First reflection point - AVS Forum
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post #1 of 20 Old 11-04-2010, 11:49 AM - Thread Starter
 
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Is it always mandatory to use an absorptive at the first reflection point? Is there any advantage to not making this point absorptive?

Thanks,
Mark
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post #2 of 20 Old 11-04-2010, 12:04 PM
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Originally Posted by mark1974 View Post

Is it always mandatory to use an absorptive at the first reflection point? Is there any advantage to not making this point absorptive?

Thanks,
Mark

Mark, it is highly recommended by all the audio experts here to treat first reflection points.
The sound that bounces from these points reach your ears at different intervals of time vs. the sound you hear directly from your speakers, which results in poorer interpretation of dialog and other sources due to resultant comb filtering.

You certainly don't HAVE to but it's recommended as most important areas to address first.
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post #3 of 20 Old 11-04-2010, 12:12 PM - Thread Starter
 
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Thanks for the reply. More specifically in some cases is it better to use diffusers there instead of absorption?
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post #4 of 20 Old 11-04-2010, 12:23 PM
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My impression is that absorption can easily (and more cost effectively) address first reflection areas. Not sure what situation would recommend the use of diffusion.

I'm not an audio expert and am only offering what I have gleaned reading a lot of threads/comments. Maybe someone who is will chime in and elaborate on this for you.
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post #5 of 20 Old 11-04-2010, 12:40 PM
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On this topic, what thickness OC703 would be most suitable for first reflection absorption? 1"? 2"? Thicker?

Or would another product like Linacoustic, OC705, or maybe acoustic cotton be more suitable?

--Drew


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post #6 of 20 Old 11-04-2010, 12:56 PM
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Drew, if you take a look at Ethan's products as an example (his Microtraps and RFZ panels) range from 1.25"-2" thick. These products are recommended on the site for first reflections, with the RFZ absorbing more lower freq due to increased thickness.
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post #7 of 20 Old 11-04-2010, 01:23 PM
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Quote:
Originally Posted by mark1974 View Post

Is it always mandatory to use an absorptive at the first reflection point? Is there any advantage to not making this point absorptive?

Thanks,
Mark

The one advantage I've read about is when your speakers have superb offaxis response, that it may benefit from leaving the sidewalls untreated to expand stereo width. The reflected information is similar to the onaxis information. If OTOH the speakers have poor offaxis response, you'd want to absorb the "inaccurate" information. However, it may be that this is more for stereo than mch, sorry I really don't know much.

For the thickness of the material chosen, it boils down to your target frequencies. 1" or even 2" will do either nothing at all to very little for your bass, and it's typically the bass that is the most pressing issue in most rooms.

Corner trapping is a good goal, and the front corners are the bang for buck spots. There are many here who go nuts and do the "superchunk" traps, floor to ceiling.

Basically, it seems to me, the closer that either the listener or speaker is to any boundary, the more imperative it becomes to treat that area. For instance, if you are up against the back wall, it becomes imperative to treat that wall. If the speakers are too close to sidewalls, ditto. Too close to the front wall, ditto. Many people treat the front wall directly behind speakers to reduce SBIR.

 

 

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post #8 of 20 Old 11-04-2010, 04:32 PM
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Not all acoustic "experts" uniformly suggest absorption at the first reflection points ... Floyd Toole among them. Absorption would be called for if the off axis response of the speaker is, well, bad. You don't want that reflected back into the room. The key is to determine the arrival time of the early reflections vs the direct sound and the estimated dB delta between the direct and early reflected sounds.

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post #9 of 20 Old 11-04-2010, 05:05 PM
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Curious then, what if you have electrostatic panels like Martin Logans where there's almost no off-axis response? Assuming you kill everything going backwards and toe in like mad, could that actually be almost kinda sorta sufficient? I'm talking blind theory here and ignoring the fact that if you toe in enough, you'll get the left main bouncing off the right wall and really hose things up.
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post #10 of 20 Old 11-04-2010, 05:09 PM
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Quote:
Originally Posted by Dennis Erskine View Post

Not all acoustic "experts" uniformly suggest absorption at the first reflection points ... Floyd Toole among them. Absorption would be called for if the off axis response of the speaker is, well, bad. You don't want that reflected back into the room. The key is to determine the arrival time of the early reflections vs the direct sound and the estimated dB delta between the direct and early reflected sounds.

"Maybe someone who is will chime in and elaborate on this for you."

And there you have it, except for the diffusion vs. absorption question.
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post #11 of 20 Old 11-04-2010, 07:19 PM
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^^^^^

How about a combination of both! Absorption to deal with low frequency interference, and diffusion to deal with preserving and diffusing high frquencies keeping that larger apparent stage width. Best of both worlds! There have been a number of threads that have discussed this, and there are several products out there that deal with this; ex. Quest Perfsorber, RPG BAD panel. Best wishes!

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post #12 of 20 Old 11-05-2010, 03:54 AM
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Quote:
what if you have electrostatic panels like Martin Logans where there's almost no off-axis response?

That's, well where I come from, horse manure. Take your MLs out into your back yard, fire up some noise (or music to your taste), and stand perpendicular to the speaker (to the side of it). You're listening to the off axis response (which in the ML case, is rather poor ... to be polite).

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post #13 of 20 Old 11-05-2010, 04:20 AM
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Quote:
Originally Posted by Dennis Erskine View Post

That's, well where I come from, horse manure. Take your MLs out into your back yard, fire up some noise (or music to your taste), and stand perpendicular to the speaker (to the side of it). You're listening to the off axis response (which in the ML case, is rather poor ... to be polite).

Well, to be fair Dennis, nobody is going to be listening to their speakers outside on a regular basis so his point does have some merit.

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post #14 of 20 Old 11-05-2010, 06:43 AM
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No, it has no merit. He's saying electrostatics have little or no off axis response. That is a false statement. Taking the speakers outside is a way to prove that point due the absence of side walls, front walls, or back walls...a poor man's anechoic chamber.

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post #15 of 20 Old 11-05-2010, 07:08 AM
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Quote:
Originally Posted by mark1974 View Post

More specifically in some cases is it better to use diffusers there instead of absorption?

I would use both. Diffuse the first reflection from the nearby speaker, so that the soundstage from the left speaker extends a bit further out to the left, without the acoustical mirror effect of a bare wall. Absorb the first reflection of the far away speaker, so that sounds from the left speaker aren't bouncing off the right wall and coming at you from the opposite direction of the speaker.

Sanjay
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post #16 of 20 Old 11-05-2010, 07:09 AM
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Question....is it fair to say that generally speaking electrostatics and/or ribbon speakers have poor off axis response?
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post #17 of 20 Old 11-05-2010, 09:23 AM
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Quote:
Originally Posted by Dennis Erskine View Post

No, it has no merit. He's saying electrostatics have little or no off axis response. That is a false statement. Taking the speakers outside is a way to prove that point due the absence of side walls, front walls, or back walls...a poor man's anechoic chamber.

I'm confused. I thought he meant the off axis indoors is bad. Wouldn't the off axis response be worse outdoors with no boundary layer?

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post #18 of 20 Old 11-05-2010, 09:39 AM
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The reflection of a poor off axis response is what we are trying to avoid...hence absorb it if it is poor, so you are correct. But in order to determine if a speaker has poor off axis response, we need to measure the speaker itself in the nearfield. Dennis is going the route of the extreme poor man's method...take your speaker outside with no boundary effects and use you ears...although you kind of have to know what you are listening for.

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post #19 of 20 Old 11-05-2010, 10:13 AM
 
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Originally Posted by flyng_fool View Post
I'm confused. I thought he meant the off axis indoors is bad. Wouldn't the off axis response be worse outdoors with no boundary layer?
Dennis's point has nothing to do with whether the speaker is inside or outside. His point was simply an example to illustrate that electrostats still radiate a lot of energy sideways where at least in simplistic theory you'd expect them to cancel out and radiate no energy. If you put such a speaker in an anechoic chamber you hear ONLY the sound coming from the speaker, not coming from reflections, and outside with no boundaries is a simple anechoic chamber, what dennis called the poor-man's anechoic chamber.

If you were to take one outside and listen off-axis perpendicular to the speaker, you'd still hear a lot of sound coming directly from the speaker.

His point is simply that despite the fact that electrostats have very different directionality than other speaker designs, there is still a lot of off-axis sound happening and you still need to pay attention to first reflection points, perhaps even more significantly so because of the severe changes in FR as you move off-axis with an electrostat.

It doesn't matter where the speaker is, the off-axis response is always the same and has nothing to do with the room. Moving the speaker outside is just an example experiment to hear directly what that off-axis response is, in an isolated fashion without having all the room boundaries obscure the sound directly from the speaker.
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post #20 of 20 Old 11-05-2010, 11:37 AM
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Ah, I see now. Thanks.

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