How Does Imaging relate to Dispersion?

[Zonker92]

Generally speaking, and assuming a room is fairly reflective of sound, does broad dispersion tend to vary inversely with (or impair) good imaging, by virtue of unpredictable / undesirable reflections off the walls, floor and/or ceiling?

In this scenario, and assuming one is willing to confine one's listening position to a single perfect sweet spot, is the ideal loudspeaker for imaging one that has extremely limited ("spotlight") dispersion? And does this mean that speakers with narrow dispersion tend generally to have better imaging (in that sweet spot) than ones with broad dispersion?

I'm curious, because while I've always read that broad, uniform dispersion is desirable, I have trouble reconciling that with the idea that unpredictable reflections could tend to impair good imaging. Does this mean that one has to choose between good dispersion and good imaging, or is there a happy medium somewhere?

And what about speakers with rear and/or side radiation? How can they achieve good imaging in the face of unpredictable room reflections?

Thanks.

 

[dumptruck]

I'm trying to think of a concise response to this, but I don't think there is one...

Here are 265 pages of discussion on the topic: http://www.diyaudio.com/forums/multi-way/195124-what-ideal-directivity-pattern-stereo-speakers.html

I will say one thing though: it's very important to realize that "narrow dispersion" is not something you really find very often. Generally, all the options available transition from ~360° dispersion in low bass to somewhere 50-110° up top, so the question is how wide they should end up, and how they should get from one end to the other. Same thing for bipoles and dipoles except with the mirrored pattern (but maybe not quite the same answers).

 

[Zonker92]

Thanks. I'll take a look.

"narrow dispersion" is not something you really find very often. Generally, all the options available transition from ~360° dispersion in low bass to somewhere 50-110° up top, so the question is how wide they should end up, and how they should get from one end to the other.

Ah so; that makes sense; thanks.

 

[tomlinmgt]

I'll have a go.....knowing full well that dump will probably have his red pen in hand to mark up my reply real good.

Yes, speakers with broad dispersion patterns in a reflective room will more often than not suffer in performance due to the arrival of early reflections to the listening area. The early reflections will mask the spatial cues that exist in the recording which promote imaging and sound stage development. This is one reason ESLs, planars and ribbons (provided all are configured for dipole radiation) tend to have such good imaging and sound stage production in spite of being in a room with no acoustic treatments. As a result of their design, dipoles will have a null in output directly to the sides....commonly referred to as "dipole cancellation". The output slowly increases as you move from the sides to on axis and this dispersion behavior minimizes the influence of reflections (since they're rather weak) off of side boundary layers. Line source dipoles will have a null in output toward the floor and ceiling as well, resulting in little to no interaction with the floor and ceiling. Horns have controlled dispersion as well, but the pattern is wider and output across the dispersion patter more consistent compared to planars, ESL's and ribbons...especially with constant directivity horns which (if I understand correctly) have consistent SPL output across the entire dispersion pattern. Having this well defined dispersion pattern enables positioning of the speaker (or direction of the horn) so that interaction with the boundary layers to the side is reduced. And many horns will have a vertical dispersion pattern that's intentionally more narrow than the horizontal dispersion pattern so that interaction with the floor and ceiling is minimized. Cones and domes have very wide dispersion patterns...up to 180 degrees in the midrange....but output across the dispersion pattern will vary. Cones and domes offer the user the least amount of control wrt to being able to minimize interaction with reflective boundary layers.

Alright dump...try not to leave me with anything that'll scar. And nothing to the face, please.

 

[Valveglow]

Well Zonk, it's like this...


...I don't know. Perhaps (and I say perhaps) the best imaging comes from point source radiator type speakers. I have experienced incredible imaging from certain tiny speakers with no dynamic range, no bass whatsoever, etc...

Conversely, I always wanted to hear some of the better Tannoys and others with a Coaxial point of sound origination but with some oomph.

Many of the 7 foot tall line sources confuse me as to how their imaging would be precise, and yet there are some phenomenal (and phenomenally expensive) line source speakers.

I've owned Magnepans and always felt that they imaged very well, and yet they are much akin to some people I know, that is, bipolar...dipolar!

Personally (and please don't shoot folks, I'm not an Omar hater) I never cared for some of those Bose reflecting bookshelf speakers. Though dispersion was amazing, I often thought I was hearing (seeing?) 8 foot wide guitars, etc.

As long as my JBLs image well, I'm content...

 

[Zonker92]

Thanks; I'll read and grok.

So how best to position CD horns for imaging? Toed in to reduce wall reflections?

 

[dumptruck]

Thanks. I'll take a look.

I decided I could handle commenting on at least one tiny part of the question and edited my post :thmbsp:.

Thanks; I'll read and grok.
So how best to position CD horns for imaging? Toed in to reduce wall reflections?

Definitely pointed somewhere around the middle of the listening area, but you need some good space behind them or a lot of absorption on the rear wall for that trick to entirely work the way it's supposed to.

 

[Zonker92]

Cool; just saw that.

Definitely pointed somewhere around the middle of the listening area, but you need some good space behind them or a lot of absorption on the rear wall for that trick to entirely work the way it's supposed to.

Cool. I have a reflective (cinder block) back wall (and a wife to accommodate), but maybe some toe in would be good:

 

[twiiii]

The broader the dispersion pattern the shorter the Dc, critical distance. Dc is where the direct sound of the speaker equals the reflected energy of the room. For the spoken word the idea is to have less than 10% articulation loss. Which can usually be maintained when the Dc has been exceeded by 60 + % in distance when the system has a controlled even response without severe room resonances, reflections, with adequate room signal to noise.

So lets say you have point source speaker with a broad pattern like a big Tannoy Coax. Lets say its directivity is 6 for arguments sake and the Dc for the speaker in the room is 7 ft. So as long as you sit with in 11 ft of the speaker with out masking from to much bass, you would be happy. But let's say that you want to listen at 16 ft. Well that means a higher directivity index and a longer Dc. An Altec 511b horn should work in this case. But you must remember you have two speakers in the room producing sound at the same time interfering with each other, So you would have to cut the reverberation in the room by half in both cases. Or in an Altec case switch to the Big Mantaray horns. Gets ridiculous fast.

 

[tomlinmgt]

Geez, I think I probably edited my reply at least a dozen times.

 

[dumptruck]

I'll have a go.....knowing full well that dump will probably have his red pen in hand to mark up my reply real good.

Nah, you can relax this time. I don't have any unbearable disagreements with anything in your post, and I don't really want to get too far into this topic anyway. I don't feel personally confident in any particular "recipe" for "good imaging", it's still pretty hotly debated by the highest level pros, and there are SO MANY freaking variables involved.

Heck, you can't even really define "good imaging" without restricting it to a particular sort of stereo illusion for a particular recording and mixing process.

 

[tomlinmgt]

The broader the dispersion pattern the shorter the Dc, critical distance. Dc is where the direct sound of the speaker equals the reflected energy of the room. For the spoken word the idea is to have less than 10% articulation loss. Which can usually be maintained when the Dc has been exceeded by 60 + % in distance when the system has a controlled even response without severe room resonances, reflections, with adequate room signal to noise.

So lets say you have point source speaker with a broad pattern like a big Tannoy Coax. Lets say its directivity is 6 for arguments sake and the Dc for the speaker in the room is 7 ft. So as long as you sit with in 11 ft of the speaker with out masking from to much bass, you would be happy. But let's say that you want to listen at 16 ft. Well that means a higher directivity index and a longer Dc. An Altec 511b horn should work in this case. But you must remember you have two speakers in the room producing sound at the same time interfering with each other, So you would have to cut the reverberation in the room by half in both cases. Or in an Altec case switch to the Big Mantaray horns. Gets ridiculous fast.

Okay..this is new to me and very interesting.

"Dc" is stated as a distance value, I assume. How is "reflected energy of the room" stated?

 

[tomlinmgt]

Nah, you can relax this time.

But you're still gonna whack me with your stick some other time. Good to know. Gonna keep us noobs in line, aren't ya dump. :yes:

I don't have any unbearable disagreements with anything in your post, and I don't really want to get too far into this topic anyway. I don't feel personally confident in any particular "recipe" for "good imaging", it's still pretty hotly debated by the highest level pros, and there are SO MANY freaking variables involved.

Heck, you can't even really define "good imaging" without restricting it to a particular sort of stereo illusion for a particular recording and mixing process.

I think the one thing that is widely agreed upon is that early reflections will blur the stereo image and anything that can be done to eliminate or minimize them...be it acoustic room treatments or speaker design...will help improve all components of fidelity including imaging and sound stage production.

 

[Zonker92]

early reflections will blur the stereo image and anything that can be done to eliminate or minimize them...be it acoustic room treatments or speaker design...will help improve all components of fidelity including imaging and sound stage production.

Cool; thanks.

 

[tomlinmgt]

With the 19's positioned as shown here in the picture, convergence of the lines that would represent on-axis occurs just in front of the listening position. The outer margin of the dispersion pattern of the 811B horns is pointed to the rear corners of the room. This provides a very strong center image as well as very strong resolution of content in the area between the speakers as well as outboard of the speakers. Toed in any more and the sound stage narrows where content presented outboard of the speakers becomes much weaker in resolution and presence. Toed out any more and content in between the speakers loses resolution.

Of course, you can see my room has extensive acoustic treatments (including two absorption panels on each side wall and three absorption panels on rear wall) so I may be disqualified since you're largely discussing with a fairly reflective room in mind.

 

[Zonker92]

Ah, that reminds me of this paper from Pi speakers, which I have yet to read through:

http://www.pispeakers.com/Pi_Speakers_Info.pdf

Interesting drawing on page 7 for CD cornerhorns.

I guess I should experiment more with toe-in.

 

[tomlinmgt]

And what about speakers with rear and/or side radiation? How can they achieve good imaging in the face of unpredictable room reflections?

Thanks.

For rear radiating designs, the reflected content is outside of the time window that would make it an early reflection....so it instead becomes, you guessed it, a late reflection. Late reflections reach the listener with sufficient dwell time between themselves and the direct sound wave that spatial cues in the recording are not disrupted. As a matter of fact, many believe they can enhance those spatial cues...giving them additional "space" and "ambiance". This is why having adequate distance between the speaker and the front wall is so critical wrt getting proper performance out of dipole designs. Too close to the front wall and the rear wave arrives at the listener too soon...making it behave more like an early reflection. There is a minimum delay time in milliseconds that's recommended but I can't remember it. Dump probably knows.

 

[tomlinmgt]

And if you have a boundary layer to the rear that is fairly close to the listening position....say around five/six feet or less....you will be getting a strong reflection from that rear boundary layer that is adverse as well. Absorption panels would be very beneficial in this scenario. I speak from experience......

 

[Zonker92]

Yeah, I had some Carver ALS IIIs that radiated from the rear. They seemed to sound way better when I closed the curtains on the windows that were close behind them ...

 

[dumptruck]

I think the one thing that is widely agreed upon is that early reflections will blur the stereo image and anything that can be done to eliminate or minimize them...be it acoustic room treatments or speaker design...will help improve all components of fidelity including imaging and sound stage production.

Absolutely, but on the speaker design side, it's not always totally obvious what that means. For example, if you're going to have a significant early reflection, which of these would you want it to look like?