# Early Reflection Question



## Bob_99 (May 8, 2006)

Bryan or Ethan,

When looking at an ETC graph and there is a fairly strong reflection at 2 ms, is it possible to tell if the reflection is coming 2 feet (roughly) near the source or 2 feet from the microphone? 

:dunno:

Also, if the other side of a ceiling is an open space with just fluffy fiberglass between the joist, are the height modes diminished versus having a finished floor above the ceiling? 

:scratchhead:

Thank you.

Bob


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## bpape (Sep 14, 2006)

Hi Bob.

Height modes will be dealt with better with the larger space and more insulation. There will still be some modal issues but if it's a non-flat hard surface above, like a pitched roof, they'll be more difficult to predict.

The 2ms reflection is hard to say. It's something that has approximately a 2' farther path than the direct sound from the speaker to mic path. Could be a seat, a table, a close untreated side wall, etc.

Bryan


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## Bob_99 (May 8, 2006)

Bryan,

Thank you for your response.



> Height modes will be dealt with better with the larger space and more insulation. There will still be some modal issues but if it's a non-flat hard surface above, like a pitched roof, they'll be more difficult to predict.


From this, am I to understand that the roof above the ceiling will also affect the sound? I never would have even begun to figure that one out. Would I be better off removing the fluffy stuff and filling the joist with rigid fiberglass?

As for the reflection issue, the front of my speakers are currently even with the face of the TV. Should I move them back a bit to avoid reflection from the face of the TV? I will do some measurements tonight to see if it makes a difference but I would appreciate your thoughts about it.

As always, thanks for your time.

Bob


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## bpape (Sep 14, 2006)

Sorry - misread the post the first time. The cavity above the ceiling being larger will help with bass absrobtion. No need to do rigid - just do an extra layer of fluffy if you want.

As for speakers, you really want the faces of the speakers out in front of the TV as much as you can swing.

Bryan


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## Bob_99 (May 8, 2006)

> As for speakers, you really want the faces of the speakers out in front of the TV as much as you can swing.


 Bryan,

Would you mind explaining this? Most sites explaining speaker setup say the front of the speaker should be flush with the screen but if I were to take a guess, I would have said slightly behind the screen to avoid reflections. I'm at a loss to explain why in front of the screen.

Thanks

Bob


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## Ethan Winer (Jul 21, 2006)

Bob_99 said:


> I'm at a loss to explain why in front of the screen.


Most speakers send sound off to the sides as well as the front. The specific curve of off-axis frequency response and loudness varies a lot, but they all send some sound sideways, and even to the rear at low frequencies. So if the speakers are in front of any reflecting boundaries, less of that "sideways" sound will be reflected.

--Ethan


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## eugovector (Sep 4, 2006)

And, if you listen/watch more as a solo/duo, putting your front speakers out a little will help to keep the distance of the speakers from the main viewing/listening point equal:


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## bpape (Sep 14, 2006)

Not to mention if you have your speakers right next to your TV and flush, then the entire TV screen acts as part of the speaker baffle itself drastically changing the frequency response. Baffle step compensation circuitry is built into most crossovers and is designed specifically for the width of the speaker baffle. Changing the baffle width causes problems. This is one of the reasons you never take a 'box' speaker and build it into a wall unless you also compensate by killing the front of the wall with absorbtion.

Bryan


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## Bob_99 (May 8, 2006)

Thank you everyone for all the great information. This is greatly appreciated.

:T:T:T

Bob


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## Guest (Jul 27, 2007)

bpape said:


> Not to mention if you have your speakers right next to your TV and flush, then the entire TV screen acts as part of the speaker baffle itself drastically changing the frequency response. Baffle step compensation circuitry is built into most crossovers and is designed specifically for the width of the speaker baffle. Changing the baffle width causes problems. This is one of the reasons you never take a 'box' speaker and build it into a wall unless you also compensate by killing the front of the wall with absorbtion.
> 
> Bryan


Thanks Bryan, very much.
Now, can you (and Ethan, and everybody) help me with this dilemma? I plan to improve my listening room (as soon as I pay may depths, :daydream. I am not interesting in watching movies. Yet, I do have LCD screen between my speakers. Now, I plan to threat the front wall with 4" of 703 insulation, spaced 4" from the wall. I know about the baffle problem.
What should I do? Should I cover whole front wall with the insulation, and place LCD in front of this; should I place LCD close to the wall, and put insulation around LCD (but, since LCD is close to the wall, there is no room to put insulation on part of the wall behind LCD); or should I place 4" of insulation behind LCD, but without 4" gap of air (IOW, attach insulation directly to the wall)?
My room is 345 cm wide. On one sidewall I plan to put insulation 40 cm from the sidewall, on other sidewall insulation will be 20 cm. That leaves only 285 cm for speaker placement, and the width of LCD is 82 cm, but if I'll put LCD more close to the rear wall, I will need to leave 116 cm wide gap without insulation (or with insulation attached to the front wall), because my equipment rack is 106 cm wide. TIA
-- Mario Petrinovich


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## Guest (Jul 27, 2007)

Here is a picture, so that you can visualise what I am talking about. This is a version with the 116cm gap in insulation. The other solution would be to put 4" insulation attached to the wall, in that gap, or to cover whole wall with insulation, without any gap. TIA -- Mario Petrinovich


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## Sonnie (Apr 11, 2006)

Hi Jesus and welcome to the Shack!

It would probably be better if you started a new thread with your questions so that it will get more attention... :T


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## Guest (Jul 27, 2007)

Sonnie said:


> Hi Jesus and welcome to the Shack!
> 
> It would probably be better if you started a new thread with your questions so that it will get more attention... :T


Ok thanks, I'll start a new thread. -- Mario Petrinovich


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