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Author Topic: Smaart RTA 1dB/Octave HF Roll Off?  (Read 1282 times)

Frank Koenig

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Re: Smaart RTA 1dB/Octave HF Roll Off?
« Reply #10 on: October 16, 2017, 03:46:18 pm »

In the case of MTW, we have 1Hz resolution up to 140 Hz, and 1/48th octave resolution from 60Hz up - which is double the frequency resolution of v6.

2 Hz resolution requires a .5 s measurement window which will include reflections off of boundaries within ~ 280 ft. of the measurement location (565 ft out and back at 1130 ft/s). For practical purposes this is an unwindowed measurement. Reflecting objects that subtend a small solid angle (small and far away), however, will have relatively little effect as the sound power they contribute falls off as the fourth power of their distance (inverse square law twice, like radar). This still does not explain the difference between the RTA (constant fractional-bandwidth power spectrum) and the smoothed magnitude.

I think the first step is to determine if this is a strictly acoustic phenomenon, or some system or measurement issue. As might already have been suggested (and you likely already did), take all processing out of the system and measure the RTA at the speaker terminals or the input of the known-flat power amplifier. Also perform a "loop-back" on the spectrum analyzer with the measurement and reference inputs tied together. If that checks out, insert the electroacoustic part and see what you get. If it turns out to be acoustic, the next thing I'd try is to alter the physical arrangement, such as the measurement distance and angle. Angle SHOULD have relatively little effect as the source is nearly omnidirectional, but we're probing assumptions here.

Varying the type of averaging and smoothing makes sense, too. Ideally, the smoothing of the magnitude should match, to the extent possible, the bandwidth of the RTA filters.

Next time I'm out in a field with a woofer and my measurement rig I'll try to reproduce this. It may be a while. With interest.

--Frank
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Art Welter

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Re: Smaart RTA 1dB/Octave HF Roll Off?
« Reply #11 on: October 16, 2017, 05:33:17 pm »

I think the first step is to determine if this is a strictly acoustic phenomenon, or some system or measurement issue. As might already have been suggested (and you likely already did), take all processing out of the system and measure the RTA at the speaker terminals or the input of the known-flat power amplifier. Also perform a "loop-back" on the spectrum analyzer with the measurement and reference inputs tied together. If that checks out, insert the electroacoustic part and see what you get.
Ding Ding Ding-

We have a winner, Frank K!

The Smaart RTA function is flat when set to any of the portions of "octave".
The RTA function when set to "Log" or "Lin", rolls off the top end, on "Log" about 30 dB from 20Hz to 20kHz, or 4 dB from 30 Hz to 100 Hz as measured...
This does not occur on the Magnitude response screen, flat response looks flat  regardless of whatever you choose, although "Lin" is useless for anything below 200 Hz.

So the interesting by-line is when looking at harmonic distortion of sine waves on the RTA, the harmonics will appear to have less level (inaccurate) when looking at Log function compared to the octave functions. A 30 Hz fundamental with the third harmonic at -20 would have 10% distortion, but using the Log function would read about -24, only 6.31% distortion.

Smaart has made me a (sometimes) inadvertent "liar" when reporting speaker distortion ;^).
Well, at least I learned something new about a 10 year old software ::).

I'll be using REW for my future distortion testing, but it won't run on my laptop at all...

Cheers,
Art
« Last Edit: October 16, 2017, 05:42:43 pm by Art Welter »
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Frank Koenig

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Re: Smaart RTA 1dB/Octave HF Roll Off?
« Reply #12 on: October 16, 2017, 06:17:33 pm »

The RTA function when set to "Log" or "Lin", rolls off the top end, on "Log" about 30 dB from 20Hz to 20kHz, or 4 dB from 30 Hz to 100 Hz as measured...
This does not occur on the Magnitude response screen, flat response looks flat  regardless of whatever you choose, although "Lin" is useless for anything below 200 Hz.

Interesting. I don't get it. The RTA's filters should have constant fractional bandwidth (1/3 octave, etc.) and should measure flat on pink noise irrespective of log or linear scaling on either the amplitude or frequeny axes. On white noise they should show a rise with frequency of 3 dB/octave (or sqrt(2)/octave with a linear amplitude scale) as the amount of energy each filter swallows is proportional to its width in Hz, which is proportional to its center frequency. Or is Smaart just Stuupid in this instance?

--Frank
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Art Welter

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Re: Smaart RTA 1dB/Octave HF Roll Off?
« Reply #13 on: October 16, 2017, 07:21:20 pm »

Interesting. I don't get it. The RTA's filters should have constant fractional bandwidth (1/3 octave, etc.) and should measure flat on pink noise irrespective of log or linear scaling on either the amplitude or frequeny axes. On white noise they should show a rise with frequency of 3 dB/octave (or sqrt(2)/octave with a linear amplitude scale) as the amount of energy each filter swallows is proportional to its width in Hz, which is proportional to its center frequency. Or is Smaart just Stuupid in this instance?

--Frank
It appears Smaart V6 running on OSX 10.6.8 is Stuupid in this instance. Whether this is the way it originally was written, or a OS incompatibility problem, I can't tell, not worth loading an older OS that won't work with today's interwebs back on the old MacBook.

Anybody with different versions of Smaart should be able to quickly tell if this glitch is history, or still happening.
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Stelios Mac

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Re: Smaart RTA 1dB/Octave HF Roll Off?
« Reply #14 on: October 17, 2017, 01:37:44 am »

but it won't run on my laptop at all...
Offtipic but, Smaart is really lightweight software (so is REW), they should be able to run flawlessly on any macbook, no matter it's OS.
Was the last OSX update you did a fresh install or an upgrade? If the latter, you really should do a fresh install, it'll perform like a brand new device. It just takes some time to backup & copy all of your data, plus reinstall all software. IMHO It's worth it though.
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Art Welter

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Re: Smaart RTA 1dB/Octave HF Roll Off?
« Reply #15 on: October 17, 2017, 11:33:59 am »

Offtipic but, Smaart is really lightweight software (so is REW), they should be able to run flawlessly on any macbook, no matter it's OS.
Was the last OSX update you did a fresh install or an upgrade? If the latter, you really should do a fresh install, it'll perform like a brand new device. It just takes some time to backup & copy all of your data, plus reinstall all software. IMHO It's worth it though.
The last "upgrade" was a fresh install on a replacement of the MacBook the Ex smashed. I purchased a refurbished unit specifically so I could run Smaart V6, and it does not run "flawlessly", although it did with the OS that was current when I bought it 10 years ago.
REW won't run on the OSX 10.6.8 in the MacBook, the last OS version possible for it's hardware .

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Mark Wilkinson

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Re: Smaart RTA 1dB/Octave HF Roll Off?
« Reply #16 on: October 17, 2017, 12:28:25 pm »

The last "upgrade" was a fresh install on a replacement of the MacBook the Ex smashed. I purchased a refurbished unit specifically so I could run Smaart V6, and it does not run "flawlessly", although it did with the OS that was current when I bought it 10 years ago.
REW won't run on the OSX 10.6.8 in the MacBook, the last OS version possible for it's hardware .

REW at least, has prior versions going way back http://www.roomeqwizard.com/changehistory.html

If you're still interested in getting to the bottom of what you saw in Smaart, I'd get whatever version of REW running,
 and start from ground up with a soundcard calibration.

The more I think about your original post, the clearer it seems that the reference pink is rolling off.
I kinda dismiss acoustic rolloff because the rolloff is too smooth, and besides, if it were acoustic I'd think transfer would show it as well.
I'm also a little doubtful the rolloff is due to measuring ...windowing or averaging etc,..... but I sure as hell defer to Chris or Frank here...

I left thinking something is rolling off the pink in your setup.....whether it's the soundcard, your adapters....anything that's in line with what you're using as your reference feed.
The REW swept-sine  soundcard calibration process could pinpoint the source of rolloff precisely, as you could calibrate the soundcard alone, then add pieces of the reference chain one by one.
Soundcard cal is a cool feature I wished Smaart had....
And hey, if there's no rolloff with REW all the way through the reference feed, you know Smaart's pink has stink !

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Art Welter

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Re: Smaart RTA 1dB/Octave HF Roll Off?
« Reply #17 on: October 17, 2017, 04:23:12 pm »

REW at least, has prior versions going way back http://www.roomeqwizard.com/changehistory.html

If you're still interested in getting to the bottom of what you saw in Smaart, I'd get whatever version of REW running,
 and start from ground up with a soundcard calibration.
Mark,

Guess you missed that my laptop is OS6.8, REW starts at OS7, and the problem in the OP was sorted, it's a Smaart RTA glitch, see post #11.

Cheers,
Art
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brian maddox

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Re: Smaart RTA 1dB/Octave HF Roll Off?
« Reply #18 on: October 17, 2017, 10:04:41 pm »

Ding Ding Ding-

We have a winner, Frank K!

The Smaart RTA function is flat when set to any of the portions of "octave".
The RTA function when set to "Log" or "Lin", rolls off the top end, on "Log" about 30 dB from 20Hz to 20kHz, or 4 dB from 30 Hz to 100 Hz as measured...
This does not occur on the Magnitude response screen, flat response looks flat  regardless of whatever you choose, although "Lin" is useless for anything below 200 Hz.

So the interesting by-line is when looking at harmonic distortion of sine waves on the RTA, the harmonics will appear to have less level (inaccurate) when looking at Log function compared to the octave functions. A 30 Hz fundamental with the third harmonic at -20 would have 10% distortion, but using the Log function would read about -24, only 6.31% distortion.

Smaart has made me a (sometimes) inadvertent "liar" when reporting speaker distortion ;^).
Well, at least I learned something new about a 10 year old software ::).

I'll be using REW for my future distortion testing, but it won't run on my laptop at all...

Cheers,
Art

I guess this goes back to that First Rule Of Troubleshooting. 

First, test the test device.

If i had a nickel for the number of times THAT one has bit me...
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brian maddox
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'...do not trifle with affairs of dragons...

       ....for you are crunchy, and taste good with ketchup...'

Art Welter

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Re: Smaart RTA 1dB/Octave HF Roll Off?
« Reply #19 on: October 18, 2017, 02:36:53 am »

If i had a nickel for the number of times THAT one has bit me...
We would both be so effing rich the queen would do whatever she wanted.
Test results, +/-positive...;^)
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