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In Reply to: RE: Diffraction and front baffle and cabinet design posted by villastrangiato on April 17, 2012 at 11:59:56
You are correct that offset, baffle width and driver's bandwidth/coverage play a role in diffraction - for flat baffles on rectangular boxes. And that it can be managed to some extent using those factors. I disagree that it can be made to disappear."you'll find that the responses in figure 15,16, and 17 (the rectangular box) have the best response. The sphere with offset tweeter (figure 6) is not a practical design and you didn't specify an offset driver which makes a significant difference as the centered driver on a more practical circular baffle (figure 8) is the worst case (which is what I was referring to). "
You have made a significant error about spheres and the drivers in the Olson paper. I suggest that you read the referenced 'OLD' paper by Olson, all of it, a couple of times. If you had read it you would not have made this rather revealing error.
The reported tests used the same very small high quality driver, to ensure that it was a pistonic source. The pictures of the sphere and the hemisphere throughout the paper are offset so that you can see the single driver clearly. So Fig 6 is not a special case with an offset tweeter to give that very smooth FR.
Put a single driver on a sphere - anywhere - and it will be centred on the sphere by definition (as would any infinitesimal dot be,) no!? And, you would measure it on axis, as with all the other shapes covered.
Fig. 6 - the sphere with a single - centred - driver, has the smoothest FR / best diffraction behaviour. Despite the driver being - inevitably - centred. So that rule about centred drivers falls away - in the case of a spherical enclosure only.
Because a sphere is not a circular baffle as it does not have any edge. It will have a very smooth BDS dip, as predicted by the spherical diameter formula, so that compensation for it is easy to build for, or you can forget about it, as my 2-way spheres do. Room-gain helps.
Even a truncated sphere with a small sliced (circular) baffle is measurably superior to a 90-degree edge box and audibly so.
In the late 1970s. Enclosures of the same Vb, same tuning, stiff laminated walls for the box (3mm hardboard pressure glued to the inside walls, which took days) same amount of BAF. Gave the box every chance by offsetting the driver (not done on the sphere) and making the baffle area narrower than that of the sphere, and maximally dissimilar dimensions, sharp edges but. The mass of the enclosures were matched using a concrete block glued underneath the box encl, so motor reaction effects were at least similar. Same screws, same plastic rawl plugs. The same WR 5inch Coral Flat 5 was used.
Measured FR? no contest. Listening? not in the race. Measurements done at ANU. More than 30 years ago.
Offsetting the driver on even the small circular baffle on the sphere did help a bit more, so the production model 2's had that as well.
Now, the audible superiority back then may have had an additional cause - that curved enclosures are quieter and are self damping. The test did try to minimise the differences there, and the enclosure talk of boxes just is a given.Practical / real spheres then were truncated and thus did not have the predicted single/audible internal standing wave either. Just in case you want to bring that up. Neither did Gallo's spheres. Very good tight bass though.
It is plain to me that wide radius curved baffle edges are best. Probably not practical. Curved edges are better than sharp, and felt helps, a lot.
LBNL John Dunlavy 'respected' both the Model 2 Audiosphere with the single Coral Fl5, and the 2-way Model 3. Told me so himself.
Mind how you go.
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Edits: 04/18/12 04/18/12 04/18/12 04/18/12 04/18/12 04/18/12 04/18/12 04/18/12 04/18/12 04/18/12Follow Ups:
For the record - the paper by Olson was designed to prove his assumption that the spherical enclosure would produce the "smoothest" or most "ripple free" response. In attempting to suggest I have been in error about spherical enclosures, Timbo conveniently ignored this aspect of Olson's paper and study:
"The axial response-frequency
characteristic thus obtained was corrected
so that the volume current produced
by the mechanism was inversely
proportional to the frequency, as previously
described."
The energy lost to diffraction (bending around the spherical enclosure into free space) is maximal with the sphere enclosure - it has the highest pressure drop off. The curves presented in the linked paper are deceiving in that this heavy drop off at the lower frequencies is "corrected". In the real world, the one in which we actually build speakers, we often don't have that luxury of either "correcting" the drop off in response via a paper mathematical computation, via electronic equalization, or via the sacrifice in sensitivity that happens when high frequency rise is compensated for passively. In point of fact as should be obvious to anyone with the slightest aptitude for physics, the response of spherical shaped loudspeakers is the most dominated by the phenomenon known as diffraction. Without heavy equalization, that fault or problem would likely be the most evident to the average listener of all the enclosure types listed in the linked paper.
There is no magic bullet or "perfect" speaker design. Some designs incorporate more rigorous research and development or engineering finesse than others and that's about it.
MYTH ABOUT SPHERICAL SPEAKERS BEING THE BEST POSSIBLE DESIGN - BUSTED!
:)
that a sphere has the smoothest diffraction behaviour and the smoothest frequency response.
'Heavy Eq' needed to compensate? By the time the curve has flattened again about a 3db shelf is enough IME and that of most others who have built BDS shelf Eq.
Where did I deny that BDS losses occur with spheres or that it is theoretically more lossy? It does but slowly and smoothly. In real rooms by about 3db by where the FR response flattens again. Heavy Eq?
I am not so much concerned with FR, but with the effects on timing, interplay, expression and nuance which are what acoustic music is about. Villa knows most of the market doesn't much care for acoustic music.
Villa is shrieking to the rooftops that BDS loss is a FLAW of spheres, yet it is by definition present in almost all speakers, but far less smoothly than with a sphere. Right in there between 200 and 400 hz where much that is fundamental in acoustic music happens.
Eq'ing for BDS is built in to almost all 'practical' dynamic speakers even the big ones, 2-ways, even 3-ways, no matter what the shape of the enclosure.
Viz. A '2 and a half way' column with two or more woofers, one covering bass and mids, and one doing bass below the BDS only. OR The tweeter and mids get padding resistors up to -6db worth.
The 2.5 way might seem to be a more transparent way, than using padding R's but doesn't always get good reviews.
The easy way to compensate for BDS - from a sphere - is not heavy Eq or a parametric, but a shelving response circuit in place of or in addition to a bass tone control. Sitting in the near-field and playing a bit louder works for me. So, I'd rather have less circuitry and not bother.
With boxes the FR is no longer smooth, and the compensation ought to be more complex. But most mfrs don't bother to fully compensate the peaks and dips, if at all.
I'd rather have good diffraction behaviour with a smooth BDS curve below about 200hz, than use a box with poor diffraction behaviour.
;-)!
That I happen to have chosen to use spherical enclosures is an educated choice, and I did it because I found that they got out of the way of the music. As predicted by the science.
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Well, you can always treat the box. That's what I do.
Using a variety of techniques - some of which were outlined earlier in this thread, diffraction based response anomalies associated with the use of "boxes" can be essentially eliminated. The measurements don't lie. And one of the smoothest measuring speakers ever made for home audio is NOT shaped like.....(gasp)... a sphere.
http://www.stereophile.com/floorloudspeakers/jbl_synthesis_1400_array_bg_loudspeaker/index.html
It's not often you hear John Atkinson say things like "Good grief!" about a speaker's ruler flat response.
" Note also that the JBLs are generating full output down to 25Hz. But, good grief! Look at the in-room response above 200Hz: It is extraordinarily flat and even, falling within superb ±1dB limits other than a very slight boost at 2kHz. "
This is clear, unambiguous, and convincing evidence that the issue of diffraction related anomalies can be eliminated to the point of inaudibility in NON SPHERICALLY shaped loudspeakers. The same cannot be said of true spherical loudspeakers which in the current highest expression of the art form, still exhibit uneven response despite the assistance of full digital equalization. Theory and practice are not always on the same page.
http://www.stereophile.com/content/cabasse-la-sph232re-powered-loudspeaker-measurements
Building a limited bandwidth and limited dynamic range loudspeaker and equalizing its response to create a smooth graph and prove a point is one thing. Building a real high fidelity loudspeaker that exhibits truly exceptional performance in every respect is quite another.
The description of the first case in the linked paper is not very explicit in terms of the driver's position. It could very well be offset with the center axis of the driver not being co-linear with the sphere's radius. If one relies on both the pictures and the description - it certainly is a valid perspective and not clearly WRONG as you assert."ig. 2. The sheet
metal sphere shown a t ( A ) is 2 ft. in
diameter. The loudspeaker mechanism
Is mounted with the cone approximately
fludh with the surface. The sheet metal
hemisphere shown a t (B) is 2 ft. in
diameter with the back closed by a flat
board of hard wood. The loudspeaker
mechanism is mounted upon the zenith
of the hemisphere with the cone of the
loudspeaker mechanism approximately
flush with the surface."
Offsetting the driver could actually improve response by reducing radiation into segments of 4pi space and similarly focusing that radiation into other segments.I read this paper many years ago. And I stand by what I said. Sphere's are not practical designs. The severe drop off caused by centered placement of drivers on a sphere requires heavy equalization to produce an acceptably uniform response. More importantly, it complicates the process of integrating multiple drivers successfully. Cabasse's Sphere is a very obvious case in point. Heavily equalized coaxial speakers are not for everyone. Like anything else, there are strengths and weaknesses. And there are technical and practical challenges to getting it right. If it were as you suggest - that spherical enclosures are head and shoulders above all other types of enclosures, then the high end market would have been flooded with spherical shaped speakers not long after Olson published his paper 60 years ago. The traditional box with offset drivers and rounded edges can readily match the sphere's level of performance from a diffraction perspective (as evidenced by the last three graphed responses) WHILE incorporating a higher performance, multi driver design at reasonable cost. I've heard Cabasse's Sphere and they are certainly impressive but for what they cost - they ought to be. Given the limitations imposed upon them via the spherical shape - there are other speakers that outperform them in several respects that don't cost anywhere near as much - flatter response, deeper bass, greater dynamic range...
Edits: 04/18/12
I gotta think that the resonance -inside- a sphere would be insurmountable.
Consider a situation where the sphere is sized such that the motor of the driver occupies the central space of the sphere- then you'll have a complex set of modes caused by the varying dimensions to the motor from the inside of the sphere.
Bass is supposed to sound big. 6.5" is not a woofer size.
Thank you 3rdRock for your input. As I have been tenaciously reminded, this thread is about something other than an aftermarket treatment to remove diffraction effects so I will bugger off. The flash made them look quite pink. They are not pink. Again, thank you. Listen well.
To state as you did in earlier posts that it is 'WRONG' that a sphere gives the best FR is plainly and utterly incorrect.
You were wrong and I am asking, given that you were also quite rude to another poster in that part of this thread, that you acknowledge that at the top of this thread. I also think you should do so to the relevant poster.
Next? With a single driver on a sphere 'offseting' the driver is clearly unnecessary because the FR is as smooth as it gets on axis.
You still have not got that as one of the central facts the paper established, for all time.
I can not believe that you ever read the paper or absorbed it, your speed and narrowness of response here and now simply serves to confirm that view.
Gallo showed that spheres are practical enclosures and that they do sound very quiet. My spheres are heavy, but far lighter than many current 'high-end' wide range arrays, and single piece enclosures. But, they are easier to aim / adjust to very fine tolerances in azimuth, elevation and rotation, by eye. That's a pretty practical idea I think. And, I started treating our difficult room DIY and learning about speaker positioning in the 1980s. Also very practical, and cheap to boot.
That both sides of the high end market haven't taken them on, simply confirms for me that one of the central propositions of market theory - that consumers and suppliers are rational often fails to be true in reality. Mostly untrue is more accurate IME. Further I am no longer persuaded that the short term interests of consumers and investors should be the ONLY compass and rudder for humanity.
I pay attention to the hobby to learn, and find ways of DIYing those ideas that stand up to examination. I didn't get in to the hobby to spend lots of money but to do the very best possible job of reproducing real stereo recordings of acoustic music, within a budget. Especially of choral and orchestral music in large spaces. I hang out here to convey what I've learnt and am learning.
So I'm not very interested in the $$$$ and conspicuous consumption aspects of home audio. The high end and HT are to me good examples of a hobby that has lost its point. The high end is focussed on selling expensive low ROI upgrade components, and even lower ROI cables, as often as possible, to a market who want to reproduce recordings that I couldn't / wouldn't use to make sensible judgements, about where I'm going in the hobby. Don't wave the 'high-end' at me, it isn't convincing, put it away.
As your typical response to being corrected is to duck it and then use ad hominem both here and elsewhere within the same thread, you may not last around here. I have, you may not.
I can see why you 'left' DIYAudio. Perhaps you can't help it.
Or maybe it's just a matter of believing that you know more about wave launch than THE wave guide man, John Dunlavy. Wave-launch is what home audio boils down to. And that's a systems judgement.
Mind how you go.
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Tim,
I respect the work of John Dunlavy. But I also respect the work of people like Greg Timbers, Laurie Fincham, Peter Walker, and Lawrence Dickie - to name a few. All are or were great designers in the loudspeaker business and none of them have endorsed the use of spherically shaped loudspeakers. In fact, Mr. Walker proved with his novel approach that you could emulate much that is good about a point source without actually trying to build one. If that isn't a clear statement on spherically shaped loudspeakers, I don't know what else could be. Like so many in audiophilia, you pretty much disqualified yourself with an extreme, ridiculous suggestion that subwoofers suffer from diffraction effects when sharp edged baffles are used. And with that, I see no point in further discussion - much like at DIY Audio, where I felt no need to go any further when people like Dennis Murphy and John Kreskovsky tried to suggest that sound was being reflected back into a tube cavity loaded with a driver on one end. I've been at this too long to take instruction or lessons from total novices in acoustics or electrical engineering.
mini monitors don't, or that it doesn't matter. When I am pretty sure that they do.I perhaps did not punctuate correctly. You could have asked what I meant, but because I had dared to correct you elsewhere in the thread, you decided to be insulting.
In this thread have been repeatedly rude, insulting, smug, and not just to me, and just plain wrong on the subject of the thread. And still refuse to accept that you were wrong, and that your behaviour is unacceptable.
Spheres may be impractical to your mind, my experience is different, that they do work, and for well established reasons.
You know better, eh? The problem is that you don't know any better.
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Edits: 04/18/12
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