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Actuated Variable Accoustical Tuning for Bass Enclosures

It's lengthy, but may be worth reading
 
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OK, first-off, for my critics, this idea is not patent pending, nor do I expect it to be. This is something that I've been pondering for my own use.

I presume that most audio enthusiasts are aware that the accoustical merits of the average ported (fourth-order) sub-bass enclosure are significant enough that this type of enclosure is produced in great quantities. It would also be worthy of mention that this enclosure has one of the highest efficiency levels of any accoustic-suspension-enhanced enclosure being produced (with nearly double the output of a sealed-box enclosure) at frequency levels which are near its tuning frequency. Additionally, a characteristic of this enclosure is that the enclosure's tuning can be manipulated not only by the volume of the enclosure, but also by manipulation of the cross sectional area and length of the tuning port.

My discontent with the current usage of this enclosure is that it has a 'roll-off' of 12db per octave (this is quite a significant value) from its tuned frequency, and that any considerable deviation from this frequency results in increased cone excursion and reduced power handling of the woofer which it holds.

Previous efforts to correct the non-linearity of this type of enclosure include analog rotation of phase (which is errantly referred to as equalization) and digital processing, neither of which correct the conditions of over-excursion or reduced efficiency of the driver involved, but merely add control to the audible output.

What I propose is a system comprised of the following:

At the 'business end' of the system is a variable cross-section port, with three fixed walls and one which is movable. The movable wall is actuated by a high-speed linear motor or actuator. This will allow for rapid changes of cross-sectional area of the port.

The 'controller' for this system would be easily incorporated with the "mistrack" buffer currently employed by many currently available source units, and would essentially 'predict' the moment at which a certain frequency will arrive, sending its respective actuation signal to the port actuator accordingly.

The controller can be tuned or 'programmed' to nearly any accoustical environment by monitoring the playing of certain frequencies and having the controller seek an optimum setting for that frequency.

The benefit to this type of system is that the enclosure would be constantly tuned to the frequency being produced, maximizing clarity, volume and efficiency, while minimizing the physical excursion of the driver involved. In essence, the accoustical enclosure is now able to 'play along' with the musical track at hand. I acknowledge that volumetric restraints may require that one may need to divide sub-bass and mid-bass frequencies with similar control systems in order to execute this idea properly, but the result would be phenomenal, with all bass and sub-bass drivers being operated at maximum efficiency and power handling at all times.

Note: It may be beneficial to be familiar with Thiele-Small theory and parameters if you intend to annotate this idea.
X2Entendre, Apr 24 2003

$37,500 a pair http://www.stereoph...howarchives.cgi?382
[thumbwax, Oct 04 2004, last modified Oct 05 2004]

[link]






       How about aluminum cabinets to complete the nicely written spiffy ensemble? See link while you're dropping crumbs from this flaky crescent everywhere.
thumbwax, Apr 24 2003
  

       How long, do you suppose, would the entire worst-case (largest motion) excursion of the articulated baffle take?   

       Could a crossover network that is already doing time-alignment delays for the mid and high frequency drivers, assuming a 3-way system, reasonably be engineered to also handle the look-ahead ("prescience") signal to the baffle actuator? Would that be better because it would be self contained to that enclosure and the specific signal arriving at that enclosure? Could the actuator be a voice coil mechanism? Would that be faster or more accurate than a linear or stepper arrangement?   

       How critical would the seal be between the articulated and static surfaces of the port be?   

       How much does port tuning rely on the length, or depth, of the port as opposed to the cross section (or is it only a matter of total port volume)? If not, would an irising shutter arrangement work better to control the strict cross section? Could a series of such shutters arrayed throughout the total length of the port control both the cross section and the effective port length by selectively closing and opening a given shutter depending on the inbound signal? Could a conically-shaped port, that controlled both it's cross section, length and volume by having the overlap, at the small end of the cone, articulated, be of any merit?   

       Finally, would a mechanical system with a mass great enough to handle the workload be able to ever move fast enough not to add a clumsy quality to the sound? Would the total transient response, or rise time, ever be good enough, even with the look ahead you propose, or would a slide-whistle effect occur?   

       I have no idea what Thiele-Small theory is. Sorry this is such a messy annotation, I'm in a hurry to go to sleep and had to dash it off.
bristolz, Apr 24 2003
  

       Rather good. For the sake of (possibly) added coolness, might the port be circular, and its area varied in the same way as a camera iris?
angel, Apr 24 2003
  

       [bristolz], the cross-sectional port area must be varied over the entire length of the tuning port in order to accurately manipulate the tuning characteristics of this particular type of enclosure. The irising shutter configuration would effect the tuning, but the results may be un-predictable. The seal could be anything ranging from an oil-soaked to a high-speed-grease-saturated material such as felt, or even a closed-cell foam material. Time alignment relays seem out of the realm of this idea, as they require action 'after the fact', and this application requires anticipated mechanical actuation. I cannot see a voice coil arrangement being more accurate, but if governed properly, it might as fast as a linear motor. I am not able to tell you of how a conical port would react, as I have no experience with them.

[angel], from an engineering standpoint, I feel it would be rather difficult to manipulate an entire column's cross-sectional area with an iris-type movement. I think that this application would need to rely upon linear actuation.
X2Entendre, Apr 24 2003
  

       Yes, I //have absorbed the full concept of this idea//. Did you see the fourth-order Revel Salons link within the article? Most outstanding speakers I've ever heard and as close to lossless as one could hope to get, BTW. I have a feeling there would be dampening effects with the iris design limiting spread of wavelengths - part of the article which serves as a link is dealing with the rubber net grille which, to my minds-eye, seems to have an effect on the listening experience, though I've not heard this particular set firsthand - only the Salons.
thumbwax, Apr 24 2003
  

       [bristolz], the mass of the mechanism would be contained within the enclosure, and the mechanism's volume would not affect the linearity nor quickness of response. I cannot imagine that a musician would be able to manually produce tones at a rate which would exceed this proposed system's abilities.

[thumb], sorry if I seemed to skip your anno earlier... I was trying to give a good answer to [bristolz]. I shall take another look at your link and find the material you have referenced. Thank you for the croissant!
X2Entendre, Apr 24 2003
  

       "I cannot imagine that a musician would be able to manually produce tones at a rate which would exceed this proposed system's abilities."   

       Perhaps not but a synth or a recording engineer can.
bristolz, Apr 25 2003
  
      
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