Testing acoustics - waste of time?

My modest cellar bunker is little by little getting ready for use, and it seems it's not at all as bad acoustically as my worst fears were.
Now, is it a waste of time to run (was it white or pink?) noise through the system, and record it with a Zoom H2 in various spots in the room, and then study the curves with Span or similar? It would, I assume, point the worst spots of the room, maybe the best as well, but with what kind of reliability/accuracy?
Buying a dedicated measuring mic/software is not an option, but I could use my AKG 3000 or Rode M3  recording the noise with a laptop, if you think that would be better. 
 
As I mentioned, the room sounds surprisingly decent in the most important spots, but I'd like get some confirmation to what I hear.

 
You might want to play around with Room EQ Wizard.  I'm sure you can get useful results for a mic you already have.  The software is free and will tell you just about everything about your room.
 
Hang out a bit at https://www.gearslutz.com/board/studio-building-acoustics/ to learn what's useful and affordable, and what's over the top.  One of the stickies describes how to measure a room.
 
The quality of what you hear varies greatly depending on where place your monitors and your ears, as you seem to have noticed.  The first stage rule of thumb is to face along the length of the room, in the center width-wise, 3/8 of the way back from the front/end wall.  Monitors in an equilateral triangle as far from the side walls as you can get them.  You want symmetry, so it wouldn't be good to have a huge opening in your left wall and not on your right.
 
So what are the dimensions and characteristics of your room ?  Surface materials, etc.
 
 

There's actually not much point in describing the room, as there are no alternatives to the positioning of things. I'd just want to find out how good or bad it is the way it is now. If it's bad, I'll just have to live with it. But anyway:
 
The room in total is 7m x 5m but it's split at 3,5 m to separate the other half for storage room. The closet-wall splitting the rooom is filled with clothes in hangers. So the music-hobby half is about 3,5m x 5m, work spot is in the middle of the short wall. I can roll the table and monitor speakers about 1m further from the wall when I start, say, mixing, which puts me sitting approximately in the middle of the room. It's partly of light gravel blocks, partly concrete, only 2,05m high.
I've added absorbing panels (2 cm thick) on the ceiling and covered the walls with the same panels. I don't know what that material is called in english, could be "wood fibre panel". Magazines from the 30's-40's, paperbacks from the fifties and comic books from the sixties cover the ceiling corners around the room, all walls have unsymmetrical shelves with books, CD's etc. and two sofas are in the back.
 
As mentioned, it sounds surprisingly decent, but I'm interested in mapping the less obvious weaknesses.

Room EQ Wizard is the place to start - not ARC.
 

It sounds to me like you have a pretty good space with absorbing materials scattered throughout. If you want the budget way around it, sit in the spot you most likely will be monitoring from, run some sample sine waves through your monitors at various frequencies and listen. Then slowly move around the room listening for dropouts in volume and loud spots. These are your problem areas from standing waves and reflectins that are in phase with each other. You can mark these areas off with tape, so you know where they are. As long as you are aware of them, and avoid monitoring from them, you should be all right. I know for me, just knowing where they are helps me avoid them. One of my dropouts around 200hz occurs right in front of my computer keyboard. After I tweak, I back up about a foot and sit tall to monitor, so I know the room isn't affecting what I hear.

Don't forget you can also listen to beautifully recorded reference CD's in your environment and get used to how they sound.  In a way you can eliminate the room from the picture all together.

Kalle Rantaaho
There's actually not much point in describing the room, as there are no alternatives to the positioning of things. I'd just want to find out how good or bad it is the way it is now. If it's bad, I'll just have to live with it.

 
Maybe it's best not to know .
 
I wouldn't let anything discourage you from making music, just be aware that if you create a piece that sounds great in your room, it may not sound good anywhere else.  Concrete is a great reflector and your ceiling is pretty low.
 

The room in total is 7m x 5m but it's split at 3,5 m to separate the other half for storage room.

 
The following is just an example of the fun you can have.  I don't know how accurate your measurements are, but I entered them into Ethan Winer' Mode Calc app and here's what came out :
 

 

 
These are (some of) the frequencies that may give you issues as they are ones at which your dimensions will cause standing waves.  Ideally, they're as spread out as possible.  In your case, for example, around 250hz you have three modes close together, so you can expect oddities around there.
 
You can sweep a sine wave at different frequencies while moving around the room to give you some idea of what's going on (that's more or less what Room EQ Wizard does). 
 

The closet-wall splitting the rooom is filled with clothes in hangers.

 
Can you open it towards you ?  Full width of the room ?  That can certainly help.
 

 
As mentioned, it sounds surprisingly decent, but I'm interested in mapping the less obvious weaknesses.

The only thing that would be practical to do is put some absorption at the primary reflection points on the side walls (and ceiling but you don't really have room for that).  Have someone hold a mirror on the side wall and when you can see your monitor in the mirror, that's where to put the absoption.  Absorption panels on legs or wheels (gobos) are versatile.
 
What kind of music are you playing and at what volume ?  It's good that your room isn't square or worse cubic.  It would be interesting to see some graphs from REQW just for fun if you're inclined to spend some time learning how to use it, then measuring at different locations.  Probably not worth the trouble unless you're really interested.
 

I've used REW5 extensively.  Click on Generator, set that to 40, walk around your room and you can hear where all the bass frequencies are piling up.  The worst place is probably where you'd want to construct and place a bass trap or 2.
 
To shoot your room you need a calibrated mic, like the SPL meter from Radio Shack.  I managed to get my listening position flat to +/- 5db, which I am told is "good".  To improve that would require doing more stuff, like hanging a 4ft by 6ft panel stuffed with 2" of Roxul SafeNSound from the ceiling over my listening position ... which I am not doing, because I stupidly used REW5 to give myself a hearing test ... and failed.   

I JUST FINISHED...
relocating all of my studio equipment....
had a minor flood, had to move stuff anyway,
now that THAT is taken care of, took the opportunity to make a new layout of my studio space.
moved my mixing area, centered on a wall,
whereas before it was in a corner..
this made a BIG difference.
 
moved my treatments around,
mostly by ear and line of sight, common sense stuff..
with my newer small nearfields (JBL 305's), i went with a slightly wider triangle (48" center to center)
and in my space, this seems perfect.
nice wide sweet spot.
 
 
i used the IK Multimedia ARC 2 software,
at about 80db,
i dialed in 14 test points and the curve was not nearly as intense as it was previously,
obviously moving my gear around and really getting things centered,
made a big improvement.
 
very close to flat with the ARC2 plugin,
i checked my older mixes that i had been happy with,
and they ALL needed minor tweaks,
in both the low end and high end.
 
after a couple of hours of comparing level-balanced pro mixes with my best self mixes,
i'm happy with the outcome.

this move, gave me a chance to setup a new guitar recording layout as well....
i've decided, that my old Roland 1x12 cab, that i have opened up in the back, and that i have placed my best driver in (Celestion Heritage G12H-55), sounds best facing the wall and miced from behind....
a really rich sound, i'll be experimenting a lot with this shortly.....
my newer AVATAR closed back 1x12, with an older rare driver in it (late 70's Celestion G12C-30) sounds best miced from the front.

so those two, plus my Demeter Silent Speaker Isolation cabinet, comprises my current guitar setup.
i have a 2nd Roland closed back cab, with a Carvin British series driver in it, that is not being used currently.
 
 
basically,
i have cheap stuff,
but make the most with it.
 

Ok. I did a quick starter test.
I downloaded a high quality white noise wav from here
 
http://www.audiocheck.net/testtones_whitenoise.php
 
I imported it in a SONAR project. 
Then I played the noise through my monitor speakers and recorded it with AKG c3000b microphone at my listening/mixing spot.
 
Analyzing with Voxengo SPAN the curve has a clear dip at 100-150 Hz and 1000-1500 Hz, the latter of which is already visible in the manufacturers frequency chart, so it depends (mostly) on the microphone. Otherwise the curve is not straight, but forms an "evenly wavy" horizontal line. 
If that is reliable info, I assume the only thing to do is use an EQ to correct that when mixing (and trying to remember to switch it off when exporting :o).
 
I'll do some more elaborate measurings when I understand enough about Room EQ Wizard. I'm a little unsure, though, if it's worth the trouble without a proper measuring microphone.