Mic positioning in a room

[midoban]

Hi, i'm putting up a modest home studio for both recording and monitoring (computer) in the same room, the room is a 3.5x3 meters rectangle.

i've been reading a lot about positioning absorbers, diffusers and bass traps, but i havent managed to find any info about where to put the mic (singing/playing spot) in relation to all the acoustic treatment. i know that there is some trial and error to be done, but still what are the basic guide lines for this?

the kind of info i'm looking for is something like - the vocalist/guitarist is best facing an absorber/diffuser/bass trap and with his back to an absorber/diffuser/bass trap...

Thanks!

[takman]

well, you can about it a couple different ways...

but the whole idea here is that you don't want to be sitting ( listening position) in a peak or null. These would show up in a frequency response graph as high points and low points.

for example lets say you measured the room. at 100hz you had a really big dip.....you wouldnt hear that particular frequency while mixing, which is bad......but if you move either the listening position, speaker placement, or both...you might either change the frequency response, or physically move yourself out of that dip.

Low frequencies are the most difficult issue to deal with in a room. They have the most energy. And how they interact within the room determines where your listening position will be. Room modes ( low frequency resonances) are a direct result of the physical size of the room.

The best way to go about this, if possible is to start from the beginning. With the room completely empty, you place a speaker in a floor to wall corner of the room, and position it so that fires diagonally across the room. Then you place your test mic in the ceiling to wall corner diagonally opposite to the speaker. This will excite all the modes of the room.

Then, you will know all the problem low frequencies. This is basically what a room mode calculator does. But actually testing it, is the most accurate way to go about it, because room mode calculators dont take into account a lot of real world conditions within the room.

So, you know what problem frequencies you have, because you tested the room and it shows on the frequency response graph. Also the waterfall graph will show these resonances in time, which is very useful......because it shows you how long these resonances take to decay.

So, lets say you know your problem frequencies. For example, 60, 80, and 100hz. They show up on the FR graph as large peaks or nulls....and on the waterfall as long decay times.

If you dont have the luxury of taking everything out of the room. Then you can use a room mode calculator to find the problem modes/frequencies.

Now, generally speaking, the listening position will be somewhere between the front wall, and the center of the room. It will however NOT be in the dead center of the room, because there is always a peak or null there. This is where the whole "listening position should be 38% from the front wall" thing comes from. And of course, you will want to be centered between the left and right sidewalls.

So you can setup you monitors and mic in the general area where you think they will be. Centered between left and right walls, and somewhere between slightly forward from the dead center of the room to the front wall.

now, since you know the problem frequencies.... say in our example, starting with 80hz, you can run a test tone @ 80hz ( REW has a test tone generator)...and move around a bit and see where it sounds the loudest, or in the case of a null, see where it seems to disappear. then move on to the next frequency.....Basically, what you are doing is trying to map them out. And trying to figure out the place where they have the least effect.

Once you've done that, then you can re-test, and see how the FR graph looks. If you want it better, start moving things around again. There will be some trial and error here. Ideally you should end up where it shows the Flattest response on the graph.

Now, regarding the problem frequencies. Both testing and the room mode calculator will show lots of modes/problem frequencies......we wont be concerned with them all, just the strongest ones that stick out from the others.

Then once you have your listening position worked out, you can start installing bass traps. Once you finished that, you can test again, and see if you need more bass trapping....The more you add, the smoother the low/mid frequency response should get...and it should show less modal ringing on the waterfall as well.

Then you can move onto the ETC