I am experimenting with checking frequency and amplitude values in my mixes, trying to confirm what I learned in a video tutorial.  I use both Sonar and Ableton Live 9.7 for my music.  Though the DAW track meters are similar, the include Spectrum in Ableton Live, the QuadCurve EQ in Sonar, and Voxengo Span all behave (and display) differently.  Mainly, it's all in the slope, I believe.
 
Since Voxengo Span is the only one having an adjustable slope, what is the correct value for it?
 
I've read it all depends on what you're checking.  But all in all, if a pure, 1kHz sine tone at -6dB is inserted into Span, then should the peak reside at -6dB?  After all, Sonar's track volume meter shows a steady value of -6.0dB.  At Span's default slope setting of +4.5, that sine tone peaks (the actual "peak" of the fundamental) at -24.7dB.  Block size is 2,048.  Adjusting the block size increases its amplitude in the meter, but even at 65,536 samples, the amplitude is only -9.6dB peak.  Setting the slope 0, i.e. linear, results in a -9.4dB peak.  Still not -6.0dB!
 
Do spectrum analyzers display the RMS amplitude of the frequency?  With that in mind, Voxengo Span also will show you the RMS value down at the bottom, left corner.  For the aforementioned sine to at -6.0dB, Spans shows -10.6dB for RMS, -12.9dB for the peak of the fundamental, and -6.0dB for the peak (bottom middle Span).

What gives?  Please help.  Thanks!

The correct setting is whatever you can understand.  It is not a reliable peak meter or RMS meter.  It is there to visualize problems in the frequency response.  The peaks are averaged out based on the FFT (block) number.  So a sine wave of 1000Hz will likely contain an average of all the surrounding frequencies until the next block.  A bad answer is just don't worry about it. Use the right tool for the right purpose.  

Spectrum analyzers basically split the frequency range into a number of equally wide bands that tell you how much energy is in each band. 
 
[EDIT: I lost my brain earlier - you don't get the correct reading because of windowing.]

Thank you for the reply, drewfx1.  The reason why I ask is because I am trying to confirm what a Groove3 tutorial has instructed.  I began typing up this huge post before realizing that the data coming out of Voxengo Span is totally different (amplitude wise) than other spectrum analyzers.  Obviously, I want to compare apples to apples.  I would like to "straighten out" the data before starting another thread with the issue.

I agree with Daniel in that accurate level measuring is not its thing. There are other things that are better for this.
 
The main one for me is the slope of the analyser. I like pink noise to be a straight horizontal line. Sometimes (on some analysers) it is not and the slope needs to be set so the pink noise ends up with no slope.

In terms of apples to apples it can be difficult. If every parameter was the same then you should get the same result. Unfortunately there are lots of parameters that are involved (including the sample rate of your project) and not all parameters in a given spectrum analyzer plugin are visible or controllable or even called by their proper technical names. Some plugins also have additional features like the "slope" in Span which I think just progressively boosts higher frequencies in the display - you probably want to set it to 0dB.

Jeff Evans
I agree with Daniel in that accurate level measuring is not its thing. There are other things that are better for this.
 

It should always measure what it measures accurately. Unfortunately you have to know everything that's going on (which they don't necessarily tell you ) and understand exactly what it's telling you. 
 
So you either stick with one tool and get to know how it responds or you learn to use a perhaps less friendly, more technically oriented tool that requires understanding to know exactly what it's telling you.

Thanks Daniel and Drew!
 
Overall, I'm not unfamiliar with the way spectrum analyzers work, and historically I have always used them to check the frequencies in a mix and allow me to visualize what my ears tell me (and my ears don't seem very musical these days).  In other words, I can see the frequencies to which to cut or boost, if necessary.  And that's what I use spectrum analyzers for...checking the frequencies...not necessarily checking the amplitude of said frequencies.  One exception to that though: checking the amplitudes in a mix relative to one another in the same EQ or spectrum analyzer tool.
 
I'll just go ahead and mention my dilemma here instead of another thread.
 
You see, having watched this Groove3 tutorial entitled "Top 10 EDM Don'ts," I find myself dumber for having watched it.  The author, whom I think does a great job in his explanations and training, instructed how to adjust levels of the kick's, and the bass's, fundamental frequency using a spectrum analyzer as opposed to using the standard, track VU meter with a peak/max-level indicator.  He doesn't advocate to not using the track meter to set levels, but it just wasn't mentioned (or shown) in this particular video.  He also advises to listen and check professional songs in the spectrum analyzer to see how they stack up.  Seems like decent advice except for one, big problem:
 
The fundamental frequency of a kick or bass note does not equal the peak amplitude "held" in a track's meter.
 
For example, a bass line sample is playing and looping.  It's track's audio max-peak indicator says -3dB.  It's way too loud for the mix, so the track's volume fader is adjusted so the indicator reads -8dB, let's say.  So in most simplistic cases, that's it; you've set the bass level.  But following his advice, you then check how loud the fundamental frequency of the bass sound is so you can compare it to the kick drum, let's say, and you now find that the amplitude is nowhere even close to what the track level meter says.  So how the hell can you actually "mix" with this advice??!?

For anyone with a Groove3 subscription, I encourage you to watch the 2nd video in the "Top 10 EDM Don'ts" video series to see what I'm talking about.  Also, as a primer to his methodology for "normal" level setting, check out the 8th video in "Top 10 EDM Do's" series.
 
EDIT: To rule out the possibility of one of the aforementioned samples having a transient or a click that would force the VU meter to peak at a louder mark, I verified that the sample did not have such a sound inside of it, zooming into the waveform to verify.

2:43AM
Since Voxengo Span is the only one having an adjustable slope, what is the correct value for it?

FYI, Melda Production's free MAnalyzer also has an adjustable slope.

It is definitely not the only one with adjustable slope.  Feed in pink noise and adjust for a flat line. The slope needs to be set for 3dB on Span for the flat line. I think it defaults to 4.5dB
 
Although from the Fab Filter website mentioned below by timidi there is this information:
 
The Tilt (Slope) setting tilts the measured spectrum around 1 kHz with a specified slope, expressed in dB per octave. The default setting of 4.5 dB/oct results in a natural looking spectrum, resembling best how loudness is perceived by the human ear.
 
Levels wise I must say I just did some tests and it is dead on accurate. I know bitflipper has been praising its level accuracy and I would say he is right for sure. It is showing exact match for peak and rms levels compared to all my other metering options.  What I do like about Span is it has many metering options too which is also nice including K system levels where 0 dB VU on the meter agrees to either K-12, K-14 or K-20 ref level.