Need advice on the execution of an aperiodic vent

IG81

Member
2008-02-22 1:21 pm
Hi,

I am building aperiodic enclosures for a pair of Philips AD7060 6" fullrange drivers I have.

The initial sealed box, unstuffed, should yield a Qtc of 0.84. I expect that with stuffing, this figure should be down to around 0.8. My goal is to get it down between 0.65-0.7 with an aperiodic vent. I would like to execute this in a well-controlled, tuneable and/or progressive manner in order to adjust response until I'm satisfied and am juggling between a few options.

First, I thought of drilling ~½" holes progressively and inserting punched open-cell foam cylinder in them. As a progressive procedure, that would work great and have well-controlled resistive material density, but I am questioning the suitability of open-cell foam, as it seems a biit too loose, what do you think?

Second, I could go the gutter-mesh + fiberglass sandwich way, but how to make it tuneable? Somehow change the exposed surface area or change thickness of sandwiched material for a same area?

Thanks,

IG
 
While more work than the other techniques (i've tried them all), the foam behind the array of holes works best for me. You do have to have the right kind of foam (i fortunately have lots -- a specific part of the paking from Apple service boxes). I've been spreading it around by using as packing material on outgoing shipments.

dave
 

IG81

Member
2008-02-22 1:21 pm
While more work than the other techniques (i've tried them all), the foam behind the array of holes works best for me. You do have to have the right kind of foam (i fortunately have lots -- a specific part of the paking from Apple service boxes). I've been spreading it around by using as packing material on outgoing shipments.

dave

I too am collecting packaging foam, from a pharmacy in the same building that I work in. They are blocks of ~10"x6"x4" (iirc) of rather dense, grey, open-cell foam. Lots of cool things to do with those.

After giving it some thought I've settled on a circular fiberglass + gutter mesh sandwich, adjustable from the exterior. I can see how I can make it to be either too little or too much, so I should have the whole usable range in between to work with. Part of the reason I settled on that is that I bought a 20$ 4" DeWalt hole saw, and I'll be damned if I'm not gonna be usin' it! :D

IG
 
I too am collecting packaging foam, from a pharmacy in the same building that I work in. They are blocks of ~10"x6"x4" (iirc) of rather dense, grey, open-cell foam. Lots of cool things to do with those.

Sounds similar. Mine are ~3" x 1.25" x long

After giving it some thought I've settled on a circular fiberglass + gutter mesh sandwich, adjustable from the exterior. I can see how I can make it to be either too little or too much, so I should have the whole usable range in between to work with.

I have found that long & skinny (ie a slot) works much better with the gutter mesh. It is not stiff enuff to adequatly squish the fiberglass in the middle of the circle.

dave
 

IG81

Member
2008-02-22 1:21 pm
Sounds similar. Mine are ~3" x 1.25" x long



I have found that long & skinny (ie a slot) works much better with the gutter mesh. It is not stiff enuff to adequatly squish the fiberglass in the middle of the circle.

dave

At the moment, I plan on a 4" diameter hole through the back baffle, directly behind the driver, with a square piece of 3/8 ply glued inside the baffle in which will be the actual vent hole of 3" diameter. I will install the 4" diam guttermesh - fiberglass - guttermesh sandwich recessed into the baffle, held on by a 4" OD - 3" ID 3/8 ply ring. This is not a really big area, ~7in^2, but my design is not too compromised to start with, so I thought I could still get by with it. I can make a test sandwich before drilling into the baffle to see how it works at this diameter, compression-wise. Then I'll make me a turkey sandwich cause all this talke made me hungry! :)

I should be able to get some work done tonight, I'll post some pics.

IG
 
Hi,

I am building aperiodic enclosures for a pair of Philips AD7060 6" fullrange drivers I have.

The initial sealed box, unstuffed, should yield a Qtc of 0.84. I expect that with stuffing, this figure should be down to around 0.8. My goal is to get it down between 0.65-0.7 with an aperiodic vent. I would like to execute this in a well-controlled, tuneable and/or progressive manner in order to adjust response until I'm satisfied and am juggling between a few options.

First, I thought of drilling ~½" holes progressively and inserting punched open-cell foam cylinder in them. As a progressive procedure, that would work great and have well-controlled resistive material density, but I am questioning the suitability of open-cell foam, as it seems a biit too loose, what do you think?

Second, I could go the gutter-mesh + fiberglass sandwich way, but how to make it tuneable? Somehow change the exposed surface area or change thickness of sandwiched material for a same area?

Thanks,

IG

I'll be following this with interest, I have a set of ad8080's - the 8 inch version of your speakers - how did you work out what size box to use - have your found the t/s parameters somewhere or did you measure the speaker?
 

IG81

Member
2008-02-22 1:21 pm
I'll be following this with interest, I have a set of ad8080's - the 8 inch version of your speakers - how did you work out what size box to use - have your found the t/s parameters somewhere or did you measure the speaker?

I measured the T/S parameters myself. I found few useful references to this driver online, which would have been for interest mainly, as it is recommended to measure parameters of a specific driver, especially old ones, for better accuracy.

I use WinISD for simulation. I simply modeled a sealed enclosure with a higher Qtc than what I want, in order to have a smaller box, details in the first post of this thread. The aperiodic part of the design can be modeled with the leakage factor "Ql" I believe, but I do not really know how much of this metric to input. I will design it by trial and, quite possibly, error. :) I guess when my design is complete, I could compare with the WinISD sim to see how much Ql is needed to represent my actual aperiodic vent.

The same technique I use to measure T/S parameters will be used to measure the whole system's Fc and Qtc with varying amounts of fiberglass until I am satisfied.

For Dave: I found a section on one of your pages about different Philips FR's a while ago, but cannot re-locate it. It seems you have a bunch of these pages I discover through a link once in a while, that are not obviously accessible through the main website page. Not complaining here; I find it makes it kinda interesting! :)

IG
 
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IG81

Member
2008-02-22 1:21 pm
Welcome to www.oudio.nl !

You may have seen this already but i thought id post it in case you were interested.

Click on brochures and then educational. The ones of interest are the loudspeaker brochures which contain box building plans and measurement graphs.


I'm having a lot of difficulty finding t/s data for my 8080/m8's so i'll probably end up measuring as well.

Thanks for the link. I have seen a bunch of links to that site lately, but all dead, I guess they pointed to an older version/location. Some of these, or similar, Philips documents I also found on MFBfreaks.

Measure them yourself, it's worth it. One can only guess as how a speaker might have aged. That and factory specs are generaly flawed as well.

IG
 
For Dave: I found a section on one of your pages about different Philips FR's a while ago, but cannot re-locate it. It seems you have a bunch of these pages I discover through a link once in a while, that are not obviously accessible through the main website page.

The section was never finished, so what was done sits idle. It is a composite of edited down eBay adverts (ones i ran).

Vintage Philips

dave
 

IG81

Member
2008-02-22 1:21 pm
For every listing on that page, i have at least 10 more that just need editing and collation.

dave

Wow! :) All Philips or counting other brands?

I tried my 3" ID sandwich and, although the center is not as compressed as the outer edges, it's not bad at all. Have not tested the whole system yet, as I still have to install my back baffle gaskets and a couple of other small details, but I think it should work fine. I hotglued the 4" guttermesh circles to the 3/8 ply rings and it's pretty solid, keeping a decent enough tension on it.

IG
 

IG81

Member
2008-02-22 1:21 pm
I got done building my AD7060 aperiodic boxes, and tuned the aperiodic vent. Here's how it turned out.

Here is the front view of the aperiodic vent. 3/8 ply, cut into a 4" OD 3" ID ring, under which is glued a guttermesh circle.

vent front.jpg

It compresses the fiberglass disc over a 3" ID circle, cut into a small piece of 3/8 ply glued inside the back baffle, via 3 screws.

vent back.jpg

back baffle vent.jpg

The pink fiberglass I used has a 2" nominal thickness, which is what I started with, not knowing what to expect, being my first time doing such a vent. Initial simulations in WinISD gave me an Fc ~75Hz and Qtc ~0.84. With the damping materials in the box, it turned out measuring ~72Hz and ~0.79. Turned out that the 2" thick fiberglass acted almost as a closed element. I tried 1" thick, which got Qtc down to 0.75. Then tried about ½", and got a Qtc of 0.65, pretty much what I was aiming for. Second enclosure got down to 0.67 with about the same amount of fiberglass, close enough.

This was a very educational process for me, I'm glad to have done it. Here are some more pictures of the enclosures:

Back of the driver with ductseal to damp the stamped steel frame:

back driver.jpg

Shot of the insides. I made a guttermesh tube behind the driver to leave a clear path to the aperiodic vent.

with gasket.jpg

Front and back shots.

front+back.jpg

front.jpg

I have also installed a pressure screw, from the centers of the back and front baffles, to act as a brace for the big panels.

I listened to them a bit, while they don't sound bad, a few things will need to be done. Some BSC most definitely. Maybe some foam damping around the whizzer. Perhaps a phase plug. Bass did not sound quite right, but in-room placement was far from ideal at the time. If I get them to sound good enough for me, I'll finish the cabs nicely.

Thanks to all who helped out!

IG
 

IG81

Member
2008-02-22 1:21 pm
Congrats on finishing your speakers :cool:

If you have time and equipment i'd appreciate seeing a couple of quick measurements please.

Thanks!

All I can measure for now is in my last post, except for an impedance curve up to a few hundred Hz. I'd really like to get myself a nice microphone to measure FR on the computer. Maybe I'd have to get a better soundcard as well. I never quite investigated that before, as I'm just starting to build things more seriously. Would also be fun to measure my commercial speakers.

IG
 
Hi IG81,

I have seen articles in the past on tuning aperiodic enclosures and they always seem to focus on minimizing the impedance peak at resonance. Which I guess would also mean tuning for the lowest Q. Does this make sense ? Is this also what you are doing. Is there a Q minimum when playing with the stuffing after which the max impedance begins to rise again ?