Image Depth?

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Saturn94

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Image Depth?
« on: 30 Nov 2010, 06:22 pm »
I've heard it said that image depth is increased when you pull the speakers out from the wall behind them.  So which is more important, the distance the drivers on the front are to the wall behind or the distance the back of the speaker is to the wall behind?

For reference I'm refering to closed back speakers, not open back or open baffle speakers.

Thanks.

Nuance

Re: Image Depth?
« Reply #1 on: 30 Nov 2010, 06:46 pm »
I've heard it said that image depth is increased when you pull the speakers out from the wall behind them.  So which is more important, the distance the drivers on the front are to the wall behind or the distance the back of the speaker is to the wall behind?

For reference I'm refering to closed back speakers, not open back or open baffle speakers.

Thanks.

Both.  The distance the drivers are from the back (and side) wall(s) will effect reflections, sounds stage width and depth and imaging.  The distances the rear ports are from the wall will effect bass response. 


jsalk

Re: Image Depth?
« Reply #2 on: 30 Nov 2010, 07:11 pm »
Speakers almost always benefit from being set away from the wall or corners.

There are a couple of aspects to consider here.  First, the farther the speakers are away from the wall, the later reflections off of room surfaces arrive at the listening position.  If the distance to the rear wall is short, human hearing cannot tell whether those are room reflections or part of the original signal.  So these early reflections can serve to "smear" the image.

There are two ways to deal with this issue.  Acoustic panels placed at first reflection points can absorb those early reflections and keep the sound pristine at the listening position.  Where are these first reflection points?  It is easy to find them with the help of a mirror and an assistant.

While seated in the listening position, have your assistant slide a mirror along the side walls until you see a reflection of the speaker in the mirror.  This is a first reflection point.  Repeat this for the rear wall and perhaps the ceiling to locate all of these first reflection points.

The second way to deal with these is to position the speakers at least 4 feet from the nearest surfaces.  This is not possible in most cases since the rooms are not large enough.  But if it is, reflections from surfaces about 4 feet or more from the speaker will be interpreted as room reflections and not as coming directly from the speakers.  So no acoustic treatments may be required.

The other aspect of this question relates to the amount of midbass energy you experience.  If the speaker is placed close to a wall, these frequencies will be reinforced.  This will vary from room to room and from speaker to speaker.  For example, we have found that ported speakers are more sensitive to this effect than transmission line speakers.  But if your speakers are sounding a little on the "boomy" side, pulling them away from the rear walls and corners will likely improve bass performance.

I hope this helps.

- Jim

Big Red Machine

Re: Image Depth?
« Reply #3 on: 30 Nov 2010, 07:39 pm »
I was going to say that! :lol:

Saturn94

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Re: Image Depth?
« Reply #4 on: 30 Nov 2010, 07:47 pm »
Both.  The distance the drivers are from the back (and side) wall(s) will effect reflections, sounds stage width and depth and imaging.  The distances the rear ports are from the wall will effect bass response.

Thanks.

Let me make sure I understand correctly; (1) imaging/depth is affect by how far the front of the speaker is from the walls, not the space behind the speaker, and (2) bass response is affected by how close the back of the cabinet is to the wall (at least for rear ported designs).  Do I have this right?

Saturn94

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Re: Image Depth?
« Reply #5 on: 30 Nov 2010, 08:02 pm »
Speakers almost always benefit from being set away from the wall or corners.

There are a couple of aspects to consider here.  First, the farther the speakers are away from the wall, the later reflections off of room surfaces arrive at the listening position.  If the distance to the rear wall is short, human hearing cannot tell whether those are room reflections or part of the original signal.  So these early reflections can serve to "smear" the image.

There are two ways to deal with this issue.  Acoustic panels placed at first reflection points can absorb those early reflections and keep the sound pristine at the listening position.  Where are these first reflection points?  It is easy to find them with the help of a mirror and an assistant.

While seated in the listening position, have your assistant slide a mirror along the side walls until you see a reflection of the speaker in the mirror.  This is a first reflection point.  Repeat this for the rear wall and perhaps the ceiling to locate all of these first reflection points.

The second way to deal with these is to position the speakers at least 4 feet from the nearest surfaces.  This is not possible in most cases since the rooms are not large enough.  But if it is, reflections from surfaces about 4 feet or more from the speaker will be interpreted as room reflections and not as coming directly from the speakers.  So no acoustic treatments may be required.

The other aspect of this question relates to the amount of midbass energy you experience.  If the speaker is placed close to a wall, these frequencies will be reinforced.  This will vary from room to room and from speaker to speaker.  For example, we have found that ported speakers are more sensitive to this effect than transmission line speakers.  But if your speakers are sounding a little on the "boomy" side, pulling them away from the rear walls and corners will likely improve bass performance.

I hope this helps.

- Jim

Thank you for the great explanation. :)

I do understand how early reflections affect imaging/depth depending on how close the speaker is to the walls.  What I specifically wanted to know, at least in terms of image depth and how it relates to speaker position relative to the wall behind the speaker, is which distance are we talking about, the distance from the front of the speaker to the wall or the distance from the back of the speaker to the wall?

My guess is that the distance from the front of the speaker is the critical one, but I don't know that for sure, hence my post. :)

Big Red Machine

Re: Image Depth?
« Reply #6 on: 30 Nov 2010, 08:05 pm »
Well, the front of the speaker is the source of the sound, and it wraps around the speaker and bounces off the baffle, the sides, walls, rear wall, etc. at lower frequencies.

Nuance pointed out that if the port is in the rear and is too close to a wall it can introduce aberrations to the bass you don't want to hear as they are not in synch with the driver on the front I believe.  (out of phase?)

It may be easier to pull the speakers toward you in a small room and go with a nearfield triangle and treat the side wall reflections first.  If you still get boomy corners due to bass build-up then traps in the corners may be a next step.  My room is only 13.5 wide and I trap with triangles floor to ceiling that are about 16 inches deep.  They work!

Also, do not set the speakers equally from each wall.  I have a 6 inch offset to one side and am around the 62% back position.  This helps reduce the effect of room modes we cannot escape.

I go with answer A.  I was typing this response while you were typing yours.

Saturn94

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Re: Image Depth?
« Reply #7 on: 30 Nov 2010, 08:19 pm »
Well, the front of the speaker is the source of the sound, and it wraps around the speaker and bounces off the baffle, the sides, walls, rear wall, etc. at lower frequencies.

Nuance pointed out that if the port is in the rear and is too close to a wall it can introduce aberrations to the bass you don't want to hear as they are not in synch with the driver on the front I believe.  (out of phase?)

It may be easier to pull the speakers toward you in a small room and go with a nearfield triangle and treat the side wall reflections first.  If you still get boomy corners due to bass build-up then traps in the corners may be a next step.  My room is only 13.5 wide and I trap with triangles floor to ceiling that are about 16 inches deep.  They work!

Also, do not set the speakers equally from each wall.  I have a 6 inch offset to one side and am around the 62% back position.  This helps reduce the effect of room modes we cannot escape.

I go with answer A.  I was typing this response while you were typing yours.

Fortunately side reflections don't seem to be much of an issue for me.  The left speaker is slightly over 4ft from the center of the speaker directly to the left side wall.  At the first point of reflection on this wall is a large window that covered with heavy curtains.  The right speaker is about 12ft from the right side wall, so again, the side reflection doesn't seem to be an issue.

So my question was restricted to the distance to the wall behind the speaker and which affects image depth, the distance from the front of the speaker or the distance from the back of the speaker.  Thinking about it, I guess I'm wondering how much the cabinet depth gets in the way of image depth (if at all)?

jsalk

Re: Image Depth?
« Reply #8 on: 30 Nov 2010, 08:32 pm »
First reflections will impact the sound regardless if it is from a rear wall, side wall or ceiling.  The easiest thing to do in your situation is to kill the first reflections off the rear wall (since the side walls seem far enough away).

In a rear ported design, you want the speaker far enough from the rear wall so that midbass frequencies are not reinforced creating "boomy" sound.  This has nothing to do with image depth, but is a consideration in all speaker set-ups.

In terms of image depth, in order to create the optimum situation, you either have to kill the first reflection off the rear wall or move the speakers out from the rear wall far enough so that reflections are interpreted as reflections.  The farther you move the speakers from the rear wall, the later reflections will arrive at the listening position and the deeper the "room" will sound.  In that regard, image depth can be enhanced by moving the front of the speaker farther from the rear wall.

- Jim

Saturn94

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Re: Image Depth?
« Reply #9 on: 30 Nov 2010, 09:05 pm »
First reflections will impact the sound regardless if it is from a rear wall, side wall or ceiling.  The easiest thing to do in your situation is to kill the first reflections off the rear wall (since the side walls seem far enough away).

In a rear ported design, you want the speaker far enough from the rear wall so that midbass frequencies are not reinforced creating "boomy" sound.  This has nothing to do with image depth, but is a consideration in all speaker set-ups.

In terms of image depth, in order to create the optimum situation, you either have to kill the first reflection off the rear wall or move the speakers out from the rear wall far enough so that reflections are interpreted as reflections.  The farther you move the speakers from the rear wall, the later reflections will arrive at the listening position and the deeper the "room" will sound.  In that regard, image depth can be enhanced by moving the front of the speaker farther from the rear wall.

- Jim

I forgot about the ceiling, but the room has a cathedral ceiling with at least 7ft or more of space directly above the top of the speakers.

Do you have an idea of what distance is required to perceive the reflection as a reflection and not part of the original sound?  I thought I read a post some time back that the reflected sound should have to travel at least 5 ft farther than the original sound, that would mean a minimum distance of 2.5ft from the front of the speaker.  Does this sound correct?

BTW, my listening position is about 8ft from the speakers.

Thanks. :D

jsalk

Re: Image Depth?
« Reply #10 on: 30 Nov 2010, 09:29 pm »
I forgot about the ceiling, but the room has a cathedral ceiling with at least 7ft or more of space directly above the top of the speakers.

Do you have an idea of what distance is required to perceive the reflection as a reflection and not part of the original sound?  I thought I read a post some time back that the reflected sound should have to travel at least 5 ft farther than the original sound, that would mean a minimum distance of 2.5ft from the front of the speaker.  Does this sound correct?

BTW, my listening position is about 8ft from the speakers.

Thanks. :D

I am not entirely positive, but somewhere between 4 to 5 feet is probably correct.

- Jim

Saturn94

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Re: Image Depth?
« Reply #11 on: 30 Nov 2010, 09:55 pm »
I am not entirely positive, but somewhere between 4 to 5 feet is probably correct.

- Jim

Thanks.  I'm at about 4.5 feet (about 2.25ft each way), so I'm probably right on the border.  I'm might try to see if I can pull the speaker out another 3 inches (I'm hitting my limit already) to see what happens; that would put me right at the 5ft mark.

Stercom

Re: Image Depth?
« Reply #12 on: 30 Nov 2010, 10:47 pm »
Excellent discussion. Proper positioning of your speakers is one of the most effective yet cheapest ways to improve your sound.

ThorsHammer

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Re: Image Depth?
« Reply #13 on: 1 Dec 2010, 11:31 am »
I have ST/RTs and was consistently having trouble getting clear bass...even low E guitar strings seemed to be a bit muddy and have too much resonance. I know my room had a lot to do with the problem...hardwood floors and can't really pull the speakers more than 2' from the rear wall and wall treatments really aren't an option for me.

I finally found an easy solution that improved the sound significantly...but I'm not sure why. I'm using some F6 Athena floor standers as rears and they have a grill that's about 9" wide and 2.5' long...I took those grills and placed them in back of the SongTowers, leaning them against the backs so they were about 20" behind the port where they hit the ground after some experimenting with different positions. This eliminated my problem and they can barely be seen from the front side of the speaker.

I don't know why this was so effective. Maybe the material diffuses the sound wave before it hits the reflection points enough so that it minimizes the problem. But, it certainly was a fast cheap fix and they can be brought closer or farther away by simply adjusting the angle.

pelliott321

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Re: Image Depth?
« Reply #14 on: 1 Dec 2010, 01:27 pm »
I also believe that there is a lot of psycho-acoustic stuff going on.  If there is physical depth to look into beyond the speaker the mind will push the sound there especially if there is something interesting to look at while listening.

Brucemck

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Re: Image Depth?
« Reply #15 on: 1 Dec 2010, 02:37 pm »
For not much money you could purchase two RPG binary diffusor panels that were 2" thick by 2' wide by 4' high, covered in fabric that coordinates with your decor.  You'd place these vertically on the wall behind your speakers to absorb and scatter that reflection point.

A second panel or two, mounted horizontally on the wall directly behind your head, would be nice too.

http://www.silentsource.com/diffusors-rpg-bad.html

http://www.silentsource.com/diffusors-rpg-bad-arc.html

They make a large contribution to imaging.


HT cOz

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Re: Image Depth?
« Reply #16 on: 1 Dec 2010, 02:46 pm »
Has anyone called to see how much they actually cost?

oneinthepipe

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Re: Image Depth?
« Reply #17 on: 1 Dec 2010, 05:12 pm »
There is a lot of good information on the Acoustics Circle.  For short money, you could build some broadband absorption panels/bass traps, which will not only improve bass but will also clear up the mid-range.  OC 703 panels can be purchased wholesale for a reasonable price, and the materials for frames and for covers is cheap. 

Saturn94

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Re: Image Depth?
« Reply #18 on: 1 Dec 2010, 06:10 pm »
Thanks.  I'm at about 4.5 feet (about 2.25ft each way), so I'm probably right on the border.  I'm might try to see if I can pull the speaker out another 3 inches (I'm hitting my limit already) to see what happens; that would put me right at the 5ft mark.

I have to revise my distance estimates.  I completely forgot about the role my 60" plasma plays in this.  It takes up most of the space between the speakers and sits 9 inches out from the wall (it's on a stand).  So a good deal of reflections I'm getting are coming off the TV.  Perhaps later I might consider having the TV wall mounted to place it further back since I cannot place the speakers any farther into the room.

Imaging is certainly not lacking in my setup, but I'm curious how much improvement I can obtain.

For not much money you could purchase two RPG binary diffusor panels that were 2" thick by 2' wide by 4' high, covered in fabric that coordinates with your decor.  You'd place these vertically on the wall behind your speakers to absorb and scatter that reflection point.

A second panel or two, mounted horizontally on the wall directly behind your head, would be nice too.

http://www.silentsource.com/diffusors-rpg-bad.html

http://www.silentsource.com/diffusors-rpg-bad-arc.html

They make a large contribution to imaging.



Interesting.  Not sure how much they may help in my situation as there isn't much "blank" wall space behind my speakers.  As mentioned above, most of the reflective surface is from the TV between the speakers.

I guess if I really wanted to get fancy, I could mount the TV on the wall and put up retractable curtains to cover the TV, just like a old fashion movie theater. :D

Kinger

Re: Image Depth?
« Reply #19 on: 1 Dec 2010, 06:36 pm »
Have you ever tried throwing a soft thick blanket over the TV during critical listening sessions?