monitors placement? (where to measure from)

[bubblefish]

ok i would appriciate some opinions on this if anyone has the time...

below are three wave form i have recorded in my soon to be new studio.

they show the rooms responce to ethans bass tone responce test. measured at three different locations.

the test for those unfamiliar with it involves playing a sine wave from 40hz -300hz and measuring the recorded responce.

I did this as mentioned above in three different locations following different advice gained from the internet:

1. from here


The goal in determining the position in the room is to excite as few of the standing waves as possible. A number of sources have suggested formulas to find rough placement. Here a couple I’ve seen. Where x is the distance from the side wall to the center of the speaker and y is the distance from the back wall to the center of the woofer. 1) x = (0.277) x (room width), y = (0.450) x (room width). If this puts the speakers too far out into the listening room use x = (0.277) x room width, y = (0.353) x room width. 2) x = (0.276) x (room width), y = (0.618) x (ceiling height).

(the formulas high lighted in bold show the two formulas i used to get posistion 1 and two respectively below)



and also from ethans winers forum where i read that one should stive to locate the listening posistion 38% of the rooms length back from the front wall.

anyway using these different ideas i set up my system and recorded the results using a ECM8000 mic in an empty room with no treatment of any kind.

its amazing to see the difference one can produce by simply moving the monitors and listening postion aound. however the different approachs led to some very different locations for the set up of my studio.

BTW studio dimentions are 11'5" length- 9'7" width- 8'4" height. and I,m firing into the longest dimesion of my room

the 38% posistion has me up tight against the front wall and I,m struggling to keep the speakers a foot away from the front wall to do this and maitain a equilateral triangle betweens my self and my monitors i have to bring them closer to gether about 2' 6" apart

however the distance from the back wall to my head is 7'7"

posistion 1 have the speakers well towards the centre of the room ( about 1/3 of the
way in)and my listening position only 3'5" from the back wall

posistion 2 have the in a more managable posistion with my head 4'4" from the rear wall.


luckily though i am pretty free to set up the room in what ever manor is most suitable so I'm asking for your takes on this given the room layout and measured responces.

I hope its clear enough anyway.




anyway I,m hopeing that some one here might be able to help me interpret these results abit better to me posistion 1 looks flatter over all but also a little quieter perhaps. where as having it at 38% gives a big hump around 190Hz
anyway i would be glad to hear others takes and views on all this.

cheers
kieran

[knightfly]

Don't have much time right now, but check this out -

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=2441

The 38% rule can get you into trouble; for small rooms, as you've found out, it's very tight - for ANY room, it's dangerously close to 37.5 %, which is the 4th harmonic null point. Safer to go 40-42%, or in your case find another NON-null position further back. The harmon axial mode calculator can help there, if you have Excel on your computer. (See the above link)

Sounds like you need to find a wider stance for your speakers, one that does NOT put them in side-to-side nulls; then, a bit of math to find how far back from the centerline between speakers your head should be for equilateral triangle; finally, SCREW the equilateral bit and get your head in a location that is neither peak nor null, following that harmon spreadsheet.

60 degree monitoring doesn't have to be cast in stone, but staying away from peaks and nulls is a VERY GOOD THING... Steve

[bubblefish]

cheers steve that helped alot,

getting out of the triangle fixation in part was the key. And that exel program is a great help as well

cheers

kieran

[bubblefish]

hi steve one more question, i'm working from the exel program and your explainations of drawing the room mode grid o place the speakers.

how ever I,m not sure of where i should measure from with regards my monitors:

should it be:

from the front of the tweeter?
in the middle of the tweeter and the woofer (at the front)?

from the back and or side of speaker cabinet to wall?

its just this can effect measurements by up to 12" at times which i thought might be serious and undo all the effort put into finding the best locations in the first place.

whats your method when using this system?

also would you set the whole thing up slightly off say 4-6" off the central axis of the room to aviod the big nulls there yet maintain the symetry between the listening posistion and the monitors?

cheers
kieran

[knightfly]

Measure from the center of the dome-shaped voice coil cover on the woofer and you'll be as close as possible; we're concerned with room modes and SBIR here, all of which are lower frequency artifacts so the woofer is king... any HF stuff will be more affected by smaller interferences, even things like mic booms, etc, and will change with miniscule head movements etc.

Also, measure to SOLID surfaces NOT the face of absorbers (although the faces of PANEL type absorbers WILL change modal responses a measurable amount... Steve

[bubblefish]

thanks steve

thats a great help as was /is that exel program

all the measurements are to solid surfaces as I,m only just starting to place absorbsion as of today.

thanks for taking the timeout to help its appreciated.

cheers

kieran