# Audibility threshold of early reflections



## toyotafan (Jul 31, 2008)

I have some early reflections coming from the floor at ~2ms and ceiling at ~5 and 8ms. The IR graph below is from my center channel which is on a stand under a large projection screen that's fairly close to the floor. I realize speaker placement is sub-optimal.

At what point do early reflections become audible? Do the reflections in the graph below warrant treatment? 

I worked with Bryan Pape several years ago on my room treatments which consist of the following:
Front wall - 2" OC703 with triangular bass traps in both corners floor to ceiling.
Side walls 1" OC 703 full coverage first 5' from front wall, 1" up to 48" for the rest 
Rear wall 2" full coverage with facing to reflect high frequencies
Ceiling - no treatments


----------



## bpape (Sep 14, 2006)

If they're within the first 20ms and louder than 20db down you should address them.


----------



## toyotafan (Jul 31, 2008)

Thanks Bryan, both are -10 to -15db down and <20ms. Next question is how to treat them. Does the frequency of the reflection dictate how thick the acoustic treatment needs to be? 

I have a 2'x4'x4" panel I used for testing and when I held it up to the ceiling it pretty much eliminated the ceiling reflections but when I tried it on the floor it only reduced that reflection by about 30%.


----------



## bpape (Sep 14, 2006)

Floor reflection is probably much lower in frequency and may not be able to deal with it. Ceiling sounds about right. Yes, not all reflections are in the same frequency range that you find to be that intense.

Can't really treat the floor anyway unfortunately.


----------



## phazewolf (Feb 5, 2012)

Have you thought about trying to but the center channel above the screen instead of where it is now? Sense you can't treat a floor but the ceiling can be dealt with that may help with the issue.

Maybe if you can just try it and see if it helps or not sense you have a spare trap your just out time if it does not work out well for you.

You could even use a thicker trap on the ceiling then what you have and by moving the center away from the floor it should decrees how loud that reflection is if my understanding if correct.


----------



## bpape (Sep 14, 2006)

Good thought - but... You'll have a reflection on the ceiling and floor regardless of if it's high or low. Plus, you wan to put the center channel in whichever position is closer to being in the same place as the L and R speakers, which is pretty much always below. This is because you can't adjust for differences in location up and down like you can front to back using time delay.


----------



## toyotafan (Jul 31, 2008)

Center channel above the screen is a bad idea, at least in my room. The tweeter would be too far off axis.


----------



## toyotafan (Jul 31, 2008)

The real problem I'm trying to resolve is at certain times vocals will have a hollow sound. I'm pretty sure it's from reflections either from the ceiling or floor. How can I determine what frequency the reflections are at?


----------



## bpape (Sep 14, 2006)

The treatments used, if I remember correctly, had to be made thinner due to space considerations. That's a piece of the problem. The rear wall treatment would generally be much thicker.

It could be somewhat ceiling reflections which I would address with 4" panels as long as it's not in the way of the PJ beam.

Got pictures of the front of your room?


----------



## toyotafan (Jul 31, 2008)

I don't recall any space constraints on the rear wall. Projector is shelf mounted so 4" on the ceiling is no problem. 

Front wall. A friend of mine took this with a D-SLR before the front speakers were installed.
http://home.avvanta.com/~sesmith/temp/theater front1.jpg

Front wall with the center channel on a stand under the screen. Unfortunately it's really dark, I can't get a good photo even with the lights up all the way.
http://home.avvanta.com/~sesmith/temp/theater front.jpg

Rear wall.
http://home.avvanta.com/~sesmith/temp/theater rear.jpg

Drawing of room.
http://home.avvanta.com/~sesmith/temp/theater11.jpg


----------



## bpape (Sep 14, 2006)

Then sure, you could do 4" panels spaced a couple inches off the ceiling in the reflection zone.


----------

