new bedroom-mixing-room, null-problem

[khe86]

hi,

here is markus from switzerland. This is my first post here, and i`m looking for some help/tipps.
i have to say my english is not very good, please let me know if i write something that doesent sound right

i moved to a new appartement and have room for mixing and sleeping
the roomdimensions are: 5m x 3,15m x 2,37m (l x w x h)
the room is completly square, no angled walls.
there is a door on the 5m-wall, and a big window-front, approx. 1.8m x 1m (l x h) starting 1m off the floor. my mix-position is facing to this wall. the bed is on the back of the room.

http://dl.dropbox.com/u/5298811/RoomTre ... ayout1.jpg

what i have done so far:
- four massive superchunks from floor to celling in the room-corners.
- a big cloud over the mix-position (1.8m x 1m x .10m, with ca 0.15m air-gap)
- a big corner-absorber on the window-wall between celling and windowwall, 0.20m (two layers of rockwool)
- side-absorbers for first-reflection-points (0.1m + 0.1m airgap)
- my monitors, adam a7 (eq-flat) are on massive, sand-filled monitor-stands, as near on the wall as possible (SBIR) (tweeter-hight: 1.16m)

http://dl.dropbox.com/u/5298811/RoomTre ... _klein.jpg

now i have done some measurements.
and here are the first problems:
there is big null between 65Hz and 85Hz.
the center-frequenzy of this null is moving when i move the microphone (ecm8000), but its not moving when i move the monitors away from the wall.
this tells me that this null is caused by reflections from the back of the room and not SBIR, right?

http://dl.dropbox.com/u/5298811/RoomTre ... e/fg-1.jpg
http://dl.dropbox.com/u/5298811/RoomTre ... e/wf-1.jpg

according the excel-sheet in the sticky-thread here ("are my speakers in a null?") none of the frequencys are shown in my measurements, so i think my speakers /mic are not in a null.

what can i do about this moving null-frequency between 65Hz and 85Hz?
i think i have to do more basstrapping in the back of the room. but what kind of bass-trapping can affect these frecencys efficently?
for example, 20cm rockwool + 10cm airgap isn`t very efficient at these frecuencys, right? (ca. 0.5, porosus absorber calc.)

maybe an idea: there are two room-modes at ca. 60Hz and 100Hz. i`ve read somewhere that room-modes can steal energy from neighboour-frequencys. is this true? so maybe two helmholzresonators tuned to 60 and 100hz may help to fill this dip?

or would a sub be an opinion?

here is a single REW-measurement:
http://dl.dropbox.com/u/5298811/RoomTre ... sung1.mdat

and here a set of measurement with only the right speaker. here i am moving the mic-postion / monitors to check if the nulls are caused by sbir or reflections from the back of the room
http://dl.dropbox.com/u/5298811/RoomTre ... Refl..mdat

any help, critics or opinions are much appreciated!

thanks a lot, and best greeting from switzerland,
markus

[Soundman2020]

Hi Markus, welcome!

Please follow the forum rules, and upload all your pictures to the forum, instead of linking to an external site.

Thanks!



- Stuart -

[khe86]

hi stuart

sorry for that! here i uploaded the pics, but the REW *.mdat-Files are not allowed and a zip-folder would be to big. is it ok if i leave this measurment-link?

thanks a lot!

best greetings,
markus

[Soundman2020]

Hi Markus,

the center-frequenzy of this null is moving when i move the microphone (ecm8000), but its not moving when i move the monitors away from the wall.

If you see different nulls and peaks as you move the mic around, but they don't change when you move the speakers, then you are looking at different modes! In other words, you are not "changing the center frequency of the null", but rather you are seeing nulls for different modes.

The peaks and valleys occur in different parts of the room for each mode, so what is happening is that you are moving the mic to the places where each mode happens to peak of cancel itself out.

In other words, you have modal issues! (But I think you already knew that... )

according the excel-sheet in the sticky-thread here ("are my speakers in a null?") none of the frequencys are shown in my measurements, so i think my speakers /mic are not in a null.

Actually, you have a lot of modes in that area:

54.7 hz (0,1,0 Axial)
64.6 hz (1,1,0 Tangential)
68.9 hz (2,0,0 Axial)
72.7 hz (0,0,1 Axial)
80.4 hz (1,0,1 Tangential)
87.9 hz (2,1,0 Tangential)
90.9 hz (0,1,1 Tangential)

They all involve different walls, so there doesn't seem to be any single constant there: it seems to be a general lack of bass trapping. How did you build your superchunks? What type of insulation did you use?.

One interesting thing: your speakers are located almost exactly at half the room height: not sure if that has anything to do with the issue, but it might.

Another point: I would put some absorption over those windows in front of you: it wouldn't surprise me if you are getting comb filtering and reflections off that.



- Stuart -

[khe86]

hi stuart,

thanks a lot for your answer!

the two super-chunks in the back of the room are made with ISOVER TP1 (62.5cm x 125cm x 10cm) >= 5 kPa*s/m2, cutted in triangles (four out ouf one plate),
the two super-chunks in front of the room are made with SWISSPOR ROC TYP 1 (60cm x 100cm x 10cm ) >= 5 kPa*s/m2, cutted in triangles (four out ouf one plate) (link: http://www.swisspor.ch/index.php?sectio ... Page&id=76)
these front-superchunks have also 10cm-plates of this ROC Typ1 behind the triangles to make the Super-chunks deeper.
all superchunks are room-height, and some thin plastic/fabric is wrapped around them.

unfortunalety, i lost the measurments while i was installing the superchunks (to show the effect of them), but as far as i remember, the influence was very big. mostly in the ringing-time in the waterfall-diagramms, but not so much in the frequency-plots.

i have done some tests with covering the frontwall (windows) with TP1 (but no airgap), but this haven`t done much. but i will try it again with airgap and more TP1.

yes, the speakers are nearly at half of the room-height. i moved them up (ca. 10cm) in one of the measurements, but the measurements are not much different.

i will try to do some more (temporary) basstrapping in the back of the room for testing. but the layers of rockwool has to be very thick (20cm) and there must be a large air-gap (20cm) to dampen these low-frequencys, right? (i will play around with the porosus absorber calc).
or are there any otherer acoustic modules for these low-frequencys which work better and doesen`t eat up so much space?

thanks a lot stuart!

best greetings, markus

[Soundman2020]

or are there any otherer acoustic modules for these low-frequencys which work better and doesen`t eat up so much space?

If you are certain that you have a problem at a very specific frequency, then you could build a tuned trap, such as a panel trap or a Helmholtz resonator of some sort: Probably a panel trap would be better for those low frequencies. Basically, it is just a large panel of some type of material (usually wood) mounted over a sealed cavity with insulation inside, and a small air gap between the panel and the insulation. The material and cavity are chosen specifically for the resonant frequency that you need. For example, a sheet of 1/2" plywood over a 6" cavity would be about right for a 60 Hz. problem. 1/4" plywood over a 4" cavity would be right for 100Hz. Etc. The position of the trap in the room is also important: unlike absorption, which is velocity based, a panel trap is pressure based, so it should go right up against the wall, in the position where the pressure peak is highest for that particular resonance.

There are also other types of tuned resonant trap that you can build, but the panel trap is simplest.

- Stuart -

[khe86]

hi stuart,

i will read more about this panel-traps, thanks for the information!

i played around with the perforated panel absorber calculator and found a configuration which may works for my problematic frequency-range (see picture).
what do you think about this configuraiton?

i will do some systematic measurements across the walls and corners to find the highest pressure-points of the frequencys which are causing the moving null at the listening-position.

more question will follow i think

thank you!
cheers, markus