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[George Reiswig]

I am having a hard time finding EASE/GLL data for my speakers, so I'm thinking about learning to make these measurements myself using Open Sound Meter software, my UA Apollo interface and...well, that becomes question #1.

1. I have a choice of microphones I could use: I could use an Avenson STO-2 mic, but since most of the test setups that I see are done outside, with the mic on the ground, it got me wondering if one of my Crown PZM-30FS mics might not be as good or better in some regards because of the PZM part? Or is that just introducing another variable that I should not?

2. Is there an established standard for this measurement that would make my measurements more helpful to others? E.g. what decibel level is registering at the mic, what distance from the speaker the mic is placed, etc.?

I want to make (or obtain) measurements for my dB Tech IG4T speakers, my Bassboss VS-21 Mk2, and my Electrovoice PXM-12XMP speakers. I managed to find the data for my Yamaha DXR-10s on their web site.

Thank you!

[Rob Timmerman]

dB has a GLL for the Ingenia IG4T available for download on the product page: https://www.dbtechnologies.com/docs/299/2349/IG4T_EF3_V1.1.gll.zip (one of the downloads at https://www.dbtechnologies.com/en/products/ingenia/ingenia-ig4t/ )
 
The official measurement guidelines that are used in creating GLL files are published at https://www.afmg.eu/en/we-are-loudspeaker-company-do-you-have-measurement-guidelines-creating-loudspeaker-data-files-gll

[George Reiswig]

dB has a GLL for the Ingenia IG4T available for download on the product page: https://www.dbtechnologies.com/docs/299/2349/IG4T_EF3_V1.1.gll.zip (one of the downloads at https://www.dbtechnologies.com/en/products/ingenia/ingenia-ig4t/ )
 
The official measurement guidelines that are used in creating GLL files are published at https://www.afmg.eu/en/we-are-loudspeaker-company-do-you-have-measurement-guidelines-creating-loudspeaker-data-files-gll

Thank you very much, Rob!

[John Schalk]

It is frustrating when you can't find GLL files for your own speakers but you're in luck in that dB Tech has a GLL file for your main PA speaker.  As far as subwoofers are concerned, unless you are wanting to look at the prediction in EASE Focus for a specific sub frequency at a specific point (listener) in your design, you can use a GLL file for a sub that is of the same general type.  Frankly, since you only have one main subwoofer (for now!) I'm not sure how much predicting you need to do The sub is going to go in the center, if space allows, or on one side if it doesn't.

If you are interested in modeling different subwoofer deployments then I recommend using Meyer's MAPP 3D software.  Meyer's 900 LFC is a front loaded, single 18" powered subwoofer so it makes a nice building block for trying out spaced pairs versus center clusters as well as more complex designs.  The acoustic principles are the same since your sub is also a front loaded (vented box) speaker.  Sure, the actual frequency response and SPL levels will be different, but the interactions of multiple boxes will be the same.

[George Reiswig]

I think looking for GLL files likely is not what I thought it was. In any case, I don’t have PC’s so I’m having a hard time viewing the files anyway.

What I am really trying to do is phase- and time-align my sub to the mains, and try to ensure they are working together properly. My thinking is that if I can establish a baseline measurement that applies to when they are equidistant from the listener, then I could apply additional delay based on distance and temperature in the field.

Another thing I thought of that may make this point moot is that I might need to test this with the mixer and whatever effects (e.g. EQ) applied, in case that is affecting phase by adding latency. IDK.

[Frank Koenig]

What I am really trying to do is phase- and time-align my sub to the mains, and try to ensure they are working together properly. My thinking is that if I can establish a baseline measurement that applies to when they are equidistant from the listener, then I could apply additional delay based on distance and temperature in the field.

If that's all you want to do one of the ~$100 measurement mics from Rational Acoustics or Parts Express will do fine. Lay it down on a piece of flat material (plywood) to keep it out of the dirt. You're dealing with 10 ft wavelengths so it's not too critical. The bigger problem is your measurements getting confounded by nearby reflecting surfaces.

I use the Rational Acoustics mic for this and keep the good measurement mic at home. BTW, except for the top octave, where the cheap mic is a little hot, they match almost perfectly.

Establishing a "reference delay" is exactly what I do. Saves time and is good enough for throw and go.

--Frank

[John Schalk]

I think looking for GLL files likely is not what I thought it was. In any case, I don’t have PC’s so I’m having a hard time viewing the files anyway.

What I am really trying to do is phase- and time-align my sub to the mains, and try to ensure they are working together properly. My thinking is that if I can establish a baseline measurement that applies to when they are equidistant from the listener, then I could apply additional delay based on distance and temperature in the field.

Here is a link to Merlin's subwoofer alignment process.  He shows how to perform an alignment "at the shop" to set a baseline which you can then adjust for distance out in the field.  He has helpful videos for each step in the process too.  I would recommend having your mixer included in the measurement path if it is performing any DSP functions.  For instance, I mix on the X/M 32 platform, and I use the polarity and delay features on the Outputs of the mixers to do my alignments so I include my X32 Rack in the signal path when I am checking my system's alignment in my driveway.

https://www.merlijnvanveen.nl/en/study-hall/166-subwoofer-alignment-the-foolproof-relative-absolute-method

[Frank Koenig]

I'll add that in a pinch, if you're in a small room lousy with modes, you can get a decent sub alignment using nearfield measurements. The magnitude may be off but the phase will be close enough and a whole lot better than nothing. Place the mic alternately between the grilles of the sub and the LF part of the top and carry on as unusual. Apply additional delay as needed based on the sub-top distance difference at the listening position.

--Frank

[George Reiswig]

Thank you to everyone for contributing here. I’ve managed to start capturing some data that seems to show good coherence.

I still am curious as to whether a good PZM might be a good way to do these baseline captures, versus placing a measurement mic on the ground? Maybe at the wavelengths we are talking, boundary effects of the ground on a measurement mic don’t really matter? 

[George Reiswig]

Thank you to everyone for contributing here. I’ve managed to start capturing some data that seems to show good coherence.

I still am curious as to whether a good PZM might be a good way to do these baseline captures, versus placing a measurement mic on the ground? Maybe at the wavelengths we are talking, boundary effects of the ground on a measurement mic don’t really matter?

So I did a bit of an experiment, using my Avenson STO-2 as the measurement mic, and a Crown PZM-30S as the reference signal. Up to about 2kHz, they were in excellent agreement. And I know the Crown is not flat above that point. So...I guess it doesn't matter, really.

[George Reiswig]

Follow-up question on measuring the VS21 “vented and horn-loaded” subwoofer:

I cannot effectively get an impulse estimate of distance to subwoofers, and there since the driver itself is not loaded in the front, it’s hard to tell what delay I should tell the software to compensate for distance between the microphone and speaker when measuring it. Is there a reasonable point on the cabinet that I should be measuring to, then applying the relevant temperature/elevation calculations for calculating a delay to apply? E.g. to the middle of the cabinet where the driver sits, or to the front lip of the horn, or…?

[Mac Kerr]

Follow-up question on measuring the VS21 “vented and horn-loaded” subwoofer:

I cannot effectively get an impulse estimate of distance to subwoofers, and there since the driver itself is not loaded in the front, it’s hard to tell what delay I should tell the software to compensate for distance between the microphone and speaker when measuring it. Is there a reasonable point on the cabinet that I should be measuring to, then applying the relevant temperature/elevation calculations for calculating a delay to apply? E.g. to the middle of the cabinet where the driver sits, or to the front lip of the horn, or…?

You can't really get an implulse response of a subwoofer in its passband. A 100Hz wave is about 10' long, where on that is the peak? You can get an impulse outside the passband by arbitrarily using a higher frequency, but that really won't tell you the reality of below 100Hz. It is better, in SMARRT or some other FFT analyzer to get the response of the top speakers and then look at the phase response of each speaker and adjust the timing so the phase curves match up through the acoustic crossover range.

I also can't see any reason you couldn't use a PZM for this ind of set up.

Mac

[George Reiswig]

You can't really get an implulse response of a subwoofer in its passband. A 100Hz wave is about 10' long, where on that is the peak? You can get an impulse outside the passband by arbitrarily using a higher frequency, but that really won't tell you the reality of below 100Hz. It is better, in SMARRT or some other FFT analyzer to get the response of the top speakers and then look at the phase response of each speaker and adjust the timing so the phase curves match up through the acoustic crossover range.

I also can't see any reason you couldn't use a PZM for this ind of set up.

Mac

That’s what I am trying to do, which is why I’m asking the question about where to measure to for this type of subwoofer. If I use Merlijn van Veen‘s method of relative and absolute measurement, in his illustration he places the grills of the main and sub equidistant. Which makes sense for a sub that is built with a front-facing cone. But this doesn’t have that, and I’m trying to do this as accurately as possible.

[John Schalk]

That’s what I am trying to do, which is why I’m asking the question about where to measure to for this type of subwoofer. If I use Merlijn van Veen‘s method of relative and absolute measurement, in his illustration he places the grills of the main and sub equidistant. Which makes sense for a sub that is built with a front-facing cone. But this doesn’t have that, and I’m trying to do this as accurately as possible.

For Merlin's method, you should still line up the grills of your speakers.  The goal of his method is to phase align the main and sub speaker when they are next to each other so that, when you are in the field and the speakers are spaced well apart, you can use a simple distance measurement to determine the additional delay that is required.

Assume you are out at a gig.  You stand in the place where you want to align your main(s) with your sub.  You aim a laser measure at the left main speaker and note the distance.  Next, you aim the laser at the Bassboss sub, what part of the sub are you measuring to?  The grill, of course.  Does that make sense?

[Frank Koenig]

That’s what I am trying to do, which is why I’m asking the question about where to measure to for this type of subwoofer. If I use Merlijn van Veen‘s method of relative and absolute measurement, in his illustration he places the grills of the main and sub equidistant. Which makes sense for a sub that is built with a front-facing cone. But this doesn’t have that, and I’m trying to do this as accurately as possible.

This is just a reference delay. It really doesn't matter what point relative to the speaker enclosure you use so long as you use the same point later when calculating the delay. The face of the grille is an easy-to-remember choice.

On impulse responses: You certainly can obtain an accurate impulse response of any speaker, including a subwoofer, but its main practical use for us is archiving measurements. It's a sequence of real, as opposed to complex, numbers and is a complete representation of a single input and output of the system modeled as a linear-system. All other linear-system representations can be derived from it.

The frequency-domain representation (log-magnitude and phase for us audio folks, usually) is far more useful for time aligning speaker boxes with each other as Mac and others often remind us.

You can convert between the time- and frequency-domain representations using the Fourier Transform although there a few computational pitfalls to be aware of.

--Frank

[George Reiswig]

For Merlin's method, you should still line up the grills of your speakers.  The goal of his method is to phase align the main and sub speaker when they are next to each other so that, when you are in the field and the speakers are spaced well apart, you can use a simple distance measurement to determine the additional delay that is required.

Assume you are out at a gig.  You stand in the place where you want to align your main(s) with your sub.  You aim a laser measure at the left main speaker and note the distance.  Next, you aim the laser at the Bassboss sub, what part of the sub are you measuring to?  The grill, of course.  Does that make sense?

That does make sense, I believe. IOW, the important factor here is which surface you're taking your distance measurement from. Not so much "where does the sound start at this speaker" measurement. And I'm guessing that at the wavelengths involved in the crossover region, a few inches off for the subwoofer won't make too much of a difference? If my understanding is right, you'd have to be almost 2' off at 100Hz in order to be out of phase by 60 degrees?

On impulse responses: You certainly can obtain an accurate impulse response of any speaker, including a subwoofer, but its main practical use for us is archiving measurements. It's a sequence of real, as opposed to complex, numbers and is a complete representation of a single input and output of the system modeled as a linear-system. All other linear-system representations can be derived from it.(SNIP)
--Frank

Sorry, Frank...I think we're talking about two different things. I'm not talking about shooting an IR of the speaker. I'm talking about "Impulse" in the sense that Open Sound Meter is using it. As I understand it, it's basically the way the software estimates how far away the speaker is by timing a point in the reference signal to when that same point registers at the measurement mic. It's THAT which I can't get the software to register when it comes to a subwoofer, for the reasons that Mac Kerr said above, I believe. So I believe what Merlijn van Veen‘s method is doing is basically putting both speakers in approximately the same equidistant plane from the mic (at least as far as a laser measure would regard it), then using the full-range speaker to measure that impulse (timing) and applying it to both the full-range speaker and the sub.

[Frank Koenig]

Sorry, Frank...I think we're talking about two different things. I'm not talking about shooting an IR of the speaker. I'm talking about "Impulse" in the sense that Open Sound Meter is using it.

Ah, very good. I'm not familiar with Open Sound Meter but faced with the problem of determining an absolute delay in a narrow band of frequencies I would use a tone-burst test-signal (which could be a physical test signal or a "virtual" test signal in a numerical model of the system based on the impulse response) with a carrier frequency at the frequency of interest and an envelope such as a raised-cosine (Von Hann) window. Then compute the envelope of the response of the system to the tone-burst by taking the magnitude of the analytic signal* corresponding to the real response. Then compute the time difference between the peaks of the input and output envelopes. (The width of the window controls the time-frequency uncertainty tradeoff.)

I wonder if this is what Open Sound Meter does? The above approach for determining delays gives generally robust results even for "messy" real systems. After a few years of fooling with this stuff I've come to think of tone bursts as often the best conceptual test signals and representations for many audio problems. Tone bursts are a good model for the waveforms of percussive musical instruments, for what it's worth.

The group delay of a system at a particular frequency can be thought of as the delay of the envelope of a tone burst with a carrier at that frequency, just as above. The group delay is minus the slope of the phase and that is what we attempt to match, along with the phase, when aligning speaker pass-bands. So it all fits together.

*An analytic signal is one in which the amplitude of its Fourier transform is zero for all negative frequencies. The imaginary part of the analytic signal in the above example can be derived from the real system response by means of a Hilbert transform. You can think of the above method of getting an envelope as creating a complex signal whose imaginary part is a frequency-independent 90 degree phase shifted version of (or in quadrature to) the real part, which is the original signal. Then take the magnitude of that complex signal. Probably more than anyone wanted to know about penguins...

--Frank

[Art Welter]

And I'm guessing that at the wavelengths involved in the crossover region, a few inches off for the subwoofer won't make too much of a difference? If my understanding is right, you'd have to be almost 2' off at 100Hz in order to be out of phase by 60 degrees?

George,

Yes, a few inches won't make any audible difference in phase response at 100Hz.
At 72 degrees F, sound travels 1130' (feet) per second, 1.13' per millisecond, 0.885 ms per foot.
1130/100=11.3' for one wave length, 360 degrees, 10ms
360/60 =6
11.3/6=1.88'
You could correct a 60 degree phase misalignment in the crossover region by moving the "late" speaker forward physically by 1.88' or applying a delay of 2.44ms to the speaker that arrives "too soon".

At 100Hz, 5ms is equal to 180 degrees of phase change,  a polarity reverse for alignment can avoid adding excess delay.

Art

[Frank Koenig]

At 100Hz, 0.5ms is equal to 180 degrees of phase change,  a polarity reverse for alignment can avoid adding excess delay.

Decimal point, Art 

[George Reiswig]

Decimal point, Art 

Yess! That sure made me go “hmmmm!”

[John Schalk]

So I believe what Merlijn van Veen‘s method is doing is basically putting both speakers in approximately the same equidistant plane from the mic (at least as far as a laser measure would regard it), then using the full-range speaker to measure that impulse (timing) and applying it to both the full-range speaker and the sub.

No.  Merlijn's method places the main and sub speakers next to one another to remove any physical separation from the phase alignment calculations for the initial alignment.  I believe that Open Sound Meter's method of delay finding uses the impulse response peak, just like Smaart.  This works well for fullrange speakers but is generally not useful with subwoofers.  Once you have the two speakers placed next to each other and have captured an impulse response to set the delay time to the main speaker, the remainder of the process is a bit more art than science.  You have to adjust the delay of one of the two speakers until their phase traces are in alignment in the crossover region.  I'm not going to attempt to explain the process any further because I am still learning how to recognize what I see in Smaart and to take the correct actions myself.

There are quite a few YouTube videos on this topic, some better than others.  I recommend that you search for videos from Nathan Lively and Michael Curtis.  Both of them have videos that run through the basics of using OSM, but both of them have also switched to using Smaart for their in depth analysis.  I took a look at OSM, but decided to buy a license for Smaart primarily because I knew that I would need help learning how to use the software and posting questions to a FB group was not good enough (for me).  I paid $450 for a license of Smaart LE during Rational's December sale.  I looked at it as just another "equipment" purchase.  My biggest challenge with measuring systems is doing it often enough to learn and retain what I've learned.  I can't blast pink noise in my neighborhood too often!

[George Reiswig]

No.  Merlijn's method places the main and sub speakers next to one another to remove any physical separation from the phase alignment calculations for the initial alignment.  (Snip)

Thanks again, John! Lots of patient explanation on this forum, which I appreciate. I have looked at Michael Curtis’ videos, and between that and reading (and watching) Merlijn’s stuff on his process, I probably have just about enough knowledge to be at the peak of the Dunning-Kruger curve.

With respect to your quote above, it feels like we were saying the same thing, but perhaps not since you said “No…”. Are you saying the speakers have to be co-located for this to work? I was thinking that as long as your mic is omni, and both speakers are on the ground and equidistant from the mic, it should work. Practically, putting them together makes sense, but I’m trying to understand theory here, so…

[John Schalk]

With respect to your quote above, it feels like we were saying the same thing, but perhaps not since you said “No…”. Are you saying the speakers have to be co-located for this to work? I was thinking that as long as your mic is omni, and both speakers are on the ground and equidistant from the mic, it should work. Practically, putting them together makes sense, but I’m trying to understand theory here, so…

Yes, I think that the speakers have to be co-located if you are going to try following Merlijn's process.  Think about is this way; your sub and main speakers are both self-powered and have built in DSP features.  Therefore, there will be some amount of latency in each signal path, main & sub, which is difficult to measure electronically.  By that I mean, we can easily measure the latency in our digital consoles or in an external DSP unit like a Driverack, but unless your powered speakers include a processed output, it's hard to measure it.  This is why you can't use a simple physical alignment of the drivers in your speakers to time align them.

Going back to the start, the purpose of co-location is to provide a known distance from which you can make adjustments later on when the system is deployed out in the field.  I suppose you could locate your main and sub 6' apart, and exactly the same distance from your PZM mic, but then you'd have to remember that you aligned your system with 6' of separation as the default.  Why would you do that?

Well, that is what I think I know about this process.  But bear in mind that Michael Lawrence has written that main to sub alignment is probably the most over discussed topic in sound system optimization and not so important as many folks tend to believe.  I can recommend his book, "Between The Lines." 

[Art Welter]

Yess! That sure made me go “hmmmm!”

I just put the mistake in to see if you were paying attention

[ProSoundWeb Community]