'Perceive' Contruction Diary

[tktran]

must say that I have egg on my face. I've completely misjudged these drivers and been rude to tktran when his advice was actually the right stuff.

No offense taken Shin.

Just because I'd spent time on ONE particular ROUGH ported solution and dismissed it, I'd actially failed to try the finer details and perhaps a more indepth look at its possibilities... However without a really thorough investigation of ported I'd quite clearly missed details - and probably still am.

I think this is a common mistake in any kind of scientific enquiry. It's easy to draw the wrong line of conclusion when we think we've exhausted all possibilities.

From personal discussions with Troels I know that he is extremely rigorous in his methodology, sometimes even a bit pedantic. So I had no reason not to believe him when he said he had found a good configuration for the L22RNX4P.

When he cannot find a good solution, he just says so. eg.
'I couldn't get as good a sound with the parallel filter as my series filter, so I gave up' .But I've never heard him concluding or championing that "series filters are therefore better".

To me that comes with age and experience. Again, no offense intended, but you and me- we're both the same age and fom the "first time" thread we've both revealed that we've only just started in this hobby. We have a lot to learn...

One more thing I can say about the L22RNX4P is that if you haven't investigated a notch filter in the low pass, to tame the primary 4Khz resonance and further hash, perhaps it's worth a try. Yes the peak is a few octaves away from your crossover point, but without a notch filter Troels found it audible on pink noise testing. Notching it out lead to improvements in test tone material, and may lead to improvements in bass-mid integration when listening to program material.

[soongsc]

Originally posted by ShinOBIWAN


Thats my take on things so far.

Blue is the driver.

Green is the straws.

Grey is the driver back brace connected to the duct walls.

Yellow is a 3" thick layer of MDF

Red is bracing that holds the duct in place and fully encapsulates it and connects it with every wall of the cabinet.

Am I close?

I get sort of lost when there are lots of text, but this picture shows somthing like what they do in wind tunnels to smooth aiflow. The only difference is, in a wind tunnel, the air is circulating, the the speaker, the air is moving back and forth.

[soongsc]

Originally posted by ShinOBIWAN


I'm not using the XO that I outlined in the beginning, like many things they were scrapped in favour of a digital XO.

Time/phase alignment can be achieved in many ways, so I wouldn't pay credence to anyone that says there is only one way.

Originally posted by soongsc



Did you test the phase characteristics (minimum phase) along with the frequency responses for each driver after they were in the enclosure or before?

Just be aware that time aligned is phase aligned, but not vice versa unless you get to minimum phase alignment. Time/Phase aligned XO does not mean that the acoutics will be time/phase aligned. Acoustics will can be time/phase aligned at specific points only, unless you can achieve the same acoustic radiation pattern throughout the audio range.

[ShinOBIWAN]

Originally posted by soongsc






Just be aware that time aligned is phase aligned, but not vice versa unless you get to minimum phase alignment. Time/Phase aligned XO does not mean that the acoutics will be time/phase aligned. Acoustics will can be time/phase aligned at specific points only, unless you can achieve the same acoustic radiation pattern throughout the audio range.

Yep understood. Already knew that but thanks anyway.

Time alignment is the physical matching of acoustic centers relative to the listening position ie. sloped/stepped baffle, offsets etc.

Phase is the matching of the drivers radiation patterns.

And the XO's phase gets completely changed when it hit the drivers.

[ShinOBIWAN]

Lo Scott,

I've redone the sketch with the changes you have suggested, a few things that I'd like to ask though:

I've changed the reflector to a 'guide' since I really wish to mount the 215 GMF as close to the ATC as possible and it seems that this would work better, am I mistaken?

The aperoidic venting is confusing me, where exactly does it go and what does it look like?

[ScottG]

Ola!

Everything looks good except:

1. The reflector should be like it was b4 but with a slighty larger diameter (as large as, or slightly larger than, the diameter of the Duct/Port) - AND a more aggresive (triangle-like) angle. Don't worry about it being close to top panel - all other surfaces but the reflector will have 10+ coats of the NASA paint (which absorbs freq.s between 10 Hz to 20 kHz with little airflow resistance).

The reflector has 3 basic functions:

I. Reflect higher freq.s into absorptive boundries. Important with the thin diaphram on the Supravox. Otherwise of little importance.
II. Provide a dispersive airflow. Minor importance, and this relates to the question about being near a boundry like the top panel.
III. Breakup standing waves, particularly those that would generate between the back panel and the driver (and in the Duct/Port). This is by FAR the #1 reason for this device - so my use of the term "reflector" is a bit of a misnomer.

2. The aperiodic vent is as follows (and your UK supplier should have it):

http://www.madisound.com/cgi-bin/ind...11910&pid=1607

I think the interior diameter is 4 inches, don't remember what the exterior diameter is.. Don't believe the bit about increasing the volume 20% - the effect in some circumstances is like this, but you should be working with your max 50 liters (or a bit more if you can..).

In a perfect world we would have a supercomputer to model the interior standing waves and see where the placement of the "vent" should be.. but we don't. So, experienced guessing (when considering esthetics) leads me to say that it should be on the back panel 20% up from interior's bottom, centered.

3. The grey brace doesn't have to be that large a diameter - (though if it was you could probably find some pvc pipe (plumbing section) that was close to being the perfect "fit" for the magnet). If you do go the PVC route then you need to fill it with something like sand, or a lot of caulk (to dampen it). (I've done the latter b4 and it works rather well.)

4. The vertical bracing looks good UNLESS you have a "thin" profile tube. If however the sonotube is fairly massive (and non-resonant) then its perfect. (BTW, inspect the sonotube b4 purchasing - it needs to be ROUND not oval - and make sure the seller will cut it to the length you want.. I personal had a @@@@ of a time just cutting that cheap tubing I provided a link to - I shudder to think what real sonotube would be like to cut.) Also consider at least one horizontal brace (for the box, not the Duct/Port).

[wigginjs]

I don't mean to sound like I'm trying to be a party pooper, but have you considered this? Very diminishing returns? This seems like an extremely time consuming and VERY complicated box construction. Forgive me for oversimplifying, but how much better than your standard sealed box with aperiodic vent will this really be? This stuff has pretty much left my league as far as knowledge goes (and definately as far as experience goes), but I often find that when I'm working on a project (even outside of audio) there is usually a simple elegant solution to a given problem. This seems like a very complicated solution.

[soongsc]

Originally posted by m0tion
I don't mean to sound like I'm trying to be a party pooper, but have you considered this? Very diminishing returns? This seems like an extremely time consuming and VERY complicated box construction. Forgive me for oversimplifying, but how much better than your standard sealed box with aperiodic vent will this really be? This stuff has pretty much left my league as far as knowledge goes (and definately as far as experience goes), but I often find that when I'm working on a project (even outside of audio) there is usually a simple elegant solution to a given problem. This seems like a very complicated solution.

I second this comment. Normally, unless you have the mathematical part of all this simulated using finite element method to truly indicate the differences, it is much better to let the design grow via evolution process so that you can identify the improvements in each stage of the design. What we normally would do is try a certain technology on an existing product wto see if the difference is identifiable or not, if yes, then take the next step.

[ScottG]

Originally posted by m0tion
I don't mean to sound like I'm trying to be a party pooper, but have you considered this? Very diminishing returns? This seems like an extremely time consuming and VERY complicated box construction. Forgive me for oversimplifying, but how much better than your standard sealed box with aperiodic vent will this really be? This stuff has pretty much left my league as far as knowledge goes (and definately as far as experience goes), but I often find that when I'm working on a project (even outside of audio) there is usually a simple elegant solution to a given problem. This seems like a very complicated solution.

I've actually tried this "formula" with fullrange drivers (high eff. low mms), and yes it does make a LARGE difference in the passband we are discussing (..and higher in freq. as well if your into fullrange drivers). With most HiFi drivers though - you could be correct that diminishing returns outweigh complexity (..don't know). Additionally, its not as difficult to build as you might think. I personally think that there are many cabinets that are FAR more complex (like Jensens with bitumen or lead lining) that can often yield significant audible benefits over your traditional bass reflex, (making them worthwhile despite the complexity). To me the real complexity isn't the cabinet build, but rather the cabinet finishing (..black laquer seems like such a simple yet elegant finish.. elegant it is, simple it isn't).

In any event, if Shin' wants to proceed with this design I'll try to be here to help-out. If he wants to dump-it in favor of somthing else, then thats cool too. Either way, I just hope that he gets the sound he is looking for (..or even better than what he presently "perceives" possible).

[soongsc]

Originally posted by ScottG


I've actually tried this "formula" with fullrange drivers (high eff. low mms), and yes it does make a LARGE difference in the passband we are discussing (..and higher in freq. as well if your into fullrange drivers). With most HiFi drivers though - you could be correct that diminishing returns outweigh complexity (..don't know). Additionally, its not as difficult to build as you might think. I personally think that there are many cabinets that are FAR more complex (like Jensens with bitumen or lead lining) that can often yield significant audible benefits over your traditional bass reflex, (making them worthwhile despite the complexity). To me the real complexity isn't the cabinet build, but rather the cabinet finishing (..black laquer seems like such a simple yet elegant finish.. elegant it is, simple it isn't).

How do the results of this "formula" show up in listening tests and measured data?