reflections etc.

[steelie]

When people talk about treating the primary reflection points in their rooms are they referring to the spots directly behind the speakers or in front? Sorry if this is an obvious question. My RF-7s are three feet out from the rear wall and if I stand behind them and directly in the middle I hear an extroardinary amount of built up bass. Obviously, I don't typically stand behind my speakers to listen to them but should this be treated? Can I use a low-lying bookcase to break up the sound or do I need some sort of panels for this job?

Also, the RF-7s are toed in to point toward the sweet spot and about two feet away from side walls. Since they have a 90 degree dispersion pattern out of the horn, should I use that angle to figure out where the reflections are off the adjacemt side wall? Do you have to treat the entire vertical length of the wall or is it enough to treat the area around the horn's height?

Thanks for any help.

[WMcD]

I'm kinda working on the issue myself.

Standing behind the speaker, you're hearing the effect that the treble horn is doing a good job of projecting forward. You are hearing the bass alone.

Do buy The Master Handbook of Acoustics at Borders.

The treble horn does a good job of keeping the sound off the walls. Walls are still a problem. From what I read, about 20 percent of the walls have to be treated with absorbers and or diffusers before there is an improvment. That is quite a bit of real estate.

Best,

Gil

[DrWho]

The first reflection point is the spot on the wall where you would be able to see the speaker from the main listening location if there was a mirror there. The easiest way to find this location is to sit in your listening chair and then have friend move a mirror around the wall until you can see the tweeter of your speaker. Do this for both speakers and on the cieling too. Don't forget that you'll be able to see each speaker on both side walls. This should give you a total of 6 first reflection points. (2 for each side wall and 2 on the cieling).

As far as the bass buildup in the front of your room behind the speakers, I've found that treating this vastly improves the bass performance of your speakers. Ideally, you'd want to use bass traps behind your speakers in the corners, or maybe some masonite panels (a quick search through this forum should find you lots of info on that). A bookshelf in the front might help diffuse the build up, but I've had a similar situation and all it did was move the buildup more towards the corners of the speakers. Whether or not this shift causes an improvement at the listening position depends on too many factors to list. In my situation, I noticed no difference.

Because our rooms are fully enclosed and small in comparison to the size of the wavelengths, whenever we have a buildup of bass in one location, we have a corresponding null in another. By reducing the buildup, we are then reducing the null which effectively increases the bass output...it's almost like we're moving the buildup to the listening position.

BTW, what are you experiencing that is causing you to consider acoustical treatment? Do you hear problems at the listening position in the same frequency range as the buildup at the front of your room?

[steelie]

Thanks to both for your help. Room treatment is just the next evolution in improving the sound in my room, which is large by my standards: 18 by 30 with a ceiling that slopes from 8 to 14 feet in the middle.

I'm satisfied with the amount of deep low bass we get but I'm not happy with the mids. Running a frequency scan showed that with my sub, I'm at plus or minus 5 db from 20 to 200 hz. There is a gap in certain frequencies above 200 hz, though, which occasionally drops about 10 db - sometimes more although I'm not sure what this depends on. The highs are fine.

I suppose this could just be the RF7s being RF7s or perhaps it's the source components, I really don't know. Mostly I'm happy with the setup. Imaging and soundstage is good. I'm just thinking about the next step to try to improve the system.

Thanks again.

[dbflash]

Good Post.

I am in the process of getting material for my room.

Once I find the reflection points with the mirror trick how much material do I use. Do I use Auralex foam even from the tweeter half way up the wall towards the ceiling? How far from the tweeter towards the sweet spot should I make the foam panel.

Danny

[DrWho]

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On 11/19/2004 4:09:17 PM steelie wrote:

Thanks to both for your help. Room treatment is just the next evolution in improving the sound in my room, which is large by my standards: 18 by 30 with a ceiling that slopes from 8 to 14 feet in the middle.

I'm satisfied with the amount of deep low bass we get but I'm not happy with the mids. Running a frequency scan showed that with my sub, I'm at plus or minus 5 db from 20 to 200 hz. There is a gap in certain frequencies above 200 hz, though, which occasionally drops about 10 db - sometimes more although I'm not sure what this depends on. The highs are fine.

I suppose this could just be the RF7s being RF7s or perhaps it's the source components, I really don't know. Mostly I'm happy with the setup. Imaging and soundstage is good. I'm just thinking about the next step to try to improve the system.

Thanks again.

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At what frequencies are the mids dropping out? Also, how did you setup your test? Were you using test tones and radio shack meter, or pink noise with an RTA? There are correction values for the RS meter which is why I ask. Also, the use of test tones isn't the best approach to measuring system response...If that's how you measured, then I'll get back to you on the problems with it and some ways around it. The 10dB drops that you're recording are not flaws in the RF-7.

When considering the right direction to take in acoustical treatment, you will need to decide the characteristics of your room. Is it dead? too live? echoey? bright? harsh? dull? etc etc...Then you'll have to go hunt down the correct treatments and go from there. Here's some reading to get you started:

http://www.acoustics101.com

BTW, what is the basic layout of your room? is it a dedicated room? where is all the furniture and do you have your speakers on the long or short wall? How many "main listening" positions are there?

[DrWho]

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On 11/19/2004 5:46:54 PM dbflash wrote:

Good Post.

I am in the process of getting material for my room.

Once I find the reflection points with the mirror trick how much material do I use. Do I use Auralex foam even from the tweeter half way up the wall towards the ceiling? How far from the tweeter towards the sweet spot should I make the foam panel.

Danny

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This is where the test and check method becomes important in acoustics. Obviously, a 1" x 1" panel isn't going to do much of anything. At the same time, covering that entire wall from corner to corner and floor to cieling is actually going to hurt the sound. One thing to keep in mind is to make the panel large enough to cover the same first reflection point from each listening position. For example, if multiple people are listening in your room and they all sit on one couch, then make sure the panel covers the mirror spot of the most left seat and the most right seat. I personally would just use a 4' x 4' panel centered on the first reflection point of the main listening position. However, I would first do a temporary install before I made it permanent.

Btw, here's another great acoustics resource:

http://www.ethanwiner.com/acoustics.html

Ethan Winer is definetly on the forefront of acoustic science.

[steelie]

At what frequencies are the mids dropping out? Also, how did you setup your test? Were you using test tones and radio shack meter, or pink noise with an RTA? There are correction values for the RS meter which is why I ask. Also, the use of test tones isn't the best approach to measuring system response...If that's how you measured, then I'll get back to you on the problems with it and some ways around it. The 10dB drops that you're recording are not flaws in the RF-7.

-- My newest frequency chart shows the biggest drop occurring at 160 Hz. It's about -12 db compared to the rest and then immediately goes back to normal at 200 Hz. It drops to -5db at 250 Hz and then back up to 0 at 315 Hz. The other significant drop is -5 db at 800 Hz. The variance is minimal until I get to 2500 Hz, where it drops to -12 db.

I tried unsuccesefully to download free test tones from the Internet so I ended up buying the Rives Audio CD, which includes test tones corrected for the RS meter.

BTW, what is the basic layout of your room? is it a dedicated room? where is all the furniture and do you have your speakers on the long or short wall? How many "main listening" positions are there?

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--The room is not dedicated to music/HT, although my SO might argue with me considering that it takes up a large part of the room. There are openings to other rooms that I've had to live with. I have the system set up on the short wall b/c the long wall provided horrible sound when I tried it. I think ... just a guess here ... that the way the ceiling slopes up affected the sound of the system when it was on the long wall. I'm not sure how to answer the main listenig position question. I always just considered the spot creating an equidistant triangle from the two front speakers as the main listening position and everything else is just to the left or right of that.I suppose there are three, although we have room behind us for a few more if we ever expanded. (Highly unlikely in this room)

[DrWho]

Well the dips you're experiencing are from standing waves. Do the values change significantly when you move the meter around? One thing you might want to do is move the meter around when conducting the the test and record the loudest SPL you measure. Then compare the two charts and look for any similarities between the two. The reason for this is because it does take time for a standing wave to form and with a test tone, this happens very quickly. However, music very rarely plays the same frequency continuously for a long enough time for the standing wave to occur. By moving the meter around and recording the loudest SPLs around you, then you are getting a better idea of what you're actually hearing. Any consistent trends between the tests will be specific areas of concern.

If you'd like your own testtone generator, check out http://www.nch.com.au/tonegen. The demo will run out, but if you uninstall and reinstall, it'll resume function.

I can send you a zipped file containing 6th octave test tones if you don't want to go through all the trouble making your own. I tried uploading them to the forum, but it kept timing out. PM me your email addy and i'll send it right over. While I'm at it, I can send you an excel spreadsheet that automatically compensates for the RS meter and draws a graph of your frequency response as well.

The slope in your room...what is the orientation in relation to your speakers when you had it on the long wall and right now? I'm picturing your wall sloped up from the speakers when it was on the long wall and right now, it is sloping left to right. Does your wall slope back down on the other side? In most cases, sloped walls are a very good thing as long as the stereo symmetry doesn't get screwed up.

[steelie]

Thanks for the offer, I appreciate it. That's just what the Rives Audio CD allows me to do, along with a spreadsheet. Would creating my own test tones allow me to do something else?

When my setup was on the long wall, the ceiling sloped down left to right and right to left, with the peak in the middle.

Now, on the short wall, the ceiling slopes up from the speakers, reaches its peak, and slopes back down to 8 feet on the other end of the room. To me, this setup gave the sound more detail. It was real muddy the other way around.

Thanks again. I really appreaciate the help.

[DrWho]

Well this is along the lines of what I'd do in your room. Everything is close to scale, 1 foot = 20 pixels.

Purple = LENRD Bass Trap

Blue = Diffusive Panel

Grey = Absorbtive Panel

Red = reflection angles of untreated room

I could create a schenematic if you put your speakers on the long wall too if you wanted, but I think your current layout is the best acoustically. If your main listening position is drastically different, then obviously you'd have to move panels to compensate for that. You might also want to put diffusive/absorbtive panels on the cieling part that is going up as well (which would help reduce echo bouncing back at ya).

[steelie]

Wow! I don't know what to say, really. I'm amazed. That's pretty intricate considering the small detail you were working with. Thanks for working that out for me.

One thing I've been wondering in terms of analyzing the room response. When I walk around the room with my SPL meter, I'm looking for hot spots on the walls, right? I can easily find variations in the open room but the purpose is to look for spots that can be treated, so really only the walls and ceiling, right?

Also, the room is surrounded with wood paneling from the floor to a chair guard. The remainder of the wall is drywall, covered with art, etc. I get no rattling or any such issues from the paneling but how does it differ from just clean drywall. Does it create a harsher sound or does it tame the peaks? The paneling isn't something I can easily change so I have no frame of reference here.

Thanks again for that great detail. It's much appreciated.

Edit: The area centered directly behind the speakers is treated with a diffusor of sorts, right? We mentioned a bookshelf or masonite panels earlier in the post.

[DrWho]

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On 11/25/2004 2:52:19 AM steelie wrote:

One thing I've been wondering in terms of analyzing the room response. When I walk around the room with my SPL meter, I'm looking for hot spots on the walls, right? I can easily find variations in the open room but the purpose is to look for spots that can be treated, so really only the walls and ceiling, right?

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When dealing with the lower frequencies, you want to find hotspots that aren't at your listening position. Absorbing these peaks should help to reduce nulls at the listening position. I never thought about it, but along the walls and cieling really is the best place to do treatment. Corners are especially important to treat because the geometry naturally results in bass build up.

If anything, I would say that your wood paneling helps the acoustics of your room because it provides another tonality seperate from the drywall. I find it ironic that this is mentioned now because just recently, we were moving the warehouse and offices at work to a new location and there were two offices of identical size...one with drywall and one with wood paneling like you describe. When completely empty, it was amazing how different the rooms sounded...the wood room sounded warmer versus the pingy sound of the drywall. It's a general rule in acoustics that large surfaces of the same material is something to avoid.

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On 11/25/2004 2:52:19 AM steelie wrote:

Edit: The area centered directly behind the speakers is treated with a diffusor of sorts, right? We mentioned a bookshelf or masonite panels earlier in the post.

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Sorry I didn't mark any area on the diagram centered behind the speakers, I just did the corners. If I were to go with anything there, it'd be a combination of a large diffusor with absorbtion behind it. Something like a large masonite panel stuffed with rigid fiberglass. One thing you may consider is building an array of helmhotz resonators. Build the array such that the combined surface acts as a large diffusor. Ethan Winer talks about the Helmholtz Resonator in that link I provided...make sure you build for varying frequencies as it will sound unnatural if you do too much at just one.

You mentioned a bunch of paintings in the room. One thing I've seen done is to mount the pictures on the face of acoustical panels...It almost looks like an extended frame kind of thing. It looks even cooler when using slanted panels so that the picture is tilted down.