In-Room THD - AVS Forum
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post #1 of 13 Old 09-02-2014, 11:22 AM - Thread Starter
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In-Room THD

As "we" all "know", the THD values measured for the electronics can be vanishingly small. The Speaker and the Room dominate what you think you thought you heard.

Assuming adequate electronics, operating at a reasonable level, say, 85dB spl, nothing overdriven or obviously strained, what kind of values for THD might I expect to see for in-room measurements?

What levels might represent Excellent Good Fair and Poor. I assume there will be different values for low, mid and high frequency reproduction.

Measuring tools I have available are UMIK-1 and REW.

Does the room itself (forget the refrigerator or dog barks) contribute to THD ?

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post #2 of 13 Old 09-02-2014, 11:39 AM
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Quote:
Originally Posted by RayDunzl View Post
As "we" all "know", the THD values measured for the electronics can be vanishingly small. The Speaker and the Room dominate what you think you thought you heard.

Assuming adequate electronics, operating at a reasonable level, say, 85dB spl, nothing overdriven or obviously strained, what kind of values for THD might I expect to see for in-room measurements?
Mostly due to the speakers and the room. 1% (-40 dB) @ 90 dB SPL is really good for speakers. Depends on frequency. Most drivers have noticeably increasing distortion at their lower frequency extreme.

Expect worse.

Here is an example of a really good inexpensive speaker:

http://www.soundstagenetwork.com/mea...y_primus_p162/

Here are a bunch of speaker tests:

http://www.soundstagenetwork.com/ind...d=16&Itemid=18

Some of the Audioholics speaker tests also have THD masurements.

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Does the room itself (forget the refrigerator or dog barks) contribute to THD ?
Nonlinear bending of the drywall, and various rattles and shakes.

You might want to prepare for serious testing with some slow sine sweeps, which should make anything that is prone to shake to do so. Correct before trying any serious meausrement.
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post #3 of 13 Old 09-02-2014, 02:24 PM - Thread Starter
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That's a long list of speakers.

Too bad they don't show the THD curves below 50db and sometimes 45dB.

Anyway, it makes me look pretty good

Mine, at about 90dB spl at the listening position, with some odd discontinuity up high:



And taken at more pedestrian listening levels:


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post #4 of 13 Old 09-02-2014, 02:41 PM
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Using REW, I get between 0.2% and 0.3% through the mids and treble regions (600-20k), 0.35% - 0.5% (45-600hz), measured around 85 SPLdb.

You might want to focus more on the odd numbered harmonics (3rd, 5th, 7th order) than THD.

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post #5 of 13 Old 09-02-2014, 02:51 PM
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Quote:
Originally Posted by RayDunzl View Post

Mine, at about 90dB spl at the listening position, with some odd discontinuity up high:


I get that odd rise also, and in about the same place. Coincidence?

Click image for larger version

Name:	rew thd.jpg
Views:	32
Size:	94.8 KB
ID:	242658

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post #6 of 13 Old 09-02-2014, 03:26 PM - Thread Starter
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After looking more closely:

That's where it drops from 2 to 1 harmonics in the measurement...

10th harmonic gives up at 2kHz fundamental

3rd harmonic quits at 6.6kHz fundamental

and the second harmonic (and THD calculation) quits at 10khz fundamental



So, it's a bandwidth-restriction aberration in the measurements.

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post #7 of 13 Old 09-02-2014, 03:36 PM
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Quote:
Originally Posted by RayDunzl View Post
3rd harmonic quits at 6.6kHz fundamental
Since the 3rd, IMO, is the most distasteful of them all, we shouldn't give too much credence to the data after 6600hz then.

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post #8 of 13 Old 09-02-2014, 03:48 PM - Thread Starter
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Originally Posted by jim19611961 View Post
Since the 3rd, IMO, is the most distasteful of them all, we shouldn't give too much credence to the data after 6600hz then.
In REW, yes...

I try to view data with the same level of suspicion that I view opinion.

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post #9 of 13 Old 09-06-2014, 02:48 PM
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Quote:
Originally Posted by jim19611961 View Post
I get that odd rise also, and in about the same place. Coincidence?

Attachment 242658
PC power supply noise and/or motherboard usb design issues. Get a linear power supply and it will be mostly gone. UMIK measurement on normal PC can not be trusted.
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post #10 of 13 Old 09-06-2014, 03:39 PM
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Originally Posted by coli View Post
PC power supply noise and/or motherboard usb design issues. Get a linear power supply and it will be mostly gone. UMIK measurement on normal PC can not be trusted.
Well said. Here are my distortion measurements of my mic using REW:



This is without playing anything. We see that "nice" harmonic spray.

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post #11 of 13 Old 09-07-2014, 07:44 AM
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Quote:
Originally Posted by amirm View Post
This is without playing anything. We see that "nice" harmonic spray.
What happens at -80db isnt significant in terms of obtained measurement data.

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post #12 of 13 Old 09-07-2014, 07:55 AM
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Originally Posted by jim19611961 View Post
What happens at -80db isnt significant in terms of obtained measurement data.
I don't believe that view is accurate either. On my old PC, I get a mysterious and consistant bump after 15khz. On my new PC, that bump is no where to be seen.
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post #13 of 13 Old 09-07-2014, 07:59 AM
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Quote:
Originally Posted by jim19611961 View Post
What happens at -80db isnt significant in terms of obtained measurement data.
The levels are not calibrated in that measurement so the absolute numbers are not correct. That aside you are right that for acoustic measurements, it is not consequential. For "THD" measurements however which is the topic we are discussing, the mic is adding its own distortion so you are not just measuring your speakers.

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