When H4 and H5 are the same line - that's a sign you are measuring noise, not THD. Even H3 joins at higher frequencies.
Looks realistic, even the H5 peak is there. I would not mind anything below 0dBSpl. Maybe some smoothing to get more precise readings.
Thank you for your interpretation @IamJF
My preliminary tests suggested that in an ordinary indoor environment when the noise floor is ~30dB, the stepped sine is the most accurate view of distortion.
For quick and dirty view of a distortion, a repeated log sweep can be used as a proxy.
Measurements involving only a single measurement of 1M samples, the distortion measurements, loses a LOT of accuracy.
When I was reading the REW manual, I had incorrectly inferred that only electronic equipment eg. DACs, amplifiers, would benefit from stepped sine or repeated log sine sweep.
More to come...
My preliminary tests suggested that in an ordinary indoor environment when the noise floor is ~30dB, the stepped sine is the most accurate view of distortion.
For quick and dirty view of a distortion, a repeated log sweep can be used as a proxy.
Measurements involving only a single measurement of 1M samples, the distortion measurements, loses a LOT of accuracy.
When I was reading the REW manual, I had incorrectly inferred that only electronic equipment eg. DACs, amplifiers, would benefit from stepped sine or repeated log sine sweep.
More to come...
I am constantly amazed how much time and effort John puts into the REW help file (along with the program of course). Copied from the help file:
The plots are derived either from analysis of the impulse response or from stepped sine measurements. Impulse responses measured using logarithmic sweeps separate distortion from the linear part of the system response. The distortion components appear at negative times, behind the main impulse. Analysing the frequency content of these components allows plots of distortion harmonics to be generated. The longer the sweep, the better the distortion components are separated from each other. Sweeps of 256k or shorter should start at 0 Hz to prevent an artificial rise in distortion at the lowest frequencies due to the reduced low frequency bandwidth of the resulting impulse response. When measuring a system with high distortion levels use a long sweep setting (e.g. 1M or higher), at shorter sweep lengths the harmonics may affect each other giving misleading results. A spot check can be made at frequencies of interest using the RTA and the signal generator. If discrepancies are observed consider making a stepped sine measurement instead. The noise floor of log sweep distortion measurements rises with frequency. For the lowest noise floor with sweep measurements use multiple sweeps, but note that requires the input and output to be on the same device for reliable results.
Although much, much slower than a log sweep the stepped sine measurement can measure low distortion levels much more accurately than a sweep, particularly at high frequencies and for higher harmonics. Stepped sine distortion measurements show distortion components up to the ninth harmonic, THD and the noise floor, in the same way as the sweep-derived results, but also include THD+N (total harmonic distortion plus noise and non-harmonic distortion) and N (noise and non-harmonic distortion) alone. Note that the noise floor plot shows the spectral content of the noise measured with no signal playing. The 'Noise' in the N and THD+N shows the summed level of all non-harmonic distortions and noise across the frequency span for each test frequency. It consequently sits much higher than the noise floor plot. For stepped level measurements the X axis can be dB SPL, dBFS, dBu, dBV, dBW, V or W showing either the generator or input level.
The plots are derived either from analysis of the impulse response or from stepped sine measurements. Impulse responses measured using logarithmic sweeps separate distortion from the linear part of the system response. The distortion components appear at negative times, behind the main impulse. Analysing the frequency content of these components allows plots of distortion harmonics to be generated. The longer the sweep, the better the distortion components are separated from each other. Sweeps of 256k or shorter should start at 0 Hz to prevent an artificial rise in distortion at the lowest frequencies due to the reduced low frequency bandwidth of the resulting impulse response. When measuring a system with high distortion levels use a long sweep setting (e.g. 1M or higher), at shorter sweep lengths the harmonics may affect each other giving misleading results. A spot check can be made at frequencies of interest using the RTA and the signal generator. If discrepancies are observed consider making a stepped sine measurement instead. The noise floor of log sweep distortion measurements rises with frequency. For the lowest noise floor with sweep measurements use multiple sweeps, but note that requires the input and output to be on the same device for reliable results.
Although much, much slower than a log sweep the stepped sine measurement can measure low distortion levels much more accurately than a sweep, particularly at high frequencies and for higher harmonics. Stepped sine distortion measurements show distortion components up to the ninth harmonic, THD and the noise floor, in the same way as the sweep-derived results, but also include THD+N (total harmonic distortion plus noise and non-harmonic distortion) and N (noise and non-harmonic distortion) alone. Note that the noise floor plot shows the spectral content of the noise measured with no signal playing. The 'Noise' in the N and THD+N shows the summed level of all non-harmonic distortions and noise across the frequency span for each test frequency. It consequently sits much higher than the noise floor plot. For stepped level measurements the X axis can be dB SPL, dBFS, dBu, dBV, dBW, V or W showing either the generator or input level.
@ernperkins
I wrote about that in a previous post
I hadn't heard anyone, except @Hörnli, say that the a single log sine sweep was misleading for driver distortion measurements. Just don't do it, he said.
My guess that we all did it because we were impatient. A full range stepped sine sweep with 24 points per octave takes HOURS, not seconds!
.
I wrote about that in a previous post
I hadn't heard anyone, except @Hörnli, say that the a single log sine sweep was misleading for driver distortion measurements. Just don't do it, he said.
My guess that we all did it because we were impatient. A full range stepped sine sweep with 24 points per octave takes HOURS, not seconds!
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Actually, it takes about 4 minutes... it just seems like hours... 24 steps x 10 octaves x 1 sec/step = 240 seconds
It isn't misleading - it just has higher noise floor! As long as your distortion is high enough it's measured properly - like H2 in your measurements.
As most people seem to not be interested in low level harmonic distortion (<0,1%) a long enough single sweep can do fine.
And did I ever mentioned get a good mic and silent room?
So far no one has recognized the one major limitation in @tktran303 's measurement endeavor, in log sine sweep and stepped sine alike.
Just the usual waffling about. Peppered with audacious bragging.
DIY is done. I was there as it happened.
Just the usual waffling about. Peppered with audacious bragging.
DIY is done. I was there as it happened.
The major limitations:
1. To accurately capture distortion, you must be able to accurately capture SPL. On an (quasi-IB or IEC baffle) 31.6cm is far enough, but in an enclosure, that is not. The microphone needs to be in the far-field) which is ~twice the longest dimension of the enclosure.
1a. Microphone model not shown (microphones can, and DO affect distortion measurements)
1b. Background noise not shown (background noise can, and DOES affect distortion measurements)
Your silence helped me meditate on this on, and it came to me. Thank you @Hörnli
1. To accurately capture distortion, you must be able to accurately capture SPL. On an (quasi-IB or IEC baffle) 31.6cm is far enough, but in an enclosure, that is not. The microphone needs to be in the far-field) which is ~twice the longest dimension of the enclosure.
1a. Microphone model not shown (microphones can, and DO affect distortion measurements)
1b. Background noise not shown (background noise can, and DOES affect distortion measurements)
Your silence helped me meditate on this on, and it came to me. Thank you @Hörnli
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