# active vs passive room treatment



## venkataraman manu (Dec 11, 2011)

Hello, fellow Shaksters,
Greetings as I start another topic of interest.This is with regard to the effects, need and ( or otherwise ) deployment of active or passive room correction.I have a feeling that with the advent of real quality room acoustic management like Audyssey ( it's latest avatar xt32, it is really good:R ) will we able to say bye to the tedious and expensive passive treatment of a room?! 
It would be convenient and cost effective, esthetically more appealing if we could go only with the dynamic treatment that Audyssey multi position room correction does than to go with a mere passive treatment of absorbers, diffusers.Or should we go with both? Wish I could know the answer:dontknow:
I know what Audyssey ( I have used all their avatars.Latest is best_ imo )_does to sound in an untreated room like my living room as a standalone management .It has my large fabric covered recliners doing duty as absorbers and bass traps though:heeheeoes a fine job of tuning of all ranges of sound that meets my ears as I sit and I am very happy with the results.Immersive and enveloping audio bubble really. But is it enough or is there more to it? 
I wish to know your views on this now that I am nearing the end of my HT construction.Could save me a pile:bigsmile: 
Thanks :wave:


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## bpape (Sep 14, 2006)

Audyssey can only address frequency related issues in the room. It cannot correct for decay time or for reflections which is where treatment comes in (in addition to addressing some frequency response issues thereby reducing the need for EQ.

Also, Audyssey setup (or any EQ) can only really optimize for one seat. The rest are addressed based on what can be done for all seats that have the same problems. If one chair has a peak and another has a null at the same frequency, EQ cannot fix that.

Bryan


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## venkataraman manu (Dec 11, 2011)

Hi, Bryan,
Thanks for your views.I agree with you that the best position calibration is best in primary calibration seat, however much they say at Audyssey that it makes 8 seats equal in audio perception.
But I think that the time domain correction that Audyssey does by effecting delays to all the speakers is simple and effective and _ imo_ it is actually responsible for the enveloping sound field that it generates.Does passive correction work on the time domain?
Best:wave:
Manu


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## robbo266317 (Sep 22, 2008)

You should always try and optimise the sound before adding any equalisation.


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## venkataraman manu (Dec 11, 2011)

Well, I get the point, thanks:wave:


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## bpape (Sep 14, 2006)

Yes. it will certainly address getting the timing of the speaker correct in relation to each other, though again, it can only do so for one position.

What I was really referring to was the decay time in the room. The frequency response may be nearly perfect but if 100Hz down is only 20-25db down after 1 second, it's still not right and a lot of things are still being masked.

Bryan


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## venkataraman manu (Dec 11, 2011)

Thanks Bryan, I am going to apply some acoustic treatment outlined by Auralex when I sent them my room dimensions.Check out my attachment and let me know your views on it.As I live in South India, there are not many dealers on diffusers or absorbers so I am planning to put absorbers on dIY basis, using rockwool 2" for absorbers and 4" rockwool for bass traps.I bought thro eBay the Auralex q'fusers.The placement I intend to go with Auralex diagram.What is your take on it?
Best
Manu :wave:


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## venkataraman manu (Dec 11, 2011)

Thanks Bryan, I am going to apply some acoustic treatment outlined by Auralex when I sent them my room dimensions.Check out my attachment and let me know your views on it.As I live in South India, there are not many dealers on diffusers or absorbers so I am planning to put absorbers on dIY basis, using rockwool 2" for absorbers and 4" rockwool for _bass traps_.I bought thro eBay the Auralex q'fusers a box of 14 pieces of about 2ft square.The placement I intend to go with Auralex diagram.What is your take on it?
Best
Manu 
Attachments
Seetharaman Venkataraman Home Theater.pdf (319.6 KB, 0 views)



Read more: active vs passive room treatment - Home Theater Forum and Systems - HomeTheaterShack.com


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## bpape (Sep 14, 2006)

Not able to access your attachment.

Bryan


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## venkataraman manu (Dec 11, 2011)

Hope you see tha attachment now!
Manu


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## bpape (Sep 14, 2006)

Personally, I would go thicker on the side wall reflections and skip the fusors on the back in exchange for at least 2-3 6" panels. You can use the Q's high around the perimeter and flanking the absorption on the rear wall.

Bryan


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## venkataraman manu (Dec 11, 2011)

Hello, Bryan,
2-3 6" panels, Bryan? You mean in addition to bass traps? I am going to have them in all 4 corners using 4" rockwool on a DI Y basis, floor to ceiling.
You think I should deploy the q'fusers on the ceiling above my seating position? I also see Auralex wants to have the Wave6 diffusers on the top in addition to the back wall. I thought they would do a better job at the back wall where the front speaker's sound output would fall. I also am thinking of making the diffusers using mdf as material.What are your opinions on it? Maybe deploy them instead of wave6( not available here in India ) 
Thanks for your views and gimme more :clap:
Manu


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## bpape (Sep 14, 2006)

I think you're going to get more benefit from the thick absorption in the middle of the rear wall. You can skip or at a minimum face the corner ones to minimize mid/high frequency absorption.

In your situation, high around the perimeter, ceiling reflections, and rear portion of the side walls would be the places I would do the diffusion.

Bryan


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## venkataraman manu (Dec 11, 2011)

Thanks for your thought, Bryan!
Now, my gear is a 9.2 system and my query is whether that would need a change in the acoustic treatment of my HT room that we are discussing about as opposed to stereo 2 channel listening and acoustics treatment thereof?
Since there are so many speakers, 9 to be precise in my case ( plus the two subs) with about 18 each of 1 and 2 reflection points to be controlled with absorbers and diffusers and bass traps, it gets complicated as opposed to two channel room treatment.
Could you comment on that?
Thanks
Manu


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## bpape (Sep 14, 2006)

I'm not as concerned about the reflections from the surrounds other than on the front wall. 

2 channel and HT are 2 very different things. The overall amount of absorption would not be as high in a 2 channel space. Corners and reflections are still needing to be addressed and the front wall should be 100% dead for HT and won't hurt the 2 channel.


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## venkataraman manu (Dec 11, 2011)

I was surprised that the Auralex made no mention on the treatment on the front wall except the bass absorbers. You have mentioned 100% dead front wall! That is difficult with the 55" display occupying space and the front high and center speakers mounted there too!
By the way, is there any simple way of measuring the decay( I guess it is same as the reverberation time?). Not too great on the use of REW.I am a surgeon, see! :dontknow:
Manu


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## bpape (Sep 14, 2006)

Do as much of the front as you can. Sure - there are always a few things in the way.

REW is the easiest way to measure the decay time in a room.

Bryan


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## bsaaudio (May 4, 2012)

If I might jump in on this thread… acoustical treatment plans short of actually measuring the response of a room as energized by the actual system is a futile attempt to achieve success. There is a significant gap between predictive modeling and real world measurement. Never set in stone any acoustical treatment plan before actually understanding what it is the room is doing when energized by a system at the intended listening position and the direction of the source. 

Considering that home theater speaker systems are generally near field devices, I see more of a potential problem with seating locations that are in the far field which exposes your listening position to weight more heavily towards the influence of room reflections. In many instances, perceivable improvements can be made by simply moving into the near field of the front speaker array if practical. At the very least it is first and best place to start in creating a true sweet spot for a room. 

In the near field the typical 6db reduction of volume level per doubling of distance is not observed which translates into more direct energy exposure. Our brains process direct and reflected energy as a single source within a time window of 50 ms so it makes sense to reside in the near field where the direct sound has a greater influence on the cumulative effect. 

Forgot to add that anything I might add is certainly up for rebuttal.


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## bpape (Sep 14, 2006)

While I would agree that it is important to understand what the room is actually doing, to say that without knowing 100% doing any treatment is futile is a bit extreme. There are absolutely known things that can be addressed regardless. Decay time can absolutely be predicted. Reflections need to be addressed (though how to address them can certainly change pending measurements), Known problems such as 70ish Hz suckouts from 8' ceilings, the need to address the front wall to prevent surround reflections, etc. all can and should be done regardless.

Bryan


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## bsaaudio (May 4, 2012)

Of course you can predict LEDT's,RT60's, Room Nodes the question remains are you relying on predicitons or real world verification to approach a room's fate? I think you stated better that things change and that was my intended context given a manufacturer simply gave a fellow shackster a plan on paper without critical judgements of what his room is actually giving him.

You mentioned the practice of treating the front walls primarily for surround reflections. Is this really a common problem or is the reason for front wall treatment primarily done for the front screen array boundary effect as a obstacle to our mutlichannel happiness?onder: 

On a tagent issue. I'm very surprised at what FFT based impulse respone measurements are giving me in the LFE band pass as far as arrival times. For example a subwoofer may be physically 8 feet from the measurement microphone, but the impulse response is showing a much longer propgation time that can't be explained. I've heard "rumors" that placing the measurement microhone on the floor takes the floor reflection out of play and can yeild a more accurate data as to how to handle and manage the LFE bandpass in the system. Any thoughts?


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## bpape (Sep 14, 2006)

Putting the mic on the floor is great if you are going to be listening with your ear on the floor.  Seriously, the path length is very different there than to your ear so while it may take the reflection out, it's not at the right distance. You must measure where you listen. If there are bounces going on that make it react as though it's a different distance, then those need to be corrected.

Front wall is one of those things that should be done regardless. Now whether you need 2" or something thicker should be determined by a measurement in some cases. Regardless, it's required. We know reflections need to be handled. Absorption is usually best. That said, do we use 2", 4", 4" spaced off the wall? 4" is safer but many people simply don't want something that thick or are in real rooms where width issues are problematic and simply can't tolerate it. Again, reality and usability need to be blended with theory and measurement.

RT60 is only for large room acoustics. I'm looking at more around T30 in most residential spaces. That is one thing that does not change. It can be accurately predicted IF you account for all surfaces, furnishings, construction, people, etc. If not, then you're not making an accurate or appropriate prediction.


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## bsaaudio (May 4, 2012)

Of course if 1st order reflections in the mid to high bands from surrounds warrant it, but short of that, there are no reasons to simply make it common practice for every room. 

The old thinking was that boundary effects in the low bandpasses from the front speakers were the primary reason for treatment, but field studies for example by Pat Brown with SynAudCon found that agressive acoustical treatment on boundaries (quarter spacing test) changed the measurement slightly, but not enough to call it a solution to smoother response. In summery the bottom line is that free space placement of loudspeakers and/or horn loading the components is the best path to smoother response from a system design perspective. In small room application-HT, good luck! you can't really avoid boundary effect short of in-wall designs. Bass traps in the corners to tame modes (as indicated by Aurlex) seems to be the primary concern for front wall treatment short of geometry corrections of the room. 

In many of the case studies I've read by Sam Burkow of the SIA Smaart Live fame (the commercial transfer function software I use by trade) He comments that too much is made regarding decay times (RT). His point is that two rooms with identicle decay times can sound radically different due to the distribution of the reflections. According to Burkow, he considers decay time the smallest piece of the acoustical puzzle and the least important primarly because decay time is a function of room volume and overall absoption rather than distribution. I guess that would also apply to RT30, 20 or whatever interval of time you are evaluating rate of decay at various frequencies. 

This sort of plays into my original point that you have to deal with the real room rather than simply apply predictions and expectations to form standardized ways of approaching home theater because they are small rooms and can be predicted. For every mathematical equation that predicts, there are caveats that go against the prediction. 

Shortly, I'll start a new thread on the idea of why mic placement on the floor for measurement is indicated in certain situations. Its interesting and directly relates to transfer function measurement with tools like REW. :T


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## bpape (Sep 14, 2006)

Reflections on the side do need to be addressed. More a matter of how and how much.

You've gone from making statements on a recommendation on one room to making statements that people are trying to make global recommendations on all rooms.

How big a part decay time plays is also related to what the room is to be used for. Did Burkow make those statements in relation to multi-channel home theater - I don't believe so.

Be very careful taking snippets of things like that out of context and trying to apply them across all situations - exactly what you're saying others are trying to do by making generalized recommendations without measurement. 

Pot, meet kettle :whistling: :huh:


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## bsaaudio (May 4, 2012)

Yes, Dr Floyd Toole has written and commented that side wall reflections are signficant problems. They are for lack of a better description the most important to deal with but not the only things to deal with. I'm not sure what your point is exactly regarding side wall treatment???? 

The context of this thread at least from the way I'm posting from is not "side wall" but "front wall" treatment. More specifically the fact that Aurlex submitted a schedule of materials to an indiividual that only included corner bass traps on the front wall and nothing else. 

Somebody (your associate if I'm not mistaken) then interjected implying that front wall treatmeant by way of practice is something that is usually done. I don't know that...a blanket statement as I took it. The recommendation of Aurlex conflicts with the blanket statement because it their recommendation lacked any front wall treatment beyond the bass traps in the front corners. 

Secondly, Sam Burkow did in fact make his "snipet" within the context of a case study of a HOME THEATER in spite of what you may believe. The case study involved transfer function measurement that made a direct colleratation of predicted/calculated modal responses that were at the center of subjective problem the HT owner was complaining about.

The owner fell victim to inadequate treatment and left it in a state of "unbalance". The modal responses were always there, but became even more perceived once the thin abosorption panels changed the perception of the room to the owner. It went from "bad" to a "different bad" when the panels were added. 

I guess Burkow could have complely left the context of the case study to simply inject something about large room acoustics as related to RT60 of large rooms. Maybe you have a better insight on why he might do that? I'm at the mercy of what I'm reading. 

I think his "snipet" should be weighted with precision and how he gives it. Decay time is the smallest piece of the puzzle (not the only piece). I took his snipet in context of what RT is all about. Absorption and room volume are the primary varriables that affect room decay and are mathetematically expressed by those factors alone. 

Take any room, and heavily load absorption on one end or one half of the room and you can enact changes to the decay of the room. You can then distribute the same treatment differently yet you have the same amount of treatment, the same volume of room, and the calculated decay time should not change. Burkow was not qualifiying his statement by large or small room acoustics but specifically what RT measurement is.

Third, your associate made another blanket statement that T30 not T60 is used for small room acoustics. The reason is that you can't obtain a T60 measurement in a small room is because the decay is so quick you run into noise floor to accurately asses the rate of decay down to -60db which is standard for looking at reverberation time anyway. 

T30 and T20 are used to EXTRAPOLATE down to a -60db decay rate so essentially you are desicribing the same thing but different ways to get there depending on what the room allows for measurement. T30 is not a measurment exlusively to HT rooms because they are small. but merely a work around to see the decay down 60db where the energy is imperecievable or without consequence if you will. 

And finally, the decay rate of the room is void of subjective preference as Burkow alludes. Nobody goes into a room and says we need to target this specific decay rate because it "sounds the best". 

You shouldn't find it out of context to look at field studies of boundary effect. You place any speaker near a boundary and you have issues to deal with. Pat Brown's study was interesting because he actually showed response curves of a speaker placed directly against a boundary where aggressive treatment was made (front wall and floor) and then compared the same speaker in the free field. He looked at many different speaker types including near field monitors. Home theater speakers are essentially near field devices. 

Even with acoustical treatment (aka on the front wall) where you might intuitively think its warranted because the speakers are commonly and unavoidably found near signficant boundaries there, the bang for the buck to treat the wall is marginal compartive to free space placement which would have a greater positive effect. If recording studies go out of their way to place monitors in the free field (both near field on the console and inserted in the front wall) maybe we should pay attention or at least do what we can to place speakers as far away from boundaries as practical and then assess how to correct the issues.


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## bpape (Sep 14, 2006)

I apologize for the snide remark - it was meant as a joke. 

I was truly asking if the remarks you referenced were for home theater. Not trying to say that they weren't - I just wasn't familiar with that work.

I would disagree that if you take the same amount of treatment all piled in one area of the room and then distribute it throughout the room (or in more targeted areas) the decay time will not change.

I understand well the boundary interaction you're speaking of on the front wall and the same exists on the side walls and the ceiling.

I would agree that the company in question (Auralex) did a disservice by not addressing the front wall but not specifically for the reason of boundary interactions but for stopping reflections from the surround channels from contaminating the front soundstage. Again, not saying in any given situation that the boundary interaction might not also need to be addressed but that simply then goes to how thick one needs to go potentially - not whether it needs to be done.


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