Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums > Audio Measurement and Testing
Measuring Loudspeaker THD
Matthias McCready:
I am also still quite curious about measuring THD in speakers for several reasons:
1) I had a D&B C4 rig I mixed on for a little over a year. It was a fine rig, but it was designed for the room it was in, and the deployment was rubbish. I got a new rig, and the D&B C4 was fortunate enough to go to a better home, where its coverage pattern is close to perfect. When I went a listened, the system sounded just phenomenal, there was clarity I simply never had (at a similar listening distance). When I asked the installer what they done, he mentioned that all of the HF drivers were quite old and tired, and needed replacement.
2) I know a sine Wave can audibly reveal problems with a driver, so THD should provide a data point for this.
----
So for those of you measuring THD, what are you looking for in the data?
----
An additional but related question I would have, is should I be measuring my amp channels?
With modern amps (presuming the right DSP is put in) are they are either working or not? Or is this something that I should be measuring as a potential factor?
---
Thanks again for help with all of the questions!
Matthias
Chris Grimshaw:
I'd say that modern amps are smart enough to know if they're working or not.
Old iron amps might have their class AB bias drift around a bit over the years, possibly resulting in crossover distortion (NB - nothing to do with speaker crossovers, it's a slightly "sticky" zero crossing on the amp output) if the bias runs too low and it ends up class B.
With regards to speaker health, I'd run a sine sweep at -10dB from maximum output, and see what you get. I'd look for:
- Frequency response within 3dB of original measurement across the band
- Harmonic distortion within certain limits. Maybe +/-50% of the original measurement. ie, original = 10% THD, current = max.15%.
I haven't done much/any research into how thoroughly-broken-in drivers behave compared to new, but I think the above guidelines are sensible with regards to deciding whether something needs replacement.
NB - Mic position is flexible, but must be consistent. Putting the mic up against the grille, for example, is okay, so long as you put it in the same place next time you do the measurements. As you move the mic further from the cabinet, other acoustic variables can contaminate the measurement.
Chris
PS - I'd also do a measurement at any bass reflex ports, which might help you spot internal rattles.
Matthias McCready:
--- Quote from: Chris Grimshaw on August 27, 2022, 07:16:12 AM ---I'd say that modern amps are smart enough to know if they're working or not.
Old iron amps might have their class AB bias drift around a bit over the years, possibly resulting in crossover distortion (NB - nothing to do with speaker crossovers, it's a slightly "sticky" zero crossing on the amp output) if the bias runs too low and it ends up class B.
--- End quote ---
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?
--- Quote from: Chris Grimshaw on August 27, 2022, 07:16:12 AM ---With regards to speaker health, I'd run a sine sweep at -10dB from maximum output, and see what you get. I'd look for:
- Frequency response within 3dB of original measurement across the band
- Harmonic distortion within certain limits. Maybe +/-50% of the original measurement. ie, original = 10% THD, current = max.15%.
--- End quote ---
NB - Mic position is flexible, but must be consistent. Putting the mic up against the grille, for example, is okay, so long as you put it in the same place next time you do the measurements. As you move the mic further from the cabinet, other acoustic variables can contaminate the measurement.
Chris
PS - I'd also do a measurement at any bass reflex ports, which might help you spot internal rattles.
[/quote]
So far I have been using the same mic positioning I have been for Smaart Traces; which is based on the baffle size of the speaker, and taken on axis of the loudspeaker.
----
So far it has been an interesting project.
1) In one system I found what appear to be many drivers that need replacement, in addition to boxes that were not designed to be arrayed together.
2) In another system, I found a few of the boxes had the HF horns that were out of polarity, and I don't think those boxes are biamped. This goes to show, how important it is, even for high-end brands, to test boxes BEFORE they get installed.
Chris Grimshaw:
--- Quote from: Matthias McCready on August 28, 2022, 07:04:36 PM ---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?
--- End quote ---
I'd say that's a sensible assumption with modern amps.
--- Quote from: Matthias McCready on August 28, 2022, 07:04:36 PM ---So far I have been using the same mic positioning I have been for Smaart Traces; which is based on the baffle size of the speaker, and taken on axis of the loudspeaker.
--- End quote ---
Fair enough.
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
John Roberts {JR}:
--- Quote from: Matthias McCready on August 28, 2022, 07:04:36 PM ---
2) In another system, I found a few of the boxes had the HF horns that were out of polarity, and I don't think those boxes are biamped. This goes to show, how important it is, even for high-end brands, to test boxes BEFORE they get installed.
--- End quote ---
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
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version