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Measuring Loudspeaker THD
Matthias McCready:
--- Quote from: Chris Grimshaw on August 29, 2022, 07:32:41 AM ---
IMO, for health checks, you could put the mic to the grille and eliminate pretty much all acoustic effects. Sure, the frequency response curve will be weird, but as long as you stay consistent, it'll be easier to spot changes that are due to driver age, rather than someone-has-moved-a-nearby-flight-case.
Chris
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These are good points. My measurement distance is loosely based of the Tracebook Project (Merlijn Van Veen & Nathan Lively). Their project gives a recommended distance, so that frequency response can even out. However the difference is that the loudspeakers I am measuring are staying rigged; I know this is not ideal and was not sure how well this would work, but for the most part I have not had too much ripple variance and the differences tend to be speaker differences, rather than the room from what I can tell.
--- Quote from: John Roberts {JR} on August 29, 2022, 08:57:31 AM ---You might want to check with the mfr to confirm the reversed polarity is in error. It might improve behavior in the crossover transition region.
JR
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For that situation, there were 9 D&B 24S boxes. 7 of them had an identical phase trace, and 2 had an identical, but deviated phase trace that started around 1k, and then went about 180 degrees out of phase, comparatively. So two boxes are different, in the exact same way, than 7 other boxes.
John Roberts {JR}:
--- Quote from: Matthias McCready on August 29, 2022, 09:23:41 AM ---
For that situation, there were 9 D&B 24S boxes. 7 of them had an identical phase trace, and 2 had an identical, but deviated phase trace that started around 1k, and then went about 180 degrees out of phase, comparatively. So two boxes are different, in the exact same way, than 7 other boxes.
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sounds like a mistake to me...
JR
Riley Casey:
Measuring at the grill as already suggested is the correct solution for knowing what the speaker system but at the grill you no longer have a speaker system you have several speaker components that all need to be measured separately for useful results. Identify a location on axis with each driver exit and take a separate measurement for each driver. It doesn't matter that you are getting overlap from other drivers, what matters is that each set of measurements remains the same across all the speaker systems. In my experience its unlikely that mechanical speaker driver failures produce relatively low levels of harmonic distortion. A torn cone driver or diaphragm produces high levels of distortion very quickly. For isolating that kind of issue in a multiway flown speaker system simply running a sweep tone thru the box and listening to a mic on a fish pole on headphones would probably be most effective . Its probably not terribly useful information to know that speaker seven in an array has THD of 20% when the others are at 5%. You want to know if its a woofer or a mid driver.
--- Quote from: Matthias McCready on August 28, 2022, 07:04:36 PM ---Point in case I had an L'Acoustics LA12X die on me two-weeks back, it let me know that it had "an internal short-circuit and would be powering off." ;D
Is it dangerous for me to assume that any differences in frequency and distortion (presuming everything is the same on the front end) is due to measurement position (room interaction) and the loudspeaker?
...
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Chris Grimshaw:
--- Quote from: Riley Casey on August 29, 2022, 01:49:19 PM ---You want to know if its a woofer or a mid driver.
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Riley,
See attached. REW gives harmonic distortion by component vs frequency. The two graphs are the same box at different power levels. SPLs are somewhere near correct.
Chris
Matthias McCready:
--- Quote from: Riley Casey on August 29, 2022, 01:49:19 PM ---Measuring at the grill as already suggested is the correct solution for knowing what the speaker system but at the grill you no longer have a speaker system you have several speaker components that all need to be measured separately for useful results. Identify a location on axis with each driver exit and take a separate measurement for each driver. It doesn't matter that you are getting overlap from other drivers, what matters is that each set of measurements remains the same across all the speaker systems. In my experience its unlikely that mechanical speaker driver failures produce relatively low levels of harmonic distortion. A torn cone driver or diaphragm produces high levels of distortion very quickly. For isolating that kind of issue in a multiway flown speaker system simply running a sweep tone thru the box and listening to a mic on a fish pole on headphones would probably be most effective . Its probably not terribly useful information to know that speaker seven in an array has THD of 20% when the others are at 5%. You want to know if its a woofer or a mid driver.
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These are great points.
In an ideal world, THD, probably should be done per driver, and field magnitude phase traces should at minimum be taken on the ground plane in the same exact spot, to lessen the impact of the room on the measurements (meaning the position of the mic/speaker in the room).
---
My specific goal for this project, is to perform annual inspections on multiple installed PA systems; we are adding two more rigs within the coming 6 months or so. So in this situation I am willing to forgo some accuracy for the sake of efficiency. If something shows up as funky, then I can investigate further (measure individual drivers, pull the box down, go through the box etc). So as long as my THD data is good enough to show me that a driver in the array box 7 is bad (which THD by frequency would give a good hint), I don't necessarily need to immediately know which one is bad.
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