So I got Sonarworks Systemwide along with its calibrated reference mic and decided to use the mic to play with REW. It allows me to input the Sonarworks Ref Mic calibration file, which is fantastic. First few runs in an untreated room and I see a comb filter with a ~10dB notch about every 1.3kHz. This is easily seen when the frequency response is plotted on a linear scale. According to my reckoning this is due to interference between a direct line-of-site ray and another taking a reflective path that is 5" longer (1/2 wavelength of 1.3kHz). I'd upload a plot that shows this but I can't get image attachments to work here from Chrome or Edge
 
So, where in the world is this extra 5" coming from? First, I'm using Yamaha MSP7s, which have a crossover at 2.4kHz. By far most of the comb filtering occurs above this, so I'm looking at tweeter issues, which are relatively directional. And acoustic foam will work well in isolating/filtering those frequencies. After messing around with different options, it turns out the reflection is coming off my table! When I place acoustic foam on the table the frequency response becomes remarkably flat for the most part, and neglecting bass frequencies (which is another problem). The table is where I have my laptop, monitor, mouse pad/keyboard, where I rest my elbows, etc. This is a problem! I need all those things while I do work. I can't get rid of the table. The nearfield monitors are pointing at my ears. But I can't put acoustic foam all over the place since that'll keep me from using the equipment.
 
How do you all deal with this? All the studio pictures I see have a table between the monitors and the seat spo they should be having the same issues. How are those reflections dealt with? 

fret_man
 
So, where in the world is this extra 5" coming from?

my first question would have been ... how far are your speakers from the wall?
 

fret_man
it turns out the reflection is coming off my table! When I place acoustic foam on the table the frequency response becomes remarkably flat for the most part

that would have been my second guess. and finally somebody raises here what mastering engineers keep saying that reflections off the mix desk cause phase issues.
 
I always found it interesting that people using flat mounted touch screens in front of them never worried / encountered this ... perfect reflective surface
 

fret_man
How do you all deal with this? All the studio pictures I see have a table between the monitors and the seat spo they should be having the same issues. How are those reflections dealt with? 

of course you can't do without the desk (or computer in your case), so one option would be to have a second pair of speakers at a different distance, possibly closer, and maybe also at a different angle. that's what I have been using for a long time now and even though I'm very accustomed to my "mains" I wouldn't want to miss that second pair which reveal different things.
 
 

Nice job figuring out the source of your trouble! That's the hard part.
 
As for what to do about it, first make sure your monitors aren't sitting directly on the table. Raised stands / angling the monitors upward a bit might help. 
 
If you have any room at all on your desk in front of the monitors, something like diffusion also might help. You've probably seen the purpose-built panels that look like randomly sized blocks. Some guys put various objects in front of the monitors so that the surface isn't completely flat.
 
Anyway, I suspect a practical solution can be found in your case. Let us know how things go ... thanks for posting!
 
Cheers,
Eddie

Oh, I should also say that my monitors are on IsoAcoustic stands which sit on Auralex MoPads which, in turn, sit on the table. I don't believe I'm getting any significant mechanical sound coupling thru the table to the monitor.

Well it seems you've got that part covered!
 
Depending on how sensitive your aesthetic sensibilities are, you could try making a small absorptive pad that would stick directly under the tweeter horizontally, preventing the desk reflections. Just a thought. Would look weird, but hey. Sound is what matters, right? Might be worth experimenting.
 
Cheers,
Eddie

And to add to post pictures they need a web page link so open a account with imgur
I use a Pine desk for this reason. Its soft wood not hard so less reflective. And if your desk is real cluttered that acts as a diffusor :)

OK, got image u/l to finally work.

Here I artificially separated the two curves by 40dB to make them easier to see. The top curve is due to the sound reflecting off the table surface. The bottom curve is after I covered most of the table with acoustic foam (ignore the problem at 14.5kHz for now - that's another problem I need to track down). You can clearly see the comb filtering affect every 1.3kHz on the top trace and how removing the table top reflection solved that problem on the bottom trace. 
 
This seems like such a fundamental problem in all studios. In every picture I've seen if you draw a line between the engineering seat and the monitor there's stiff horizontal surface there. There's gonna be reflections! And they muck with the frequency as shown above in blue. Why don't I hear anyone talking about this? Isn't this a universal issue?