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Old 28th March 2007, 08:14 PM   #21
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Default Diffraction, Stored Energy, and Illusions

Quote:
Originally posted by Cloth Ears


A fellow audio enthusiast at the Melbourne Audio Club also has found this, but actually as a result of building his last pair of speakers. He says that he has pretty much given up the idea of building anything else and finds box speakers are now far too colored to go back to.

A small indication of them is here at Bob F's hybrid ribbon speakers. I'm sure I can get more details if you wish.

I do understand your intent is to use available cone drivers/co-axials, so I won't OT this thread anymore.
Not off-topic as far as I'm concerned. The whole subject of dipole and cardioid speakers needs much more work and attention. For one thing, almost everyone is using idealized models for polar patterns that don't pay enough attention to diffraction effects at the edges of the baffle. As mentioned below, a flat baffle is merely a special case of a conical horn - and like a horn, suffers from diffractive energy storage which falls in the critical 0-2 mSec time window.

Similarly, speakers with vertical-array drivers suffer from non-coincident arrival time, although this at least is a one-shot error versus the slowly decaying succession of reflections created by edge diffraction. Although difficult to measure (it just barely appears in MLS waterfall displays), the ear is extremely sensitive to this succession of closely-spaced echoes. If you're curious, they can be auditioned directly by playing pink-noise at a modest level, and walking around the front of a conventional speaker. If you listen closely, you'll hear what appear to be small tweeters spraying noise off the corners of the cabinet - this is at a maximum at a 45-degree angle with respect to the front panel.

I think of panel-edge diffraction being a bit like the partially-reflecting Brewster windows in a gas laser, which reflect light back and forth as the energy is built up with each pass inside the laser. In a speaker, thank goodness, nothing builds up the energy with each pass, but the successive reflections do take a while to die down below audibility.

Quote:
Originally posted by ScottG
Ah Lynn.. my favorite in the HiFi "world"..

While I'm generally no fan of horns .. consider that a good bit of a cone driver's bandwidth at higher freq's is in fact horn loaded. The larger the driver the lower the freq.. and the worse it is - Bastini's in particular. Just about any coax has a similar reality. Worse still, the "horn" for these cone drivers is moving around.

Heck.. even a baffle acts as a horn.

Also note that most pro drivers have increased distortion at lower spl's near the top of their passband (at higher freq.s) due to larger VC's (when compared to better hifi drivers). Both linear and nonlinear. Additionally, while they do exhibit less thermal compression.. they often suffer from mechanical compression that won't allow a lower freq. response anywhere near their max spl rating.

Despite the above.. I'm pretty much lock-step with your views. I just like hi eff. drivers better, more energetic/dynamic and IF they are of lower mass then they seem to offer greater clarity. Also, I've noted that linear decay is critical at least 9-10 db when factoring time - the shorter the better. Oddly though, HOW you achieve a cleaner decay is extremely important. Drivers with high internal loss and only moderate or worse ability to react to change, despite having *overall* cleaner decays (i.e. are "cleaner" further in time and level) sound significantly less clear. Likewise, drivers with acoustic resistance (particularly near time acoustic resistance), often via box stuffing and panel friction, kill ambiance and make the sound more monophonic (and "attached to the speaker").

I however specifically do NOT prefer a dipole pattern above the modal region - it creates a "sameness of sound" from recording to recording.. and in comparision to a really good enclosure falls short on depth. My preference is for a radial in the midrange and a wide dispersion pattern higher in freq. approaching 180 degrees (..when considering the front wall in a listening room). This does make several demands on both the "box" and the driver..
Good reminder that a cone has horn-like behaviour at the top of the passband, with the de facto driver being a ring radiator close to the voice coil, a hard dust cap, or an unfortunate combination of both.

This image certainly calls to mind the famous RCA LC-1A quasi-coax driver and the clever dimples that HF Olson put on the cone. When I was at the last Rocky Mountain show, I found out the principals of Cogent actually owned (!) a pair of LC-1A drivers, and I urged them - in the strongest terms - to put them back in production.

The patents on all of the classics - LC-1A, 604 Duplex, and the Tannoy Dual Concentric - ran out decades ago, so there's no reason at all they can't be built with much, much better materials. My dream driver would have a hemp-composite cone, diamond-diaphragm compression driver, and field-coil magnets. (Oh, did I mention that particular LC-1A is the RCA lab prototype and uses field-coil magnets?)

I've been thinking quite a lot about dispersion characteristics - most horn-fans don't know it, but unless they've got an all-horn system, the dispersion is not in fact constant with frequency. The direct-radiation bass unit radiates over 360 degrees (omnidirectional), then gradually narrows down to 180 to 90 degrees (depending on crossover point), then hands off to a horn with the specified radiation pattern (typically 120 to 90 degrees), which then narrows further as the frequency increases (unless it's a constant-directivity horn, which have their own problems). The narrow sidelobes that appear in the polar-pattern curves also appear as ripples in the time-domain, frequency-domain, and impedance curves - this is a consequence of antenna theory, where errors in one domain must appear in the other domains as well.

A conventional direct-radiator box also has a 360 to 90 degree transition, but the frequencies are much higher. The typical dome tweeter is 180 degrees over most of its useful range, and the direct-radiator woofer, just like the direct-radiator woofer in a horn system, makes the 360 to 180 to 90 degree transition over its working passband. And like a horn system, the radiation pattern gets very sloppy and complex at and above the crossover frequency, as one or more main lobes move up and down with frequency, nulls sweep close to the listener position, and many small sidelobes are created by cabinet-edge diffraction. Most of the "phase trims" we like to do with the HF crossover are actually doing little more that moving the main lobes up and down, and pushing the nasty-sounding null zone further away, or at least not on top of the floor reflection.

With a dipole or cardioid system, we're not starting with 360 degree radiation from the bass box, but 180 degrees instead. (The choice of subwoofer - dipole or closed-box - is a separate issue.) There's a better chance of keeping the radiation pattern fairly constant, especially if we twiddle with felt lining behind to the driver to adjust the rear energy level (thus choosing between figure-8, limacon, or cardioid radiation patterns). Not to mention standing waves from inside the box no longer exist - now we just have to contend with standing waves on the cone, which occur several octaves higher. This is where Bud Purvine's ENBL patterns, or other termination techniques, come in handy.

Room size is an essential consideration here. When the sound is several wavelengths smaller than the smallest room dimension, the listener can separate the sound into direct-arrival and total-room-energy components. At wavelengths approaching room size, direct-arrival and total-room-energy merge together. What sounds unnatural are the direct-arrival and total-room-energy spectra diverging from each other, since this is something that rarely happens in nature (or a concert hall).

As mentioned earlier, the ear/brain/mind processes sound in different "slots" using cross-correlation techniques (the first 20 or so reflections are compared to the direct arrival), and can compare the direct-arrival spectra to the earliest reflections. This happens automatically all the time - in fact, it's working when you're asleep or awake, constantly analyzing the environment for threat or safety. The environment-processor is not only connected to the frontal lobes (the "adult" sense of self - responsible for planning-ahead) but to the limbic system, which reacts with emotion and activates the fight-or-flight system.

When a hifi system tampers with the early-arrival information, the ear/brain/mind discounts the artificiality and categorizes the sound as just another artificial creation, mimicking life but most certainly not the real thing, just as you'd never confuse a TV picture or movie with real-life. When you get that hair-raising "spooky" feeling with a hifi system, that means early-arrival information is arriving intact, and the auditory illusion of being somewhere else is working. The machine is really generating an illusion, something HDTV and IMAX are still a long way from doing successfully.

The reason that movies and TV shows "pull you in" is the light hypnotic trance generated by the quiet, darkened room - a modern replica of the environment where thousands of previous generations listened to stories told round the tribal campfire. For a really fun experience, listen to the ZBS radio-dramas on a top-quality system - the illusion of "being there" is far more intense than watching a movie, as your ears open up to the environmental sounds of Morocco at night, and the narrator weaves the story-line.
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Old 29th March 2007, 12:44 AM   #22
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Lynn,

It is really good reading your words again - some people just have a way of putting their ideas across so that others can understand them.

Quote:
Originally posted by Lynn Olson
When you get that hair-raising "spooky" feeling with a hifi system, that means early-arrival information is arriving intact, and the auditory illusion of being somewhere else is working. The machine is really generating an illusion...
I can get this effect on some tracks in my home (in spite of pair of sealed woofers and a pair of Elacs x-ed at 120Hz - all boxes). But the tracks are all fairly light (Diana Krall, Keb 'Mo, Aaron Neville, and a recording I did of my brother and brother-in-law one night at a cafe). You can get the "you are there" feel with some of these. But I'm looking to the time my dog does more than look up (and you can hear him thinking "you're trying to fool me again, aren't you?") when I put on rainstorms or fireworks I've recorded off the internet. I guess I won't be able to play them again after that - but he's currently my arbiter of realism .
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Old 29th March 2007, 01:30 AM   #23
DougL is offline DougL  United States
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Lynn;

Good to hear that you are on the mend.
I am a fan of your tube amps, and look forward to your new speaker.

With the Coax driver, do you think the Dipole to monopole / waveguide transition is important? There has been discussions of extra realism with a second tweeter.

I'm following this with interest.

Doug
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Old 29th March 2007, 02:33 AM   #24
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Dear Mr Olson, Iv'e learned more usefull information from your web site than most that Iv'e run into on the web. Many thanks for the time you have invested in making your site so coherent and accesible to a layman such as myself. Get well soon!
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Old 29th March 2007, 03:09 AM   #25
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Default Re: Diffraction, Stored Energy, and Illusions

Quote:
Originally posted by Lynn Olson
don't pay enough attention to diffraction effects at the edges of the baffle
This is true of horns too. The ones pictured below have a larger and more realistic soundstage than any box speaker I have used. Note the mouth rolls away from the listener at the edge.
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Old 29th March 2007, 07:36 AM   #26
BudP is offline BudP  United States
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Hi Lynn,

I have some questions about drivers and mounting surfaces, for anyone with information they would care to share.

1. Would it be helpful to narrow the null zone between front and back sides of the drivers and would this benefit or detract from their activities after mounting?

2. Assuming that it would not be beneficial to narrow that null zone, would it help to slow the back wave down with a light mass damping?

3. If the mass damping on the back is beneficial would speeding up the front even further be beneficial?

4. Has there been any even semi rigorous evaluation of mounting board shapes and perhaps phase directing side panels. used to force a wider dispersion of the front wave VS the back?

5. Assuming the existence of technology for making the edge termination effectively invisible to an expanding wave front and it's lateral wave creator, are there analogs in other wave propagation devices, that are used to bend and shape emitted waves, that we might draw upon to refine our ideas about mounting boards and cone drivers.

6. Does anyone know what drivers Bob Carver used in his large dipole speakers and are there any sites devoted to this speaker where folks are dissecting Bob's thoughts?

Bud
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Old 29th March 2007, 08:36 AM   #27
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Quote:
For one thing, almost everyone is using idealized models for polar patterns that don't pay enough attention to diffraction effects at the edges of the baffle.
One advantage of open baffles is indeed the minimised edge diffraction effects.
When a wave is travelling from the driver to the edge of the baffle much less of it is reflected since it is absorbed by its inverted "twin brother" from the back side of the baffle !

Regards

Charles
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Old 29th March 2007, 10:57 AM   #28
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Default Re: Diffraction, Stored Energy, and Illusions

Quote:
Originally posted by Lynn Olson
My dream driver would have a hemp-composite cone, diamond-diaphragm compression driver, and field-coil magnets.
Hi Lynn,

Sorry to hear about your leg! Hope you mend quickly. I had a nasty fall not far from you in Niwot when I lived in Colorado. Fortunately, I landed squarely on my right kidney and didn't break anything.

I'm somewhat surprised to see you lusting after a diamond diaphragm given your stance on carbon-fibre and kevlar woofers. The same high Q resonant behaviours happen with stiff CD diaphragms. I reckon some people can hear this hence the continued popularity of aluminium over titanium and berylium. The polypropolene of CD driver materials, if you will.

Jeff
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Old 29th March 2007, 11:23 AM   #29
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Default Re: Re: Diffraction, Stored Energy, and Illusions

Quote:
Originally posted by jeff mai


This is true of horns too. The ones pictured below have a larger and more realistic soundstage than any box speaker I have used. Note the mouth rolls away from the listener at the edge.

Jeff,

Where did you source your horns from if I might ask? What are you using as a driver? is that an Altec compression driver?
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Old 29th March 2007, 01:29 PM   #30
JoshK is offline JoshK  Canada
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I was just going to ask the same thing.
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