Sloped baffle

Looked up quadrature and unless you're masochisticly inclined to imaginary vectors, this might be easier...

http://sound.westhost.com/ptd.htm

Ngjockey,

I have looked at this site for very many years. The Soundwest site has enough errors to mislead someone relying upon it for 'basic information' and a bit of the math.

Specifically:
In its Section one, the author does not understand a tweeter is still not time coherent when its wires are flipped over to invert its polarity (paragraph 3). He goes on to mis-represent the amount and degree of cancellations between mid and tweeter when the tweeter is not in the right position (below Fig. 5). What he presents instead is a graph showing TWO IDENTICAL, PERFECT, FULL-RANGE DRIVERS interfering, not a graph of one mid crossing over to one tweeter.

In its Section two, the information in the paragraph below Fig. 10, about phase shift and its audibility on square waves, is just plain wrong (even stating we can't hear it, then giving real examples of how we can hear it).

In Section four, on the audibility of phase distortion, not only is he wrong about its audibility, but he goes on to present an argument based on sound coming from live instruments.
He does not get it that we want to PRESERVE whatever phase relationships exist in the music, no matter where we sat, no matter where the recording microphones were placed. Can you spot the big flaw in his argument based on hearing live music? I have seen this exact bad-logic presented on many other forums as the main reason not to bother with making speakers time coherent.

In his Conclusions, he claims the room acoustics and bad recordings will hide much of what should be gained from making the speakers time coherent. To me, that makes it obvious he's never lived with time-coherent speakers for any length of time.
He mentions how a little pair of speakers in his workshop will reproduce a square wave at one frequency if he holds the mic in just the right place. I can see he does not recognize those speakers likely still have a phase shift of 360 degrees at some frequency, and how that will make a CONTINUOUS square-wave signal still appear square.
He does not remember that 360 degrees of shift at some frequency means the previous square-cycle is then projecting/delaying some of its frequency-components INTO THE NEXT CYCLE, and so on. He should have been examining only the first half of the very first square-wave cycle-- its first up-and-down only, to figure out what a speaker is doing.

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His are the answers I find quite common on the web, but not in most of the professionally-reviewed papers published by the AES. Their important papers on speaker design can be purchased by anyone as their three Audio Anthologies books. There are still errors in too many of those, but one must know calculus and physics quite well to find them.


I think the general public should not take a writer's claims about audio design for granted, unless the writer also presents the scientific concepts and logic behind those concepts, and WHY those have to be correct. Which is what I've endeavored to do.

Best regards,
Roy

In light of Roy's feedback to the sound.westhost.com material, here is (what seems) a much better website to read up on quadrature signals.
The article deals with complex signals but it is not complicated - the graphics make it much easier to understand.
if you don't want to read the article, scroll to Fig 10 directly & you will see why 2 signals in quadrature (i.e. separated by 90 degrees of phase) add up to a constant i.e. adding 2 signals in quadrature does not give you another signal; rather it gives you a scalar/just a number.
As Roy was saying earlier on - this can happen ONLY with a 1st order cross-over where the phase difference between tweeter-mid, mid-woofer is 90 degrees & when these signals add up at the listener's ear they appear as tho' there was no additional delay thru the x-over.
http://www.dsprelated.com/showarticle/192.php

That's how I was presented this subject. A good link, thank you. Made me flash back to all the horrible homework involved. And then, as the math of physics became ever more advanced during grad school, one wound up using this math daily...

Best,
Roy

Here are my answers to important questions posed earlier, and some clarifications.

To the OP: Psag, you originally asked if a sloped baffle is important. Speaker designs that avoid this are instead using the phase shifts of their crossovers to make sure there are no cancellations/suckouts in frequency response. That is about all their designers look for/measure during the design phase, since they do not make any measurements in the time domain.

I think those designers would have an easier time developing their high-order crossovers if their drivers were first stepped back from each other, as on a sloped baffle, and they got rid of the sonic reflections off their front surfaces.

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Bifwynne, at the beginning, you asked "perhaps someone could explain in layman's terms what causes speaker to operate out of phase. Does it have something to do with the use of caps and chokes in the x-over? Or perhaps the attribute of a dynamic speaker creating its own back EMF by reason of the voice coil moving in a magnetic field??

Incidentally, do all these electrical dynamics operating in tandem cause the electrical phase shifting that gives most amps a headache? "

Let us begin with the phase definition. If a speaker's woofer and tweeter were out of phase more than 'a bit', they would show a dip or even a complete suckout in frequency response, at or near their crossover point, with the microphone placed where your ear would be. As we see from Stereophile's tests, most speakers do not have this issue. So all of those must be "in phase", "phase coherent", "phase linear", or "phased aligned" As I explained earlier, that does not mean they are time-coherent speakers. As a reminder, the opposite IS true: time-coherent speakers are always phase coherent.

What makes the phase go weird?
-- In the speaker cabinet, it is from the drivers' locations/no stepped baffle, and having too many drivers per frequency range.
-- Any crossover circuit's inductors and capacitors delay the signal or advance it, respectively. Resistors do neither. A simple first-order crossover circuit has an inductor going to the woofer, and a capacitor on the way to its tweeter. At their crossover point AND ALL other frequencies, the time-delay created by the woofer's inductor is precisely offset by the time-advance created by the tweeter's capacitor. This is not possible with higher-order crossovers, because the values of their more-numerous inductors and capacitors cannot offset each other.
-- The back-emf from any driver is also a contributor to time-delay in its lower-range, whether woofer or tweeter. Thank you for pointing this out. I should have mentioned this earlier. That back-emf situation is altered by the type and size of the cabinet behind a woofer, and the size of any rear-chamber on a tweeter, and from ferrofluid in its magnet gap.
-- Any cone or dome breakups change the arrival-time as we go up the scale, but mostly we would hear ringing, sibilance, maybe 'dirt' being added to the music. Regardless, the best cones will not show a loud ringing at some frequency (as with most metal cones available in 2014) nor have a ragged frequency response in their upper ranges.
-- And yes, all these phase shifts will talk back to the amp. However, the crossover circuit's design is the primary cause of large swings in a speaker's impedance curve, above 100Hz. Those variations are 'electrical phase shifts' only. These swings in impedance do not reflect the acoustic phase at one's ear- no direct correlation.
The amp gets a headache because large swings in impedance means its output voltage (the pressure it puts on its electrons) is no longer sync'd up with WHEN those electrons are allowed to move by the crossover parts (inductors and capacitors). When the values of those caps and inductors do not offset each other, the result is exactly like pushing a child on a swing at the WRONG time.

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Bifwynne, on the first page, you speculated on the effects of mics, of recording and mastering, processing, playback, etc.

Each of those areas has unique problems, which do not sound like phase shift from a speaker. Each process produces a time delay in the highs and sometimes the lows, but only a speaker can put phase shifts (plural) across the main tone range. Also, whatever that signal is, I see no reason for home- and studio-speaker designs to distort it more.

On that same page you asked
"How are small speaker manufacturers able to design speakers without the benefit of the R&D budget, engineers, and testing facilities that some of the larger manufacturers have at their disposal?"

For me, it's been knowledge, education, and longer, much wider experience. My talent seems to have been expressed as an ability to make the cognitive leap between seemingly unrelated factors, which then made one more link to hearing vs. measurement. All of this has led to me not needing an anechoic chamber (I can always go outdoors for that). I also found the fancy digital test gear gave misleading and often incorrect numbers, compared to analog test gear.

When a designer does not really understand the fundamental physics of how and why drivers move and respond as they do, nor how crossovers delay the signals, then their only recourse TO IMPRESS their board of directors, is the anechoic chamber/digital route, for that is what the AES and any university would also advise those board members responsible for hiring 'a great designer'. Such a designer then blames the sonic differences between his and other speakers as 'we all hear differently'. His board of directors and all reviewers and editors gladly go along with that bullcrap.

We all certainly listen for different things. But here we have found, that as a speaker is made more and more time-coherent, everyone AGREES on the sounds heard in each and every tome range. They all hear 'the bass' in the same way, etc.

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Ohlala, on page one, the possibilities of off-axis cancellations you mention turn out to be non-issues on music, especially when the cabinet is not large, and has little sonic reflections from its surface.

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Timlub, on page one, your speaker design is only phase coherent at its crossover point, not time coherent, as you may know. Your electrical crossover slopes work well because they are combining with the phase shifts of your particular woofer and tweeter, which I am sure you suspect. Thank you for sharing your experiences! Appreciated.

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Bifwynne, you ask too many (good) questions! On page one you ask,

"here the ultimate Q. How can one tell whether a speaker is time and phase coherent? Critical listening? Reviewer comments? Bench test?

If critical listening is that important, the real challenge for us is, as many have written, that it is not easy to meaningfully audition speakers. So what's a person to do?

I'll ask again, how important is time and phase coherence? FWIW, ... really more as an FYI, ... Paradigm's web site states that its 'speakers have phase coherent crossovers designed so that the summed output of the drivers is completely and accurately rejoined.' Is that hype? It is true at all frequencies?"

On my website, I have suggestions on how to audition speakers. I know these work. They are simple, taking only time and effort. The time-coherence part of the audition sounds like clarity and depth, and when time-coherent speakers are designed with the best parts, the musicality is greatly improved.
With the very best, you find yourself never, ever thinking about 'the sound of the bass' or 'the highs'. Instead, you subconsciously always focus on the music and how it is being played, and its emotional and physical connection to you.
When a speaker is time-INcoherent, the music is fragmented, leaving you to hear only 'the details' and 'the soundstage' or 'the air', or 'the impact'. Right now, I see only Green Mountain Audio, certain models from Thiel, and Vandersteeen as making time-coherent speakers. The Audio Machina company is part-way there. With any others claiming time-coherence, I've seen no proof on their websites, or in Stereophile tests.

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Ivan_nosnibor, I appreciated your thoughts, thanks. However, the time delays in your digital crossover circuits are fixed time delays for each driver, when the real problem is the amount of time delays are different at each frequency. You remark on hearing perhaps the highs 'imaging closer to you' on non-time-coherenet speakers, with the mids 'not projecting as far into the room', and so on.

I have found instead it is about the lack of depth in the highs, caused by the smearing of a late-arriving mid, and so on down the musical scale. WHEN the highs arrive is not WHEN you hear the image, but only a portion of that image. One example is hearing the esses and tees of the singer's voice arrive from the tweeter's location above the mid, not from the mid driver's location, where the main part of her voice comes from, listening with eyes closed. That is one sound of a tweeter arriving too soon. It can also sound like the band is leaning forward, for want of a better word. It can sound like the rhythm section is behind the beat (as they would be in those speakers).

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Almarg,

Your described a square wave as "the summation of an infinite number of sine waves, one being at its 'fundamental frequency"' (the frequency with which its pulses repeat), plus others at every odd multiple of that frequency (i.e., the 3rd, 5th, 7th, etc. harmonics). The amplitude of each harmonic decreasing as its order (i.e., its frequency) increases." This is all true, but only of an ongoing series of square waves. The analysis is somewhat different when we examine just the first up-cycle, without even the first down-cycle following it. Just an FYI, seemingly never mentioned on the internet nor in textbooks.

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Mofimadness, the Loudspeaker Design Cookbook is generally excellent, but all previous issues got the concepts of phase time-coherence somewhat wrong. It has been awhile since I looked over a copy, so I can't remember where the problems showed up. I advise to take its advice with a modicum of salt.

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Bfwynne, the Revel 2 and Magico have oodles of phase shift, mostly from their crossovers. What you are seeing in the Stereophile tests is just as John Atkinson says- the mid and woofer take longer for their sounds to arrive. What is not readily apparent is how the phase (time delay) is changing at EVERY frequency. Otherwise, one could fix the Magico and Revel 'problems' by moving their tweeters back, etc. Actually, Almarg gave you a very excellent answer.

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Usermanual, you ask about us proving we are time-coherent.
1) This would not change our sales.
2) It cannot be done in a singular graph or 'scope image useful to a layman, by anyone including us. This is not a case of sour grapes- please read my letter to sixmoons regarding the issues with measurements. Note some of my graphs do not line up correctly with my text on their website.

In the 1994 Stereophile test on our Diamante three-way, remember JA always measures at 50 inches, right in front of a speaker's tweeter. That makes ANYONE'S mid and woofer too far away, relative to the tweeter.

JA then moved his mic straight down, to get farther from our tweeter, closer to the mid and woofer, looking for our claim of time coherence. You see our step response get sharper, more compact. But our frequency response goes to heck because he is now going VERY far off-axis of both mid and tweeter. Again, this test was done in 1994. In the intervening twenty years, every aspect of our sound, and of any measured performance, has improved.

Above all, trust your ears more than measurements and reviewers. My letter to sixmoons shows why this has to be so.

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This covers page one, I think. Perhaps page two will be much, much shorter.

Best,
roy