Directly above the listening zone. Ideally this would be a first reflection point and would help towards overall creation of a reflection free zone when used with absorption/diffusion in front, sides, and back as well.
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Jetter it is a bit difficult on ceilings but you have to find the early reflection point. To do that you sit in the listening position and have somebody else hold a mirror against the ceiling and walk slowly between you and each speaker. when you can see the speaker's reflection in the mirror that would be that speakers early reflection point. Then do it for the other speaker and place one panel for each speaker centered on those points. The trick is to find a way for that person to do that. Either a ladder that you have to keep moving or a mirror on a long handle. The mirror has to be flat against the ceiling. The other way to do it is with a laser. You shine the laser on the mirror and when it hits the speaker you have it. Have fun!,
One more point just for future reference. Line sources or linear arrays the best of which extend from the floor to the ceiling do not send almost any energy to the floor or ceiling. If they are dipoles they don't send almost any energy to the sides either. The only reflections you have are to the front and rear walls and only the reflection off the front wall is early enough to matter.
thyname, thanks for the link.
mijostyn thank you for the explanation.
I can see the obvious physical problem of holding and moving the mirrors while sitting in the listening position. It does seem that having a ladder and friend (or wife/significant other), LOL, to hold the mirror would be pretty essential.
I haven’t validated the calculations for accuracy, but quick scan of the numbers look good for this calculator. One of my friends sent it: http://www.acoustic.ua/forms/calculator4.en.html
You may want to experiment with position in and around the calculated values. Some ideas below on holding the panels in place for listening tests.
If you want to go cheap, I have used something like this to hold things against the ceiling when I don’t have helping hands, or I don’t want someone standing in the way of the sound. The trick is to fully extend it just slightly off vertical (so it is slightly longer than the distance to the floor), lock it, then push the base towards vertical so it has enough pressure hold whatever you are trying to hold in place. https://www.amazon.com/Mr-LongArm-3212-Pro-Pole-Extension/dp/B00004YUPO/ref=sr_1_8?keywords=extensio...
If you want to get a bit more sophisticated, https://www.amazon.com/FastCap-Support-System-144-Inches-3HAND5/dp/B000067S12/ref=sr_1_12?keywords=d... is even better. Well worth the $70 for a set of two. You can hold an acoustic panel in place with one on each corner.
I measured from the speaker face to my listening position by using plumb lines to place tacks in the ceiling, ran string through then, creating the angles, then made the center of the panels halfway the distance.
See my system pics for details. They hang about 10" from the ceiling (9 foot ceiling in this space)
Just an update: for those following.
I installed two GIK Acoustics 242 panels for the ceiling today.
I posted one picture under my System.
Thanks everyone for your help on this thread. I am very happy with the results.
Special thanks to @atdavid. We don’t have to be enemies. This is a hobby for me. We can both enjoy. Our own way, even if it is different