Stereo with four speakers

[slinger]

I'm still pissed. Back feels wonderful though. I think I have found the answer

[karatestu]

I still have this idea in my head. It was fuelled by some threads I found on diyaudio.com about directivity, reflections, corner horns etc etc. Soffit mounted corner horns was a good one No, I don't want horns. Haven't got the space for one thing.

If you mount a speaker in the top corner of the room there are allegedly some very nice advantages. I don't mean getting a small bookshelf and putting it in the top corner and angling it down to where you want it. That has some issues due to early reflections amongst other things.

If you were to make an enclosure in a tetrahedron shape (look that up) and mount it so three of it's faces are right up to the two walls and the ceiling with no gaps then you are left with only one triangular face facing down and roughly in to the centre of your room. If your walls are solid like mine then it could be done by fabricating a triangular baffle with three sides of equal length and fitting it across the corner . The other three sides of the enclosure are provided for you by the existing walls and ceiling.

Well this gets your speakers out of the way for one thing and you could even run the speaker cables behind the ceiling to a certain degree depending on where your electronics are and how many speakers you have. Yes i know that there is no semi omniness going on here but hey, sometimes the compromise may be worth it.

Now, what are the advantages of doing this ? From what I gather there are a few. If the triangular baffle (the only one visible) is equal across the corner then this baffle can be seen as just an extension of walls and ceiling. Getting the driver(s) right in to the corner and close to the walls and ceiling is very important so we don't want large drivers with their high VAS which requires a large enclosure volume Imagine trying to put up a 12 incher up there with it's 80 Litre plus (in my case 160 Litres) cabinet requirement. Not going to work.

So we need a mid bass or full ranger which has a lowish VAS and Qts which will still work in a sealed enclosure. So for example a 5.25" with not too high a Qts or VAS will get the driver to be able to be right up in the apex. That will make for a very small baffle but if going two way we need room on the baffle for a tweeter. A full ranger or coaxial would be much easier but they bring their own compromises which you can either live with or can't. Compromise, compromise, compromise. Much of this speaker designing lark comes down to that.

So we have basically a soffit mounted speaker which means there is no baffle step loss. Plus we get what is effectively a 9 dB boost in the bass because we aren't losing it to baffle step and we are firing in to eighth space (pi/2 steradians) .If we had wall mounted speakers say in the centre of the wall we would be firing to half space as the wall plane divides the space we would have if the speaker was in the middle of the room in to two halves. If we mount the speaker half the way up the wall but in the corner we are now firing in to quarter space as the imaginary sphere has now been cut in to quarters. So keeping the speaker in the corner but now moving it to the ceiling apex we are now firing in to eighth space because the sphere has been cut in to eight .

From each proximity to a boundary (two walls + ceiling ) we get a 3 dB boost so 9 dB in total. Very nice. This is why we have to chose the QTS and VAS of the mid bass carefully. Too high a qts will probably lead to too much bass so experiments would start with a lowish qts driver which when the 9dB boost is added turns out about the right amount of bass.

So the idea is best with not too large a driver but the bass boost will save the day (bass wise). No 12 inchers, semi omni or multiple drivers of the same type in this idea sadly but the trade off just might be worth it for some (me included). There is talk of controlled directivity being used here by the use of horns and wave guides to stop any early reflections but I think that was more for when you try to do this with a normal bookshelf speaker and mount it up in the corner.

The space that the drivers fire in to is in effect a very large tetrahedron shaped horn with dimensions that equal those of your room. But depending on how big your room is it means that your speakers are quite a way apart. I think this would likely work better in a smaller room like that I have at home (3.8M x 4.2M). Also with high frequency coming from the corner apex's the image is likely to be up high. Could be a problem for some. I don't know about me yet until I try it

So we get no early reflections from the front wall because the speakers are effectively mounted in it. There may still be early reflections from any furniture you have in the corner of the room near the speaker. Late reflections past 40 msec are ignored by the brain and heard as echo so don't spoil the image. Plus we don't get destructive cancellation from sound bouncing off the front wall. Room modes will still be evident but changed in frequency and hopefully magnitude. The advantages are mounting and making it more and more appealing.

So back to my stereo with four speaker idea - four speakers, one in each corner apex mounted as described above alternating LRLR channels may be worth a go. A bit of a departure from semi omni with multiple drivers but I am considering giving it a go. Only one baffle to make for each speaker and I have all the necessary drivers although the qts of the little 5.25 inchers may be too high.

Phase issues aside (if there does indeed end up being any) this may be the ultimate in room filling sound (for me and my family) with numerous sweet spots. You can still be closer to one speaker than another but there isn't a way around that until somebody invents something incredible. I may try it and love it but miss my current speakers and their advantages too much.

S

[Geoff.R.G]

Lots of theory there Stu but I have only one thing to say, suck it and see.

[karatestu]

That's what I thought Geoff. A few things I have tried in the past were against the advice of so called experts and things turned out to be subjectively better for my ears.

Theory has it's place I suppose but I am wary of becoming another theory spouting cut and paste idiot who doesn't really know the reality. I do find the subject of acoustics extremely interesting but until you actually try it then there is no way of knowing which parts are true and which are not. And there will be no measurements or hypothesis so nobody will take any notice of my findings anyway

[Geoff.R.G]

That's what I thought Geoff. A few things I have tried in the past were against the advice of so called experts and things turned out to be subjectively better for my ears.

Theory has it's place I suppose but I am wary of becoming another theory spouting cut and paste idiot who doesn't really know the reality. I do find the subject of acoustics extremely interesting but until you actually try it then there is no way of knowing which parts are true and which are not. And there will be no measurements or hypothesis so nobody will take any notice of my findings anyway

The Wright brothers didn't have very much aerodynamic theory but they flew, Turing didn't have a computer as we know it but he devised programming, the first nuclear reactor was based on an idea that could easily have been wrong and made the University of Chicago glow in the dark. Pioneers don't have theory to work on, they just get on and try, they note what works and what doesn't and try again, and again, and again until they start getting it right.