all the data you need to create a spinorama!

Some notes:
  • Although the spinorama calls for 10-degree increments, I like making vertical measurements at 5/10/15 degrees as some speakers vary significantly within these small changes in vertical listening position.
  • If you want your data to be as clean as possible, you can set a delay between each measurement so you give yourself enough time to get out of the reflection zone. In practice, this takes forever and I almost never see my body add noticeable reflections, so I just sit down behind the speaker to rotate the turntable.
  • You might be tempted to label your horizontal angles as 'left' and 'right' and vertical as 'up' and 'down'. Don't do this if you plan on importing into VituixCAD, as it will require you to use +/- instead.
  • For curved baffles, you might need to use some kind of wedge to support the speaker at the right angle. I usually just use some thing books
  • Make sure your turntable doesn't go too far past the edge of the table your rig is on, lest the whole thing topple over when your reach far angles.
  • If you want to share off-axis measurements straight from REW, it can help make the data more legible if you alternate solid curves with dotted curves. You can make curves dotted by going to Controls>Trace options.
  • We'll be simulating the off-axis bass response in VituixCAD. It's not perfect, but it works decently for spinorama purposes. The wider your gate, the more accurate those measurements will be.
  • If you found some of your off-axis curves are really messy, you can check the impulse response window to see if reflections are coloring the sound. If you find you need to shorten your time window, make sure you change the window for all your measurements, or else your spinorama will have issues. Under 'IR Windows' shorten your right window a little bit, then select 'Apply Windows to All, Keep Ref Time'
  • Again, you don't have to make a full spinorama. If you wanted to save a little time, or don't want to deal with balancing a speaker for vertical measurements (say if measuring a tower), you could simply measure the angles needed for the early reflections curve without flipping the speaker on its side. For the early reflections curve, you just need all the horizontals, plus +40,+50,+60 and -20,-30,-40 for the verticals.
Almost done! Now we just need to import this data into VituixCAD to create the real deal spinorama.
 
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napilopez

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6) Creating the Spinorama in VituixCAD

You don't need VituixCAD to create a spinorama, but it makes it a lot easier. VituixCAD creates a spinorama automatically upon importing the relevant data, and it can simulate the off-axis bass measurements we ignored in previous sections.

(Note: this guide was written with VCAD version 2.0.65.0)

First we need to export the data out of REW with the proper formating. If you ignored my suggestions in the previous section, you'll need to label your files as +/- horizontal and vertical angles. The VCAD default prefixes are 'hor' and 'ver' but we can change that; I prefer H and V because I'm lazy.

Order doesn't matter, just that they are labeled as positive and negative. VCAD will ignore duplicate angles (such as V0 and V180), so it's up to you whether you want to keep them in there or not. I delete them so I can just choose which duplicate I want to use.

I will also remind you that all your angles should use the same gate, or else the directivity information gets screwed up without some workarounds.

We begin in REW by going to File>Export> Export all measurements as text. These are the settings I use:
  • Use range of measurement
  • Use resolution of measurement
  • Use custom smoothing: No smoothing (smoothing should be added at the end)
  • Use REW export format (recommended)
1605301936473.png

I recommend exporting these files into their own folder called 'Spin angles' or 'For VituixCAD' or whatever. Because this will export all the measurements in your REW session, I also recommend navigating to the folder after export and deleting anything you do not strictly need to create the spin: i.e, anything that's not the necessary angles and the corrected nearfield bass response. Delete any test measurements, repeat measurements, etc from this folder.

Now we open up VituixCAD. Before we can do anything else, we'll need to go to the crossover tab, and link the amp and driver (just draw tap from one dot to the other). VituixCAD is a speaker simulation app, and we're basically asking it to imagine that all these measurements came from a single driver.

1618178274653.png


Next, we're going to set up VituixCAD to format data the way we want it.

Click on the Options menu. When the window pops up, you want to make sure all the settings are kosher for a proper spin. Here's what I'm using:
1618178712090.png

Simply pressing the "CTA-2034A" button should make it match the spin standard right away. But some things to note:
  • Set "plane keywords" to whatever you are labeling your files. I just use "h" to denote horizontal and "v" to denote vertical. The default is "hor" and "ver," so you'll want to switch that if you're using a different naming scheme. The CTA button won't affect this setting.
  • Make sure mirror missing angles is checked (it fills in missing horizontals on symmetrical speakers).
  • Make sure "Listening Window DI" is checked, as this is more common than on-axis DI for spinoramas.
  • Make sure the Power & DI section matches the above image. Although you can mess with the Angle Step setting if you'd like -- VCAD will interpolate the results if you choose a different angle step.
  • I'm using 1000 mm listening distance because that's the distance I measure at. The spin standard is 2000 mm, but it shouldn't have a big impact on your results either way.
Now return to the "Drivers" tab of VCAD. Right-click on the "Power & DI" panel and select 'Traces.' Make sure all the Spinorama curves are selected (this is just for previewing the data, we can export it later). These are the standard ones we use, although lately I personally have also started to include the 'Horizontal ERDI' as well to account for the fact that the spinorama does not distinguish between vertical and horizontal data.

1618185316536.png

Now let's generate the spins. Although we already showed how to merge the nearfield and farfield respones in REW, we will do it again in VituixCAD to generate a complete spin.

Go to Tools> Merger, or press F4.

First, let's import the corrected bass response. Tap on the folder icon in the 'Low frequency part' section and open the bass file we just exported. On the bottom right of this section, make sure 'No baffle loss' is selected. By default, it is set to spherical baffle loss, but we need to switch it as we have already corrected the bass in a more accurate way.

1618186644676.png


Let's import the farfield data next. Select the folder icon under the 'High Frequency' part of the window. Navigate to the folder where you've exported all your measurements. Select all the spinorama angles, and double-check that nothing except for the angles is selected before you import.

Next, we are going to scale down the bass response to line up with the high-frequency measurements. First look through the high-frequency measurements and click on H0 to preview the measurement. It should be checked as 'axial,' but if not, make sure it has that checkmark.

1618187214327.png


Then scale down the low-frequency measurement until it lines up. You can simply tap on the 'Scale' text box and press your keyboard's down arrow until the two measurements line up, or enter the value manually if you remember it from before.

Now in the 'Transition' box to the right, select where you want to merge the data. As noted earlier, somewhere between 200Hz and 800Hz should work, depending on how well the measurements line up. You may have to experiment with this to see what gets you the best results, as it may vary depending on your gate, but in practice, I usually select between 300 and 600 Hz.

Next to 'Blending,' select how smoothly you want to merge the measurements. I usually blend them across 1/2 an octave, though 1 octave works too.

In the "Output" segment at the bottom right, select "create merged responses" and "feed speaker". Select TXT as your file format. Then click Save on the bottom right.

Chose a place to save your merged frequency responses. I usually keep them in a subfolder for the speaker I'm measuring and call it 'V Merged'

If all goes well, VituixCAD should tell you it's done after a few seconds (it may also warn you that a duplicate direction wasn't loaded, which is fine). Return to the main window, and.... congratulations! You should now see an honest-to-goodness Spinorama in the 'Power & DI' section. You can double-click on this window to see a larger version.

2021-04-11_20-51-33.jpg


Now you should save the project file. From here it's up to you how to present the data.

If you just want to use VCAD's spin and call it a day, you can right-click on the spin and select 'export image.'

If you want to change the colors, you can right-click on the image and select 'Traces,' then pick the colors from the 'Line Color' column. If you want to smooth the data, you can do that from the right side of the window where it says 'smoothing.

Personally, I prefer to deal with data presentation in REW. It makes the measurements prettier and makes it easier to present consistently.

To do so, make sure smoothing is disabled, then simply go to File>Export> CTA-2034-A data. Find a folder to keep the spin curves; I recommend making a separate folder as there will be 16 resultant curves. Set the file name to something reasonably short, since REW has short character limits.

Under 'Save as type' select '.txt.' This will create all the traditional spinorama curves, as well as the early reflections breakdown (ceiling, floor, side, rear, and front reflections), total horizontal and total vertical reflections, as well as horizontal and vertical ERDI DI.

Now navigate to the folder where you saved the files, and drag and drop them all into REW. Note: You will need to add an offset to the Directivity curves in order to make them visible next to your other measurements. You can use whatever value works, but I recommend a multiple of 5 (say 45 or 50 dB) so it lines up cleanly with REW's grids.

You can then of course change the colors to whatever you want them to be. If you want to make any of the curves dashed, say the predicted in-room response, you can do that from the trace options menu.

Lastly, I consider consistent scaling to be extremely important. As noted earlier In REW, we can use the 'Capture' button in the All SPL window. Select 25/dB per decade as your aspect ratio to make sure your images are always at the right scaling. While VituixCAD also has the option to export an image at specific aspect ratios, for whatever reason, it doesn't seem to be consistent.

HDI-1600 Spinorama.png


Some notes:
  • You can visualize directivity from the 'Directivity' window of VituixCAD. Right-click to see more directivity views, including polar maps and SPL plots; you can also opt to normalize the graphs or add contour 3dB lines to the polar maps
  • Sometimes the off-axis bass just doesn't look right for whatever reason. In these situations, I prefer to cut off the data below 200Hz. To do this, import the spin data into a new REW window, then select 'Export all measurements as text.' This time select 'use custom range' and cut off the data below 200Hz. Then re-import these truncated curves into REW, which you can present alongside the merged on-axis.
  • I like to apply 1/24 smoothing in REW.
  • You might want to rename the spin curves from VituixCAD's defaults, as many of them are rather long.
  • Dark mode looks cooler :)
And that's it! Hopefully this extremely long guide was helpful to get you on the way to creating a spinorama without having to buy a Klippel NFS or anechoic chamber. Good luck, and feel free to ask me any questions!
 
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#7
It's very informative thread.
I have a question. I'm familiar with near field and quasi measurements but is it possible to make CLF 3D Balloon using near-field + quasi measurement? (not mentioning the time needed to do countless of manual measurements)
 
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napilopez

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That's it. I'm finally done! Pardon for all the words and potential typos, but I do believe that this is the only comprehensive guide to creating spinoramas using freely available tools on the web. I wish I had had such a wordy guide when I set out on this process a couple of years ago. It would have saved me a lot of headaches.

Maybe someday I'll make an 'abridged' guide, but for now I wanted to leave as few stones unturned as possible.

Tagging @Dennis Murphy since you asked me about this a while a back. I know you already know a lot of the stuff here, but if you ever wanted to give REW and VituixCAD a go for measurements instead of praxis, here's your guide.

It's very informative thread.
I have a question. I'm familiar with near field and quasi measurements but is it possible to make CLF 3D Balloon using near-field + quasi measurement? (not mentioning the time needed to do countless of manual measurements)
I've never used CLF, so now clue unfortunately.
 

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#9
  1. Measure the diameter/area of the port. If it's a flanged port, you can use either the 'throat' or 'mouth' of the port, but I lean towards the former.
  2. Set your microphone flush with the port's throat or mouth, whichever diameter you measured earlier.
Thanks so much for doing this! It's great. Question regarding ports: what if it's a slot port or more complicated, a down-firing slot port like the Wharfedale 225? Where would the mic be positioned?
1618229372861.png
 
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napilopez

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Thanks so much for doing this! It's great. Question regarding ports: what if it's a slot port or more complicated, a down-firing slot port like the Wharfedale 225? Where would the mic be positioned?
View attachment 123581
In that case you just need to get the microphone as close as possible to the port or even a bit inside it and use the eyeball it method. I've tested a few weird ports this way and it's worked out just fine.

If you're unsure, you can then somewhat verify the measurement makes sense by moving the speaker away from walls and measuring the speaker from a few close up distances, say 1 - 2 feet. this will lessen the impact of reflections enough that you can get a decent idea of the speaker's bass contour. Although reflections will begin to have an impact as this point, your summation should stilll line up decently with the non-gated response from this close.
 

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#11
In that case you just need to get the microphone as close as possible to the port or even a bit inside it and use the eyeball it method. I've tested a few weird ports this way and it's worked out just fine. You can then verify the measurement makes sense by moving the speaker away from walls and setting the mic to only a foot or so away from the speaker; this will lessen the impact of reflections enough that you can get a decent idea of the speaker's bass contour. Although reflections will begin to have an impact as this point, your summation should stilll line up decently with thenon-gated response from this close
Thanks for your help!
 
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napilopez

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Now... who will be the first newcomer to try? :)

As mentioned, you don't even need to do a full spin. Honestly just doing an on-axis + a few horizontal angles already tells us so much.
 

sweetchaos

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Now... who will be the first newcomer to try? :)
I'm in the process of moving, but I'd really like to measure 3 of my speakers:
1. Jbl 305p mkii (active)
2. Genelec 8040b (active)
3. Psb imagine xb (passive)

Plus 2 more from my bro:
1. Edifier r1850db (active)
2. Yamaha hs5 (active)

Plus 2 more from my friend:
1. Kef R300 (passive)
2. Elac DBR62 (passive)

I have the Dayton Audio UMM-6 measurement microphone. Anyway, once I settle down in a month or so, I'll see what I can do. :)
 

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#15
Now... who will be the first newcomer to try? :)

As mentioned, you don't even need to do a full spin. Honestly just doing an on-axis + a few horizontal angles already tells us so much.
I’d like to. I have a large-ish yard, the issue that comes with that is running power, and even though I’m in a suburban/rural area, I have a decent amount of noise (I hear motorocycles about a mile away on the main freeway as I type this; honestly, sorry for anyone that rides, but they are too damn noisy).
 
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napilopez

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I’d like to. I have a large-ish yard, the issue that comes with that is running power, and even though I’m in a suburban/rural area, I have a decent amount of noise (I hear motorocycles about a mile away on the main freeway as I type this; honestly, sorry for anyone that rides, but they are too damn noisy).
You probably shouldn't worry about the noise floor all that much for these types of measurements.

However loud it may be, it's almost certainly not worse than the absolutely godawful noise floor in my backyard :). Behold!

Noise floor.png
That's the on-axis for the HDI-1600. I seemed to have measured during a particularly noise moment.

For reference: My yard is only a block away from a highway, it's adjacent to a police parking lot, I live in an industrial part of brooklyn with constant machinery sounds around me, and the yard has three giant walls reflecting that sound. The day I measured the HDI-1600 also happened to be insanely windy, and those sounds are also amplified by the giant walls.

Actually a good idea to add a note about that in the setup portion as well. REW is really quite amazing at listening through the noise.

My old place where I made most of my measurements was way quieter by comparison, but in practice, there's not much of a difference for spinorama purposes and nearfield measurements.
 

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You probably shouldn't worry about the noise floor all that much for these types of measurements.

However loud it may be, it's almost certainly not worse than the absolutely godawful noise floor in my backyard :). Behold!

View attachment 123705 That's the on-axis for the HDI-1600. I seemed to have measured during a particularly noise moment.

For reference: My yard is only a block away from a highway, it's adjacent to a police parking lot, I live in an industrial part of brooklyn with constant machinery sounds around me, and the yard has three giant walls reflecting that sound. The day I measured the HDI-1600 also happened to be insanely windy, and those sounds are also amplified by the giant walls.

Actually a good idea to add a note about that in the setup portion as well. REW is really quite amazing at listening through the noise.

My old place where I made most of my measurements was way quieter by comparison, but in practice, there's not much of a difference for spinorama purposes and nearfield measurements.
Wow. That is a lot of noise!

I may try to make a spinorama for my Fostex 6301. It's tiny and easily mountable on a tall microphone stand. Single driver and sealed. Should not be too hard for a first spin project.
 
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napilopez

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Wow. That is a lot of noise!
Right?? I couldn't believe all the stay low frequency energy when I moved here and thought something must've been wrong with my microphone.

I may try to make a spinorama for my Fostex 6301. It's tiny and easily mountable on a tall microphone stand. Single driver and sealed. Should not be too hard for a first spin project.
Do it!!
 
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ernestcarl

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Right?? I couldn't believe all the stay low frequency energy when I moved here and thought something must've been wrong with my microphone.



Do it!!
Ceiling is pretty low inside and I'm sick of moving furniture out of the way so I may have to do it elsewhere. Huh! I just realized my brother or sister's empty basement might work fine if they can tolerate the weird noises... :p Definitely will do it within the following summer months... but I will need to do some preliminary practice test runs in my cramped space first.
 
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