Originally Posted by terry j
you mean 'time aligning' or any other applicable term? (methinks I have to use the correct terms
Can you expand on this some more?? Presumably it would involve measurements of the signal, so any accompanying graphs would be fantastic.
Originally Posted by karlsaudio
So the closer you can get the sub to the main speakers, the better? Or as long as the effective spacing keeps the 1/4 wavelength of a frequency slightly above the crossover frequency?
As I added the qualification above, the more common 'time alignment' (a term Ed Long trademarked from back in the 70's) is actually a misnomer. I had a few esteemed mentors and instructors who held us accountable for saying what we meant and meaning what we said, which meant that "signal alignment" was the proper term. For as they correctly observed: you can align signals with respect to time, but you cannot align time!
An important distinction indeed.And a bit of 'baggage' I carry with me, as i too am inclined to use the term 'time aligned', provided all know the distinction...
First, it is ALWAYS advantageous to have all energy sourced from a single co-located location. That is the impetus behind the co-axial, co-entrant and Synergy horns, and all of the various means to align both the acoustic origins of multiple sources. In doing so, destructive interference (polar lobing and comb filtering) as well as group delay is effectively rendered moot.
The devil is, as they say, in the practical details of how to overcome the limitations in our manufacturing processes.
In regards to the effect of spacing on sources, the greater the inter-driver spacing (real or virtual), the lower the frequency where the destructive interference due to their superposition (combining) begins.
One such common manifestation of this effect is known as SBIR - speaker boundary interference response - where the energy reflected from one or more boundaries superposes and crates an undesired null(s).
And for the purposes here, the interaction of spaced sources applies on several levels and is fundamental to the study and understanding of reflections in a bounded space as well as the interaction of individual drivers which reproduce overlapping passbands. So the sources can be real (as in two or more actual drivers) as well as real and virtual, as with a speaker placed near a boundary where its reflection functions as one or more additional spaced source(s) depending on the number of proximal boundaries.
As far as how this can practically be done (or at least come somewhat reasonably close...)...realizing that the lack of co-location offers tradeoffs no matter how one does it that result in either the sources being aligned with respect to time at a point, or, at best, in a plane (and hence the more common vertical alignment of drivers affording such a relationship -at least- on a limited horizontal plane)...
But the easiest way to align the signals is with the impulse or the ETC. I prefer the ETC as it generally is displayed as a more precise spike rather than as a lower Q 'loop' (assuming we used the impulse response to verify that both signals direct arrivals are in polarity and are both positive).
Using the ETC of both sources (you might run it individually for each source and identify which peaks correspond to each source, and then they may be run with both sources driven simultaneously),you can use the displayed results to tell you both what the time differential is between the sources, and also directly determine read the time offset with a small adjustment.
Let me explain.
Using the 'standard' configuration for generating the ETC, we use the hardware loopback to compensate for hardware propagation delay (the additional time it takes the hardware to 'get around' to executing the test signal). As a result, the time of each arrival is accurate.
So one , once having identified the sources, can simply use subtraction and determine the time offset.
For those who are a bit lazier, we can also display this time differential directly. Many heretofore are want to use the "set time = 0 to IR peak" for general measurements, not realizing that it negates the loopback correction and offers little more than a neat response with the direct arrival time set equal to 0.
But here it is useful. For by setting the first arriving signal equal to time=0, we can then directly read the time offset for each arriving source. And while this will NOT help with locating reflections, it can help with delay settings. And in this case we can read directly the required amount of signal offset in terms of time that we must use to align the signals.
For arrayed elements (for which this is typically used), it is simply a matter of dialing in the amount of delay into a precision microsecond delay unit.
(Please note also, that for this discussion I am intentionally avoiding array topologies which will determine the spatial dispersion and orientation of the polar lobing due to the interaction of the separate spaced sources' overlapping passbands, but the diagram modeled in EASE below should illustrate that - pictured for 2 spaced sources positioned horizontally. The lobes will rotate corresponding to the physical relationship of the sources, in other words, if the 2 spaced sources are positioned vertically with respect to one another, simply rotate the diagram 90 degrees, and if they are position at 45 degrees relative to each other, the lobing will be oriented as illustrated by the diagram when it too is tilted at 45 degrees....)
But in any case, now that everyone is on the same footing and all are totally confused
, we can use the ETC response to align the sources provided one is able to adjust the amount of signal offset electronically. And when the signal arrivals are precisely aligned, you will observe an increase in the magnitude of the (now singular) arrival peak that indicates the summing of the energy magnitudes.
Note too, that as the time offsets between the sources get closer, the windowing of the ETC response can be adjusted by 'zooming in' so that you can see the differences with very great acuity. So instead of looking at the differences from the equivalent of 30,000 feet, you can zoom in and examine the difference from say, a foot away. A feature that makes the ETC invaluable for all sorts of examination. Sort of like having a telescope that if looked at in one direction shows you large orders of magnitude, while looking in the other end becomes a microscope illuminating things orders of magnitude smaller - a feature loudspeaker designers appreciate in analyzing very small scale behavior, reflections in the throats of horns, etc..
Is that simple enough?
If anyone desires more detail or additional info, feel free to PM me.
Oh, and anyone desiring more info and a much more precise method if interfacing subs with full range elements, please refer to Charlie Hugh's comprehensive procedure
(don't let it scare you!). Its not that difficult using a time capable tool like REW, ARTA, or Easera)
If any one needs more info or clarification, please feel free to PM me.
And since the topic of modes came up...
BTW, for think who think they will eliminate modes with speaker placement (if only), you might refer to this site
. It has problems in that the 3space diagram is confusing as heck as it appears as if it is an optical illusion as far as its orientation, but it is one of the only modal calculators that actually provide a glimpse into the actual 3space distribution of modes, rather than the archaic and misleading 2space graphs generally provided.
And note too, that there are no such 'things' as separate axial, oblique and tangential modes.
This is a conceptual fantasy that has become real to the point that we talk about axial modes as distinct from tangential modes as distinct and separate from oblique modes. Much like we can simply focus on or manipulate one to the exclusion of the other.
Thus our language imparts connotations that are not relevant to reality, and instead create additional problems by our thinking we can solve one aspect (or mode) without there being a corresponding shift in the others.
This is complete and utter mistake. They are ALL aspects of the SAME phenomena. It is simply as if we look at the behavior from above the room, or from the side of the room, or from the end of the room. What we see are NOT 3 different events!
They are simply 3 perspectives! Trying to separate them is like trying to imagine a bar magnet with just a positive end and no negative end. Or a one sided coin, with obverse side.
And just like the fundamental problem encountered in the infamous Descartes "mind-body" problem that typifies much of the problem with western dualistic logic; once we name the 'aspects', we can conceptually then manipulate them as if they have an independent existence all their own independent of the other 'named parts'. We can mistaken talk of them as of they function independently of one another. It is like saying that your arms can go the movies while your legs spend the afternoon soaking in the hot tub. It sounds great, but it is in fact, nonsensical.
Yet we do it all the time, especially in audio and acoustics.
And this is what we get with many acoustical phenomena that are 'made one' via the Gabor Analytic, applied to acoustics (and also electronics) where is is known as the Heyser spiral (for those interested). Here one can see a grand unification where we can plainly see that all of our measurements are but different perspectives of the exact same complex behavior.
But I digress into a fascinating world which is at first confusing, but quickly becomes a tool facilitating amazing clarity and opening up many new realms for exploration. But that assumes we have a greater understanding of the behavior of the 'thing itself' and use the measurement tools to illuminate and increase our understanding of the actual behavior rather than simply ascribing magical properties to a measurement. It's also a world where you quickly discover that the mystical world of imaginary numbers simply means that they represent the same behavior from a different point of view! Suddenly the 'imaginary' (a VERY
poor choice of descriptive terms!) become Very real, and very significant. (It wasn't until I saw that that what I had learned to manipulate over several degrees in physics FINALLY assumed meaning, which up until them we merely memorized and learn to manipulate without ANY semblance of an idea as to what they really correlated. And once the Analytic was provided (which, BTW can be measured and displayed in 3 space with the TEF analyzer), the light instantly went on and it still constitutes one of those significant epiphanies....
Sorry... but the main point is to realize that modes are a SINGLE complex behavior where the energy ebbs and flows like a giant blob, varying with frequency/wavelength. Or think of it like the weather, where regions of high and low pressure alternate and ebb and flow about. But in any case, lose the flat land perspective of separate axial, oblique an tangential modes!!!! It is a 3 space distribution of high and low pressure energy regions distributed in a bounded space. And hopefully the Hunecke site will help you to visualize the 3space distribution - as there is no 'height mode' and help you to realize that what we commonly term as distinct and separable axial, tangential and oblique modes are simply different perspectives on the same unified behavior that is contributed by the relationship between the wavelengths and the various dimensions of the bounded space.
Oh, and sorry for mentioning subjects that go beyond the product descriptions on the boxes of stuff available at Best Buy, as is so lamented by some in few other threads.