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Author Topic: Do Low SPL measurments Scale Up?  (Read 2093 times)

Mike Karseboom

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Do Low SPL measurments Scale Up?
« on: January 17, 2014, 11:03:45 am »

I tried to research this but could not zero in on any discussions.  The question is can I take FFT measurments of a speaker system's response at low "volume" or spl and  expect the results to still apply as the system output level is increased.


This would be so I can perform measurement activities prior to a show while others are in the room and not be so obtrusive.  Also, just testing at home where high levels might annoy neighbors.


This would be using Systune with  pink noise and a single measurement mic.   I realize  I would have to play the pink noise loud enough get above any ambient noise or at least take the ambient noise into account.  Also, I generally have excess  rig for the gig so I am not trying to project past the point where the speakers get into to significant power compression or anything clips.   This would be in small halls with capacities of about 200 people and I would typically be measuring a PA system I bring  in.


From what I have seen in my very limited experience, Systune seems to produce similar phase and magnitude results at both low and high sound levels.  That is, leaving everything the same but just changing pink noise volume with the main fader level on the mixer.  I can get what appear to be similar measurements with the system turned down so low I can barely hear it.


If I can expect the measured results (valid, meaningful, right, etc. or not)   to be the same at very low spl as they are high spl, then I would just measure at low volumes.  I don't really like listening to that pink noise either!  Or are there other acoustic factors that only come into play as the spl goes up?


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John Roberts {JR}

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Re: Do Low SPL measurments Scale Up?
« Reply #1 on: January 17, 2014, 12:33:35 pm »

At low level room ambient noise can corrupt measurements, at high level and after heating up, device output linearity can suffer. Between the two things "should" scale. But I'm not the speaker guy here.


JR
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Chris Tsanjoures

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Re: Do Low SPL measurments Scale Up?
« Reply #2 on: January 17, 2014, 01:50:08 pm »

sound systems are (generally) linear, however our human hearing mechanism is not. If you have an analyzer available - make several transfer function measurements at varying drive levels. You will see that the frequency response of a system doesn't change as level rises or lowers. However, the way we perceive the sound does (think Fletcher-Muson curves). As JR said, noise interference will be more obvious the lower in level you measure, so as long as you are above noise floor you can use the coherence trace to rule out or otherwise understand what sort of anomalies you are seeing that are a result of noise etc.
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Mike Karseboom

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Re: Do Low SPL measurments Scale Up?
« Reply #3 on: January 18, 2014, 01:43:06 am »

I played around with this some today and came to a couple of conclusions.  Not sure if they are valid or not but this is what I observed:


1)  Phase alignment seems to be possible at low levels.  Focusing in on a sub to high crossover at 100Hz gave very similar phase graphs at very low levels and moderately high levels.


2) Measuring magnitude at low levels it was difficult to get good coherence.  The basic response was there but with a lot of "jittering" around.  It required more spl on the pink noise than I would have thought to smooth things out and get a relatively steady graph.  I never did calibrate with an SPL meter so can't give actual numbers.  But I had to get the pink noise up high enough to where it would definitely not go unnoticed.


I guess this is OK as I think for my very limited expertise in this area just being able to check for any gross timing problems near the crossover and perhaps being able to apply a corrective delay is the best I can hope for.  Looking at magnitude responses, other than out in the open in "freespace", is incredibly confusing to me.  The  frequency response curves I get indoors are so far from flat or smooth I just can't base many decisions on them. 
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Tim McCulloch

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Re: Do Low SPL measurments Scale Up?
« Reply #4 on: January 18, 2014, 01:24:27 pm »

I played around with this some today and came to a couple of conclusions.  Not sure if they are valid or not but this is what I observed:


1)  Phase alignment seems to be possible at low levels.  Focusing in on a sub to high crossover at 100Hz gave very similar phase graphs at very low levels and moderately high levels.


2) Measuring magnitude at low levels it was difficult to get good coherence.  The basic response was there but with a lot of "jittering" around.  It required more spl on the pink noise than I would have thought to smooth things out and get a relatively steady graph.  I never did calibrate with an SPL meter so can't give actual numbers.  But I had to get the pink noise up high enough to where it would definitely not go unnoticed.


I guess this is OK as I think for my very limited expertise in this area just being able to check for any gross timing problems near the crossover and perhaps being able to apply a corrective delay is the best I can hope for.  Looking at magnitude responses, other than out in the open in "freespace", is incredibly confusing to me.  The  frequency response curves I get indoors are so far from flat or smooth I just can't base many decisions on them.

1)  Correct,  Phase is time, and other than having enough level for each pass band, the actual magnitude in SPL is mostly irrelevant... except that...

2)  Coherence is a determination of how much "trust" is put into the displayed magnitude at that frequency.  You need enough magnitude to get over ambient SPL, but often only by a few dB.  In fact I often mix in pink noise with pre-show music (it fills in the 'gaps' in the music) to get more coherency in the parts of the spectrum for which the music does not provide sufficient content.  Most of the time punters can't tell the pink noise is in there.  The default setting for coherence blanking is usually sufficient.  If you have gaps in coherence, look at the phase trace, too, and see if the lack of SPL is being caused by a cancellation.
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Re: Do Low SPL measurments Scale Up?
« Reply #4 on: January 18, 2014, 01:24:27 pm »


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