Vas is basically (BASS-ically ha ha) stiffness. The genius of Neville Thiele and Dick Small (and another whose name is on the tip of my brain) was to realize the physical aspects of a speaker could be modeled as electrical circuits. This took speaker design from cut-and-try black magic and folk shamanism into a realm of methodical engineering.
Vas is the physical stiffness of the suspension (cone surround + spider) transformed/multiplied by the area of the cone. That yields a stiffness number which is equivalent to a certain volume of air. In other words, a Vas of 1 cubic foot means the suspension is effectively as stiff the air in side a 1 cubic foot box. So from that we come to
There is not really a "better"; it really depends on the application. Vas matters at small signal and large, it usually doesn't change drastically until you hit near excursion limits.
I don't remember the formula offhand, and since it's raining I feel to lazy to go look it up in a copy of Small's thesis which I (ahem) acquired at one point. IIRC Vas = stiffness * area squared *some constants. So in this case with less area it would mean the 18" suspension is stiffer than the 21"
The magnets have nothing to do with Vas nor Fs. They affect the mangetic field B, which then multiplies by the length of voice coil wire L in the field to give total magnetic strength. That controls the electrical Q, Qes. The mechanical Q, Qms, is affected by physical properties of the suspension and such. The magnets physical shape can affect this, more so in midranges and tweeters. Half the Vas means the suspension is stiffer; the fact that Fs is a bit higher would indicate the moving mass is somehow less-different cone? Coil? Both? Hard to say without model numbers or something.
I mostly agree with the points presented in your post but wanted to clarify a details. First, Vas is proportional to the suspension (mechanical) compliance Cms, which is inverse stiffness: Vas = Cms Sd^2 rho c^2. Here, Cms Sd^2 = Cas (the acoustical compliance).
That is not quite correct. The use of electrical/mechanical circuits to describe the motion and impedance of a transducer was already well-established by the time Olson wrote "Elements of Acoustical Engineering" (1940). The relation V = C rho c^2 is also fundamental to theoretical acoustics and did not originate with Thiele or Small. The unique contributions of Thiele and Small were to refine and simplify some of the equations describing a transducer in various types of enclosures (sealed, vented, passive radiator). The fundamental contributions are all noted by Small in his June 1973 vented-box paper. The important names are van Leeuwen (1956) (the first complete vented-box analysis and only available in Dutch), de Boer, Novak, Keibs. In particular, Thiele's famous 1961 paper was based on the theory presented in Novak (1959).
It seems to me that the importance of Vas is to determine the suitable enclosure size for a given driver. The "compliance ratio" alpha = Vas/Vb is typically chosen somewhere between (say) 0.5 and 2. So, larger Vas means you'll need a larger box.
Just to clarify - Thiele and Small realised that speakers could be modelled using filter theory. Speakers were modelled using standard electrical circuit theory well before then.
Was the other name John Benson, perhaps?
Good point. Small credits de Boer (1961) with providing Butterworth and Chebyshev (filter) alignments, and also Theile (1961) with a similar but more practical and comprehensive study of vented systems. Small also remarks (in 1973) that neither de Boer's nor Thiele's paper was widely read. So, Small's role seems to have been to restate (and present in a very clear, easy-to-understand way) the earlier work of Thiele and others. Finally, Small (1973) remarks that Benson (1972) is the most comprehensive "small signal treatment of vented box systems". Benson is my preferred reference for the basic theory. Beyond that (treating the port as a transmission line, including enclosure eigenmodes, or treating semi-inductance and creep effects in the transducer) there is no standard reference.
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Ah yes! Bigger = softer, like a pillow. Well, maybe not all bigger pillows are softer ha ha
Yes also, as is so generally true in science the pivotal luminaries stand on the shoulders of giants. I guess it would be better phrased that Thiele & Small really made the analogies vastly more useful to speaker engineers, especially as computers came into being and you could program them to find frequency response and so on. Still missing a name, Australian, I'm recalling a series of articles in Amalgamated Wireless Australia with a ton of equations and derivatives. And I'd add the names of Chris Strahm (LEAP) and Doug Rife (MLSSA) for making really the first widely/easily accessible simulation and measurement devices. Maybe not as luminous as the others, but really kicking things to another plane of design technique; it was literally my prayers being answered.
As for Jim Novak, I actually worked for him. He was a cool guy-a bunch of us snuck off to a Pink Floyd concert one afternoon, and he simply called us into his office and in effect said "boys will be boys and I get that it was Pink Floyd but don't do it again eh?" RIP Jim!
As indicated in Post #64, the fundamental contribution that Thiele and Small made was to use filter theory to develop a model that was effectively equivalent to that of the classic electro-mechano-acoustical circuit (except that voice coil inductance was not included).
The traditional electro-mechano-acoustical circuit can of course also be used for computer simulations (and is indeed preferable because Le is then taken into account).
The AWA Technical Review articles were written by John Benson, the name suggested in Post #64.
Right. You're thinking of Benson. His papers are conveniently assembled in book form. This book is absolutely my go-to reference for the classical box analysis (but copies are pricey now)
Catalog Record: Theory & design of loudspeaker enclosures | Hathi Trust Digital Library
That's awesome. In the 70s? I'd love to hear more stories.
Here is an interesting historical account of Novak's work from a forum member: Vented box tuning
there seemed to be some understanding prior to Thiele and Small
including Villchur and Kloss
Albert Thuras bass reflex patent - August 1930
https://acousticsresearchcentre.no/wp-content/uploads/2015/07/Thuras-bass-reflex-principle.png
Rudy Bozak's 27 inch full range speaker for the 1939 World's Fair
http://4.bp.blogspot.com/-rCB1wW_3onc/UMgURdR6MaI/AAAAAAAAdM4/rs8wiLvExgw/s1600/27-2.jpg
http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=36506&stc=1&d=1231747430
including Villchur and Kloss
Albert Thuras bass reflex patent - August 1930
https://acousticsresearchcentre.no/wp-content/uploads/2015/07/Thuras-bass-reflex-principle.png
Rudy Bozak's 27 inch full range speaker for the 1939 World's Fair
http://4.bp.blogspot.com/-rCB1wW_3onc/UMgURdR6MaI/AAAAAAAAdM4/rs8wiLvExgw/s1600/27-2.jpg
http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=36506&stc=1&d=1231747430
Is this definition correct. Many people have said something different on here and all confusion broke loose. Is it the volume of air that when COMPRESSED in a cubic meter exerts the same force? Or is it the total volume of air. If it was just the total volume of air then a higher VAS would be looser because more air means more cushion. If it was amount of air in a given space then higher vas means stiffer.
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So why does something like this with its massively powerful magnet have such a high vas?
Dayton Audio PS220-8 8" Point Source Full-Range Neo Driver
Is it because the cone is light?
Dayton Audio PS220-8 8" Point Source Full-Range Neo Driver
Is it because the cone is light?
It's because the driver diaphragm suspension has a relatively high mechanical compliance of 1.41 mm/N.