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Multi-Way Conventional loudspeakers with crossovers

Vas Question
Vas Question
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Old 10th September 2014, 06:37 PM   #21
arjunm009 is offline arjunm009
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Originally Posted by fastbike1 View Post
Vas and box volume are virtually never the same.
If I design an enclosure which has internal volume equal to that of Vas, and also suppose it is suitable for the driver, what will happen
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Old 10th September 2014, 06:59 PM   #22
Ron E is offline Ron E  United States
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Originally Posted by arjunm009 View Post
If I design an enclosure which has internal volume equal to that of Vas, and also suppose it is suitable for the driver, what will happen
Vas is inversely related to the stiffness of the woofer suspension, and Vb is inversely related to the stiffness of the enclosure. The ratio of Vas and Vb (known as alpha=Vas/Vb) is called the compliance ratio. It doesn't really matter what the ratio is, as long as you understand how the ratio influences response shape to get your desired result.
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Old 11th September 2014, 01:49 AM   #23
cT equals piD is offline cT equals piD  United States
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Originally Posted by Ron E View Post
It doesn't really matter what the ratio is, as long as you understand how the ratio influences response shape to get your desired result.
How about an acoustic suspension woofer? Supposedly for such a woofer, alpha must be greater than or equal to 3. Also I was wondering if it is possible to identify an acoustic suspension woofer by its T/S parameters.

Vilchur's original woofer absolutely needed alpha > 3. I guess acoustic suspension isn't so prominent anymore.

Bet I stumped you all with these questions
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Old 11th September 2014, 10:38 PM   #24
Ron E is offline Ron E  United States
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Originally Posted by cT equals piD View Post
How about an acoustic suspension woofer?
Well, if you want a Qtc of 0.7 and use a woofer with a Qts of less than half that, you have designed an "acoustic suspension" system
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Old 12th September 2014, 12:40 AM   #25
cT equals piD is offline cT equals piD  United States
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Originally Posted by Ron E View Post
Well, if you want a Qtc of 0.7 and use a woofer with a Qts of less than half that, you have designed an "acoustic suspension" system
I was referring to an acoustic suspension woofer driver, not the system. An acoustic suspension woofer has a Vas much greater than that of a non-acoustic suspension driver of the same diameter. In other words, in a sealed system, the acoustic suspension woofer is acoustically suspended to a greater degree or the restoring force comes mostly from the air cushion rather than from the woofer's spider and surround.

Like I said, a Vilchurian () woofer in a sealed system with alpha equal to one couldn't handle any power at all because of an inadequate restoring force.

It seems like there aren't any commercial Vilchurian woofers around these days, nevertheless for example David Weems in one of his books that I have seems to take the view that alpha = 1 (in a sealed system) should be reserved for relatively small drivers such as 4 inch dia. or less.

The idea I believe is that compliance of most large woofers (> 4 inch dia.) is going to be too great if alpha <1 in a closed box and thus severely limit power handling. The restoring force is too weak relative to motor strength.

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cT

Last edited by cT equals piD; 12th September 2014 at 12:42 AM. Reason: booboo
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Old 12th September 2014, 05:46 AM   #26
GM is offline GM  United States
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It seems like there aren't any commercial Vilchurian woofers around these days........

True, you need a really low Fs + really high Vas to get down low in a compact box. Altec had a number of drivers with Vas as high as 1 k L, but to get down to 32 Hz requires a ~12 Hz Fs.

GM
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Old 12th September 2014, 07:46 AM   #27
Lynn Olson is offline Lynn Olson  United States
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The insight of Neville Thiele was transforming both electroacoustic (driver) and enclosure characteristics into electrical analogs, and using standard filter-design techniques to synthesize the required response shape. Since he was working on Australia's color-television project at the time, he was familiar with the frequency response, phase response, and group-delay characteristics of commonly used filters.

(NTSC, PAL, and SECAM analog television require filters with well-defined characteristics in transmission and reception, or color will appear obviously distorted. NTSC, in particular, requires very accurate filters with smooth group delay throughout the entire transmission chain, from camera to receiver. It should be remembered at the time of publication of Thiele's first papers in Australia, the European PAL system was in the early stages of lab development, and still six years from public introduction. NTSC was in regular use in the USA, but the high cost of receivers limited viewership to less than 5% of the public.)

It's unfortunate that Neville's work took more than 10 years to appear in the American Journal of the Audio Engineering Society, but it finally appeared alongside a condensed version of Richard Small's doctoral thesis. This is how Thiele/Small theory made its belated appearance to the world of audio design in the early Seventies. I still have the original issues of the AES Journal sitting in my library (back when I was a member of the AES).

Once you convert everything to an electrical analog, it's obvious that driver compliance (normalized to Vas) and box volume are in series with each other (in a closed box). So it doesn't matter where the restoring force comes from; either the driver suspension (if a true infinite baffle is used, like an opening to the outdoors), or mostly from the air in the box, the original AR-1W approach, or any combination in-between. It's simply a 2nd-order highpass filter, with (total) compliance as one of the reactances, and the moving mass of the cone as the other reactance.

In practical systems, the majority of damping is provided by the amplifier, not the mechanical resistances of the suspension. Although vacuum-tube amplifiers of the Fifties had a higher source impedance (or lower damping factor) than modern transistor amplifiers, nearly all commercial designs employed about 15 to 20 dB of feedback, and had source impedances in the 0.2 to 0.5 ohm range, which makes them near-voltage sources for purposes of T/S calculations.

The greater insight of Thiele and Small was analyzing a vented-box as a 4th-order highpass filter, with driver compliance and moving mass forming two of the reactances, and enclosure volume and equivalent mass of the vent forming the other two reactances. In the Fifties, you could build a few boxes, measure them, and arrive at a reasonable 2nd-order filter, but the chances of getting a 4th-order filter correct by cut-and-try are very slim. That's the reason that Fifties' vented alignments sound the way they do; there was no guide from filter theory, no agreement on how to measure a box at low frequency, and no set of standardized driver parameters.

In other disciplines of engineering, electrical analogs between moving mass (inductance), spring compliance (capacitance), and friction (resistance) arrived earlier. It just took a bit longer for it to arrive in the sphere of loudspeaker design.

Last edited by Lynn Olson; 12th September 2014 at 08:08 AM.
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Old 12th September 2014, 07:56 AM   #28
David McBean is offline David McBean  Australia
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Originally Posted by Lynn Olson View Post
In other disciplines of engineering, electrical analogs between moving mass (inductance), spring compliance (capacitance), and friction (resistance) arrived earlier. It just took a bit longer for it to arrive in the sphere of loudspeaker design.
???
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Old 12th September 2014, 08:21 AM   #29
Lynn Olson is offline Lynn Olson  United States
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The papers of the 1950's are not easy to read compared to the clarity of Neville Thiele's first paper, and the greater clarity of Richard Small's papers ten year later. The lack of standardized measurement protocols didn't help. Harry F Olson and the Western Electric team were using electroacoustic analogs in the Thirties, but did not come up with a generalized theory of enclosure and driver design with the scope and power of T/S theory.
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Old 12th September 2014, 11:37 PM   #30
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Quote:
Originally Posted by GM View Post
It seems like there aren't any commercial Vilchurian woofers around these days........

True, you need a really low Fs + really high Vas to get down low in a compact box. Altec had a number of drivers with Vas as high as 1 k L, but to get down to 32 Hz requires a ~12 Hz Fs.

GM

I have to admit to not having the faintest idea of what a Vilchurian woofer is. So GM or anyone else here who knows, would you please explain to me what a Vilchurian woofer is? I'd really appreciate it...

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