Pin point imaging isn't for everyone


A subject my posts touch on often is whether pin point imaging is desirable, or natural. While thinking about wide-baffle speakers in another thread I came across this quote, courtesy of Troels Graveson’s DIY speaker site. He quotes famous speaker designer Roy Allison:

I had emphasized dispersion in order to re-create as best as I could the performance-hall ambiance. I don’t want to put up with a sweet spot, and I’d rather have a less dramatically precise imaging with a close simulation of what you hear in a concert hall in terms of envelopment. For that, you need reverberant energy broadcast at very wide angles from the loudspeaker, so the bulk of energy has to do multiple reflections before reaching your ear. I think pin-point imaging has to do with synthetically generated music, not acoustic music - except perhaps for a solo instrument or a solo voice, where you might want fairly sharp localization. For envelopment, you need widespread energy generation.


You can read Troel’s entire post here:

http://www.troelsgravesen.dk/Acapella_WB.htm

This goes, kind of, with my points before, that you can tweak the frequency response of a speaker, and sometimes cables, to get better imaging, but you are going significantly far from neutral to do so. Older Wilson’s were famous, and had a convenient dip around 2.4 kHz.
erik_squires
@david_ten wrote: " Duke, thank you so much for your detailed posts. Very helpful!"

Thank you David, my nerdy tangents aren’t always welcome, very glad to hear you found these helpful!

@mijostyn wrote: "This is a major reason line source dipoles sound the way they do. They minimize reflected energy in a way no other type of speaker can match."

Imo line source dipoles have many things in their favor, and minimizing early reflections is certainly one of them. Imo their backwave energy is also uniquely beneficial.

First off, the backwave of a dipole is spectrally correct, which is a really good start. Then assuming the speakers are fairly far out into the room, the backwave can actually REDUCE the small-room signature I alluded to earlier! Let me explain:

The ear/brain system judges the size of a room by the time delay between the first-arrival sound and the "center of gravity" of the reflections. When we have a significant path-length-induced time delay on the arrival of the backwave energy, the ear/brain system interprets that as "we’re in a pretty big room". So less "small room signature" is super-imposed on the soundstage in the recording! Imo this is an example of "reflections done right".

(The highly counter-intuitive implication here is that MORE reflections [in this case the backwave energy], done "right", actually result in hearing LESS of the room you are in and MORE of the soundscape on the recording!)

Mijostyn again: "Horns can be made to do almost the same thing by controlling their directivity."

Yes! Horns can definitely reduce the amount of energy in the early reflections AND generate a spectrally-correct reverberant field, through uniform pattern control. (Imo gotta use the right kind of horn the right way to avoid audible colorations.)

I really like the liveliness of good horns but probably like the timbral richness and sense of immersion in the recording’s soundscape from a good dipole speaker even more. So my best horn systems have a rear-firing array dedicated to generating a spectrally-correct, relatively late-onset approximation of the backwave of a dipole speaker. There are still things that a good line-source dipole does better, but imo the additional "backwave" energy tightens the race in some areas.

Duke
Sure Duke but I think there is one caveat. You don't want reflected energy off the front wall coming right back at you. I wish I could draw a picture here but essentially you want the reflection to take the long way around the room. So with a properly toed in dipole the rear sound would head towards the front wall angled towards the side wall. Then it would bounce off the side wall and head towards the rear wall to your side around you. This gives you that late reflection from around the room that makes you think the room is bigger. If sound heads toward the center of the front wall and bounces back right at you it really confuses the image just like a blurry photograph. It also diminishes the sensation of a 3rd dimension. I place acoustic tile on the front wall to prevent this reflection towards the center. It only works for frequencies above 250 Hz but that is enough to do the job. This is the only place I use room treatment. 
Mijostyn wrote:

"Sure Duke but I think there is one caveat. You don’t want reflected energy off the front wall coming right back at you."

You are absolutely correct!!

"I wish I could draw a picture here but essentially you want the reflection to take the long way around the room. "
YESSS!!!

"So with a properly toed in dipole the rear sound would head towards the front wall angled towards the side wall. Then it would bounce off the side wall and head towards the rear wall to your side around you."

That’s what I do, whether it be a dipole or one of my quasi-bipolar horns. With my current generation of quasi-bipolar horns, the "backwave" energy is angled up towards the ceiling, further increasing the reflection path length, and allowing good results even when the speakers are quite close to the front and/or side wall.

"This gives you that late reflection from around the room that makes you think the room is bigger."

Right on!! The reflections need to come from all around, not just from the same general direction as the main speakers (which is actually the worst possible direction). And once you have the long time delays from these long path lengths plus the reflections coming from all around, the ear/brain system is really good at picking out the "hall ambience" reflections on the recording.

(Some people mistakenly think this is just a room effect, but it cannot be because how can a modest home listening room sound like it is the size of a concert hall? It must be that the concert hall ambience on the recording has been unmasked, rather than that the small room’s signature has been enhanced.)

"If sound heads toward the center of the front wall and bounces back right at you it really confuses the image just like a blurry photograph. It also diminishes the sensation of a 3rd dimension. I place acoustic tile on the front wall to prevent this reflection towards the center. It only works for frequencies above 250 Hz but that is enough to do the job."

Now you are teaching me something I had not thought of! What you describe makes sense, and I will give it a try next chance I get. THANK YOU!!

Are your acoustic tiles absorption or diffusion?

Duke
learned something new today - THANK YOU Mijostyn!
Go figure. They are absorption. These are the ones I use. Dirt cheap.
https://www.parts-express.com/sonic-barrier-fwp122-studio-acoustic-foam-wedge-panel-12-x-12-x-2-black-12-pack--260-547
Remember I have Acoustat 2+2s 8 feet tall and twenty inches wide. I put a single vertical row of seven tiles alternating the pattern. They are placed at the deflection point on the wall. The easiest way that I know of to find it is stand right up against the back of the speaker right in the middle. Have someone move a mirror back and forth horizontally across the wall slowly and have them stop when the reflection is centered on the listening position. Mark that point on the wall. That is the deflection point and your tile pattern should be centered there. Height and width are up to you and depends on your speaker's dispersion. 
I first tried double sided carpet tape to stick the tiles on the wall. The tape would not stick to the foam. Next I put a drop of viscous cyanoacrylate glue in each corner and right in the middle, sprayed the wall with accelerator and slapped the tile to the wall. Worked great! I had marked out the wall with pencil and a 4 foot level so I knew where each tile would go. If you ever want to take the tile down the residue will sand right off.

Mike