Help with understanding the measurements (Distortion)

[djigibao]

I measured the frequency response (REW) of the speakers with three different amplifiers, and then looked at the measurements in the "Distortion" tab. There I noticed different readings. I would kindly ask if you could explain what these measurements mean? Can the quality of the amplifier, i.e. the signal, be determined from this?

Here are the pictures:

Amp N

Amp S

Amp L

 

[DVDdoug]

It seems to show a difference and it's more difference than I'd expect. And I think your levels are not matched which could be skewing everything.

And it looks like you're not calibrated because there's no way you've got noise around 0dB SPL. And assuming the noise is below audibility you're probably not hearing the distortion either. Except perhaps those bumps in the low frequencies which are likely the speakers. (Speaker distortion is worse than amplifier distortion unless the amplifier is clipping.)

BTW - I've never heard distortion from anything that wasn't broken or overdriven. It's not a spec I worry about, except you can often easily over-drive a "small" woofer into audible distortion and of course you can over-drive an amplifier.

 

[Keith_W]

Funnily enough, I just made a post explaining distortion in speakers here. You might find that thread interesting reading. In particular, pay attention to what I said about what volume you take your measurement at. Distortion is SPL dependent. Measure too loud, and you get more distortion. Measure too soft, you get less.

If you are comparing distortion measurements from an amplifier, a lot of people will say that it is better to take an electrical measurement from the amplifier's output. But this misses the interaction between the amplifier and speaker impedance. What you measured is the cumulative effect of amplifier distortion AND speaker distortion.

Regardless, if you want to compare the distortion between the two curves, do this:

- First, make sure that all the measurements were taken at the same volume. If it's two different amplifiers, the output needs to be adjusted with an SPL meter so that the measurements are at exactly the same SPL!
- Click on the "Overlays" button, and then the "Distortion" tab.
- Click on the "Graph Controls" (gear icon on top right) and choose THD from the drop down box.
- Hover your mouse over the top left part of the graph and change the display to %.
- Select ONLY the graphs that you want to compare.

 

[djigibao]

First 2 amp (picture) is a volume match (@75dB) and they were measured at the same time, i.e., within 15 minutes the 3 amp is measure before this two (1 week before).
Amp N is class D, amp S is class A/B and amp L is tube amp

 

[Keith_W]

If you want to compare, you have to overlay your graphs the way I described. Otherwise, you would be trying to read the data from each point and comparing. And you are displaying 3rd harmonic, THD, and noise floor. What you need to see is the fundamental and THD. So what you posted can not be interpreted, at least not without a heck of a lot of effort which I doubt anybody is interested in putting in, since it would take you a few minutes to generate the proper graph for comparison!

 

[djigibao]

Like this:

This is with "Use harmonic freq" option ON:

Green is amp N and red is amp S.

 

[Keith_W]

Congratulations! Provided you took your measurement properly, you have shown that the combination of the green amp + loudspeaker is slightly worse than red. If you look carefully, the two curves deviate the most between 100Hz to about 800Hz. And also > 5kHz.

Taken from Amir's KEF LS50 review. Keep a close eye at the impedance curve/phase angle between 100Hz to 800Hz. Read what Amir said: "Lowest impedance above 20 Hz is at 216 Hz. Phase angle is high at the same time so that is a difficult load although music energy is also rather low there."

Well, it looks as if you have shown that one amplifier deals with that impedance curve slightly better than the other.

 

[djigibao]

Here is AMP S vs AMP L (tube)

And here is AMP N vs AMP L

And here are all amps

 

[djigibao]

I made some new measurements but with 1kHz tone and with another amp/speakers

Can anyone explain what we seen here?

 

[Keith_W]

Can anyone explain what we seen here?

Not much to explain. All the info you need is in the panel on the top left.

 

[Thomas_A]

I made some new measurements but with 1kHz tone and with another amp/speakers

View attachment 468991

Can anyone explain what we seen here?

At what SPL and distance to speaker was the measurement taken?

 

[djigibao]

At what SPL and distance to speaker was the measurement taken?

SPL 75dB and about 20cm from speakers.
This was just for test to see if i can't do it right.
I put 1khz tone and RTA record.
I don't know is this is the right way to do it...
KEF LS50 + Nobsound NS-01

 

[Thomas_A]

So 75 dB at 20 cm would be ≈61 dB at 1 meter. Usually distortion is referenced to 85, 90 and 95 dB @ 1 meter.

 

[voodooless]

SPL 75dB and about 20cm from speakers.

About? At such sort distances, the inverse square law acts fast. If it’s not exactly the same distance, and exactly the same level, the measurements it tainted. Level matching by SPL meter or mic is also not good enough! Measure the voltage at the terminals to match volume.

Also, the tube amp was measured a week earlier? That is interesting, because it’s the only one not showing the resonances at 6 and 8 kHz.. could be coincidence, but I highly doubt that this is an effect of the amp. Maybe something in the room, or did you measure the other speaker?