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Dear group -I believe an aperiodic box can flatten the input Z, which might be desirable when using a tube amp with high output Z.
Are there any general rules for making dual-chamber aperiodic boxes with regard to dividing the box volume, coupling vent tuning. and damping or do these things need a 'mobility' computer model for each case?
thanks
Freddy
Follow Ups:
Freddy -I must confess to knowing nothing about dual chambers or coupling vent tuning in the context of aperiodic boxes. What is a 'mobility' computer model? I tried standing up on my chair, but that one still sailed way over my head!
By definition an aperiodic system is one with a system Q of .5 or less. A system with a Q of .5 is also sometimes called "critically damped", and exhibits excellent transient response, but will be 6 dB down at system resonance instead of 3 dB down as in a standard second order Butterworth alignment (system Q = .707).
Enclosures that use a resistive vent, such as Dynaudio's variovent or a homemade, foam-stuffed slot, are often referred to as "aperiodic". Is this what you are referring to?
Once I was asked to build a system using a particular Morel 6 1/2" woofer that had a rather high Qts (.74 as I recall). The person didn't want big boxes, but a rather small box would have an audible hump in the bass, with corresponding blurring of bass notes.
I made the boxes with a 1/2" high slot along the bottom of the front baffle, and experimented with type and thickness of foam stuffed in the slot until the bass sounded smooth and balanced. I didn't run an impedance curve - the person I was building the boxes for was quite happy and anxious to get them home. According to Dynaudio's literature, resistive loading significantly lowers the magnitude of the impedance peak at system resonance, but by how much would have to be measured.
So anyway one applicication of resistive loading would be to tighten up an otherwise boomy or wooly bass. If I were designing from scratch to use resistive loading, I'd start with a sealed box design that would yield a system Q somewhat higher than the target. This is because the resistive loading is going to lower the system Q - so if you start out with a box sized for your target "ideal" sealed system (say system Q = .7), the resistive loading may unacceptably reduce the bass output.
I did like the midrange of the resistively loaded system a bit better that that of a sealed or vented system. For really nice midrange in a box, though, use a transmission line.
Okay going back now to your original purpose - to create an easier load for your high output impedance tube amp - to the extent that the amp's frequency response is affected by the load it sees, I would think the smoother the impedance curve, the better, all other things being equal. And resistive loading would make sense.
If you use a homebrew design with a slot into which you stuff foam, get as many varieties of open-cell foam as you can. In the system mentioned, I ended up using a rather porous foam from a fabric store, as opposed to the tighter foam sold for acoustic treatment. By being able to play around with the foam, you'll be able to tune the bass by ear. In my experience, after all the calculating and measuring and careful building is done, being able to tweak things by ear just a bit always helps.
Good luck with your project!
Duke
Hi Duke -sorry to throw you with the 'mobility model' - I've George Augspurger's program referred as such since it can model multi chamber bandpass, t-lines, and horns.
what I'm getting at is flattening the speaker's input Z and I think that was one of Dynaco's sales pitches on their A25, A35, A50 etc 'aperiodic' boxes.
I suspect those had a qtc of about one - the resistively damped slot on the A25's baffle and the dual-chamber A35 (with a damped coupling slot between two chambers) damped the input-Z peak you would see with a sealed box. whether whis is important or not is arguable - in theory that would be better with tube amps, especially pentodes where the high reflected Z could put the loadline into the 'knee' region.
Dynaco also claimed this technique provided better step reponse on the woofer. their little boxes did sound pretty good but SEAS driver are decent anyhow.
Would you consider this to be nothing more than advertising hype? - with the continual search for 'the grail' it might be worth trying with a 'sealed' box woofer. I'd guess it would be empirical where one drills some holes in a test box adn piddles with damping those til the box/woofer input Z peak is reduced.
thanks
Freddy
I think there is more than advertising hype to the resistive vent/pressure relief/aperiodic loading technique. I'm not mathemetically skilled enough to begin to tell you how to model it, though. Empirical data is probably more practical, anyway.For your test box, I would suggest a slot is more practical than drilling holes. If you drill holes, you'll have to cover them up with a resistive material, and to do a good job you'll need some kind of adhesive. Therefore, changing the thickness or type of material is going to be difficult. With a slot, you simply cut a piece of foam and stuff it in. You can easily try different foams and different thicknesses. If you want, you could add a vertical brace or two crossing the slot on the inside of the cabinet to restore mechanical integrity.
I have been using pressure release boxes for over ten years.I have used the hole method,with specific locations and systematically plugged them one by one until I appraoch that delicious combination/tradeoff of extension and bass/midrange clarity.I use 1/16" dia. holes on the centers of the side walls(the center as seen by the wavefront inside the cabinet).Holes on the back at top,bottom and center.Sometimes I plan a pair of holes at the rear locations.I fine tune with a valve on the center hole.Too many holes and they make noise.This is why I no longer put them on the baffle.The top of the box is another good place if you don't forsee anyone using it's handy flat surface to put things on(arrgghh).This is a wonderful alignment yeilding surprising bass from less than optimal enclosure volumes.
JROY
My understanding of an aperiodic enclosure is that the lowering of impedence at resonance (in a closed box) allows the amplifier to put more power through the woofer lowering F3, since the resonance peak in a normal sealed alighnment acts something like a highpass filter, limiting low frequency response in that range.
Is anyone familiar with the GMAS-18 put out a few years ago by Parasound? It used an Aurasound 1808 18" woofer in a very small aperiodic damped encloser. According to a review I read it had virtually unlimited low frequency and SPL capability.-Joshua
Pressure release introduces loss into the otherwised sealed system.
F3 is sacrificed,to a small extent,but the resulting rolloff is more gradual.Lowering the magnitude of the impedance peak at resonance lowers acoustic output at resonance,helping to tame a possibly too resonant enclosure.
JROY
My understanding is that you don't need two chambers. You just use resistive damped ports. I know ACI calls their Sapphire series "aperiodic" and it is just a port in the rear that has a foam plug.
HI JD -yeah - this would be the 'foolproof' way - to use the whole cabinet and a resitively damped vent or holes.
I wanted to see what happens when doen as Dynaco did in the A35.
many thanks
Freddy
Please post what you know about them. Have you heard a pair?
Thanks.
Hi Neward -gee its been twenty-two years sunce I had the A35 - I used two per side with a Citation 12 amp, perhpas a Citation IV preamp - they e pretty nice really I liked the SEAS tweeter and when using a total of four, they could play the Basie band nicely in a modest-sized room.
The woofer was a 10" SEAS with rubber surround and the tweeter was a fabric 1" SEAS having I beleive an electrical 2nd order hi-pass filter - I don't remember any crossor on the woofer - saved moncy and the SEAS 10" had a smooth acoustic rolloff,
I think the volume was divided like that of a 'classic' dual chamber reflex - so the woofer wnet into the larger chamber of about 2/3 of the box volume and there was another chamber formed by the partition which had a small slot -perhaps 3/8" by 6" ?? with a layer of thin open-cell foam over the slot. This supposely tuned 5Hz
As Dynacp made tube amps, i think that may have been part of the advertising ploy and impetus to flatten the input Z.
Now to the death of the Dyna A35 - the Tion IV was a tube design which exhigited turn-on/off pulses and one day I accidently turned the preamp off before the amp - not good with solid-stae stuff which can basically react to DC. It took all four woofers. Dyan was gracious enough to replace all but UPS finished them with their famous
service - upon return all four cabinets were thrashed with the corners crushed - I didn't get anything from UPS.Still, in retropsct those were decent little speakers and I personlly think some of today's stuff is worse.
best wishes
Freddy
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