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Old 18th January 2012, 08:07 AM   #1
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Default How to Measure Speaker Distortion Correctly?

Well, I have been pondering over how to correctly measure the speaker driver distortion with Omni Mic kit from Dayton Audio.

I've done some basics measurements at 85-90dB SPL levels but when I read the Dayton Help section (sort of manual), the suggested SPL is >125dB in order to run the distortion test correctly.

I am not in the slightest mood to run the 89dB driver to 125dB which will ultimately destroy the driver. But I understand the reason why the manual suggests to test the speaker at such high SPL for some non-linear distortions only appear when everything is pushed to the limitation of the driver.

But do we really need to consider that kind of non-linear distortion in the how good (or horrible) a speaker would sound under most sane levels listening?

Aside from the Omni Mic I have, I might be able to try other measuring method. Is there any other kind of distortion measurements you folks use?

And it is my understanding that the driver distortion is best tested in Free Air Loading (OB) and not inside a box! Am I getting this all upside down?

TIA!
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Old 18th January 2012, 10:06 AM   #2
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The measurement distance should be small ~1" to have so high level ?
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Old 18th January 2012, 12:39 PM   #3
bwaslo is offline bwaslo  United States
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Got that backwards -- level should be LESS THAN 125dB (at higher, the mic itself distorts too badly). 90dB or 100dB is a good level, less is ok. You just have to be above the where the distortion products will be above the back ground noise.
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Old 18th January 2012, 01:17 PM   #4
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Most types of distortion in loudspeakers are SPL-dependent (distortion level increases as the SPL increases). Thus, to be able to do comparisons, you need to calibrate and measure at a specific SPL and distance (like 95 dB at 1 m).
If you need an absolute evaluation, measure at the highest level you are typically listening.
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Old 18th January 2012, 03:52 PM   #5
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Listen to Mr. Waslo. He knows what he's talking about. And knows the product.

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Originally Posted by bwaslo View Post
Got that backwards -- level should be LESS THAN 125dB
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Old 18th January 2012, 03:59 PM   #6
bwaslo is offline bwaslo  United States
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bzfcocon is right, though -- a distortion measurement without a displayed or specified level doesn't mean much. I was just referring to the ranges where you can use OmniMic. Above 125dBSPL won't work (and even at 120dB is getting sketchy).
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Old 18th January 2012, 04:43 PM   #7
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Thanks everyone for the input! I will follow the suggestion and measure at around a more comfortable SPL for the drivers but here is what I read from the Omni Mic manual.

They wanted you to run the speaker at very high levels but I am assuming the speaker as a complete speaker and not a single driver.

Quote:
This measurement will work best when the OmniMic is positioned relatively closely to the loudspeakers, so that sounds coming directly from the speaker are much stronger than those coming reflected from elsewhere. Room reflections are very detrimental to measurement of harmonic distortion.
The microphone and speaker should be held stationary over the length of several of the test sweeps (approximately 6 seconds each) previous to each graph update
At very high levels (>125dB SPL), appreciable distortion may be generated from overdriving of the OmniMic itself.
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Old 18th January 2012, 04:52 PM   #8
bwaslo is offline bwaslo  United States
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That last sentence was intended as a thing to not do.

You want the mic close to avoid echoes/reflections but watch out that the levels, when that close, don't get too high or you'll be measuring the mic rather than the speaker.

Instructions didn't want you to run at very high levels, they wanted things arranged so that direct signal is considerably higher than reflected -- turning up the volume has no effect on that ratio, only mic and speaker placement do.

You can go as low in level as you want, provided you are above ambient noise for all the distortion products you are trying to see. But most meaningful level would be around where the speakers will normally be used.
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Old 18th January 2012, 05:03 PM   #9
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Originally Posted by bwaslo View Post
That last sentence was intended as a thing to not do.

You want the mic close to avoid echoes/reflections but watch out that the levels, when that close, don't get too high or you'll be measuring the mic rather than the speaker.

Instructions didn't want you to run at very high levels, they wanted things arranged so that direct signal is considerably higher than reflected -- turning up the volume has no effect on that ratio, only mic and speaker placement do.

You can go as low in level as you want, provided you are above ambient noise for all the distortion products you are trying to see. But most meaningful level would be around where the speakers will normally be used.
Gotcha Mr. Waslo! I hope it's the case also.

I guess I can ask you more about the Omni Mic in the future? I find it very easy to use and does a lot of things for me.
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Old 18th January 2012, 06:30 PM   #10
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Quote:
Originally Posted by megasat16 View Post
Well, I have been pondering over how to correctly measure the speaker driver distortion with Omni Mic kit from Dayton Audio.

TIA!
If "correct" includes some indication of perception then you have an even bigger problem. The latest AES Journal has a paper on the "best sounding nonlinearities" - they add distortion intentionally to make the bass "sound better".

My suggestion: learn how to take good frequency response measurements at off-axis angles. That's more important than THD.
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