Measuring VAS, phase plug leaking?

Guss

Member
2003-06-22 10:58 pm
Montreal
Hi,
I am trying to measure an L12RCYP' VAS with the closed box method. The test box is sealed with caulk and the driver is mounted with some sort of a blue-tac gum (bolts are sealed too). Also, I recently tested dayton drivers with the exact same setup and everything worked well, so far so good.

If I play low frequencies, I can hear air coming out of the rig. I own two woofers and both are passing air under these conditions. I'll assume it is the phaseplug leaking for now.

Bass output is quasi inexistant and VAS measures pretty high at 9 litres, when it should be arround 5L. If the plug does leak, VAS is obviously wrong which leaves me the added mass method to sort it out... Okay then!

But once in a sealed box, isn't that leak affecting damping? or detuning a ported box? Is this a known issue, or something I'm doing wrong?

Thank you for your input,
Gaspard
 
I have an L15 and L26 and wonder the same thing.

With an open voice coil assembly and a phase plug, isn't there a leak through the driver? Under excursion in a higher Q enclosure, is there more chance of the driver itself generating a port type "chuffing" noise through the cone?

I used delta mass on the L15 and pretty much got the same Vas as Zaph.

I suppose one way to tell would be measure harmonic distortion on an infinite bafle, vs. sealed enclosure. I don't know how you would calculate what frequencies the effect might make itself known.

Cheeras,
David.
 
Some plugged drivers are certainly not air tight... the CSS FR125S is certainly an example -- it has 4 holes in the voice coil former (for cooling i'd guess) that vent the inside of the box to the outside via the phase plug gap. It could well be part of the "farting" issue that these have (primamrily with SE amps). I just consider it a warning to turn it down.

Sonce with my measures i am usually more concerned with matching than with using the measures to design a box, and i use added mass for its convenience this has never been an issue for me.

I also tend to avoid sealed or BR boxes as well so it becomes even less of an issue.

(BTW, this isn't a problem with the phase plugged fostex that already have a vented cap, any leakage having to be much more circuitous (ie around the far end of the VC)

dave

dave
 

AKN

Member
Paid Member
2005-08-01 9:23 am
In the middle of Sweden
Hi,

planet10 said:
Some plugged drivers are certainly not air tight... the CSS FR125S is certainly an example -- it has 4 holes in the voice coil former (for cooling i'd guess)

And to lower Q in the cavity under dustcap.


I also tend to avoid sealed or BR boxes as well so it becomes even less of an issue.

Hmm, how do pressures in BVR compare to BR?
 

Guss

Member
2003-06-22 10:58 pm
Montreal
Dave Bullet said:
With an open voice coil assembly and a phase plug, isn't there a leak through the driver?

For the same procedure, the dayton (5" reference series w/ plug) didn't leak and they certainly were not bass shy. Since they have a closed back, the spider is the only way out, which is not the case with the SEAS.

I feel a bit lost... my design is flexible, but the plans won't allow for a doubling chamber size right now. I'm struggling to find VAS to fine tune the mid enclosure before I nail it down.

You suggest I do not design a closed box (even with low Q, box size is arround 3 litres) and I don't want to build a BR out of this driver. Would an aperiodic design allow to somewhat overdamp the box enough to get rid of the chuffing?


Dave, I know you usually do not build plain sealed or vented boxes, but I'd really like to know what you'd do given my situation. It's too late for me to go TL or open back, any thoughts on the matter?

Thank you for your precious input, I apreciate it!
 

Guss

Member
2003-06-22 10:58 pm
Montreal
Here is the design; I'm building two sets of towers. Those use dayton mids but the layout is the same with the SEAS, it simply comes in a much smaller package. Now you can see how the chamber variations are limitted.

[IMGDEAD]http://img213.imageshack.us/img213/8903/p1010223ko3.jpg[/IMGDEAD]
aint done the woofers holes yet
 
I am planning on giving my L15 "mid" in my 3 way 25 litres. It has a Vas of 8.1L so this is overkill. With a cabinet depth of 51cm, it will be as close as I can get to an open backed mid (assuming a highly absorbing back wall), without going OB all the way.

I would have thought the enclosure is most relevant when trying to shape highpass Q and excursion for a woofer. when a mid is playing down to 250Hz (or thereabouts) excursion isn't a problem. In my design the mid HP crossover is the dominant factor for shaping HP rolloff to the woofer, therefore the enclosure Q is less relevant.

I therefore said "lets give the L15 a massive enclosure" (since the L26 needs about 90L ported to get he best out of it (L26 = low excursion plus low Q).

Your cabinet looks solid. Can you give the L12 some more space?

David.
 

Guss

Member
2003-06-22 10:58 pm
Montreal
planet10 said:

The problem is that you have a too small a sealed box?

I'm afraid any low Q sealed box wouldn't be significant enough to take care of the chuffing (if chuffing is not always audible, it still unloads the box), so I'm turning to other methods.

Dave Bullet said:

Your cabinet looks solid. Can you give the L12 some more space?

David.

I could get away with a little over 3L ; port, brace and driver accounted.

I certainly haven't bought an L12 to crossover at 250Hz, bass is an issue. If aperiodic can help I'd give it a test sim next week, but I have no idea how to design/model such a thing. I'm sorry for threadjacking my own post but If anyone knows a good reference on the matter, I'd be thankful to have a good read. I've searched the forums and tried to simulate one according to what I've read. It failed, well it seemed pretty wrong to me, port lenght shouldn't reach astronomic values right?

Thank you,
Gaspard