Can someone check my numbers?

[dft3670]

Hello all,
I am trying to find out if I am sitting in the best mix position for my room. and if my speakers are also in their best positions.

Room Size :
Width is 8ft. 6in
Length is 11ft 7in
Height is 7ft 0in

Left speaker is 28 inches from the left wall.
Right speaker is 33 inches from right wall.
Tweeter height is 48 inches from floor.
Mix position is 48 inches from right wall.
Mix position ear height is 48 inches.

The speakers are not at equal in spacings in the room as the right side of the room has a door.

I know in rooms this small the sweet spots are pretty small. What suggestions do you have as far as moving positions.
Thanks

[BriHar]

No one can answer this question for you even given those measurements. You can either listen objectively and move the elements around always asking yourself - How is the imaging? How is the soundfield? Move your head around and check if there are any nulls or peaks.
The other alternative is to get a software such as REW (free download) and a decent mic, and test.

[dft3670]

Yep. Use REW. Yep imaging seems pretty good. Move my head around to see if I hear nulls. Sure I can do that.

My question may have been poorly worded. I'm just looking for someone to do some quick math and make sure these positions are not in nulls. I fully plan on shoooting the room to get hard data and use it for final positioning.

My thought was to calculate where nulls are in a room by using the dimension of the room. Once I did that I can see where these nulls are I can try to get speaker, height/position and mix position/height in position to keep them out of these nulls. Once I am pretty sure I have everything in the best postions, I would have basic starting postitions to work with when I start shooting the room with REW.

I'm thinking I could save a bit of time in shooting my room with ballpark positioning.

[DanDan]

Mode Calculators are based on perfectly hard walls. Unless you have such a concrete bunker, calculated figures will be a bit different to actual measured ones. Useful still though to predict the approximate frequency of expected Nulls. http://www.bobgolds.com/Mode/RoomModes.htm
Play sinewaves, the Sig Gen in REW is great.
Tune to the expected mode. When you hit it you will know.
Then walk about to confirm the pattern. e.g. First Length Mode will have a peak at the Front and Back walls and a null in the middle. Second mode goes peak null peak null peak.
Map these out on your floor in masking tape.

In many rooms the listener is at approximately half height AND width. Two nulls combining. No Kick Drum.

DD

[dft3670]

Hi DanDan,
I actually used a graph you reccomended in another post where you tape the length and width null graph on a piece of paper then by coloring in the green squares you create a map where the best spots in your room are to avoid nulls. You had reccomended that you use this as a starting point. This is what I was basically trying to do. I will try masking out the floor.

Thanks

[DanDan]

A very good start. The null map is just one advisor though.
Pink Noise and RTA, moving one speaker or listener position is another quick and informative view. REW has all this on board.
Ultimately you will have to get down to it though. Measure, find the biggest two or three issues (nulls). Determine if they are modal or BIR. Moving the speaker or mic is key here. Try to compromise between the issues using location. Forget rules of thumb. Very often speakers are best kissing the front wall.
Quite often the centre can be the best listening position in a small room.
DD

[dft3670]

DanDan, One last question. When you measure a room dimensions , do you measure to and from the hard walls or from wall to wall using the base trap fronts as your boundries.

[Ethan Winer]

I'm not Dan, but I sometimes play him on this forum. Room modes are defined by the rigid boundaries.

--Ethan

[dft3670]

Thanks Ethan. That's what I thought

[DanDan]

LOL.

Some complications and teasers.

I have a back wall, concrete brick, lined with 50mm gap, then sheetrock.
I can detect two modes. A bigger lower one for the concrete boundary, another a few Hz up caused by the sheetrock I guess.

If there is a significant amount of fibre trapping on the wall, the effective room dimension changes. Sound moves slower through the fibre.

DD

[Ethan Winer]

Originally Posted by DanDan

I have a back wall, concrete brick, lined with 50mm gap, then sheetrock. I can detect two modes. A bigger lower one for the concrete boundary, another a few Hz up caused by the sheetrock I guess.

Yep, though the OP asked about bass traps. I too have seen "double modes" in rooms where a cement wall is some distance behind thin drywall.

--Ethan