Time Alignment Questions

[Guest 1080i]

Question #1: Time alignment of drivers was not built into the cabinets of our classic AR speakers, as we know. Why wasn't it? The drivers were simply fixed to the flat front panel of the cabinet. If it is so important, why wasn't it done, and why doesn't everyone do it today on modern designs?

#2: You smart people will think I'm stupid when I ask this, but here goes: When aligning speaker drivers, why are the magnets aligned and not the sound-producing surface (the cones)? I would think the surfaces that radiate the sound would be what should be aligned, but looking at designs, it seems the magnets (hence, the voice coils) are lined up. The radiating surface is where the sound comes from. If there was a 6" difference, for example, between the voice coil position and the cone of a tweeter (not likely, but for the sake of example) and you align the voice coils, the actual source of the sound could be 6" ahead of the other drivers.

I'm not a speaker designer, so I don't understand it all. Please explain. Thanks.

[Carlspeak]

>Question #1: Time alignment of drivers was not built into the

>cabinets of our classic AR speakers, as we know. Why wasn't

>it? The drivers were simply fixed to the flat front panel of

>the cabinet. If it is so important, why wasn't it done, and

>why doesn't everyone do it today on modern designs?

ACCORDING TO VANCE DICKASON, A WELL KNOWN EXPERT IN LOUDSPEAKER DESIGN, RADIATION TILT AND PHASE SHIFT CONSEQUENCES FROM OFFSET DRIVERS IS ONLY A CONCERN AT CROSSOVER FREQUENCIES ABOVE 700 Hz. THUS, FOR EXAMPLE, THE AR3a SPEAKER'S WOOFER TO MID CENTER DISTANCE SHOULD NOT BE A CONCERN WITH IT'S 550 Hz CROSSOVER FREQUENCY. ON THE OTHER HAND, AR'S DESIGNERS COULDN'T HAVE PLACED THE MID AND TWEETER MUCH CLOSER TOGETHER WITHOUT FLATTENING THEIR FLANGES IN ONE AREA.

KEEP IN MIND, MOST OF THE EARLY AR ERA DESIGNS WERE DONE WITHOUT THE SOPHISTICATED MODELING SOFTWARE WE NOW ENJOY AND ALSO WITHOUT SOPHISTICATED TIME DOMAIN MEASURING INSTRUMENTS.

>

>#2: You smart people will think I'm stupid when I ask this,

>but here goes: When aligning speaker drivers, why are the

>magnets aligned and not the sound-producing surface (the

>cones)? I would think the surfaces that radiate the sound

>would be what should be aligned, but looking at designs, it

>seems the magnets (hence, the voice coils) are lined up. The

>radiating surface is where the sound comes from. If there was

>a 6" difference, for example, between the voice coil

>position and the cone of a tweeter (not likely, but for the

>sake of example) and you align the voice coils, the actual

>source of the sound could be 6" ahead of the other

>drivers.

>

>I'm not a speaker designer, so I don't understand it all.

>Please explain. Thanks.

THE EXACT ACOUSTIC CENTER OF A DRIVER IS STILL THE SUBJECT OF MUCH DEBATE. ACADEMIA CONTINURES TO RESEARCH THIS IMPORTANT MODELLING PARAMETER. FOR NOW, I UNDERSTAND THE RULE OF THUMB IS THE CENTER OF THE VOICE COIL, WHICH BY COINCIDENCE, IS SOMEWHAT ALIGNED WITH THE CENTER OF DRIVER MAGNETS.

IT'S ALSO IMPORTANT TO NOTE THAT TIME ALIGNMENT IS A COMPLICATED FUNCTION AFFECTED NOT JUST BY THE PHYSICAL LOCATION OF DRIVERS TO ONE ANOTHER, BUT ALSO BY THE SELECTION OF CROSSOVER FREQUENCY, ORDER AND DESIGN.

IF YOU WANT TO 'READ MORE ABOUT IT' CHECK OUT THIS LINK.

http://www.livesoundint.com/archives/2003/...align/align.php

It's all about the music

Carl

Carl's Custom Loudspeakers

[soundminded]

OK, I'm going out on a dangerous limb here (frank this might perk up some discussion) and say that IMO time alignment is a complete fraud. Here's my reasoning. The theoretical problem stems from observing that if two loudspeaker drivers are vibrating at the same frequency supposedly reproducing the same musical components of sound in their crossover region, if one is slightly ahead of the other because it is physically closer to the listener and/or they take different times between the point in time voltage is applied and when the cone physically responds (propagation delay) there will be constructive and destructive interference patterns and the correct waveform will not be reconstructed. This is of course true over the entire audible range but there seems to be greatest concern at the crossover frequencies. The worst conceivable possible case of misalignment would occur when the time delay difference puts the two waves from different speakers 180 degrees out of phase at a particular frequency. To compensate for this, one driver, usually the tweeter is recessed with respect to the woofer. This is because this propagation delay is generally greater for the larger driver so it is later (slower) to respond. This gives it a head start. Some crossover network designs claim a similar result but you'd think if it were really important, a true time delay circuit like a digital delay in a bi or tri amplified arrangement would be better. This fad got started sometime around the early 1970s with the DCM "Time Window" speakers. Then everyone jumped on the bandwagon. Dick Shahanian tilted his Rectilinear V back 4 degrees. Of course this caused a change, it put you slightly off axis of the tweeter and reflected more hf components off the ceiling. The problem is that assuming this even matters (I'm not sure anyone has demonstrated this under controlled conditions where time alignment was the only variable operative and therefore validly tested) it doesn't eliminate another source of constructive and destructive interference, namely that the drivers propagate their sound from two different locations in space. This will get the same interference results. To truely align the waveform, the drivers would have to be coaxial as well and then you'd still only get it on axis. To make matters worse, burying the tweeter inside the woofer restricts its dispersion which may not matter if you are of today's school where limited dispersion is considered desirable. The AR philosophy was exactly the opposite. So what do these constructive and destructive patterns do? They cause irregularities in frequency response whose particular character changes from location to location. But even with time alignment and coaxial drivers, this will happen anyway. It seems to me, this is just one more magic bullet audiophiles grabbed on to for awhile and then let go of when it didn't kill the beast of non realistic sound reproduction. Besides, once audiophiles had all bought their time aligned speakers, designers needed a new dragon to slay so that they could come out with a new line of products to cure the new problem. Given the endless parade of products this industry has managed to market over the decades, you'd never guess that they aren't shooting at a moving target...or are they?

[dynaco_dan]

Hi Bob;

Next time you see Dahlquist DQ-10 speakers on ebay or their upper end drivers, look at the mounting boards used for them.

There appears to be a lot of thought given into mounting them, rather than on a one piece mounting surface, typical of most, but not all, speaker systems.

There may be some interesting reading on the designing of those speakers, which I have read many nice reports.

Perhaps, James, for one, may offer some input here.

[ar_pro]

It's always a pleasure to read your opinion, soundminded.

I'm begging you, though - please start using line & paragraph breaks.

[Carlspeak]

For the sake of brevity, here's another referenced link to an even more comprehensive article. The author concludes that time alignment may indeed be a good thing.

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

It's all about the music

Carl

Carl's Custom Loudspeakers

[frankmarsi]

5-18-07

Thanks for bringing me back out "soundminded", after the other day's 'smack-down', by the CSP site’s 'moral-police' regarding you and me talking out of turn about components I use along with my speakers and some of the music I enjoy and use to test my AR speakers with, but that's another story, I won't stir the pot, besides why should any of us speak of the program-material we use to enjoy our speakers, that maybe too upsetting for some individuals. And it won’t ever be interesting unless we include cartridges and what-ever……yawn... Although my A77 Mk.4 is doing great, as usual, I even almost picked-up a 1020L Pioneer last week. “Hush” here comes the Gestapo- “now- run and don’t ever not use the word “AR” when speaking on this site again”, “now go- go to another site-if you want to speak freely”. Soundminded, I’ll see you there along with certain others no doubt, bud…..d.

In terms of speaker de-fraction, re-fraction, or plain old ‘the damn wood molding is cutting off my high frequencies’, well I have a friend that has had Theil's since 1989, just after he sold his AR-11's and Allison 1’s, he claimed they're better for time-alignment dispersion. I told him he was defending the reasons he bought those speakers for in the first place, so yeah, I do agree with you S.M. I personally do have some credence in it all, but damn, it didn't become an issue for E.V. or H.K., or R.A. while designing these great AR models, why should it be now. Although I will state while I was in a 'hi-fi-salon' not too long ago, a 'heady' 30-ish dude did mention how newer designs avoid such problems of de-fraction and some are built with ‘time-alignment' in mind, yeah like I wasn't aware...dude, I told him I have speakers older than he was, and I walked away. Be that as it may, there is a site I found that sells flat foam circles that mount around H.F. drivers to absorb frequencies that spill against a baffle board, avoiding 'smearing' and similar maladies, a'la AR's "Acoustic-Blanket'- I was thinking of buying them myself. I presume there may be some validity to all this 'refraction’ stuff, but I’m gonna hold off for a while. I’ll keep you posted, perhaps on another site if necessary. In closing; is ‘time-alignment’ really noticeable to the naked ear, and can a sloping speaker baffle assist with its principal? Watch it, did I use the word ‘naked’, is that allowed on this site? Please some body help me before I sink too low.

FM

P.S. If anyone is interested in this site with the foam rings, give me a shout.

frankmarsi@verizon.net

[soundminded]

A long long time ago, I attended classes at a university in another country whose culture was very different from ours. In that culture, form was as important as substance. If a professor picked up a red piece of chalk and wrote the number 1 on an outline on the blackboard, 900 students picked up a red pencil and wrote number 1 in their notebooks. If that point was number 6 and the professor wrote it with green chalk, 900 students in the lecture picked up a green pencil and wrote it as number 6. Positions in the next higher level were fewer and were determined by a competition based on the highest test scores. Each course had one exam, a final at the end consisting of two or three essay questions. The student was expected to reproduce the professor's notes as exactly as he could remember them in both form and substance...in black ink. If all of this seems silly to you, well it seemed silly to me too. I think most moderators of message boards like this are happy to let the participants take a conversation wherever it happens to go even if it would have been more appropriate under a different category so long as it remains within what he considers reasonable bounds of civility. Personally, when I come across a topic which doesn't interest me, I just move on, I think that's what most people do. For anyone who finds this so objectionable they can't stand it...they're always free to start their own site with its own rules.

It strikes me that there is a lot very peculiar about the way audio components and systems are engineered. They always seem to put the cart before the horse. They find a theoretical problem, expound on it, and then find a magic bullet to kill it. Voila, instant breakthrough. Engineering in the real world doesn't work that way. The most important part of an engineering problem to me is defining it. That's because the definition of the problem is used to assess the success or failure of the solution. Since the problem in high fidelity audio reproduction ultimately revolves around recreating a stimulus to elicit a remembered perception of a comparable event, it seems to me the first thing you'd want to know is what are the limits of perception of the variable you are concerned with. That is rarely if ever done in audio. For example, suppose you wanted to make a more accurate television transmission system and arbitrarily decided that a limitation of the existing ones is that they can't reproduce infrared and ultraviolet frequencies. A new improved system might be more accurate in that respect but it's improved accuracy would be worthless. In fact it might be worse than worthless, not just because of the added cost but because it might entail sacrificing performance in another area which can be perceived since many designs are often tradeoffs. So before anyone begins touting a time aligned speaker design, it would be useful to know exactly what the limits of time alignment distortion are which are audible so as to have a specification as a goal. A valid measurement method to verify performance is also crucial to prove a design does what it claims. This kind if research is in the area of clinical psychology (elective my senior year, had to write a paper on the ability to resolve two closely spaced lines by the human eye at very low light levels.) Once the variables within which performance must be held to be inaudibly different from a perfect reproducer in this regard are known, then it is possible to begin discussing strategies to design speakers to perform within that tolerance. Until then, what is its purpose? how do you know when the time alignment is good enough?

BTW, the paper cited by Carl is very interesting. I've scanned it through quickly and I may have some comments about it later. But I did not see any part of it so far addressing my quesiton about what the required limits of performance are.

[Pete B]

Witold Waldman, author of the CALSOD speaker simulation package

has a nice list of references:

http://www.users.on.net/~audiosoft/spkdesref.html#Bookmark06

There are a few more classic journal articles and I have a stack of papers on the subject around here somewhere.