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REW Graph interpretation


ACV92

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So, I know there's a ton of info on the REW software and how to use.  I'm just beginning and just want an explanation as to why I should use psychoacoustic smoothing, I believe @Chris A recommended doing so, versus not using it.  Below are two graphs of my mains as measured from my single point listening position.  When I use no smoothing I want to throw everything away.  When I apply PA smoothing it makes everything look great.  To use, or not to use, that is the question.

 

5997aafe21bd8_Mains-nosmoothing.jpg.c77d3edf35c2db65791707eefde205c6.jpg

 

 

5997ab051b328_Mains-psychoacousticsmoothing.jpg.6b12ed396c0a9a6ea7db21d5dd5480f2.jpg

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IMO, to make sense of this kind of graph, the choice is NOT between no smoothing and psychoacoustic smoothing.  Instead, the choice is between a reasonable amount of conventional smoothing (like 1/6 octave smoothing, or, perhaps even 1/3 octave smoothing) and psycoacoustic smoothing.  You'll find that 1/6 or 1/3 octave smoothing looks a lot better than no smoothing at all.  Having seen years of speaker curves in the old High Fidelity magazine, I have the impression their curves used about 1/3 octave smoothing.  Even the listening room curves in Stereophile are often 1/6 octave smoothed, and the the room response used is the average of up to 20 mic positions, and averages show fewer zigs and zags than the typical single mic position curve.  I'm no expert on psychoacoustic smoothing, but I gather the goal is to adjust the curve to resemble what the human ears and brain would hear at that mic position in that room, with that sound system.   

 

 

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You are plotting only frequencies below 232 Hz.  That frequency band is dominated by room modes, i.e., the "sparse mode region" of the room. 

 

I could tell you that there is something happening at 4.2 feet and at 2.9 feet (one of them probably the distance of the microphone off the floor and/or away from the nearest wall).  So one or both of those nulls are probably determined by where you took the measurement with the microphone, which is probably not a real frequency response issue.   The other cause is simply due to the room dimensions itself--i.e., room modes.  In order to reduce the number of nulls further, more subwoofers and positions of low-frequency-capable surround channel loudspeakers will be required to "fill up the room modes" with acoustic energy at the points in the room where they can inject that energy. 

 

Otherwise, you can't boost the nulls in your top FR plot--you can only push the phase around so that it appears as if you've fixed an issue at that point.  But if you move the microphone slightly and take another measurement, that happy situation will disappear on the FR plot.

 

The smoothing issue is like Gary said: some form of smoothing is necessary to match how the human hearing system performs.  This is from the REW help facility:

 

Quote

Variable smoothing applies 1/48 octave below 100 Hz, 1/3 octave above 10 kHz and varies between 1/48 and 1/3 octave from 100 Hz to 10 kHz, reaching 1/6 octave at 1 kHz. Variable smoothing is recommended for responses that are to be equalised.

 

Psychoacoustic smoothing uses 1/3 octave below 100Hz, 1/6 octave above 1 kHz and varies from 1/3 octave to 1/6 octave between 100 Hz and 1 kHz. It also applies more weighting to peaks by using a cubic mean (cube root of the average of the cubed values) to produce a plot that more closely corresponds to the perceived frequency response.

 

ERB smoothing uses a variable smoothing bandwidth that corresponds to the ear's Equivalent Rectangular Bandwidth, which is (107.77f + 24.673) Hz, where f is in kHz. At low frequencies this gives heavy smoothing, about 1 octave at 50Hz, 1/2 octave at 100 Hz, 1/3 octave at 200 Hz then levelling out to approximately 1/6 octave above 1 kHz.

 

Looks like you can use either 1/48 octave or Variable smoothing when looking below 100 Hz--they produce the same smoothing in that frequency band.

 

For frequencies above 200 Hz, I recommend psychoacoustic smoothing within REW's EQ facility for setting up PEQs with active crossovers.  Otherwise, if using no smoothing or something rougher than 1/6 octave smoothing, you'll be chasing your tail trying to EQ irregular frequency response that the human hearing system can't detect. 

 

Chris

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Thanks for the info all.  This is only a 2 channel setup and the measurements were taken using only the mains, not the sub.  I wanted to see, or correct, any issues with the mains as far as placement, etc. and then bring in the sub and do so more testing.  Is it best to run all channels when testing, or should you include a series of independent channel testing as well?  I'll figure out a way to attach the UMIK to my camera tripod for better mic placement as well.

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1 hour ago, ACV92 said:

Is it best to run all channels when testing, or should you include a series of independent channel testing as well?

I'd strongly recommend using one loudspeaker at a time in FR measurements.  Then you can check for polarity issues in the crossover regions using a sub and a front loudspeaker at the same time.

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17 minutes ago, Chris A said:

I'd strongly recommend using one loudspeaker at a time in FR measurements.  Then you can check for polarity issues in the crossover regions using a sub and a front loudspeaker at the same time.

Thanks Chris.  Appreciated as always.

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Chris A, garyrc, canyonman I found a couple of things out with about 4 hours of testing.  My listening position is right in a couple of nulls, just under 70db and at 100db.  Ultimately, I need some bass traps and the second Umax 15 sub.  But, the biggest discovery I made was that my crossover on the Inuke3000DSP was set incorrectly.  I had tried some different slopes briefly before but had never really 'tested' them.  I was running BW 24 on the low pass as it was a suggestion that someone made.

 

With the mic I tried all of the different available types and slopes, LR, Bessel, but found that BW 12 was the homerun hitter.  What a difference.  The low end is fuller and blends much better with the old 5.5's.  I did experiment with some delay but never saw any benefit via the graphs.  It seemed to create more issues when I tried to add any.  Anyway, I've still got work to do but at least I'm heading in the right direction.  Thanks again. 

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14 hours ago, ACV92 said:

I found a couple of things out with about 4 hours of testing.

It's amazing how fast time goes when testing and tweaking.   It reminds me of my first programming efforts...where 12 hours could go by seemingly in a blink of an eye.

 

But it's rewarding when you get the system tuned up and sounding good.

 

Chris

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