Help needed: sound appears to come from "above" speakers - AVS Forum
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Old 08-17-2012, 05:47 AM - Thread Starter
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Hi,

I would like to know if there's anything wrong with my setup or my expectations.

The problem:
I was expecting that a sound source should appear to be located vertically more or less near a line that you get when you connect the two tweeters (stereo setup). Strangely I get the impression of sound emenating from significantly ABOVE this line. Just to give you an estimate I think this effect leads to a 20 to 30 degrees angle (from my ears to the sound source vs. from my ears to the horizontal line that connects the tweeters) above what I expected. Now I am wondering if this is either correct or if there's any problem with speakers, room treatment, my ears, etc.

My setup:
B&W 803D (tweeter height is about 6 inches above ear level). Distance from ears to tweeters is 10 feet. Room size is 17 feet (front/rear), 15 feet (left/right), 7 feet (ceiling). Speakers are 3 feet away from front & side (front of tweeter). The listening position is 5 feet from the back wall.

Room treatment:
Floor is covered with carpet, sides with Thinsulate (acoustic felt). First reflection points at the sides and on the ceiling are covered with 2 inches of Basotect (acoustic foam) and in each front corner is a triangular Basotect column.

Any ideas? I tried tilting the speakers forward, but this didn't have any noticable effect.

Thanks for your feedback,
NutFlush
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Old 08-17-2012, 07:06 AM
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Quote:
Originally Posted by NutFlush View Post

Hi,
I would like to know if there's anything wrong with my setup or my expectations.
The problem:
I was expecting that a sound source should appear to be located vertically more or less near a line that you get when you connect the two tweeters (stereo setup). Strangely I get the impression of sound emenating from significantly ABOVE this line. Just to give you an estimate I think this effect leads to a 20 to 30 degrees angle (from my ears to the sound source vs. from my ears to the horizontal line that connects the tweeters) above what I expected. Now I am wondering if this is either correct or if there's any problem with speakers, room treatment, my ears, etc.
My setup:
B&W 803D (tweeter height is about 6 inches above ear level). Distance from ears to tweeters is 10 feet. Room size is 17 feet (front/rear), 15 feet (left/right), 7 feet (ceiling). Speakers are 3 feet away from front & side (front of tweeter). The listening position is 5 feet from the back wall.
Room treatment:
Floor is covered with carpet, sides with Thinsulate (acoustic felt). First reflection points at the sides and on the ceiling are covered with 2 inches of Basotect (acoustic foam) and in each front corner is a triangular Basotect column.
Any ideas? I tried tilting the speakers forward, but this didn't have any noticable effect.
Thanks for your feedback,
NutFlush

Since you are only addressing first reflection points to the left and right, reflections from other surfaces such as the ceiling are very possible.
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Old 08-17-2012, 07:50 AM - Thread Starter
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Hi Arnold,

thanks for your reply.

The ceiling is in fact also covered with about 7x10 feet of Basotect (2 inches thick). All vertical walls are covered with Thinsulate (a special acoustic felt used in automobiles etc.). There are practically no "hard" surfaces.

Is there any method to find out if this phenomenom is actually caused by reflections? Or could it be caused by something else?

Thanks!
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Old 08-17-2012, 08:38 AM
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An ETC smoothed 1 octave band test may be revealing. My guess is that your 2" material is absorbing highs and upper mids, but you are still getting reflections in the 300hz-1K range. Felt absorbs even less at the lower and midrange registers

.

Red = 668hz
Orange = 1k
Yellow = 1.6k
Green = 2.6k
Blue = 4.2k

The circled area indicates the ceiling and floor area between the listener and the speakers. See how the bands are far apart? This is the kind of thing you will see when your treatment isnt broadband.

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Old 08-17-2012, 08:54 AM - Thread Starter
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Jim,

I think I understand your point. But wouldn't that mean that the vertical shift upwards would only be present for the frequencies that do NOT get absorbed? Then I would hear the lower frequencies from above (reflections from ceiling) and the higher frequencies from straight on (no reflections via ceiling), right?

In my case I hear ALL frequencies (certainly mid and high ones) from that elevated position. I cannot hear any differences based on frequency.

Hmm, any ideas? Or did I misunderstand what you were trying to tell me?
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Old 08-17-2012, 08:55 AM
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What receiver are you using? Or amplifier possibly :-p

Sent from my GT-P7510 using Tapatalk 2

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Old 08-17-2012, 09:20 AM - Thread Starter
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What receiver are you using? Or amplifier possibly :-p

Sorry, I didn't come to my mind that this could matter. It's a DENON AVC-A1SE including the upgrade board that came out later.
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Old 08-17-2012, 09:41 AM
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Quote:
Originally Posted by NutFlush View Post

Jim,
I think I understand your point. But wouldn't that mean that the vertical shift upwards would only be present for the frequencies that do NOT get absorbed? Then I would hear the lower frequencies from above (reflections from ceiling) and the higher frequencies from straight on (no reflections via ceiling), right?
In my case I hear ALL frequencies (certainly mid and high ones) from that elevated position. I cannot hear any differences based on frequency.
Hmm, any ideas? Or did I misunderstand what you were trying to tell me?

The felt is probably passing nearly ALL frequencies. You could easily perceive hearing ALL the frequencies where the felt is present and there is a reflection.



Here ^^^^ is an approximation of what 1/4" OC 703 does. Your felt is even thinner than this probably and so does even less absorption than this.

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Old 08-17-2012, 09:58 AM
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Quote:
Originally Posted by NutFlush View Post

Hi Arnold,
thanks for your reply.
The ceiling is in fact also covered with about 7x10 feet of Basotect (2 inches thick). All vertical walls are covered with Thinsulate (a special acoustic felt used in automobiles etc.). There are practically no "hard" surfaces.
Is there any method to find out if this phenomenom is actually caused by reflections? Or could it be caused by something else?
Thanks!

My bad. On rereading your post I see where you mentioned the ceiling.

I looked up the properties of Basotect and found this:




Should be better than nothing, but not exactly a perfect absorber. I would guess that your ears, not being spectrum analyzers are noticing what the Basotect doesn't absorb, which is a fair amount.

Or, you have somehow missed covering the actual reflection point. The path may be more complex than you have suspected. ETC, anybody?

I have a couple of friends who have had problems with exactly the reflection problem of which you speak. They were able to address it by fairly obvious means, but your situation may not be as simple.
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Old 08-17-2012, 10:12 AM
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Arnold, I began my room treatment with Aurolex 2" studiofoam. It probably does about the same as the 51mm graph you show. Ive learned a lot since then. Absorbing only down to 800hz-1k really colors what you hear. The rule I follow these days is if you are going to absorb, do it down to 200-300hz, or not at all. Otherwise you get a room with an uneven reflection curve.

Nice graph smile.gif

In the case of basotect, if you were to double or triple its thickness, you would get down low enough to work pretty well (4-6" thick). Something to consider Nutflush smile.gif

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Old 08-17-2012, 10:12 AM - Thread Starter
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Jim,

would reflections from the sides lead to the impression that I described? Or only reflections via the ceiling?

As I said the ceiling is practically fully covered with basotect absorbers in the area between the listener and the speakers. So I wouldn't expect any reflections from the ceiling, or could I be wrong?

Is there anything else apart from reflections that can lead to that impression of an elevated sound source?

By the way: here you can find info about basotect: http://www.plasticsportal.net/wa/plasticsEU~en_GB/portal/show/content/products/foams/basotect_properties

like this chart (the red & blue lines are relevant in my case as I have 61mm foam attached to the ceiling and 100mm on the sides):


Thanks!
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Old 08-17-2012, 10:25 AM
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It only takes one "hot" early reflection to give the impression of locality. Like Arnold said, A ETC test would give the data you need. Without it, what you might try is taking a scrap piece of your absorbent material and place it temporarily in different spots until you notice a improvement. You can also place larger pieces around the speaker and try to notice when the reflection disappears.

Some call this the blocking method, whereby you place absorbent either around the source or around the listening position at different points in order to ascertain the direction and/or angle of the problem from one of those two points. Once you make it go away, you can move your temporary panel outward in the direction of a wall or other surface until you can localize it more definitively. .

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Old 08-17-2012, 10:54 AM
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Quote:
Originally Posted by jim19611961 View Post

Arnold, I began my room treatment with Aurolex 2" studiofoam. It probably does about the same as the 51mm graph you show. Ive learned a lot since then. Absorbing only down to 800hz-1k really colors what you hear. The rule I follow these days is if you are going to absorb, do it down to 200-300hz, or not at all. Otherwise you get a room with an uneven reflection curve.
Nice graph smile.gif
In the case of basotect, if you were to double or triple its thickness, you would get down low enough to work pretty well (4-6" thick). Something to consider Nutflush smile.gif

Often you can get quite a bit better absorption at low frequencies by simply spacing the absorber out from the wall a distance roughly equal to its thickness.

Also, covering more active space will make the absorber more effective at lower frequencies even though this does not change its basic bandpass.

Covering 100 square will absorb more bass than covering 50 square feet if there is a large wall or ceiling to cover.

Also, putting the material up in strips or squares and leaving equal size or width empty spaces in-between is a more effective use of a given amount of material.
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Old 08-17-2012, 11:16 AM
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Originally Posted by jim19611961 View Post

This is the kind of thing you will see when your treatment isnt broadband.

+1 which is why we go to such great lengths to recommend broadband treatment from the start -

the thin absorber is merely going to attenuate the mid-HF specular band, but it's the lower band that will persist which has inherently more energy content and generally more of this spectra will be 'off-axis' and directed towards the ceiling in the first place (vs the more directional HF content - dependent upon source design).
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Old 08-17-2012, 11:23 AM
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Originally Posted by arnyk View Post

Often you can get quite a bit better absorption at low frequencies by simply spacing the absorber out from the wall a distance roughly equal to its thickness.

yes, porous insulation is a velocity-based absorber and needs to be placed (spaced away from rigid boundary) into areas of high particle velocity for a given wavelength in order to be most effective. there is little particle velocity with respect to the lower frequencies (longer wavelengths) right at the boundary. an air-gap is essentially a 'free lunch' in such applications of porous absorption.

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Originally Posted by arnyk View Post

Also, covering more active space will make the absorber more effective at lower frequencies even though this does not change its basic bandpass.
Covering 100 square will absorb more bass than covering 50 square feet if there is a large wall or ceiling to cover.

the important factor (besides thickness/spacing from rigid boundary) is that the absorber (just like any other 'treatment') needs to be large with respect to wavelength.

i think he's primarily concerned with the ceiling specular indirect reflection in this scenario, not attenuating modal (bass) issues. the LF modal wavelengths are inherently large and thus, require large absorbers (sq area) - along with sufficient thickness and GFR type if restricted to porous-only LF absorbers. unless one wants a highly damped room, then sq area should be minimized and the broadband absorption surgically placed in order to attenuate the indirect signal across the given listening position area.


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Also, putting the material up in strips or squares and leaving equal size or width empty spaces in-between is a more effective use of a given amount of material.

additional losses can be accomplished via this method due to the increase in exposed edges - thus, increase energy loss via edge diffraction. but if the goal is to fully attenuate an indirect specular signal, then you would not want necessarily want to do this.
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Old 08-17-2012, 11:57 AM - Thread Starter
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I thank you all for your input. Highly appreciated!

Yet, I would like to repost one of my questions: "Is there anything else apart from reflections that can lead to that impression of an elevated sound source?"

If you have any ideas, please let me know. Also tips how to verify those possible causes would be great (like the mentioned blocking with left-over absorber material).

Thanks! smile.gif
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Old 08-17-2012, 01:42 PM
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Quote:
Originally Posted by NutFlush View Post

I thank you all for your input. Highly appreciated!
Yet, I would like to repost one of my questions: "Is there anything else apart from reflections that can lead to that impression of an elevated sound source?"
If you have any ideas, please let me know. Also tips how to verify those possible causes would be great (like the mentioned blocking with left-over absorber material).
Thanks! smile.gif

I can think of other things that would cause "images" to seem to appear along the horizontal plane where they shouldnt like unequal channel output and/or an uneven speaker polar response. But on the vertical, the only other thing, and this is a reach and very unlikely, is if you had some sort of weird baffle diffraction off of the top of the speaker or one of its drivers. Perhaps something very reflective sitting on top of the speaker.

I would persue the blocking method first. You should get results fairly quickly. For a vertical problem, sit a piece of absorbent material on top of the speaker that overhangs each side by a good bit, especially the front. That should cause any speaker energy directed upwards to be attenuated.

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Old 08-18-2012, 01:37 PM - Thread Starter
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I tried the blocking method, but it didn't significantly change my acoustic impression. I used some left over basotect foam boards that extended about 1.5 feet beyond the front of the speakers toward the listening position.

I got a stronger effect just by standing behind my chair and have my ears more than a foot higher than when sitting.

But maybe my expectations are the problem here. Could some of you please describe where you locate the sound sources vertically in relation to the virtual horizontal line that connects the tweeters? You might want to use a piece of music with a singer at center stage and some instruments to the left and right. Stereo only!

Thanks in advance!
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Old 08-18-2012, 01:56 PM
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At this point, id say post some pictures of your set up.

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Old 08-19-2012, 02:00 AM - Thread Starter
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Jim,

I'd really love to hear from others first, so that I know what the baseline reference should be.
Quote:
But maybe my expectations are the problem here. Could some of you please describe where you locate the sound sources vertically in relation to the virtual horizontal line that connects the tweeters? You might want to use a piece of music with a singer at center stage and some instruments to the left and right. Stereo only!

Thanks!
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Old 08-19-2012, 02:09 PM
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Is there a "virtual heigth" setting in the reciever for the front mains? Pioneer MCACC has settings for this.
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Old 08-19-2012, 02:43 PM - Thread Starter
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Is there a "virtual heigth" setting in the reciever for the front mains?


No! eek.gif
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