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Measuring Loudspeaker THD

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Matthias McCready:
I have been embarking upon a rabbit hole, which is testing speakers.

More to the point I have been tasked at work with going through roughly 8 installed PA systems and verifying that things are working as they should. I know some of the systems are not working as they should, so I wanted to create a more comprehensive way to examine the loudspeakers and to provide data up the chain of command, "hey these need to be taken down, and parts need to be replaced."

I ended doing a lot of research and creating a 17-page procedure manual for myself, which being a week or two into the project already needs a few revisions!  ;D

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I have 3 ways I am trying to test each loudspeaker:

Procedures 1 & 2, are both conducted at 3x the baffle distance for flown systems, with 1:1 resolution, via unplugging other boxes if needed. Procedure 1 is done at least 20dB above ambient SPL, and Procedure 2 is done at about 40dB less than the rated output of the box, at the measurement mic.

1) A Smaart TF
2) a REW THD measurement
3) An amp impedance measurement, I have been able to utilize this on my L'Acoustics Rig (Enclosure Check), but I have not been able to do this with my D&B systems, via the "Load Monitoring." The D&B systems do not have 1:1 resolution, which I am guessing has a lot to do with that.


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Anyways getting on to my question:

What level of THD is acceptable?

I know this is probably a, "it depends" situation," as I presume THD is dependent upon the drivers, the design used, and volume tested; these are PA speakers after all!

Going into the project I had presumed from my reading that above 1% would be bad.

Once I began I realized just about everything I tested has above 1% above 8K or so, even new L'Acoustics X12's, which sound great!

It has been recommended to me to compare THD between models of the same type of speaker. So that is what I am trying to do, parsing through all of the measurements that I have taken so far.

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This has been helpful, as in comparing D&B Q-Subs of three in one particular sound system, one of them sounded a little funky and did not have the output of the others. When I looked at that particular sub it had 126% distortion, compared to the 30% of the other two Q-Subs. So I believe that data there would be showing me the driver (or something internal) is ready for replacement.

When it comes to top boxes though things haven't been quite so clear for me, and I feel uncertain about what to make of my data. Some boxes have 10-20% in the HF and others have 30% or 70%. For those would all the HF drivers that are not matching the lowest, say 10% need replacement? I mean it is an OLD rig so this is conceivable to me.

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Typically most boxes seem to have below 1% in the 200hz-5k range, although some models I have measure have a stereotypical 1-2% thing going on that is consistent box to box in the low mids (L'Acoustics A10). I did have a few boxes on my older systems where there was a spike of 4-6% in a low-mid frequency, which I would presume denotes a bad woofer, being that comparatively everything is under 1% in the measured range, for other loudspeakers of the same model.

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Sorry for the long post, I am hoping one of you audio wizards can help me figure this data out.

Thanks!

Matthias


John Roberts {JR}:
Not sure what your real task is, but a good idea for installed system maintenance to benchmark nominal (working) speaker measurements to compare to later.

Some smart install system amps can measure speaker current draw that can indicate gross failure, and sometimes "fixin to break".

Have fun...

JR 

Riley Casey:
REW will make impedance measurements although you'd need to create a jig that interfaced with your amp rack outputs. For tracking speaker performance from a maintenance POV impedance covers a lot of bases.

David Sturzenbecher:
I rely heavily on complex impedance for “speaker health” as quite lot can be gleaned. For this I ether use a Dayton Audio DATS system or an IAudioInterface.   This unfortunately will not alert you to a polarity issue in a biamped box (that is what I use a Smaart TF for). It is also largely a low voltage test and some issues won’t manifest itself until driven harder. For that I use Systune Pro with a LinearX VIbox, however they are no longer made.

Here are some sweeps from a recent system check.  It’s pretty obvious to spot major problems.  What is not obvious is what to do about smaller differences. These boxes were all in the air with no lift available so the issues were documented and that is about it.


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Matthias McCready:

--- Quote from: John Roberts {JR} on August 17, 2022, 02:24:31 PM ---Not sure what your real task is, but a good idea for installed system maintenance to benchmark nominal (working) speaker measurements to compare to later.

--- End quote ---

My goal is exactly as you describe, to have an annual process for going through every rig completely, and get data on every box. A health report if you will. As new systems are installed (we have 3 new PA's going up in the next year), I will be taking baseline measurements before boxes go up, which also helps to make verify that the boxes are wired in phase from the factory.  :)


--- Quote from: John Roberts {JR} on August 17, 2022, 02:24:31 PM ---Some smart install system amps can measure speaker current draw that can indicate gross failure, and sometimes "fixin to break".

--- End quote ---

All the systems are either D&B or L'Acoustics, both have this feature, however I believe 1:1 resolution is needed, which some of the systems do not have. So I will probably need to figure out something there (maybe using a single amp and channel and bringing that to each box).


--- Quote from: Riley Casey on August 17, 2022, 05:12:54 PM ---REW will make impedance measurements although you'd need to create a jig that interfaced with your amp rack outputs. For tracking speaker performance from a maintenance POV impedance covers a lot of bases.

--- End quote ---

In your mind would these be superior to measuring distortion, and would getting some data here mean that distortion is not valuable? Or would you consider distortion and impedance to be two distinctive points of Data that could give a better picture?


--- Quote from: David Sturzenbecher on August 17, 2022, 07:22:34 PM ---I rely heavily on complex impedance for “speaker health” as quite lot can be gleaned. For this I ether use a Dayton Audio DATS system or an IAudioInterface.   This unfortunately will not alert you to a polarity issue in a biamped box (that is what I use a Smaart TF for). It is also largely a low voltage test and some issues won’t manifest itself until driven harder. For that I use Systune Pro with a LinearX VIbox, however they are no longer made.

Here are some sweeps from a recent system check.  It’s pretty obvious to spot major problems.  What is not obvious is what to do about smaller differences. These boxes were all in the air with no lift available so the issues were documented and that is about it.


--- End quote ---

This is helpful, those are some tools I don't have in the kit as of yet.

Thanks!

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