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Midrange distortion on 15" PA driver
Midrange distortion on 15" PA driver
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Old 10th October 2018, 07:01 PM   #1
LightBit is offline LightBit  Slovenia
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Default Midrange distortion on 15" PA driver

Hello,

I just bought pair of Faital 15FH500 and did some quick measurements (free air) in REW. I want to use it up to 1kHz.

It seems distortion at low volume is high in midrange:

distortion_15FH500.jpg

Will break-in reduce distortion? Or is this normal?
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Old 10th October 2018, 08:32 PM   #2
ErnieM is offline ErnieM  United States
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Looks pretty typical for a PA driver. The top of the line drivers might have less but not much. I would go for drivers with demodulating rings or sleeves if running that high. It'll sound fine if you don't put alot of bass through it at the same time.
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Old 11th October 2018, 02:10 AM   #3
hollowboy is offline hollowboy  Australia
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Quote:
Originally Posted by LightBit View Post
did some quick measurements (free air) in REW [...] is this normal?
Are you actually measuring distortion?

Where I live / do stuff, I have to be really suspicious / careful with distortion and low level tests - the noise floor is often not low enough.

e.g. I'll get what looks looks like a high distortion level... then I'll realise that an idling car engine or burner for a water heater has ruined the noise floor. Outdoors, what seems like a quiet gentle breeze or distant insects can ruin it.

What your brain filters out as background (unimportant), a mic cannot.
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Old 11th October 2018, 03:30 AM   #4
puppet is online now puppet  United States
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How clean is the test equipment?
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Old 11th October 2018, 04:21 AM   #5
daqvin_carter is offline daqvin_carter  United States
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THD_REW.png

Attaching a recent measurement for SPL and clicked the Distortion tab. I would expect the fundamental to follow frequency response and not be flat. I am guessing something is not right with the method of measuring in this case.
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Old 11th October 2018, 04:32 AM   #6
Eva is offline Eva  Spain
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In a free-air measurement the noises coming from suspension and ventilation are not masked by enclosure. These noises are rich in harmonics. The measurements in free air are not represntative of driver performance placed in an enclosure. (This also implies that open-baffle applications are tricky in the sense that driver suspension and ventilation must be quiet enough.)
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Old 11th October 2018, 06:47 AM   #7
minkuni is offline minkuni  Norway
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Quote:
Originally Posted by daqvin_carter View Post
Attachment 708710

Attaching a recent measurement for SPL and clicked the Distortion tab. I would expect the fundamental to follow frequency response and not be flat. I am guessing something is not right with the method of measuring in this case.
Note that the distortion in the OPs post is plotted as a percentage relative to the fundamental, not absolute level as in your plot. The same information is contained in both presentation styles.
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Old 11th October 2018, 07:05 AM   #8
TBTL is offline TBTL  Germany
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Plot the absolute levels of distortion. If they are smooth, they are likely to be your noise floor. Another indication that you are measuring the noise floor is when higher harmonics are not lower than lower ones. Still another one: the Beta version of REW shows the noise floor.

Microphones can also add distortion, especially in near field measurements. Sometimes it already is 1% THD at 110 dB. 110 dB is easily reached in near field measurements.
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Old 11th October 2018, 07:13 AM   #9
LightBit is offline LightBit  Slovenia
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Quote:
Originally Posted by daqvin_carter View Post
I would expect the fundamental to follow frequency response and not be flat.
This is because I enabled "Plot normalised to foundemental".

Quote:
Originally Posted by puppet View Post
How clean is the test equipment?
I measured speaker with much lower distortion with same mic, but different sound card and amplifier.

Quote:
Originally Posted by hollowboy View Post
Are you actually measuring distortion?

Where I live / do stuff, I have to be really suspicious / careful with distortion and low level tests - the noise floor is often not low enough.

e.g. I'll get what looks looks like a high distortion level... then I'll realise that an idling car engine or burner for a water heater has ruined the noise floor. Outdoors, what seems like a quiet gentle breeze or distant insects can ruin it.

What your brain filters out as background (unimportant), a mic cannot.
Maybe it could be computer fans, but I positioned mic very close to speaker. I will try to measure again with different sound card and no computer fans.
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Last edited by LightBit; 11th October 2018 at 07:28 AM.
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Old 11th October 2018, 08:18 AM   #10
TMM is offline TMM  Australia
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What is odd to me is that you have a certain feature at ~480Hz on the 2nd order harmonic which suggests a tone/breakup at 960Hz or so, which is quite plausible for a 15" driver.
However, the same feature appears on the 3rd harmonic with the wrong multiple. In your plot, the features at 480Hz on the 2nd harmonic appears the same as a feature on the 3rd harmonic at 160Hz. Usually when you see a distinct feature like that in a HD plot it directly corresponds to the frequency response or ambient noise which is fixed in frequency for the entire sweep i.e. for 3rd harmonic, you would expect 960Hz/3 = 320Hz. Instead, we are at half this frequency for some reason. To me this suggests that the problem is neither the driver nor background noise. The 1% distortion on the 2nd and 3rd harmonic is definitely not white noise like computer fans - they tend to produce white noise which will bring up the noise floor of you measurement i.e. where all you higher harmonics are bunched up around 0.01-0.02%, they will come up a bit with increasing fan noise.

I would double check your test setup, in particular check that your sound card and/or software is not performing re-sampling or has some DSP feature enabled.

At 1% THD, the sweep would not sound clean to your ears either. Instead of being a clean sine wave it should start to sound a bit like a square wave due to the harmonics.

Last edited by TMM; 11th October 2018 at 08:26 AM.
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