Modeling a variovent - diyAudio
Go Back   Home > Forums > Loudspeakers > Subwoofers

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th February 2007, 04:48 PM   #1
pjpoes is offline pjpoes  United States
diyAudio Member
 
Join Date: Jan 2005
Location: New York
Send a message via AIM to pjpoes
Default Modeling a variovent

Anyone help me on understanding how to model a variovent. Do you just make a slightly leaky sealed box, and model that until you get a desired response. Anyway to correlate the numbers I put into WinISD with what I would have to do with a variovent or two. Thanks in advance, and sorry if this was already covered, I couldn't find anything.
  Reply With Quote
Old 12th February 2007, 02:52 PM   #2
Geoff H is offline Geoff H  Australia
diyAudio Member
 
Join Date: Oct 2006
Hi pjpoes. I have recently done a few variovents, or aperiodic enclosures.

The latest is a 0.8 cub ft car sub box with a 10" bass driver. I size the port to tune the cab to about 100Hz, then damp the port.

The best achievement is a 10" full range that goes down to 30 in 29L I haven't measured the 10" bass/0.8 cub feet, not as deep as the widerange. Same rule as always, bigger is better.

see:Aperiodic enclosures

There's not a lot of info out there. It's not the best cab design, but hard to beat if space is at a premium.

Geoff.
  Reply With Quote
Old 13th February 2007, 06:22 AM   #3
bjorno is offline bjorno  Sweden
diyAudio Member
 
Join Date: Mar 2005
Location: Jacobsmountain
Send a message via MSN to bjorno
Geoff H,

You referred to:

http://www.diyaudio.com/forums/show...86&goto=newpost

and I found another reference to an old Jordan WW article.

But,
A warning must be issued when reading this article: ARU Article
Arguments: chapter 7.2. The Absorption-Damped Vented-Box System by Richard Small.

b

1(1)
Attached Images
File Type: gif aru.gif (34.5 KB, 487 views)
  Reply With Quote
Old 13th February 2007, 01:16 PM   #4
Geoff H is offline Geoff H  Australia
diyAudio Member
 
Join Date: Oct 2006
Thanks for the article bjorno. Was the warning to do with raising f3 by an octave? That hasn't been the case here, otherwise I wouldn't have bothered. In fact the 8" widerange in converted cabs appear to go well below resonance. No measurements yet, I'm improving the top end, just needed some cabs for listening tests.

All my effort has been on small cabs. The ARUs were large.
It doesn't help to flatten the z curve alltogether, it kills the lower midrange.

Geoff.
  Reply With Quote
Old 16th February 2007, 11:58 AM   #5
bjorno is offline bjorno  Sweden
diyAudio Member
 
Join Date: Mar 2005
Location: Jacobsmountain
Send a message via MSN to bjorno
Default Was the warning to do with raising f3 by an octave?

Hi Geoff,

Not only but more of general nature and, sorry, this matter takes time for me to explain why, I also had to think more than once before I could reply to your question.

The flawed view is that Jordan presents the ARU concept when trying to interpret the ARU function mostly from looking at the impedance curves.
The impedance curve reflects only what the driver senses and not the system impacts that’s really the target when designing a speaker.
This shows that he at that time didn’t fully understand the acoustics behind the whole speaker transfer function.

Otherwise I don’t wish to give offence against Jordan, he is one of my favourite driver designer.

Today when good acoustic simulators are available to anyone, I think also Jordan regrets the article and for his defence, the year was 1956 when he wrote the article, nearly 10 years earlier when I bought my first acoustic literature, Leo L. Beranek, Acoustics and made my first DIY speaker.

ARU has pass band drawbacks, high excursion and poor bandwidth.

For drivers with Qts over 0.5, the alpha values shows the need for extreme high box volumes and for normal Ql values, it’s impossible to use high Qts drivers in smaller boxes.

This also implies that its impossible to align ARU’s for third order alignments, its always of forth order and that’s one of the reasons why the excursion plummet sky-high below fB.

As Jordan didn’t investigated the vent and leakage volume velocity’s, he missed the huge increase for very low frequencies were almost no SPL is produced.

I believe that of the possibilities to apply acoustic dampening, the concept is the poorest of all and the ARU article very out of date.

But I think your use of variovent or aperiodic enclosure must be very different from the ARU.
Despite reading the referred thread;

I still don’t know if your use is pure aperiodic of pure third order or intermediate type or even forth order and what parameter(s) is you aiming at to improve?

When you advised pjpoes to read for understanding how to model a variovent, I red that tread too and found a lot of other information given, mixed with confusing statements like ‘acts kind of like a combination of infinite baffle and closed box’ and so on.

Im not troubled by all the good tips given (by GM DIY-wise) in the treads when applying dampening, but what’s is explained and really what’s affected when doing it, i.e. what acoustic parameter is changed and what impact will this change have on the system.

I found all references like the ‘tide-bits’ to be out of date or simply not trustworthy.

Then I thought when advising pjpoes to that tread he will be even more confused than I got and his very first question: ‘help me on understanding how to model a variovent’ not answered at all.

b
  Reply With Quote
Old 16th February 2007, 01:39 PM   #6
bjorno is offline bjorno  Sweden
diyAudio Member
 
Join Date: Mar 2005
Location: Jacobsmountain
Send a message via MSN to bjorno
I’m here again,

Red my own post here above and I felt I have to be more specific what really brought my attention about this subject and even to try clarifying my points.

Maybe my effort to explain seems too ambitious regarding what’s right or wrong in this matter but for the moment and rather long-winded, but for the moment I have some time left over to kill.

I believe sorting out the concepts is of great concern if somebody really wants to know how various damping methods works that is addressed the often misleading word ‘aperiodic’ and what the bearings will be that affects the system qualities of a speaker.

But in the thread, I counted more than eight different ‘acoustic dampening’ types (and being of totally different alignment types), all mixed when referred to, instead for the sake of reasonable clarity, should in my opinion only be handled one by one at a time.

It was impossible for me to follow which parameter or what alignment is changed or chosen and what’s targeted when I read through the postings.

Talking about aperiodic systems, I know even de Boer; the founder of aperiodic speaker dampening was also making non-verified or non-substantial statements when he presented this concept like ‘ because when the vent is heavily dampened its volume velocity should be negligible’.

This is also known today to be of circumventing the truth. The problem here is that most of the popular information written about this subject and spread is misleading with the respect to the acoustics and this seems not to be sorted out at all.

However to defend de Boer one point, if the box shows a FR transfer function like an infinite baffle, the driver is and can be said to look into a purely resistive enclosure, thus this is partly the only similarity with an infinite baffle.

The internal losses that affects system response in a whatever sort of ported box is much higher than in a closed box and don’t compare at all.

The system response is not of an infinite baffle, but rather quite different, this caused by a volume velocity that is quite high in magnitude and at 180 degree, thus counterfeiting the driver at low frequencies.

It’s the enclosure system volume velocity that the system output depends on not what the driver senses in this case the look-alike infinite baffle acoustic returns.

All damped vent ‘aperiodic’ systems (not ARU) is generally of third order with an extra pole and zero pair and for this case when these are made equal (flattening out the impedance) the system becomes pure third but still with severe drawbacks and as Richard Small says; ‘inferior when compared with a normal vented box using acoustic dampening of the driver’’.

Other referred concepts that were brought up in the context of aperiodic enclosures like the Dynaco speakers and the BBC patented dual chamber aperiodic speaker also the Wharfedales and the Dynaudio speakers that have other alignments with respect to ‘aperiodic dampening’.

The ‘system’ acoustics the speakers here above rely on is very different and cannot be kept under the buzzword ‘aperiodic’ and find this very confusing.

The first two of the mentioned speakers; both where of a resistance coupled double-cavity vented box types of very good design.

When later examined by Richard Small and J.F. Novak the conclusion that came out was that these designs could be further optimized if the dividing parts of the area between the chambers were made more open and as if not existed at all, the overall performance would be improved.

The latter, Wharfedales and Dynaudio is more of leakage-damped closed box types with different system behaviors and parameters.

The subject of ‘aperiodic dampening methods’ got even more confusing when Zen Mod introduced this great speaker manufacturer:

The founder of North Creek Music Systems George E. Short:

He has written this:

Quote the North Creek site:

‘There was nothing published on how to use the Variovent, or aperiodic damping in general, so for my Master's Project I undertook the subject. I spent eight months doing both theoretical and lab work, and eventually developed the mathematical foundation of aperiodic damping that was consistent with my lab results. The mathematical model correctly predicted the relationship of system Q and the changes in impulse and frequency response and correlated it with the effect on resonance frequency and the impedance curve. A method of measuring all parameters directly from the impedance curve was also devised. The culmination of this work was published as my Master's Thesis, "The Aperiodically Damped Loudspeaker System", for the successful defense of which I was awarded a Masters of Science in Applied Physics.’

Now the subject of ‘aperiodic dampening methods’ got even more confusing.

Then at this time reading the posts, I found that all theories is really mixed up and the main team is lost totally regarding what is what when speaking about the system attributes that follows when applying aperiodic dampening.

I know only one person that successfully have sorted this up once and for all and that is Richard Small that no one seems to refer to, I find this very intriguing.

b
  Reply With Quote
Old 17th February 2007, 11:57 AM   #7
Geoff H is offline Geoff H  Australia
diyAudio Member
 
Join Date: Oct 2006
bjorno, thanks for your very detailed response. I had to read both several times to absorb it all.

What I think is required here is a full understanding of the term "aperiodic" which Greg pointed out in the post I referred to.

My examples certainly do not fall into the category of TJ's ARU cabinets.
They are more like the SEAS - Dynaudio designs, and bookshelf size.

Getting back to aperiodic. If an enclosure was aperiodic, ie, no frequency characteristics, of it's own, the driver would exhibit the same properties as if on an open baffle, but with the rear wave absorbed.

Or would it not.

Now an enclosure design that can flatten the Z curve over the resonance region would have to approximate an inverse equivalent circuit to that of the driver, ie reactances in one would cancel out those in the other.

So the Variovent and descendants are not aperiodic, as they are often referred as.

My first question in the referred thread, was in relation to the amount of sound energy radiated from the damped port. I resulted in finding that myself - not very much. I know that's not quantitave - my SPL meter is not directional enough to measure it.

The port on my main system does control excursion at and below resonance. The bass is very clean, uncoloured, and fast.

I have always been concerned with the amount of resistive damping in an enclosure. It results in lost energy. The drivers I am using were 103dB/watt/metre on OBs. (10dB l-pads when matched to Altec Lansing 411-8As) They are now 97dB/watt/metre. That's a small trade-off in view of the bass extension.

Other drivers I have tried match the sensitivity of advertised specs.

We still need more data in order to derive a simulator. My experiences may differ with different drivers, however, I don't feel the need for a simulator. Minimum enclosure size appears to be about 33% of the recommended sealed encl size. Less than that, the transient response and bass falls off.

An original Wharfedale 10" RS-DD (with alnico mag) squeezed into 22L (sorry Mr Briggs, I had to try it) was woeful, yet quite good in 59L. though not as good as I recall in the original Briggs designed "many slots covered with felt vented" enclosures that they were in.

Geoff.
  Reply With Quote
Old 21st February 2007, 04:17 AM   #8
diyAudio Member
 
audiosteve's Avatar
 
Join Date: May 2004
Location: Niwot, CO
Does anyone know where George Short's masters thesis paper on aperiodic modeling, referred to above, can be downloaded??

Thanks.
  Reply With Quote
Old 18th March 2014, 11:52 PM   #9
diyAudio Member
 
Join Date: Oct 2009
Location: New Zealand
Default Variovent and Fonken

Resurrecting an old, but very interesting, discussion.

I've been doing some reading up on aperiodic vents triggered by the Fonken enclosures.

Madisound has a kit for the "A26" which is the old Dynaco A25 in a modern and updated version. The Dynaco A25 sold over 1 million copies and was highly rated.

There are some Fonken enclosures that have a different size for the same driver. The smaller enclosure is supposedly working better with solid state while the bigger enclosure is supposedly working better with tube amplifiers.

Did anyone try out stuffing the port on the smaller enclosure with polyfill and measuring the effects on the Q and what the end effects on the sound quality was?
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
How to calculate an enclosure with variovent ? TomatoBangBang Multi-Way 4 16th January 2009 05:35 PM
simulating variovent with MJK sheets? Henkjan Full Range 23 13th April 2008 03:16 PM
3D Modeling?? Nathan1k9 Multi-Way 18 10th January 2007 03:11 AM
Dynaudio Variovent vs. Scan-Speak Vent amichalik Multi-Way 1 11th June 2006 02:42 AM
Variovent placement... arcturus Multi-Way 12 16th October 2004 04:46 AM


New To Site? Need Help?

All times are GMT. The time now is 05:39 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2