I just read a posting in another place about the sealed box with variovent bass loading (stuffed port with the lower port resonance completely eliminated by the stuffing).
This is said to be less reactive than a closed box, has the same 12 db/octave rolloff but has a lower resonance and lower box Q for the same volume.
Of course, if the port was completely stuffed it would be a closed box design but I wonder what treats lie in the region between ported designs and closed boxes?
Comments anyone?
Steve
This is said to be less reactive than a closed box, has the same 12 db/octave rolloff but has a lower resonance and lower box Q for the same volume.
Of course, if the port was completely stuffed it would be a closed box design but I wonder what treats lie in the region between ported designs and closed boxes?
Comments anyone?
Steve
Variovents
They work best if you've got drivers with a highish Q that you want to control- like Dynaudio's 17W75 series.
Dynaudio also used them as resistive structures in the interior of asymmetric isobaric enclosures. One of the best speakers I ever built used a 30W54 on the outside, 21W54 on the inside at right angles to the 30W, with the 21W firing through a Variovent. Astonishingly clean bass.
They work best if you've got drivers with a highish Q that you want to control- like Dynaudio's 17W75 series.
Dynaudio also used them as resistive structures in the interior of asymmetric isobaric enclosures. One of the best speakers I ever built used a 30W54 on the outside, 21W54 on the inside at right angles to the 30W, with the 21W firing through a Variovent. Astonishingly clean bass.
Steve,
This is called Aperiodic loading... not a lot of quantitative work available on the subject.
Ted Jordan published the 1st paper (i believe) in Wireless World, February 1956, "A Cabinet of Reduced Size With Better Low-frequency Performance", and the 1st commercial products were the Goodmans ARUs (Acoustical resistance Units). The Dynaco A25 & A10 are probably the 2 most successful aperiodic designs (plans now available on my website). Bill Perkins of PEARL did a lot of research on aperiodic loading during the development of his PR-2. Besides lowering the Q wrt a sealed box of the same size, its biggest benefit is a flattening of the impedance curve, including a flattening of the 1st derivative of the impedance curve. This produces a less reactive loudspeaker which most amps will find easier to drive.
I have had good success with aperiodic designs for drivers that would otherwise have had to have been fitted to impractically large boxes.
dave
This is called Aperiodic loading... not a lot of quantitative work available on the subject.
Ted Jordan published the 1st paper (i believe) in Wireless World, February 1956, "A Cabinet of Reduced Size With Better Low-frequency Performance", and the 1st commercial products were the Goodmans ARUs (Acoustical resistance Units). The Dynaco A25 & A10 are probably the 2 most successful aperiodic designs (plans now available on my website). Bill Perkins of PEARL did a lot of research on aperiodic loading during the development of his PR-2. Besides lowering the Q wrt a sealed box of the same size, its biggest benefit is a flattening of the impedance curve, including a flattening of the 1st derivative of the impedance curve. This produces a less reactive loudspeaker which most amps will find easier to drive.
I have had good success with aperiodic designs for drivers that would otherwise have had to have been fitted to impractically large boxes.
dave
Steve, (7V):
Post #11 by Navin is of special interest.
http://www.diyaudio.com/forums/show...4&perpage=15&highlight=aperiodic&pagenumber=1
Navin made other posts about aperiodic boxes. I will try to dig them up.
Post #11 by Navin is of special interest.
http://www.diyaudio.com/forums/show...4&perpage=15&highlight=aperiodic&pagenumber=1
Navin made other posts about aperiodic boxes. I will try to dig them up.
I remember seeing some things in the Madisound that looked like air filters, but were sold under the premise that they would increase the leakage of the box (or something like that) and reduce box size by up to 50% or somesuch, without the danger of port noise.
Sounds like a grand idea to me. If I hadn't just been converted to the gospel of Dipoles, I'd jump on that bandwagon.
Sounds like a grand idea to me. If I hadn't just been converted to the gospel of Dipoles, I'd jump on that bandwagon.
I understand you can use more than one in a box as well. There were two big companies that made them. Dynaudio, I believe, called them variovent. They aren't made anymore, I don't think. The other company still makes them.
Edit: I guess that other company was ScanSpeak. Here's the Parts Express page. I have seen European dealers sell them also at about the same price:
http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=296-546
Edit: I guess that other company was ScanSpeak. Here's the Parts Express page. I have seen European dealers sell them also at about the same price:
http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=296-546
Parts Express use to sell these things....think they were peerless units maybe....they were like <$8 each....don't know if they still sell them....but they can be made very easy. Even PVC with some foam or insulation would do effectively the same thing.
In "How to build great sounding stereo speakers" or whatever by David Weems, he explained that not alot of poeple used the design but it was effective in having the same properties as a sealed enclosure...ie, tight and not boomy, but did alleviate the impedence rise at the Fs or whetever. The graphs showed it to be pretty effective, made more of a rounded bump than the steep curve.
In "How to build great sounding stereo speakers" or whatever by David Weems, he explained that not alot of poeple used the design but it was effective in having the same properties as a sealed enclosure...ie, tight and not boomy, but did alleviate the impedence rise at the Fs or whetever. The graphs showed it to be pretty effective, made more of a rounded bump than the steep curve.
Here we go I found it...its a scan speak unit @ $7ea...not bad, considereing it look alot more professional looking.
SCAN SPEAK 29001 FLOW RESISTANCE VENT
SCAN SPEAK 29001 FLOW RESISTANCE VENT
leadbelly said:
Sorta. GM describes a clik test that is fairly simple.
Generally, my approach is too build as big a box as i can get away with (given that even that is too small), and create a long skinny slit, or a bunch of small holes somewhere between a 1/3 Sd to as big as Sd and then tune the box with the amount of stuffing.
I also suspect that Martin King's model is cabable of given a real good idea -- you would just have to calibrate the stuffing density in the port with real-world results. ie the math is there, but the constants required for this ap aren't.
dave
My method, that has developed over a few years, is a little different, but it seems to work.
I build an open backed box, diameter about 1.25 to 1.33Sd, and slightly longer than 1/2 the wavelength of the lowest frequency I need. I then taper the stuffing, using a little next to the driver, and a lot right at the back, finished off with a grille of speaker baffle material.
However, this is very close to a TL type design, once again it shows how the lines between enclosure types can be blurred.
Dave's method would probably work much better for a bass driver, as mine would end up with a huge box, but I like mine for mids, as it removes any back reflection wave from the enclosure
I build an open backed box, diameter about 1.25 to 1.33Sd, and slightly longer than 1/2 the wavelength of the lowest frequency I need. I then taper the stuffing, using a little next to the driver, and a lot right at the back, finished off with a grille of speaker baffle material.
However, this is very close to a TL type design, once again it shows how the lines between enclosure types can be blurred.
Dave's method would probably work much better for a bass driver, as mine would end up with a huge box, but I like mine for mids, as it removes any back reflection wave from the enclosure
pinkmouse said:
You will often find me using the term aperiodic TL (as in stuffed until the line becomes essentially aperiodic ie very little terminus output.
Dave's method would probably work much better for a bass driver, as mine would end up with a huge box, but I like mine for mids, as it removes any back reflection wave from the enclosure
I use the same technique for mids & tweeters... another trick i was 1st shown by the same Bill Perkins mentioned earlier
dave
Hey guys
I remember that i recently read a white paper or whatever report it was about a ... dual chamber acoustic loading ...
Something with a small chamber , size something like 1/2 or 1/3 VAS and the small chamber being ... connected to the secondary chamber (which is much bigger) via a variovent.
I dunno which company or DIY site it was but it was very interesting. Does anybody happen to know that particular site or article???
Cheers
I remember that i recently read a white paper or whatever report it was about a ... dual chamber acoustic loading ...
Something with a small chamber , size something like 1/2 or 1/3 VAS and the small chamber being ... connected to the secondary chamber (which is much bigger) via a variovent.
I dunno which company or DIY site it was but it was very interesting. Does anybody happen to know that particular site or article???
Cheers
The Goodmans units may have been designed by Roy George, who now heads Naim Audio's R&D.
Some of their speakers use(d) this technique, of venting a smaller enclosure to a larger one, through what Naim term as a PAR (Precision Acoustic Resistance) - they are made to high precision too, costing more to make than their in-house 8" drive units.
It certainly has some interesting properties - the speakers I use have almost immeasurable bass resonance as a result of this technique, and are amongst the most 'boxless' sounding speakers I've heard this side of Quad ESL's.
Another interesting property is a bass response that is consistent at different frequencies and levels, something rarely true of conventional boxed 'speakers.
Andy.
TJ was a 'Johnny come lately', at least WRT using aperiodically damped designs. The early theater basshorn 'compression' chambers had slotted rear covers and were tuned by stuffing them with wool to suit based on impedance plots. It appears to have been pretty much a 'cut n' try'/rule-of-thumb based on measurements affair, with final tuning done at the theater by the installers based on the WE/Altec theater basshorn info I've seen and gleaned from a conversation I had with a factory engineer during a visit to the plant back in the '60s.
Altec (and I assume others) used the procedure for ~critically damping reflex designs destined for HIFI use WRT grill cloth selection/vent area long before TJ began using his ARU. He may have been the first to try to quantify it mathmatically though, and for sure was the first to tout its advantage to the public.
The 'click' test is just a cheap/DIY version of the more elaborate testing procedure they used and is quite adequate for DIY HIFI apps.
Anyway, short of open baffles, it's the only way to do a standard box IMO if Fb is > ~25Hz. As DaveP10 alluded to, this is a great way to improve the transient response of sealed back mids/HF units if you're up to doing a bit of cutting/experimentation.
WRT doing sims, any box program that allows Ql adjustment, such as BoxPlot, can do a decent enough approximation for me in the planning stage; and I imagine MK's TL spreadsheet is very accurate, with the added benefit of telling you how much stuffing is required.
Not familiar with it, but it sounds like a double bass reflex with an aperiodic vent in lieu of a tube vent to reduce the broadband notch that occurs in a traditional design DBR.
GM
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