How do we measure distortion? - Page 2 - AVS Forum
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post #31 of 41 Old 09-24-2008, 06:09 AM
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if people think its dumb and useless do not reply

LOL,i can reply because i think that's not the dumb. i want to know as the same. okay it may be easy for someone but it's not easy for me and some friends. penngray i think like u.

augerpro thank for the good links. i'm beginner. i will try to read.
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post #32 of 41 Old 09-24-2008, 06:47 AM
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many times, the simple question that they did not answer. i think that i must ask the difficult question like this, how many does the bugs in the REW program?
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post #33 of 41 Old 09-24-2008, 09:07 AM
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Originally Posted by cjwhitehouse View Post

THD% as a pure number unfortunately tells you nothing about how distorted something sounds.

It's a bit better than nothing but it's much better to plot the individual harmonics. The 2nd harmonic doesn't sound as bad as the 3rd and higher ones. Often, a driver that sounds bad at a certain frequency will show the 3rd spiking above the 2nd at that frequency. So, in a perfect world, you'd adjust your crossover so the driver isn't playing that frequency.

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Originally Posted by cjwhitehouse View Post

Measuring in-room will also completely mess up any objective measurement of distortion level as the room will boost/cut fundamental and individual harmonic levels in a way that renders the results pretty meaningless for anyone but the originator.

You can measure with the mic right next to the driver and eliminate the room effects. The only caveat is that the cheap mics will have their own distortion once you get much over 100 dB. But you can measure both ways -- close mic'd at low levels and from the seat at louder levels and get a pretty good feel for what's going on. Or you can open your wallet and buy an Earthworks mic.

Dennis H
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post #34 of 41 Old 09-24-2008, 01:37 PM
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I, like many here would like to learn more about this subject. I've been using some basic harmonic distortion measurements to help me understand what I'm hearing. I might take a reading at a moderate volume where IMO things sound clean and then where things start sounding pushed or strained. I can then look at the 2 readings and use it to help me understand what I was hearing and thus get a better understanding. Wouldn't it be interesting if the level that you thought still sounded clean and good showed very high levels of distortion(2nd, 3rd, whatever)? I'd have to re-evaluate my hearing. I do my distortion checks at the actual listening positions in room because that makes the most sense for me as this is the actual environment and I'm not evaluating for anyone but myself. I would never use them as a basis for any real comparison to anything else.

What is the point of taking low volume distortion tests, besides establishing a base line? Aren't nearly all drivers going to have very low distortion within their passband at low levels?
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post #35 of 41 Old 09-24-2008, 05:58 PM
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What is the point of taking low volume distortion tests, besides establishing a base line? Aren't nearly all drivers going to have very low distortion within their passband at low levels?

Not necessarily low volume but low enough to keep the mic from distorting when you have the mic right next to the driver. It helps you pick XO points in the design stage. Generally, all the distortion gets bigger as you play it louder but the problem frequencies are still problem frequencies no matter how loud it is and you can pick them out at 100dB close mic'd.

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post #36 of 41 Old 09-24-2008, 06:03 PM
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I test everything at 96dB @ 1m, which is pretty high for a single driver.
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post #37 of 41 Old 09-24-2008, 07:06 PM
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Basically if there is an area with increased distortion, not necessarily high just higher than the rest, or otherwise different, it is an indicator that this trend will be exacerbated as you increase the drive level? IE you can get a good guestimate of the behaviour at louder spl's?

Augerpro,
96db close mic'd on the driver is considered relatively high levels? Nevermind. I just noticed that you stated at 1meter.
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post #38 of 41 Old 09-24-2008, 10:22 PM
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I invested a little more in my microphone, I test my tweeters at 115dB at 1/2m, if they blow up in the test, they are no good for me. Keep in mind that most of the passband winds up being around 112dB or so, I like to know where the tweeters limits are and this get me closer to understanding that, I would like to test at 10dB higher, but I don't know of any mics in my price range that can do that (~$400 or less).
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post #39 of 41 Old 09-25-2008, 08:48 AM
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Quote:
Originally Posted by armystud0911 View Post

I invested a little more in my microphone, I test my tweeters at 115dB at 1/2m, if they blow up in the test, they are no good for me. Keep in mind that most of the passband winds up being around 112dB or so, I like to know where the tweeters limits are and this get me closer to understanding that, I would like to test at 10dB higher, but I don't know of any mics in my price range that can do that (~$400 or less).

B&H has the Earthworks M23 for $440 if you call them. $550 for the M30. Also, Kim Girardin has a high SPL version of his mic for $140 or so but I don't think it will work with the standard 48V pro preamp.

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post #40 of 41 Old 09-25-2008, 10:40 AM
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Thanks Dennis, I have an Audix TR40 right now, it supposedly goes up to 128dB @1Khz, but I wasn't happy with the results when testing past 120dB, for accuracy, I lowered it 5dB more. I noticed the earthworks M23 can take up to 142dB, maybe I will finally be able to perform 130dB+ near field tests.
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post #41 of 41 Old 09-25-2008, 08:14 PM
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Yeah, the max SPL spec on those mics is at something like 3% THD so you need to keep it well below there if you're trying to measure speaker distortion.

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