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Any way to obtain an accurate and smooth distortion percentage chart for speakers?

Richarrd

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I can obtain good frequency response, directivity, and other useful information when measuring speakers outdoor, but when it comes to harmonic distortion, I encounter difficulties.
The background noise outdoors is too loud. It's not a problem for measuring high sound pressure levels, but when it comes to measuring as low as 84 or 80 dBSPL, the mid-low frequency data gets drowned out by the noise, even during the quiet hours of the night in a busy and noisy city. Moving indoors, I can obtain decent distortion parameters in the SPL mode of REW, although room reflections compromise the objectivity of the frequency response to some extent. However, REW seems to smooth it out to some degree. But when switching to the percentage mode, everything falls apart. The fluctuations caused by room reflections make the relative proportion image highly inaccurate. Is there any way for me to obtain a more accurate image?
Both an anechoic chamber and an NFS (Near-Field Scanning) system are too expensive for me. I have already completed most of the useful measurements, and if I were to invest everything just for distortion measurements, it would be beyond my capabilities.


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btw this is kh80 on 84dB. far away from correct.and hf roll off is buggy somehow,not showing on 90dB.

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Blumlein 88

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Have you tried stepped sine testing? Might help some though not quite smooth and will require some time.
 

JPA

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I know this is obvious, but I'll ask anyway... have you turned off all the noisy appliances, such as air conditioning, refrigerator, etc.?
 
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Richarrd

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Have you tried stepped sine testing? Might help some though not quite smooth and will require some time.
I know this is obvious, but I'll ask anyway... have you turned off all the noisy appliances, such as air conditioning, refrigerator, etc.?
The problem of loud noise occurs outdoors. Indoors, I am not bothered by background noise, but rather by uneven frequency response caused by room reflections and the errors in percentage calculations caused by the uneven frequency response when switching to percentage mode.
 
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Richarrd

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The problem of loud noise occurs outdoors. Indoors, I am not bothered by background noise, but rather by uneven frequency response caused by room reflections and the errors in percentage calculations caused by the uneven frequency response when switching to percentage mode.
Now it's a dilemma. If I move outdoors to measure distortion, the frequency response is flat and the percentage mode is accurate. However, I can only measure distortion as low as 1% because the outside noise is really loud. Indoors, I can measure lower distortion, but the uneven frequency response creates a bumpy distortion percentage, rendering this graph meaningless as a reference.
 

NTK

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Unfortunately, Per Dr Toole, we need a quiet (i.e. very well sound insulated) anechoic chamber (or equivalent) for loudspeaker distortion measurements.
...
Measuring non-linear distortion in a room makes a difficult situation even worse, because amplitudes of distortion products are affected by reflected sounds which are highly dependent on the venue, the loudspeaker location and the mic location. Background noise is another challenge; such measurements need to be done in a very quiet anechoic chamber or equivalent. Being able to make a measurement does not make it a useful measurement.
...
 

Blumlein 88

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The problem of loud noise occurs outdoors. Indoors, I am not bothered by background noise, but rather by uneven frequency response caused by room reflections and the errors in percentage calculations caused by the uneven frequency response when switching to percentage mode.
Yes with longer testing of a sine wave tone rather than a sweep when outdoors you will get an improvement on signal to noise. It likely won't be good, but would be better.
 
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Richarrd

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Yes with longer testing of a sine wave tone rather than a sweep when outdoors you will get an improvement on signal to noise. It likely won't be good, but would be better.
OK thank you! I will give it a try. and a good news is that I found the distortion outdoor is reasonable cause I was misunderstood the 1% distortion in mid bass,that part is actually cause by speaker itself,not by environment. still can be better in mid freq but 4 times sweep with timing reference will do better, and I will try step sine to compare!
 
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Richarrd

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Unfortunately, Per Dr Toole, we need a quiet (i.e. very well sound insulated) anechoic chamber (or equivalent) for loudspeaker distortion measurements.
REW seems to have a good algorithm to calculate harmonic distortion even in high noise environment,I will try two ways and later I will compare to data on ASR and neumann ,hope i can get some interesting result!
 

ernestcarl

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It probably doesn’t make a lot of sense to measure distortion at lower levels. What are you really trying to see? Usually I measure distortion to check when the speaker starts to strain or when objects around or connected to the speaker rattles — mainly if there’s more and more higher order rise and/or when the distortion exceeds the fundamental in the bass e.g. pass ~96dB or even 100dB. You also lose headroom with EQ boosts so that may be of interest. But, again, what would be the point of doing this at very low levels?
 

CDR&D

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I can obtain good frequency response, directivity, and other useful information when measuring speakers outdoor, but when it comes to harmonic distortion, I encounter difficulties.
The background noise outdoors is too loud. It's not a problem for measuring high sound pressure levels, but when it comes to measuring as low as 84 or 80 dBSPL, the mid-low frequency data gets drowned out by the noise, even during the quiet hours of the night in a busy and noisy city. Moving indoors, I can obtain decent distortion parameters in the SPL mode of REW, although room reflections compromise the objectivity of the frequency response to some extent. However, REW seems to smooth it out to some degree. But when switching to the percentage mode, everything falls apart. The fluctuations caused by room reflections make the relative proportion image highly inaccurate. Is there any way for me to obtain a more accurate image?
Both an anechoic chamber and an NFS (Near-Field Scanning) system are too expensive for me. I have already completed most of the useful measurements, and if I were to invest everything just for distortion measurements, it would be beyond my capabilities.


View attachment 323003

View attachment 323004
btw this is kh80 on 84dB. far away from correct.and hf roll off is buggy somehow,not showing on 90dB.

View attachment 323013
Alternative way to mitigate effect of indoor reflections or outdoor noise for THD/IMD measurements for individual drivers is to measure nearfield - typically 315mm or true <20mm nearfield (notably for distortion <150Hz). Care needs to be taken however at more elevated levels to control distortion of the measurement mic itself (as near field SPL at typical 96dB@1m will be >>100dB) as well as clipping levels / headroom. Most of the common used ‘cheapies’ (ECM8000, EMM6 which I have both used and tested) etc fall apart at these levels and THD (notably 2nd harmonic distortion) induced by the mic will outweigh driver distortion. Hence why I bought a LineAudio OM-1 which has a larger diaphraghm, at ~130Euro’s is exceptional and pro-level mic, is very linear from 20Hz-20kHz. and is clean upto 130dB (at some point I may add the earthworks M23R also for reference).
 
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