Levels- and spectra-pathing symmetrical arrays of monopoles - diyAudio
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Levels- and spectra-pathing symmetrical arrays of monopoles

Posted 6th January 2017 at 04:23 PM by Grasso789
Updated 25th May 2017 at 09:49 PM by Grasso789 (Bits)

This is about spectra-pathing ("multi-way") loud-speakers made of several monopoles, in explanation omni-directional sound sources, but I really mean direct radiators as opposed to horns, which are more uni-directional. I think about home, rehersal and club loud-speakers, not stadium ones. While spectra-pathing enables large power at wide band-width, seams between paths are usually audible, altho that could have been avoided rite from the start, at least technically. But instead of fixing problems at source, one has relentlessly used power to paint over them.

The natural solution is a symmetrical arrangement of lo-pass monopoles embracing a hi-pass one, combined with a cross-over of first order.

When Germans started to loose ground in World War II, wide-range drivers playing from Large E to 10 KHz were state of the art. Mister Eckmiller developed a co-axial two-spectra-paths driver consisting of hi-pass encircled by lo-pass piston. But for mass production just to lift treble power tweeters were added to existing designs. Due to interference between monopoles, amplitude is rarely flat. If the manufacturer wants to publish a plot of amplitude versus frequency, he chooses the rare case occuring at only one microphone position. When stereo came up, it became even worse. A common stereo has just two loud-speakers placed far apart. If this is used to play mono content, of whom there is plenty even in most stereo recordings, then certain frequencies (slitely different ones for each ear) are cancelled due to run-time difference between left with rite channel. But if such stereo is used in a chamber, AND if its loud-speakers are quite dispersive, then reflections off walls fill in missing frequencies. In cinemas and concerts power is important, too: Each recipient receives his random amplitude, but visuals and athmosphere fill in missing bits. The sum of all recipients' opinions is power, say success of the movie or record.

It took until the eigthies, that Joe D'Appolito, who had looked into spectra-pathing, pointed out, that a symmetrical woofer-tweeter-woofer arrangement may have phase difference between hi- and lo-pass as occuring in a cross-over of odd order and will still direct its beam up-front. In opposite to that, an insymmetrical woofer-tweeter arrangement with phase difference between hi- and lopass tilts the beam at cross-over frequency. Another way to get around this beam tilt is to get rid of phase difference, using an even-order also called Linkwitz-Riley cross-over. I want a cross-over of first order due to its perfect time and transient behaviour.

The only well known serious application of the D'Appolito trick is JBL's CBT (for Constant Beam-width Transducer) models, which were introduced AD2008, and for which the designers obtained a patent by treating aforementioned phase difference as delay, which in conjunction with physical dimensions and relative driver amplitudes determines beam-width.

AD2000 Don Keele had thoroly approximated, how a monopole array without phase difference between monopoles has to be in order to produce a smooth beam of certain width. His approach uses levels-pathing --he calls it shading-- but not spectra-pathing. All CBTs are symmetrical, even if only by means of using ground plane as mirror. The simplest CBT, omni-directional, is a ring made of twelve (one per hour) identical monopoles mounted close together. (The more space is wasted in surrounds and basket rims, the worse acoustic response becomes. In best case, pistons barely touch each other.) The most powerful CBT built up to date, also omni-directional, may be a pulsating sphere used to excite halls evenly.

I do not strive for a loud-speaker containing dozends of monopoles and having very wide or narro beam-width. I rather have a few monopoles making a frontal beam-width of cosine (front radiation like a dipole) in the mid-range, where it pays off. I strive to have stability and smoothness of convex baffle. I notice, that D'Appolito and CBT both use levels-pathing: In case of D'Appolito, each lo-pass monopole is half as loud as hi-pass monopole at cross-over point. A three-monopoles CBT has each outer one -6dB.

I have a three-spectra-paths "Krassolito" box, which I run as synkronous-stereo loudspeaker in my narrow living room. Left and rite midrange are L and R, while woofers and tweeter are fed L+R from a third amplifier. Mesure-l`a one Bel channel seperation at 4 KHz at an angle 1h off axis. This means a less obvious but wider stereo sound, since it is told to a calm listener mostly by reflections off walls. I am quite satisfied.
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