soffit mount/ listening position in a long room

[dynamike]

i posted this in the Design forum, but i realized it should be here.

i'm building a one room studio (recording/mixing) mostly rock music.

the dimensions are 26' 2" x 10' 5" x 7' 8"

if i don't soffit mount my monitors, my desk and listening position will be almost 10 feet back. i feel like this takes up too much 'work' space for drums and what-not.

my question is. . . if i DO soffit mount my speakers, can i move my listening position closer to the wall, thus opening up more of the room? and, is there any quick work-a-round for this, like gobos to 'fake' a shorter room when mixing?

a simpler version of the question:
is the 38% rule for listening postion only pertain to desktop speakers (ie: non-soffit/flush mounted speakers?

a sketchup of my room is here:
http://johnlsayers.com/phpBB2/viewtopic.php?t=7411

[knightfly]

Are those room dimensions what your FINAL size will be after inner layers of wallboard? I'm asking because you have different dimensions in your drawing.

The 38% "rule" is really just a suggestion - it can't be applied universally, because as you see with your long room, it wastes a bunch of space.

Instead, use the methods laid out in "are my speakers in a null", in the acoustics forum - the harmon mode calculator will show you where the nulls are for your size room, and that's where you do NOT want either your head or speakers. If you notice, 38% is DANGEROUSLY close to 37.5%, which will give you a total null at the 4th harmonic of the room's length. In the case of your room, 9 feet 9 inches is a BAD place to be IMO. Check out that thread and try to "wade through it" and you'll see some other options for keeping your mix area a bit smaller.

IF you soffit speakers (and usually it's a good idea) the baffle extensions should be as wide as you can make them - typically 4 feet wide should be minimum for best control of LF in the room. They should be at a 30 degree angle and form an equilateral triangle between 2 speakers and your head, with your head actually just in FRONT of the point of that triangle -

In your size room, placing the centers of the woofers 24" from the outside wall and angled 30 degrees will keep the speakers out of serious nulls - this gives a distance between speaker centers of 6'5", which puts the listening position at about 7 feet from the front wall (center)

Building some gobo's with live/dead sides and placing them behind you while mixing (dead side toward the mix area) will help keep your recording half more live while shortening the room acoustically - a cloud over the mix desk and some corner broadband absorbers, along with absorption at first reflection points on the side walls, and you'll be in pretty good shape. You may also want to consider building some of the angled slat absorbers shown in the SAE site (see links at top right of the page) these can reduce/eliminate flutter echo (clap your hands in an untreated, bare room if you don't know what flutter is)

HTH... Steve

[dws2468]

There has been some discussion on soffitt mounted speakers regarding the 6dB increase in low freq power, and the fact that free standing speaker cabinets try to compensate for this effect in the crossover by reducing the high freq power. I am currently building a pair of soffitt mounted speakers for a control room, which use a dual concentric Tannoy 10" driver for the mid and high, and a 12" sub for the lows - all of which are tri-amped and using an active crossover.

My question is - does the fact that I am tri-amping (and therefore not using the original Tannoy passive crossover) mean that I do not have to worry about the 6dB bass rise?

I was just going to build the thing, and then do frequency response analysis to see if any eq is needed,

[knightfly]

Because you're tri-amping, you'll have individual control over each pass-band and can "dial it in" more or less to your own taste/measurements. The increased bass effect of baffle extension isn't 6 dB across the range of bass, it's more pronounced the lower in frequency - this is why a shelving equalizer can be used on the low end if you really want it "flat". So, depending on what you're using for a 3-way crossover, you may already have all the "tweakability" you need to make it happen... Steve

[dws2468]

Hi Steve

Yes, I meant to say 6dB per octave - although I am not sure where the start point of the curve is - probably depends on room dimensions/shape. The crossovers are my own design using Linkwitz Reily alignment 24dB/oct with crossover points at 100Hz and 2kHz. Each band will have its own level adjustment, which theoretically can be used to adjust for a flat overall response, but if I use the level adjustments to correct for room anomalies, then this will shift the crossover points. I set the levels by using a sine wave signal at the exact crossover frequency - and then use a test mic to get the same level from the 2 drivers either side of the crossover. The things is, to correct for a 6dB per octave bass rise, I can't really shift the entire bass region up (20Hz to 100Hz) I really need an eq which follows the same curve. I would rather not use any graphic equalizers, but if I have to I will design a filter to match and correct whatever frequency response curve I get from measurement.

But I am hoping the mere fact that I am not using a passive crossover means that soffit mounting will already give a flat response because all energy at all frequencies is radiated into the same 180 deg space.

[knightfly]

"I am not sure where the start point of the curve is - probably depends on room dimensions/shape" - yeah, and the 10" driver, baffle size, etc -

This is so far still beyond my experience, but I've yet to hear a 2-way with a woofer that's even 8" (much less 10) that didn't sound "scooped", as in too much mid-bass and too little midrange - but I've not heard the specific Tannoy's you mention.

Somewhere around this forum I've linked to a white paper that discusses baffle step compensation, and gives a formula for LF cutoff based on the woofer's free-air response - if you don't find it with a search, post back and I'll dig it up for you.

IF you're using some sort of control amp to drive your individual power amps, and if it has a basic SHELVING EQ (bass, treble, maybe midrange) you could probably, once positioning and treatment have minimized modal peaks/dips, just turn the bass up or down to taste - but don't think this is a fix for bad room ratios or SBIR - only physics/dimensions can fix those... Steve