Off axis compromises

[temp25]

Hi OP!

I have some perspectives which may help you. My experience comes from motion picture equipment design in the past, and living with no center, as well as a 2 and 3-way center I built.

It's important to really understand when and where a center channel speaker adds high value. IMHO that's the same for auditoriums as well as homes. For theaters the key value was in keeping the dialogue centered regardless of the seating position of the patron, which could be up the middle, or far to the sides. Of course in these cases we are talking about tens of feet differences.

For the home listener the value is the same. Whether you are lying down on the couch, grabbing chips from the counter or sitting in the middle the center channel keeps that dialogue and centered effects in the center. This then makes exactly what you want the center to do important. If your listening location is always in either of two recliners right in front of the screen then a center may not even add much. So, if that's not your case, what exactly are your listening angles? For this maybe 30 degrees horizontal is the extreme. The same considerations should be given to vertical listening angles. How wide does it need to be and also whether your prime listening angle will be above the tweeter axis. Your delay and phase considerations should take this into account.

This is important to consider because as part of your decision making is whether to use a 2-way, 3-way or maybe even a woofer-assisted wide band (WAW) design. You can get away with 2-way and lower order filters if you know in advance that your actual listening locations are constrained anyway.

Last tip I have is that higher order filters make off angle nulls less severe, as does using a plate amp with built in DSP capabilities. Having made a few passive speakers successfully and having made a center channel using a Hypex plate amp I can honestly say I don't think I'm ever deliberately making a passive crossover again. Too much effort with too little return. Also, if you want to see a collection of DIY center speakers check out Troels Gravesen's site, which has maybe 6 different center channels with off-axis responses.

I agree with most of what you posted. I build center speakers sometimes, just because I want to make something. I also tend to have old boxes from previous speakers on hand. I cut them down, or slope the baffle, or maybe round over edges, etc, in order to try new ideas, or new drivers . Making x-overs for them is the part that I like the most. I had an active system using a DCX2496 many years ago. It took the fun out of making x-overs, because it was too easy, and quick.

The speaker belonging to the above measurements would make a good center. If you look at the 45 degree measurement, it's still plus or minus 2dB below 10k. It would however lack the power handling of a larger speaker.

I am working on a 3-way center. The drivers are aligned vertically. My off axis seating is under 30 degrees.

 

[temp25]

I wanted to measure the distortion of my other wave-guide tweeter. I set it on top of a speaker. The distortion is considerably lower. The results of what I did next, really surprised me. I measured off axis. I just kept turning the box till I was out past 60 degrees. I think I want to use this tweeter.

 

[raymondj]

Interesting the other waveguided tweeter is.

Joking aside, some people do like optimising the response at the 30 degree off axis position.

 

[temp25]

The other tweeter is this one. https://www.parts-express.com/pedoc...utm_medium=Impact&utm_campaign=Impact&irgwc=1

I'm not sure I understand your 30 degrees comment, unless you were thinking the same thing that I was, and that being how nice the second and third sweep looked. There's some diffraction going on for sure, but the 3k, to 5k range drop along with the top end. The tweeter faceplate is 80% exposed, and can cause some diffraction. I also measured on an 11" wide baffle, and roll-off was not as smooth. I think that if it was in a more optimum baffle width, with 1/2" round overs, it would be pretty smooth.

I'm even wondering if a horizontal break, ( think really wide chamfer. ), in the baffle of 12 degrees between the tweeter, and woofer, to tilt the tweeter off axis when at the listening axis, might be an option. Although this would aim the tweeter slightly towards the ceiling, the additional off axis roll-off vertically would reduce what reaches the ceiling anyway.

 

[raymondj]

What I did not explain at all, is the fact that if the speakers are firing straight ahead into the length of the room, it would not be unusual for a listener to be 30 degrees of axis. Depending on design goal instead of letting the optimiser work with the on axis response, you can use the 30 degree to be optimised.

I am still playing around with a large 12 inch co axial with a horn loaded tweeter, it has its smoothest response around 30-40 degree of axis so that is what I work with.

 

[temp25]

Since yours is a coax, if you tilted it vertically about 20 degrees, the side to side off axis would vary from 20 degrees to 50 degrees, but do so with only going about 30 degrees side to side movement. That's hard to put into words. The driver on axis would not be listened to on axis, except maybe if you stand up near the speaker. I could do the same with this tweeter possibly, and the woofer would benefit from the non -flat baffle too.

I get the joke, now that I read it again.😀

I found an old speaker box with big beveled edges. I'm going to measure this tweeter on it, and see how it responds.

I'm also returning my other speaker to a fully passive x-over. I already have the design, I just need to confirm that my notes are correct before assembling it. I could show some before, during, and after x-over measurements, if anyone is interested.