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In Reply to: RE: that may well be the case, Dave ... posted by Satie on August 14, 2015 at 03:21:31
I just read this thread with great interest - especially the discussion of polar patterns and "time-smear" being associated with large line-source drivers (and arrays, I guess).
Does anyone know of any psychoacoustic research that sheds light on the impact of large drivers vs. point-sources? Thinking about the physics of sound propogation from large drivers, they seem problematic, yet in practice (for me) they seem to work fine.
Isn't there a psychoacoustic "summing" effect that applies to reflections or echoes arriving within a couple milliseconds or so of the initial wave-front?
Follow Ups:
Exactly correct.
These types of large speakers have considerable comb-filtering effects relative to conventional or point-source transducers. Yet, our ear/brain integrates (sums) these extremely well and they don't manifest into a significant problem for domestic listening.
If you were to measure these systems in a free-field environment and move the microphone up/down in the vertical direction you would see a multi-lobed polar pattern with large cancellation nulls at various frequencies. Not pretty compared to more conventional systems.
Cheers,
Dave.
Here are measurements of the Flatline Design (closed after the death of the founder) ribbon hybrid speaker.
The speaker sports a 69" 5/8" ribbon with only two elastomer wedge brackets for damping.
Note the very good impulse response from the ribbon.
No sign of differing arrival times.
The FR plots on the vertical dispersion showed no difference.
The expectation of a departure from a cylindrical radiation pattern in the nearfield is not applicable to continuous line sources but only to spaced line arrays of spherical sources having a substantial vertical component to their radiation. As shown here http://www.jblpro.com/ProductAttachments/AES_Ureda_Analysis_of_Line_Arrays.pdf
Yeah, a very old test. I remember reading that issue many years ago. John's frustration with measuring these types of speakers was evident then and still is now. He's not the only one. I applaud him for making the efforts. It's very challenging to test speakers of this type and get meaningful results that won't be misinterpreted by ignorant audio forum members. :)
"All things considered, the Flatline 175 illustrates why I hate measuring ribbons or other planar speakers: the interaction between the speaker and the measuring microphone is too complex for comfort at practical measuring distances;............."
Anyways, I don't care what you interpret (or misinterpret) from those test results. The distance from a measuring microphone to the center of the line is different than to the end of the line. A comb-filtered objective result will be generated. It's the nature of the beast....if monitored at a design-axis reference point.
I'm not sure if you're attempting to argue basic physics here. I mean, it is what it is....this is not my opinion.
The subjective result is obviously a different story and I will stipulate that speaker system probably sounds fine in a domestic environment with music material. (Even though there were numerous issues with the implementation.)
Did you read section 3.5 of the Ureda paper?
I actually think the Lipshitz/Vanderkooy paper from 1986 is much better written and clearer in illustration, but YMMV.
I'm not making this stuff up. Large speakers are large speakers...they will measure like large speakers. No other result is possible.
Dave.
But I don't see the measurements that show that for a continuous planar line source.
If there were a vertical pressure gradient for a tall line source - essentially floor to cieling- to create significant output from the bottom OR top to reach the listener then it should have showed up in both these measurements of impulse and testing for changes in FR at different vertical angles. Or it should have shown up in those done by our DEQX'ers or by my REW measurements in the prior decade.
I have been hacking google trying to find the measurements that you must have seen to be so certain of this. I see many for cone arrays, dome arrays and tons for commercial sound horns that show how the cylindrical waveform falls apart above a certain frequency. You can also show it in simulations.
However, all I find for long planars is that there is barely any vertical component to the output and very near none for push pull planars/ribbons. Alcons and SLS (and BG too) claim that provides their spiral arrays completely uniform coverage without mid and high freq hot spots or dead spots.from comb filtering. Others in the industry are scrambling to come up with horn waveguides and curved reflectors to provide as close to a purely horizontal launch as possible out of their waveguides to compete with the planar line source makers. Same goal as Beveridge .
I just don't get how the physics of the wavelaunch can allow a pressure gradient to form in the vertical direction off of a long planar in the near field. And therefore how there could be a spherical radiation that would arrive at the listening height from the bottom or top. of the ribbon. All you hear is the output from the center of the ribbon and the rest of it is only there to prevent the radiation from having a vertical component and to extend low end response.
Did you look at Figure 2 (and the others for straight line sources) in the Ureda paper?
It's well described in Section 3 of his paper.The math does get a little involved, but the simplified function is noted in 3.3.
Implementation of the crossover is key in real-world systems, and much depends upon the frequency range the line source is driven. I suspect that's what you're noting with your "no vertical component" observation.
I hope you're not arguing that line-sources have no inherent disadvantages relative to "conventional" and/or point-source radiators. All of them have inherent advantages and disadvantages. If you add up all the pluses and minuses, a point-source radiation characteristic would be optimum in a free-field environment. But those are (essentially) impossible to construct. (At least for a full range speaker system.)
The objective in any speaker system should be to make them as small (acoustically) as possible. Any time you make them larger, things get more complicated. :)
Dave.
Edits: 08/15/15
I see that the first significant simplification in the Ureda calculation is that the distance on axis at th middle of the line is much higher than the height of the array.ao that you are an order of magnitude farther out than you would be in a home setup. The second issue is that listening is largely close to the midpoint normal to the line so the angle (alpha) is 0 or very near it.At home you have a 1.5 to 2 m source and a 2-4 m listening distance. For a full length line source at ~2m height you are at 1 to 2 times L. The ends condition is that the top and bottom are constrained by the floor and ceiling. So the floor and ceiling act as a waveguide to keep the vertical pressure drop and dispersion from developing. I don't know how much of the model holds up for a home setting of a near floor to ceiling line source at 1.5L listening distance at the midline.
This is not to say that there is absolutely nothing coming to your ears from the bottom or top of the line source. And from anywhere else on the line, but the overwhelming amount coming from ear level and the precedence rule would make transients fully unsmeared. The comb filtering effects result in a very closely spaced set of sharp squiggles on an unsmoothed FR scan. But that is less important to our hearing since our FFT breakdown of pitch and harmonics takes much longer (10-30 ms once the steady state pitch arrives) than our response to transient acoustic events (0.001/0.01-1 ms,), which is independent of pitch and harmonic content (and distortion thereof). Our pitch resolution is fairly crude like our amplitude resolution, hence the violinist's practice of vibrato changing pitch in a +/- 5% band is still perceived as a single pitch tone.
Efit: actually our pitch resolution is much much netter than our amplitude resolution at any frequency. But it takes some time and tends to average.
I will claim that in a home setting a line source is going to have an inherent advantage over point sources since the latter do not contribute their own floor bounce cues to those on the recording. That would not be an issue in an anechoic floor and ceiling room. But I don't know how you walk on an anechoic floor made of absorbers,, snow shoes?. Also at close listening distances you can get much less floor bounce (and room reflections) in the mix reaching your ears..
Edits: 08/16/15
Go look at Alcons (Germany and USA) and SLS audio as well as Meyersound who make commercial line array PA systems The first two use planar sources for the upper band which have terrifically clean waterfall plots, They measured that the planars have a 94% horizontal radiation along their length with an insignificant vertical component, and Meyersound pateneted the "Ribbon Emulation Manifold" or REM waveguide that produces a nearly all horizontal radiation pattern.
All of them challenge you to stand under their line arrays and detect any direct radiation from them.
They don't post much in the way of measurements but they have colorful pictures of the acoustic simulation software showing that the radiation is very cylindrical.
Meyersound went to the trouble of showing why discrete horn drivers in standard horns do not produce a uniform cylindrical wave.but continuous sources like ribbon drivers do. Hence the high speech ineligibility they achieve and lack of hot spots and dead spots in the radiation coverage zone
While the bass panels should be considered point sources over most of the range below 200 or so hz, the other drivers are defacto continuous line sources that produce a real cylindrical wave with hardly any vertical component. So there is no time smear.
http://www.alconsaudio.com/pro-ribbon-2/
http://www.slsloudspeakers.com/SLS%20Audio%20LASS%20Line%20Array%20Simulator%20Software.htm
http://meyersound.com/support/papers/line_array_theory.htm
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