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Need help with Acoustic Panels and Bass Trap Placement for my HT

post #1 of 17
4/16/12 at 8:05pm
Thread Starter 
My neighbor purchased $900 worth of acoustic panels and bass traps. When he moved into our neighborhood, he did not need all of them for his living room. He had 10 acoustic panels and 4 bass traps in his garage that were being unused. He sold them to me for $15 each so I jumped on it.

I brought them home the other night and immediately noticed a huge difference in clarity and detail in my setup. You can read about it in my build thread.

Here are what the panels look like





Here are what the bass traps look like








Below were my initial thoughts of where I might place them.






I ended up temporarily placing the panels (leaning them against the wall) as I sketched out in the above diagrams with the following exceptions:

I did not place Bass Traps in the top right and left corners (will need to make a shelf for that).
I put 1 Acoustic Panel behind the Projection Screen
I did not place two Panels on the Rear Side Walls
I only placed one Panel on the Back Wall.

Other than that, everything else was placed exactly as I had originally planned.

Before just arbitrarily placing the panels in the room where I think they should go, I thought I would seek advice from the experts. The only thing I'm pretty sure of is that I need panels at the First Reflection Points and I have used a tall mirror to determine where those locations are.

My room is 13' x 19' with 10' ceilings.

Any advice you can provide would be appreciated.
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post #2 of 17
4/17/12 at 5:45am
Quote:
Originally Posted by youthman View Post

Before just arbitrarily placing the panels in the room where I think they should go, I thought I would seek advice from the experts. The only thing I'm pretty sure of is that I need panels at the First Reflection Points and I have used a tall mirror to determine where those locations are.

for the LF absorbers (bass traps, if you want to call them that) - initially place them in the 3D and 2D corners --- as much surface area as possible.

in the specular region (~250hz and above), the sonic energy can be modeled like light and thus, that is why you can utilize a mirror to determine a "geometric reflection point".

the problem with this is, the mirror details you no information as to the gain or arrival time of the reflection. thus, a reflection point may be incident of a low-gain indirect reflection that does not require attenuation (though by utilizing the mirror, you might blindly place absorption there anyways).

do you have measurement gear?
a $39 omni-mic, $69 pre-amp/soundcard/phantom-power device, $30 in cables, and $0 for a free copy of Room Eq Wizard will allow you to measure the actual acoustical behavior of the space.

the time-domain takes precedence for this scenario --- via the Envelope Time Curve. the ETC response details to you how specular energy impedes the listening position. Gain vs Time.

as such, you'll see the direct signal spike first as the direct signal is a straight vector to the listening position and thus, the shortest path. since the speed of sound is a constant within your room, then the direct signal will arrive first - followed by early arriving reflections, followed by later arriving reflections, and lastly, the decay of the specular energy until all of the energy has been fully damped.

what this tool does, is detail to you the ACTUAL high-gain, early arriving indirect specular reflection paths --- and not merely all POSSIBLE reflection paths like a mirror does. this allows you to SURGICALLY place the broadband absorbers ONLY at the boundaries incident of the high-gain early arriving reflections --- which limits the amount of specular absorption you use in the room.

when you measure (one source at a time!) with the ETC, you will see spikes of indirect energy. the total time of flight (in milliseconds) can be determined, and you can work backwards to identify the total flight path (in feet) and thus, identify which boundary the spike of energy on the graph corresponds to. you can then place your absorber panel there, REMEASURE, and VERIFY that the broadband absorber is effectively attenuating the reflection as well as verifying it is attenuated across the entire listening position!



the ETC is also useful in determining edge diffraction generators, or other "non-obvious" sources of early, high-gain indirect energy. it can also be utilized for coupling issues --- as you may find energy arriving before the direct signal in the ETC (seemingly impossible), but this can be due to coupling issues and transmission through a medium (solid foundation) that has a higher transmission rate than the speed of sound in air.

for an example, please see user:fotto's thread here (starting at post#7): http://www.avsforum.com/avs-vb/showthread.php?t=1374014
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post #3 of 17
4/17/12 at 5:53am
Thread Starter 
Thank you for the wealth of information. You just verified my point that there is much more involved with this than just adding panels and traps where I think they should go. Seems there is MUCH I have to learn in this subject. Wish I knew someone knowledgeable in this area that I could invite over to help lead me through the process.
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post #4 of 17
4/17/12 at 5:56am
there is nothing inherently wrong with your original approach, just understand there are better "tools" out there than a mirror in determining the actual acoustical behavior of the space
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post #5 of 17
4/17/12 at 5:58am
Thread Starter 
Well, my mirror was only going to be used to determine the first reflection point. I had no idea how to determine where the rest of them "should" be placed and how many of them to use. I don't want the room to be too dead, but want to properly correct any issues in the room.
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post #6 of 17
4/17/12 at 6:25am
Quote:
Originally Posted by youthman View Post

Thank you for the wealth of information. You just verified my point that there is much more involved with this than just adding panels and traps where I think they should go. Seems there is MUCH I have to learn in this subject. Wish I knew someone knowledgeable in this area that I could invite over to help lead me through the process.

yes of course, there is more and lots of science to it.

Meanwhile however I can assure you that just doing what you did will improve the sound very very significantly
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post #7 of 17
4/17/12 at 6:44am
Thread Starter 
Quote:
Originally Posted by windwaves View Post

yes of course, there is more and lots of science to it.

Meanwhile however I can assure you that just doing what you did will improve the sound very very significantly

You are correct. I did arrange them in my room without "installing" them on the walls. Here is what I posted in my build thread.

Well, I spent a few hours in the HT tonight. I demoed familiar CD's, DVD's, etc. and here is what I have concluded....Acoustic Panels are pure snake oil.

Just kidding. Stick out tongue . My findings were quite the opposite. My methodology was not scientific, I did not actually measure anything, my observation is only what I heard through my non-audiophile ears.

Placement:

I ended up placing the panels as I sketched out in the above diagrams with the following exceptions:

I did not place Bass Traps in the top right and left corners (will need to make a shelf for that).
I put 1 Acoustic Panel behind the Projection Screen
I did not place two Panels on the Rear Side Walls
I only placed one Panel on the Back Wall.

Other than that, everything else was placed exactly as I had originally planned.


Observations:

When doing the Clap Test, I still hear a slight slap echo (maybe coming from the ceiling or other parts of the wall that do not have panels) but it is not near as loud as prior to adding the panels.

Immedately, I noticed much more clarity in my main speakers and even my rears seemed to sound better. The center was crystal clear as well, dialog was slightly better (I think this can be helped with proper EQ).

I have not re-run EmoQ (which to me is kind of useless) but I will give it a try with the panels installed since I have 3 Presets I can save and I've only used 2 Presets so far.

Bluray Observations:

Book of Eli - I watched the scene where they are in the old couple's home and the crew shows up and begins to demolish the house with their weapons. Every bullet was so clear, pieces of wood flying were very distinct. Lots of detail were heard.

Music Observations:

I have always heard that room treatments are one of the best investments you can make for audio. It makes logical sense. Just thinking about the first reflection point. Your ears are hearing sound coming straight from the speaker and then a reflected sound slightly after the initial sound. This causes the sound to be muddy or muffled since your brain is confused. With panels at the first reflection point, you hear the sound coming directly from the speaker and it is much clearer.

The Soundstage was more open and wider. I was able to hear every chime, every pluck of the guitar, breaths etc. Again, very distinct and precise.

I'm not sure how much difference the Bass Traps made as I did not do a before/after test and I only have two of them and they are only in one corner of the room. I'm sure it helps but I know the bass had already smoothed out a lot since I added the 2nd subwoofer.

I brought out my SPL meter and cranked a song up to 102db (much louder than I typically listen to) and the music remained sharp, detailed and unstrained. It never got muddy, even at very high volume.

My Conclusion:

I've always known my room had acoustic issues. Lots of hard surfaces and plenty of echo that was causing the audio to not be as clear as I believed it should be.

Since they are black, they match perfectly with my color scheme.

They definitely added more clarity to instruments, surround effects.

I met up with my neighbor who lent them to me and told him to think about how much he wanted for them. His reply was that he was just glad they were being used since they were just taking up space in his garage. I got the impression that he might even give them to me but I don't mind paying for them so I told him to let me know how much he wanted for them. We will see. He did say that he bought them cheap and paid $900 for all of them ( the 10 panels and 4 bass traps that I have and the ones he is currently using in his living room (maybe 2 bass traps and 8 panels).

A few weeks ago, I dropped my Nikon D90 camera but it's on it's way back from repair and will be here on Wednesday. I'll work on getting the panels mounted so that I can take updated pics of the room with panels installed when it gets here.
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post #8 of 17
4/17/12 at 6:47am
Quote:
Originally Posted by youthman View Post

Well, my mirror was only going to be used to determine the first reflection point. I had no idea how to determine where the rest of them "should" be placed and how many of them to use. I don't want the room to be too dead, but want to properly correct any issues in the room.

then the ETC is the tool for the job - using it to identify ACTUAL high-gain early arriving reflection paths vs simply ALL POSSIBLE reflection paths as with a mirror --- surgically LIMITING the amount of broadband absorption within the room.

+1 to you for understanding this point very early on! very refreshing
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post #9 of 17
4/17/12 at 7:00am
Quote:
Originally Posted by youthman View Post

Immedately, I noticed much more clarity in my main speakers and even my rears seemed to sound better. The center was crystal clear as well, dialog was slightly better (I think this can be helped with proper EQ).

you cannot use EQ to address speaker-room interaction for non-minimum phase issues. that is why you utilize treatment to modify the room to physically address such issues.

Quote:
Originally Posted by youthman View Post

I have always heard that room treatments are one of the best investments you can make for audio. It makes logical sense. Just thinking about the first reflection point. Your ears are hearing sound coming straight from the speaker and then a reflected sound slightly after the initial sound. This causes the sound to be muddy or muffled since your brain is confused. With panels at the first reflection point, you hear the sound coming directly from the speaker and it is much clearer.

the ear-brain does not possess the resolution to determine indirect signals (reflections) arriving within the haas interval as being discrete events --- thus, it "fuses" the indirect signals with the direct signal into a "single auditory event" --- Richard Heyser called this, "time-smear distortion".

attenuating the high-gain, early arriving indirect signals allow your brain to have adequate time to process the direct signal --- something physically larger rooms naturally provide (longer time until indirect first reflections impede listening position). this will maintain accuracy of localization, imaging, etc...

when we attenuate (eg, absorb in this case) the high-gain early arriving signals, they are attenuated below the human detection threshold such that the ear-brain is NOT keying on those indirect signals for localization and imaging.

Quote:
Originally Posted by youthman View Post

I'm not sure how much difference the Bass Traps made as I did not do a before/after test and I only have two of them and they are only in one corner of the room. I'm sure it helps but I know the bass had already smoothed out a lot since I added the 2nd subwoofer.

LF porous absorption requires a large amount of surface area, as well as their effectiveness to lower (longer) wavelengths is determined by the placement of the porous insulation into areas of relatively high particle velocity. you can measure with the waterfall plot (0-300hz) to see their effectiveness --- it is not only the frequency response you are attempting to even out (due to the bass trap absorbing some of the energy such that when it is reflected back into the room, the gain is not so high and thus the constructive and destructive interference is minimized) --- but you are also looking to decrease the LF decay times (modal ringing) as well. this is a big factor in LF quality - 800ms decay times are quite common in typical, untreated small acoustical spaces, which will result in the LF notes "running together" into a muddied mess.


Quote:
Originally Posted by youthman View Post

I met up with my neighbor who lent them to me and told him to think about how much he wanted for them. His reply was that he was just glad they were being used since they were just taking up space in his garage. I got the impression that he might even give them to me but I don't mind paying for them so I told him to let me know how much he wanted for them. We will see. He did say that he bought them cheap and paid $900 for all of them ( the 10 panels and 4 bass traps that I have and the ones he is currently using in his living room (maybe 2 bass traps and 8 panels).

you can DIY for much, much, much lower cost than that - they are relatively easy to make.

glad to hear on the progress -
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post #10 of 17
4/17/12 at 7:18am
Thread Starter 
Quote:
Originally Posted by localhost127 View Post

you can DIY for much, much, much lower cost than that - they are relatively easy to make.

glad to hear on the progress -

I'm buying them from him for $210. Don't think I could make them for that price. He paid full retail for them I'm sure.

Quote:
Originally Posted by localhost127 View Post

you cannot use EQ to address speaker-room interaction for non-minimum phase issues. that is why you utilize treatment to modify the room to physically address such issues.

The more I'm learning, it seems it's best to use treatments to change the sound as opposed to trying to manipulate the sound with an EQ in my Pre/Pro. Is that correct?
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post #11 of 17
4/17/12 at 7:51am
Quote:
Originally Posted by youthman View Post

The more I'm learning, it seems it's best to use treatments to change the sound as opposed to trying to manipulate the sound with an EQ in my Pre/Pro. Is that correct?

you can use EQ to modify the direct signal, but you do not use EQ to modify the resultant frequency response AT the listening position as dictated by the speaker-room interaction.

the frequency response at the listening position is a summation (superposition) of the direct and indirect signals. as the indirect signals take a longer flight path, they arrive and sum with the direct signal constructively (peak) and destructively (null) into a comb-filter interference pattern.

you'll notice many speakers have their frequency response measured in an anechoic chamber, which is a measurement of the direct signal only (no reflections and thus, no room interaction). so, the speaker may measure flat in the anechoic chamber but once it is placed into a bounded acoustical space, you now have the summation of the indirect signals which causes the interference pattern in the frequency response as measured at the listening position.

attenuating the high-gain early arriving indirect signals not only plays into the psycho-acoustics of intelligibility, localization, and imaging, but attenuating of those high-gain reflections also decreases the amount of constructive and destructive interference in the frequency response.

here is an example that is meant to detail just such superposition.
the ETC (time-domain) is on the left, and the frequency response is on the right. you can see the direct signal in the ETC arriving first, and then the resultant reflections from the room boundaries. the frequency response on the right shows the interference pattern of the summation (superposition) of the indirect signals with the direct signal.

you can see that as the high-gain reflections are attenuated, the resultant interference from superposition is reduced.

now, the intent of this image is NOT telling you to go and cover the entire room in absorption (we've had a few users on this forum erroneously go off on tangents thinking that is the case) --- it is merely a statement of concept to demonstrate how the frequency response changes as a resultant of attenuating the high-gain indirect reflections. it is meant to show you how time-domain and frequency-domain are related.



we identify the high-gain indirect reflections in the time-domain (with the ETC), and as they are attenuated, then that will automatically reduce the interference pattern within the frequency response.
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post #12 of 17
4/17/12 at 5:53pm
Quote:
Originally Posted by youthman View Post

The more I'm learning, it seems it's best to use treatments to change the sound as opposed to trying to manipulate the sound with an EQ in my Pre/Pro. Is that correct?

It is true that you don't want to use EQ to fix problems it cannot. However, EQ is mandatory as part of smoothing your low frequency response. It is just that you have to apply it in the right order. And that order, calls for other techniques before you even start with treating the room. Here is the article I wrote recently on how to optimize low frequency response: http://www.madronadigital.com/Librar...imization.html

The techniques there are quite a bit more powerful that placing a few corner traps. See how little difference this poster got with acoustic products in low frequencies: http://www.avsforum.com/avs-vb/showt...3#post21900503

This is not to say acoustic products are not useful. But if you can at all, as the article explains, try to reduce the impact of the room using other means. Then what is left over can be treated with acoustic material.

BTW, your theater looks very nice! Congrats on going down the journey of making it sound better.
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post #13 of 17
4/17/12 at 6:58pm
Quote:
Originally Posted by amirm View Post


The techniques there are quite a bit more powerful that placing a few corner traps. See how little difference this poster got with acoustic products in low frequencies: http://www.avsforum.com/avs-vb/showt...3#post21900503

And here is an example of my low frequency response by ONLY using bass traps and Audyssey, which is included in many mainstream receivers:


No additional EQ needed via additional components. So, YMMV based on your room and you want to approach it.
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post #14 of 17
4/17/12 at 7:24pm
Quote:
Originally Posted by fotto View Post

And here is an example of my low frequency response by ONLY using bass traps and Audyssey, which is included in many mainstream receivers:


No additional EQ needed via additional components. So, YMMV based on your room and you want to approach it.

Thanks for chiming in Floyd. As I noted in my article, automatic EQ systems are a form of EQ, albeit, one that you cannot control manually. So I do recommend testing them to see if they improve the situation. Unfortunately they don't always improve things even if they do level the response. I show the results of some tests related to that in the article.

As to your graph, when I look at where you post it, you had noted that it was smoothed to 1/3 octave. As Dragon noted, you need to go 1/12 for low frequencies. I see that you post a full frequency response at 1/12 after that but not one zoomed into bass frequencies as you do above (and I do in my article where I focus even narrower to 80 Hz). Do you have that with and without Audyssey by chance?

As a coincident, someone just post their measurements of adding bass traps and disappointing results they got there: http://www.avsforum.com/avs-vb/showt...6#post21919906
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post #15 of 17
4/17/12 at 7:43pm
Quote:
Originally Posted by amirm View Post

As to your graph, when I look at where you post it, you had noted that it was smoothed to 1/3 octave. As Dragon noted, you need to go 1/12 for low frequencies. I see that you post a full frequency response at 1/12 after that but not one zoomed into bass frequencies as you do above (and I do in my article where I focus even narrower to 80 Hz). Do you have that with and without Audyssey by chance?

The two graphs you are looking at in the post you linked show FR freq response at 1/3 smoothing, and the bass response is with no smoothing. If the bass response graph would have had smoothing applied, it would be noted in the table below the graph as either 1/3, 1/12 or whatever instead of a straight line indicating no smoothing.

Now, following is a graph of before and after bass response with/without the Audyssey EQ applied (again, no smoothing). This doesn't appear to be same set-up as above (I believe x-overs are different) but you get the picture nonetheless. This has been a while ago so hard to keep track of all the measurements taken.
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post #16 of 17
4/17/12 at 8:01pm
Thanks. So without EQ, you had 15 db variation. With EQ it is down to 10 db.

Is this with a single sub? And how does it look at other seats?

Apologies to OP for the side track .
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post #17 of 17
4/17/12 at 8:50pm
I don't have measurements handy for the 3 seats across first row, but I have measured it and it is surprisingly similar without wild swings. I have one sub along front wall that's about 1/4 room width off the right wall (around 3').

Now, since your 3 hours behind me you can keep posting, but I'm going to bed.
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