Beyond the Ariel

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If I play two identical loudspeakers spaced apart with a mono signal (same to both speakers) and measure directly between them I will see comb filtering? Why would that be?

I agree completely with Dr. Geddes.

This is strange to me. Comb filtering was the first big problem I encountered, decades ago when I started measuring speakers in my living room. SPL meter and graph paper in hand, I didn't understand why the response was so peaky, up and down. That's when I learned about comb filtering and what happens when you measure two speakers at the same time. Ever since, I've been cautious about it.

I don't understand how you could NOT get comb filtering with a single point, fixed measurement.
 
Lynn,
Thanks love purple. The subs are the Rhythmic FG12/sealed/audiophile subs listed on the site. The amps allow for numerous tweaks not always found on your typical sub such as damping, rumble filter etc. Check out Rythmik for all the details. I 'm happy with subs set at 50Hz/ 24db, 20 hz cut, High damping. I also have extensive GIK room treatments . My wife tolerates my madness .

Fred
Sealed/ no eq/ beryllium driver (I also have the standard 745 and yes there is a difference)

I understand the desire not to use a 1st order HP. However with a Slagel autoformer it can work.Hey, it doesn't hurt to try- More info available on the Intact Audio site/ forum.
 
This is strange to me. Comb filtering was the first big problem I encountered, decades ago when I started measuring speakers in my living room. SPL meter and graph paper in hand, I didn't understand why the response was so peaky, up and down. That's when I learned about comb filtering and what happens when you measure two speakers at the same time. Ever since, I've been cautious about it.

I don't understand how you could NOT get comb filtering with a single point, fixed measurement.

Well, I was using a 1/3 octave RTA, so the frequency resolution was a bit limited. Still, the thing went up to 16 kHz, and down to 32 Hz, so we could see what the overall FR looked like. Also, this is in a big room (factory floor), but no damping anywhere.

But ... the mike has to be on centerline with a 1/2" (10mm) accuracy, or the HF region will be a mess. And the speakers really do have to phase and amplitude-match pretty precisely, as well as crossover parts (particularly capacitors) that have good matching.

In a speaker as rough as an old-school Altec 604 Duplex, or anything with a simple 1970's-style crossover, no chance this will work. If speakers with rough responses could accurately phase and amplitude-match, sure it would work, but in the real world speakers with rough responses are that way due to bad cabinet reflections, uneven driver response in the crossover region, breakups in the passband, or a whole bunch of other problems.

These problems tend not to phase-match across a speaker pair. And if the phase does not match (within a few degrees), the amplitude summing will be all over the place, along with dubious image quality.

A little story: the first speaker I designed at Audionics in 1975 was the TLM-200, a gigantic 4-way system that was given to me when the previous designer high-tailed it out of town without a forwarding address. The stupid thing had the drivers and cabinets already selected (holes pre-cut in production-quality cabinets that were 5 feet high), but no f*#%ing crossover at all. It was either design the crossover or find a new job; take it or leave it. Some time later, armed with the aforesaid RTA, B&K mike, and large box of resistors, inductors and capacitors, I finished the project. Sound quality was OK for a first effort.

The cabinets were in mirror-image pairs, with the drivers offset but in a vertical line (again, I didn't design it, it was my job to make it work). Once the things were done, the imaging was pretty decent, and the response was flat within +/- 1.5 dB, not bad for 1975.

A few months later, our little band at Audionics arrives at the Chicago Summer CES show, but the half-wits at the factory shipped two LEFT speakers. Faces immediately fell when we unboxed what we thought would be the right speaker. Well, now what? We fired them up, looking a bit strange, but in those days most US-made speakers had drivers casually arranged all over the front face, and were not sold as mirror-image pairs, so maybe we could get away with it.

Until we listened to them. To the dismay of all, but particularly myself, they had no imaging at all. That is, you heard sounds come from hard-left and hard-right, but everything else was a diffused blur. Center sounds were spread all over the place. It was so bad I thought the drivers were out of phase or something, but I pulled them out and checked them and the crossover wiring very, very carefully.

This was the night before the show, and we were really starting to sweat now. The terrible image blurring, it turned out, was solely due to driver placement on the front face; they measured just fine, and all the drivers and crossovers were correctly phased. Not only was I shocked, but this was a real crisis for us; it was the debut of our new speaker line, and we had hours to go before the show opened in the morning. The latest we could hold off the crowds was maybe noon the next day.

So we called the idiots responsible and demanded they ship the correct matching speaker ... open the damn box and check, OK guys? ... and make a rush trip to the Portland Airport freight department for overnight shipment. Not cheap at several hundred dollars, but the exhibit room was several thousand, so this had to happen RIGHT NOW, or we risked a very expensive failed show. Not a good thing in a company with 20 employees.

Bright and early next morning, we rush out to the Chicago Airport freight department in a rented van, collect the missing matching speaker, and worriedly set it up in the demo room. To the enormous relief of all of us, the overnighted Right speaker was a perfect match for the already-good Left speaker. Imaging went from non-existent to spectacularly good. Phew! Disaster very narrowly averted. Audionics would live to fight another day, and the show turned out well for us, with good reviews and the dealers happy with our new product lines.

But that whole experience is seared into my memory. It would never have occurred to me to intentionally audition a pair of Left speakers; everything we did at Audionics was always mirror-imaged, following UK custom of the time (we were the authorized US agents for Radford at the time, and I owned KEF mirror-imaged speakers at home).

The TLM-200, despite the insane complexity of the crossover (the KEF drivers were not very flat and required tuned notch filters), did have pretty flat response, and there wasn't much bad happening when you measured it 10, 20, and 30 degrees off-axis. In fact it was not easy to see the differences if the speaker was measured off-axis on the left or right side; diffraction artifacts are difficult to see on a 1/3 octave RTA, so you really couldn't see the problem with the measuring gear we had at the time.

My conclusion after this near-disaster was diffraction (in the horizontal plane) has a strong influence on image quality and overall spaciousness, but unfortunately is difficult to measure. So the best approach is to reduce it as much as possible, consistent with reasonable costs of production.

The null measurements mentioned in the previous post, along with the in-phase summation, really only work on speakers that are flat to begin with, and have very accurate phase-matching across the pair. All the speakers I did at Audionics, and afterward, have always been like this, but I've noticed poor phase-matching seems to be pretty common in the US high-end business (I still see it 40 years later), and I guess we were the exceptions. I just kind of took it for granted that qualified designers, like Laurie Fincham at KEF, and the people we met at BBC Research Labs, knew all about this already, since the Brits take image quality seriously.
 
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I too take diffraction seriously- I'm honestly amazed how little effort is put into absorption. It's not hard to effectively eliminate diffraction with some foam or felt, but it remains almost entirely ignored in most circles. Commercial goodies, I understand, absorption is tough to manage cosmetically, but DIY.... we should be using even our ugly tools.
 
I too take diffraction seriously- I'm honestly amazed how little effort is put into absorption. It's not hard to effectively eliminate diffraction with some foam or felt, but it remains almost entirely ignored in most circles. Commercial goodies, I understand, absorption is tough to manage cosmetically, but DIY.... we should be using even our ugly tools.

As anyone that has ever seen my speakers can attest, I have never even though about not using the Ugly Tools, or for that matter the Beautiful Tools, "....would you care for another pound of glimmering Polyester Tone Sauce (Bondo) for your Monitors, my dear Lady?"

Best Regards,
TerryO
 
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If one uses something like Holm for measurements you can see the arrival times of each speaker in a stereo pair. If you simply move the mic until the two impulses match, then there is no comb filter - as long as the pair are matched, as Lynn says. But matching has never a problem for me.

Diffraction free cabinets need not look bad, but it does eliminate many common cabinet making techniques.
 
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Thanks Lynn, that give me something to think about. When I've measured the Gedlee speakers I never thought to check for comb filtering - wish I had, now. If Earl is not finding, why would I? Curious - but chance missed.

My first speaker measurements where I learned about comb filtering were 6.5" full range drivers in a small apartment in Montmartre. Reflections were certainly abundant, tho I didn't know much about them at the time. Reflections should reduce comb filtering, right? At the moment I have no "serious" system to test, so will dig back thru some old measurement for a look at any combined L&R measurements.

Some years agoa number of us did a null test between left and right to get an idea of direct versus reflected energy in our listening rooms. It's in a thread that I can never find.
 
Fred
Sealed/ no eq/ beryllium driver (I also have the standard 745 and yes there is a difference)

I understand the desire not to use a 1st order HP. However with a Slagel autoformer it can work.Hey, it doesn't hurt to try- More info available on the Intact Audio site/ forum.

Thanks Greg... I only wondered if you pursued even a bit of EQ, 'cause Gary Dahl found it helpful with his Be Radians when not using his Raals on top.. Anyway, it's cool that you got great technical help with such iron winders/vendors like Dave Slagle.. And for a minute there, I thought J Mascis had those baffles custom made for him. He loves purple too.
 
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Or a room with highly diffuse and decorrelated reflections, like my lava cave. No great loss of energy like open air or an absorbant room, but no correlated reflections.

I don't see how that would not have comb filtering direct sounds. Unless they are masked by the reflections. Wouldn't the reflections have to be fairly early to do that?
 
"Or a room with highly diffuse and decorrelated reflections, like my lava cave."

I have a bunch of diffussors or scatterers and have been using them in my room. I think they make a HUGE difference in the tonal balance and cleaning of the resulting direct sound - so I think it might be the way to go if we have to listen to our systems in a small room - lol
 
"Or a room with highly diffuse and decorrelated reflections, like my lava cave."

I have a bunch of diffussors or scatterers and have been using them in my room. I think they make a HUGE difference in the tonal balance and cleaning of the resulting direct sound - so I think it might be the way to go if we have to listen to our systems in a small room - lol

I absolutely agree. A proper mix of absorption for the early reflections and scattering for the later reflections can have a large impact.
 
Outside in free space no boundaries.

So basically a room that is not a room.

De-correlated reflections will not create comb filters. That's what "de-correlation" means - two uncorrelated signals cannot interfere with each other. Only correlated signals can do that. Partial correlation means a less deep comb filter - from very deep to nothing as correlation goes from 1 to 0.