"Line" sources and perception

[Few]

Hi all,
I'm sipping and listening to some fairly tall approximations to line source loudspeakers. Design details can be found at:
https://www.diyaudio.com/community/threads/diy-planar-magnetic-open-baffle-woofer-array.301091/ (long thread)
https://www.homebuilthifi.com/project/18876 (cut to the chase)

I'm writing (perhaps in the wrong forum--I went back and forth) to ask for possible explanations for an observation I have found surprising.

Context: The speakers are positioned in front of a 20 foot wide wall (now fitted with acoustic absorbers not seen in the photos). The rest of the room is about 60 feet long, and the whole environment is sufficiently reflective so that most folks would say "don't bother." Concrete floor. Left wall is all glass. Sheet rock on all walls. The room sounds reflective.

I designed tall dipolar loudspeakers, and positioned them, so that at the listening position, there would be a null aimed at the side-walls, and the floor and ceiling bounces wouldn't be intrusive because the high frequency sources are 8 feet tall in a room with 10 foot ceilings. I'm happy with the result, but a bit confused by the fact that the system sounds more real farther from the loudspeakers (where there is more room-contributed energy) than it does at the designed "sweet spot." When I'm 50 feet away cutting onions, I keep looking up to figure out how John Coltrane got a key to my living room. When I'm in the closer (to the speakers) sweet spot, I enjoy a very good audio experience but it's less convincing than it is where a reflective room contributes more. This is the reverse of what I expected: Best experience in the near-field, tolerable experience elsewhere.

If anyone has insights, maybe I can increase the awesomeness throughout!

Thanks in advance,
Few

 

[Duke]

I designed tall dipolar loudspeakers, and positioned them, so that at the listening position, there would be a null aimed at the side-walls, and the floor and ceiling bounces wouldn't be intrusive because the high frequency sources are 8 feet tall in a room with 10 foot ceilings. I'm happy with the result, but a bit confused by the fact that the system sounds more real farther from the loudspeakers (where there is more room-contributed energy) than it does at the designed "sweet spot." When I'm 50 feet away cutting onions, I keep looking up to figure out how John Coltrane got a key to my living room. When I'm in the closer (to the speakers) sweet spot, I enjoy a very good audio experience but it's less convincing than it is where a reflective room contributes more. This is the reverse of what I expected: Best experience in the near-field, tolerable experience elsewhere.

If anyone has insights, maybe I can increase the awesomeness throughout!

It is possible that the spectral balance of your system is improved by the off-axis contribution, which would be proportionally greater at longer listening distances. My guess would be that the spectral balance is a bit "warmer" at distance.

You can think of the in-room reflections as "carriers" for the reverberation tails on the recording, and nice long reverberation tails contribute to the sense of immersion in the acoustic space of the recording. When the direct-to-reverberant sound ratio is too low, as can happen at close range with line-source-approximating speakers, the richness and spaciousness of a good recording is not very well presented.

I'm a dealer for dipolar line-source-approximating electrostats, and I manufacture multi-directional loudspeakers, so I have some experience with the genre.

The speakers are positioned in front of a 20 foot wide wall (now fitted with acoustic absorbers not seen in the photos).

With bi-directional loudspeakers whose "rear-firing" energy is spectrally correct, I am not a big fan of absorbing the backwave. I like about 5 feet distance from the "front" wall, and then diffusion rather than absorption at the first reflection zones.

 

[Blumlein 88]

Wow! Those are some beautiful speakers you made there. My first thought would be to swap the position of the panels and the woofer towers. But you may already have tried that.

In my experience most speakers sound better in a large room, and often from further away. I think it is because low end peaks and dips are reduced and you are really in a large space not simulating one from the recording. I mean even a small jazz club will be closer to your 60 foot room than most people's listening rooms.

So Coltrane is really from way over yonder and sounds like it, while listening up close it is likely the recording was pretty up close and it sounds like hifi more than real.

I'm assuming you don't wish to cover your walls and windows with acoustic treatment. I would suggest a quadratic diffuser behind your setup between the two doors might help some in what you want to accomplish.

Sorry that doesn't help much with your dilemma.

 

[DVDdoug]

You didn't give the depth of your room but at 20 feet wide it might be bigger than the average living room. Real live music sounds better in a bigger room with longer reflection & reverb times and "good acoustics". Maybe your room is large enough to benefit from the reflected sound. And of course, you get a higher proportion of reflected sound at a greater distance from the speakers.

 

[wwenze]

It used to be that people thought to get the best sound you need to get a speaker that measures flat anechoically and then put it in a place where there is also no reflection

Now we understand that the preferred sound is to actually take that perfect speaker and then put it into a normal room. In other words, we are so used to reflections that we prefer them. For example a small amount of reverb makes music sound good.

 

[Sokel]

It used to be that people thought to get the best sound you need to get a speaker that measures flat anechoically and then put it in a place where there is also no reflection

Now we understand that the preferred sound is to actually take that perfect speaker and then put it into a normal room. In other words, we are so used to reflections that we prefer them. For example a small amount of reverb makes music sound good.

I hope that's true.My space is 38 by 12 foot with the ceiling at 10 foot.The speakers are in front of the 38 foot wall (not restrangular,is paragon so there's no parallel walls).
Even with absorbers,diffusers,etc (professional treatment,not yet finished) the RT is constantly between 600-700 ms.
I have accepted I'll never go under 500 .
Sounds good but all these years of constant hearing that 400 is the standard is like OCD

 

[Duke]

I hope that's true.My space is 38 by 12 foot with the ceiling at 10 foot.The speakers are in front of the 38 foot wall (not restrangular,is paragon so there's no parallel walls).
Even with absorbers,diffusers,etc (professional treatment,not yet finished) the RT is constantly between 600-700 ms.
I have accepted I'll never go under 500 .
Sounds good but all these years of constant hearing that 400 is the standard is like OCD

Don't get hung up on the numbers. The long reflection path lengths of that 38-foot dimension are what's causing your long delay times, and long reflection path lengths are NOT a bad thing!

Also, an RT60 of whatever value is NOT the holy grail, in my opinion, because it doesn't separately describe the early and late reflections. Imo what is more desirable is to have fairly weak early reflections, followed by a lot of spectrally-correct energy in the later reflections.

By using highly directional speakers and/or sitting closer to them such that the direct-to-reverberant ratio is increased, you can approximate the listening-position conditions of having a shorter RT60.

 

[Few]

Thanks for the thoughtful responses and generous comments. I'm using a minidsp system for crossovers and equalization. I compensate for the dipole cancellation effects at lower frequencies, and the imperfections in the native response of the drivers (home brew and Dayton). The equalization is based on windowed measurements at the primary listening position, so the system should be "flattest" there. I agree that the absorbers behind the speakers are not ideal, but the room is so reverberant that I wanted to add some absorption somewhere, and behind the speakers made the most sense to me. If I were a bachelor I'd move the speakers farther from the wall behind them, but my wife is being incredibly tolerant by allowing these towers into the living room, so I have to make some compromises. I've thought of finding a way to make it easier to move the speakers out for critical listening, but adding sealed subwoofers is my first priority.

My current arrangement (for the last year or so) has the mid-treble towers on the inside and the woofers on the outside (the opposite of what is shown in the photos in the second link). The image at the hot seat with the configuration shown in the photos was broader--more expansive. The imaging with the mid-tweeter towers on the inside is more focused, but less expansive. I regain some of the spaciousness by combing the current arrangement and listening farther away.

Part of the reason I'm surprised by my perceptions is that the simplistic picture of cylindrical waves propagating away from finite length line sources becomes an even poorer approximation as the listening distance increases. The "lobiness" becomes more prominent. Maybe the lobes get perceptually averaged out, or swamped, if enough boundary reflections are included and that's why things sound so real at a surprising distance?

Clearly more data points are needed. Thanks for the interest, in any case.
Few

 

[dasdoing]

I am wondering..... is the mid-treble in an equilateral triangle or at least close to it?

 

[Few]

Yes it's a slightly narrow equilateral triangle at the primary listening location. The woofers (when positioned on the outside as they are now) are actually at the equilateral locations.

 

[sarumbear]

Hi all,
I'm sipping and listening to some fairly tall approximations to line source loudspeakers. Design details can be found at:
https://www.diyaudio.com/community/threads/diy-planar-magnetic-open-baffle-woofer-array.301091/ (long thread)
https://www.homebuilthifi.com/project/18876 (cut to the chase)

I'm writing (perhaps in the wrong forum--I went back and forth) to ask for possible explanations for an observation I have found surprising.

Context: The speakers are positioned in front of a 20 foot wide wall (now fitted with acoustic absorbers not seen in the photos). The rest of the room is about 60 feet long, and the whole environment is sufficiently reflective so that most folks would say "don't bother." Concrete floor. Left wall is all glass. Sheet rock on all walls. The room sounds reflective.

I designed tall dipolar loudspeakers, and positioned them, so that at the listening position, there would be a null aimed at the side-walls, and the floor and ceiling bounces wouldn't be intrusive because the high frequency sources are 8 feet tall in a room with 10 foot ceilings. I'm happy with the result, but a bit confused by the fact that the system sounds more real farther from the loudspeakers (where there is more room-contributed energy) than it does at the designed "sweet spot." When I'm 50 feet away cutting onions, I keep looking up to figure out how John Coltrane got a key to my living room. When I'm in the closer (to the speakers) sweet spot, I enjoy a very good audio experience but it's less convincing than it is where a reflective room contributes more. This is the reverse of what I expected: Best experience in the near-field, tolerable experience elsewhere.

If anyone has insights, maybe I can increase the awesomeness throughout!

Thanks in advance,
Few

Has there been any measurements of them?

 

[dasdoing]

Yes it's a slightly narrow equilateral triangle at the primary listening location. The woofers (when positioned on the outside as they are now) are actually at the equilateral locations.

generally, when having first reflctions treated/avoided, people tend to use a slightly wide triangle. Here in my room I last tried a totaly equilateral triangle again after moving, but ended up moving my seat forward like in all my past treated rooms.

 

[Few]

Yes, there are measurements embedded in the first (long) thread linked in my first post in this thread. Sorry, they're not conveniently lumped on one page.

If I can find the time to put together and install the sealed subwoofer I'm planning, I can update my measurements and perhaps try both the narrow and wide triangles.

I have also assembled a dummy head (named Ron Johnson) for making binaural recordings. I'd like to see if putting the microphones in a few key listening positions captures what I hear directly. I'm sure the tonal balance will be obscured somewhat by the headphone listener's headphones, and whatever imperfections are inherent in the mics, but maybe the spatial cues will be captured. If nothing else, it'll be fun to try.

Few

 

[hex168]

Nice project! One factor may be that your line sources are not continuous and some distance is needed to reduce comb filtering.

 

[Few]

No doubt! On that first thread I devised some measurements that make it easy to visualize the kinks in the radiated wave resulting from the gaps in the radiators. Take a look around post #170 here.

Construction constraints made the gaps unavoidable, and I did try to minimize them. If I ever rebuild the drivers I'll likely go to greater lengths to make the radiators quasi-continuous. I don't think I'm likely to make on continuous 8 foot tall driver, though! One wrong move, and HOURS of construction effort is out the window.

Few

 

[Few]

In case it’s not clear which graphic I was pointing to, here’s the clearest presentation of the idea. Picture the mid-tweeter line source along the left edge, oriented horizontally (in and out of the page). So that axis maps onto height above the floor. Each horizontal trace in the graphic is a separate impulse response, measured at a slightly different height above the floor than its neighbors. So the horizontal axis can be thought of as time, And signals at the right edge arrive later.

I made enough measurements to span more than one of the planar drivers (to include the gaps between them). So the big ridge at the left edge is the main part of the impulse, followed by a bit of junk. The interesting part is the semi-flat “floor,” which shows an interesting crisscross pattern. I discovered with Bolserst’s help that each ridge in that pattern corresponds to the discontinuities separating the mid-tweeter panels. The discontinuities show up as hiccups in the radiated wave. The amplitudes of those hiccups drop off with distance, so I don’t think they’re a huge deal at the listener’s position, but they certainly are imperfections. They’re definitely washed out at my onion-cutting location, at least 40 feet away.

Few

 

[hex168]

Nice analysis. My gut feel was that the comb filtering would be a non-issue beyond 12 feet or so and needing 40 feet would be a surprise. I guess we're back to the hypothesis of it's the ratio of direct/reflected sound. You could test to see if it sounds better, closer up, by adding some temporary bounce speakers.

 

[restorer-john]

@Few I really like the creative use of bulldog clips for maintaining tension.

 

[Few]

Ha! The clips mechanically hold the front and back garolite panels together, but the diaphragm is held by adhesive to one panel. The “surround” attached to the kapton diaphragm (the clear film surrounding the kapton) is intended to help maintain tension. It also allows more excursion than the stiff kapton would on its own. It’s worked pretty well over time but one of my desktop versions of these panels has lost tension. The towers described here have needed little maintenance. Just a couple of solder joints have needed a second touch of heat.
Few