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AP Enclosures-The Aperiodic Cookbook

An Aperiodic enclosure is one in which the woofer breathes through a resistive membrane to control it's cone characteristics. In laymen's terms, it makes a woofer in a tiny enclosure act as if it were in a large, sealed enclosure. These membranes can be either on the backside of the woofer enclosure, OR, covering the cone of the woofer (Autosound 2000 Wave mods). Some mfgrs have pre-built AP kits (USD, Image Dynamics, etc) in which you assemble the enclosure your self and they give you everything you will need for specific woofers, short of the wood. Since these kits are geared more toward a professional installer (I am not doubting anyone here!!!), this tutorial will focus on building your own Aperiodic enclosure since a lot of you are just like me---hardcore DIY'ers.

Physical characteristics---APís are the smallest types of enclosures you can make (short of free-air), and in vehicles where space is at a premium they work wonders at getting great bass response into the car. Basically, the actual enclosure needs to only be large enough to fit the woofer and the membrane, THAT'S IT. The less air space you have in the enclosure, the better. I make most of mine cone-shaped so they taper inward at the rear to further decrease the air volume. Standard MDF, resulting in a square box, is perfectly acceptable too. Unless of course you are adept to working with fiberglass....

So, what is this "membrane" anyway? It is basically a sandwich of resistive material (opposes air flow) and the most common material used is fiberglass insulation (the stuff in your attic). 'Glass insulation comes in different "weights" (densities), such as 1lb., 1.5lb., 2 lb., etc. The standard Owens-Corning insulation available at most home improvement stores is 1.5lb. This is the same stuff I use b/c it isn't very dense and is easily pliable.

What makes a membrane is a layer of this stuff sandwiched between two sheets of perforated metal mesh. The mesh needs to be fairly stiff, and I always take care to make sure the holes line up on both the inner and outer layers so the resistive material only impedes the airflow.

I use 1/8" thick perforated aluminum with 3/16" holes (purchased from a welding shop in town). Metal mesh speaker grills can also be used, and hell any type of perforated metal can be used as long as the holes aren't massive (try to stay below 1/4" diameter here.)

So how does it work? Open your pie-holes and breathe in and out real fast a couple times. Now take your audio T-Shirt your wearing and put it over your mouth, and do the breathing again with the same force. Notice the difference? Yes you did. The membrane slows down the moving air mass caused by the woofer cone, exerting air compliance on the cone just like an enclosure does. This simulates the same environment as a large sealed enclosure.

How does the membrane affect the woofer's performance and how do you "tune" it? If you asked this Q, pat yourself on the back! The membrane affects the impedance curve of the woofer.

In a sealed box, a woofer will exhibit a LARGE impedance swing at the box's resonant frequency. This accounts for almost all of the "boominess" problems we have discussed with some sealed box woofers that do not have the benefit of proper EQ. The membrane serves to equalize (flatten) this impedance curve, giving the sub a very even and predictable frequency response. The trade off here is a decrease in output in the bottom octave. Autosound 2K uses a passive device with their Wave Mods called the ANN (Acoustic Normalization Network), which is designed to apply a boost in output to make up for this bottom-octave deficiency. Basically, you can do the same thing with an EQ, so we won't get into that.

The tuning of an AP membrane, is done by varying the thickness of the resistive material layer. The thicker it is, the denser it becomes, and the less air is allowed to flow thru it freely. Seems simple enough, right? We will get into HOW to tune it later.

What makes woofers good candidates for AP boxes? Well, as you guessed, not all woofers will work correctly in this enclosure. While several physical factors affect AP performance (you need a lightweight, stiff cone, good power handling, and SVC subs seem to work better too), the single most important spec to look for is the Q factor of the sub. While Qes (electrical Q) and Qms (mechanical Q) can vary widely, and either by itself won't "make-or-break" this decision, it is...drum roll please...the woofer's Qts (total system Q) that determines the AP candidates! ANY WOOFER WITH A QTS OF .45 or LESS CAN WORK VERY WELL IN AP DESIGNS! I made that bold so it will stick with you guys. So, if you want to try your hand at this type of enclosure, seek the Qts spec of the woofer. If it is less than or = to .45, you have a winner. You will find that it is mainly the subs designed for ported enclosures that fit the bill, as subs designed for small sealed boxes (you know the ones...massive, heavy cones, stiff suspensions, heavy voice coils, etc) have a Qtc of .6 and higher. These subs tend to have a boominess that cannot be overcome with resistive damping and frankly, they sound like ass in an AP, so forget about em.

OK, lets get a little deeper here. Many AP gurus differ in opinion on how big the membrane should be. A2K feels the membrane should be the exact size of the woofer. USD feels the membrane should be larger in sq" than the cone area of said sub. Other guys who make their own go slightly smaller than the woofer. So you wanna know what I do, right? Well, most of my APs have been done in the front of vehicles where space is at a premium, I studied their designs for a while and came to the conclusion that they were using very dense (like 3lb. insulation) material and their membranes HAD to be large so they could flow the proper amount of air and avoid strangling the sub. I figured that by using a material that was less dense, the membrane could be made smaller with identical results.

I was right! I make my AP membranes exactly 1/2 of the cone diameter of the sub, and use 1.5lb. Density fiberglass insulation and the aforementioned perforated aluminum. It works exactly like it should, and many AP techies can't believe it. But we won't get into that.

Something else I should touch on is this; remember when I said some AP's are designed to go "over" the cone, and some go "behind" the cone? Well, I am of the belief that the most efficiency is lost with the "over-the-cone" designs, necessitating mega power to make up for the lost volume. I prefer to have an unobstructed full-size cone firing freely into the listening area. Because of this, my only experience with AP is with the "membrane-behind-the-woofer" design. So, we will stick to this design, cool?

The enclosure needs to be ONLY big enough to mount the woofer in it and have a space for the membrane. I prefer to have the membrane centrally located directly behind the woofer so it exerts even forces on the cone. Sometimes this is physically not possible, but it will still work with limitations (Another topic perhaps???).

So, you know what makes a woofer an AP candidate, you know your enclosure must be very small, and you know what an AP membrane is and what it does. So let's do a cyber-install!

First off, the enclosure must be mounted in an area where the membrane is in a separate compartment than the cone, or severe cancellation will occur. Same exact technique as a free-air (infinite baffle) set up. If your woofer goes behind the back seat, the cone should fire into the cabin, and the membrane should vent into the trunk. Furthermore, the trunk needs to be sealed off from the cabin as good as possible to prevent air leaks from front to back. If your woofer is going in a door, the door panel should serve (and be built) as an airtight partition between the cone and the door interior, and the membrane should fire into said door interior. In kicks, hatchbacks, vans, and SUVs, the membrane must fire OUT of the car (Yes, thatís right, outside the car!). Like I said, the chamber the woofer is in and the chamber the membrane is in must be totally separate. Hatchbacks, SUVs, kicks, etc. do not have a separated inside compartment to use here, so an external vent must be built. When this is done, care must be taken to make sure the membrane is weatherproof, either by using a layer of gortex over it (which I donít like b/c it isnít very breathable) or by designing covers which don't impede air flow but DO stop water from coming in. Some guys simply spray the membrane with Scotch guard, but over time, the water will still soak thru the insulation and make a big mess (and poor performance).

So, for simplicity, we will stick to the old "mounted behind the seat" routine, using the trunk as the membrane chamber. Let's say you want to do 2 12" subs in AP fashion here. Build the box so the front face fills up the whole trunk opening, like normal.

Make the box just deep enough to clear the woofer magnets by 1/2", and in fact, make two separate enclosures just wide and high enough to clear the woofers. On the back wall of each enclosure, cut out a hole for the membrane. We will use a 6" round membrane, as this is 1/2 the diameter of the sub. I like to make the membrane round, but you can make it square or even rectangular if you want. For my round membrane, I will cut out a 6" round hole. Next, take your trusty router and your 3/8" Rabetting bit and router out a recess on the outside of the hole. Go about 3/8"deep with it. This gives you a recess to mount your membrane inside of. Add 3/8 + 3/8 + 6 and you get 6.75" for the diameter to cut out two discs of perforated metal (be it steel, aluminum, a couple old mesh grills, whatever). Make sure you trace the template onto the perf'd metal so that all the holes line up!!!

Next, grab your sheet of Fiberglass insulation (you know the stuff---paper backing on one side, itchy 'glass on the other, about 3" thick uncompressed, etc.) and a pair of scissors, set one perf'd metal disc on the paper side, trace the circle, and cut out a "disc" of insulation. Repeat this step twice, since we have two membranes to build. Then take the paper layer and peel it off, leaving just a thick disc of 'glass. I like to start with about 1" thick layer (uncompressed) of glass, so eyeball a 1" thick section and peel the insulation apart. You will notice the 'glass is striated (composed of several thin layers), and this will come in handy during tuning, so keep it fresh in your noggin.

OK, on the backside of the enclosure, drop one perf'd metal disc into the routered hole you cut for the AP, set the layer of 'glass (1"thick) you just cut on top of this disc, and then set the second disc of perfed metal over the 'glass. Use a pick tool to go thru the 'glass and make sure the little holes are all lined up nicely. Press the mesh down, compressing the 'glass layer, and run about 8 screws evenly spaced into the wood rim RIGHT THRU the mesh itself. You just made an aperiodic membrane (WOO-HOO!). Make sure you built the box using proper techniques (silicon, wood glue, 3/4" MDF, etc) so there is no chance of it coming apart and/or leaking air anywhere. Mount the enclosure in the car firmly in place taking care to place it as far forward behind the seat as possible (this will help when sealing off the trunk) and mount your woofage with speaker wires run out, etc.) I wont get into cosmetics of the box, as that is totally up to you (formica, mirrored plexi, vinyl, carpet, whatever) but if the box is seen, put the "covering" on the baffle before the woofs go in.

Next step is to get into the trunk and pull the trunk trim out of the sides and off of the shock towers. This will expose the unsealed areas that could leak air into the cabin from the trunk. Use pieces of cut MDF, expanding spray foam, dynamat, excess insulation, whatever you got, to fill in these crevasses and put the trunk trim back in.

I even go behind the car's body panels into the cavity around the rear windows and fill them with expandable foam too. You can also use carpeted pieces of wood screwed directly to the sides of yer enclosure to fill in gaps around the box from inside the trunk. Whatever you can do to fill these gaps, do it!

Now it is time to tune your subs!!! Connect them to the amp(s) and get a couple things handy---a CD with unfiltered pink noise tracks on it and an RTA meter. While experienced guys can tune APs w/o using test equipment, I always use an RTA b/c it is accurate and gives me a good indication of where to start from. If each sub has a dedicated amp, use a voltmeter set to AC Voltage and make sure the amp gains are set to the exact same level (everyone knows how to do this, right?). I have found that, often times the 1" thick insulation layer compressed in my "membrane" is very close to ideal from the get-go, so we should see a pretty good response from jump street.

OK, set up the RTA mic stand in one front seat, select the pink noise track and put it on "repeat", set the resolution on the RTA to 3dB per step, and look at your response (with the trunk AND doors shut, engine off). No need to blast the system, just set the RTA at the smallest range (70dB) and go loud enough to get as much of your curve on the screen as you can (it will help a ton if you make sure the whole system is playing so we can set the level of the subs to match the rest of the system too...hint-hint).

As you look at your freq response curve, remember back to what an AP does...it smoothes out the response "peak" at system resonance, and the trade off is decreased output in the bottom octave. YOu should see 25 and 31.5Hz are slightly lower (less output) than the other frequencies, and at 45, 50, or 60Hz there is a small rise in output, but the rest should be about even thru the sub range. This rise in output is what we are trying to "tune" *this is the resonant frequency of the system*. The goal is to get this peak tuned to within 3dB of the adjacent frequencies (one step on the RTA display). How do we do it? Well, it is the thickness of the insulation layer in our membrane that does it. The more insulation, the more this peak will be attenuated. BUT ALSO, the more insulation, the more we lower the output at 25 and 31.5Hz. So the next step is to assess your response in these two regions of the sub's range. If the peak is only one step higher than the surrounding frequencies, stop! You are done. If it is 2 or more steps higher, then the membrane must be thicker. Unscrew the metal mesh and add another 1/4" thick layer of insulation over the insulation already there, screw the mesh back in, and measure again. Keep in mind that a trade off is occurring, and as you decrease this peak, you are also decreasing the low end too. The idea is not to decrease the peak so much that 25Hz drops to below 9dB less output than the rest of the spectrum. If it does, stop there, remove a small layer of insulation, and we will rely on our trusty Ol' EQ to do the rest.

Bolt together the membrane and the tuning of the box is done. If, by some miracle, the woofers you choose exhibit flat response in the 45-60Hz region from the start, assess the 25Hz region. Is it very low in comparison? If so, you may actually need LESS than the original 1" thick layer of insulation. Pull out about 1/4" and measure again (sounds kinky!). You obviously have a woofer that is not exhibiting a huge resonant peak (which is great!) and you should focus on getting back some 25Hz output. Remember, a 3 dB peak in the 45-60Hz range is acceptable, b/c we donít want to decrease 25Hz too much.

Also notice the relative level of the subs to the rest of the system. I usually find I must increase the gain slightly on the sub signal to make up for the inefficiency of the AP box. This can be done with the x/o gain control OR by adjusting the gains a little higher on the amp. If you have two or more sub amps, be sure to set them equal. What you want to see on the RTA is the bass region slightly louder than the rest of the spectrum. Keep in mind that cranking the gains wide open at the amp is a big "No-No" for SQ, so try to do it before the amp by boosting the signal levels relative to the rest of the system. Most x/o's have clean gain controls giving you an easy way of applying the boost. You may even want to reduce the other outputs used a little bit so you donít need to crank the sub section wide open. This should actually be done BEFORE you mess with tuning the membrane. Sorry I forgot to mention it there. Oh well, give me a break man!

Next step while we have the RTA setup and the membrane tuning done is to tweak the EQ. While a 1/3 octave EQ is best here, a parametric EQ or a 1 octave EQ will work just fine. Whether or not a 1/2DIN dash mount is going to cut it depends on the frequency choices you have. Focus on flattening out the response of the sub range only. Start at 25Hz and apply boost to this range until you get it back even with the rest of the sub range. You may need a gentle boost of 31.5Hz too, just try not to max out 25Hz, as over-eq'ing can be a bad thing> You are basically "fixing" the signal to accommodate the roll-off effect of the membrane. Next, focus on the resonant peak frequency (most often I find it to be 45 or 50Hz) and cut it so it is flat. Only a gentle cut should be needed here, as the membrane is doing most of the work. Then, if you want, you can EQ the rest of the system and focus on removing large peaks and dips in Freq response. This will give you a good baseline to begin finesse tuning by ear later.

OK, you are done with the RTA for now. It is time to listen. What will you hear? Well, that depends on your music choice mainly, but you should hear a very well balanced system with great sub bass extension, great bass detail, and the subs should seem to simply "blend into" the sound stage (if you have one). The biggest difference you will hear imo is the bass detail. Each and every note becomes much cleaner and more focused.

Of course, the AP design is only as good as the ears that tune it, so results may vary depending on whether you know ho to tune for SQ or not. What I have described will definitely get you in the ballpark for awesome SQ, but the rest is up to you.

Who is a good candidate for this design? Well, if you listen mostly to rap or trance, forget about AP's altogether. They are no matches in the loudness dept for other types of enclosures (unless you like listening to the "quality" of the music). If you listen to Jazz and classical, there is no better enclosure for you! If you are an SQ competitor, again, it is nearly impossible to beat the SQ of this box. It will work fine for Top40 and rock, as long as being heard a mile down the road is not your top priority. Once ppl hear a properly tuned AP, they are exposed to detail and control they likely havenít heard before. I was hooked after my first audition! While I do, in fact, have a separate sub setup for SPL testing, I find myself wanting to hear only the AP's when driving, as it is such a pleasure to listen to.

So if you are into getting the best possible SQ into your ride, this IS your enclosure. While there is much more I could've gotten into with tuning it and whatnot, I tried to keep this in laymen's terms for your understanding. If you guys want to get more in-depth with Q's, just post them here and I will gladly respond.

In closing, these boxes are not for your average Joe. They are kind of complicated, but like I said, the SQ is phenomenal and the real estate it takes up is extremely minimal. The performance of this enclosure is greatly affected by other system factors, which include proper amp level setting, proper tuning, a solid front sound stage, clean quality amplification, clean signal, etc. Great SQ is a monster with many heads. This box design is one way to assure you of flat, predictable frequency response in the sub bass region with good low freq extension and greatly- improved sonic detail. Without other system considerations, the benefits of this box design may not yield the vast improvement I speak of. So again, if you arenít into sheer SQ, either a competitor or an audiophile, forget about this design.

Here is what was started with. Notice the door panel is already made to where you could mount a sub in it free-air on the MDF baffle ring (3/4"). I had a spare 12 basket, so I attached some wood to simulate a larger-than-needed magnet structure. I will use this woofer model to mold the enclosure, though with proper attention, an actual woofer can be used to mold from too.

I used duct tape to tape my woofer model up, and made sure there were NO GAPS in the tape. The important thing here is to make sure the tape is securely stuck to the mounting flange of the basket, so it is the EXACT size of said basket.

Sorry there isnít a photo of the mold being glassed, but all you do is apply resin and strips of mat to the duct tape. You end up with this. It is a conical shaped woofer mold. Here it is after I trimmed off the excess 'glass and cut the AP hole. It is just sitting in the woofer hole in the door.

Next step was to 'glass this mold into the door. I did this on the backside of the door panel. Since the door panel is plastic, I scuffed the hell out of the panel on the back so the 'glass would stick to it.

I cut a 6" diameter section of perforated metal and screwed it onto the back of the mold. Then I began adding more fiberglass to make the whole thing strong. I added enough fiberglass to make the mold almost 3/8" thick, and went ahead and 'glasses over the edges of the perf'd metal to securely hold it, making it a part of the mold too.

I skipped a step here. I added a thin layer of resin and mat to the INSIDE of my enclosure, just to make sure there were no air leaks, and this made the total cross-section thickness of the mold's walls almost 1/2" thick (and since it is solid fiberglass, that is PLENTY thick!) No flexing here! Anyways, I used Noisekiller Blue inside the enclosure to dampen it. This was brushed on in two layers. Notice my AP membrane sandwich. There is a later of yellow insulation in there, and I completed the "sandwich" from the inside (just easier in the doors) by screwing the second perf'd metal disc in from the inside. This makes it MUCH easier to do membrane-tuning, because you donít have to pull the whole door panel off anymore to disassemble the membrane. Just pop out the woofer, adjust the insulation thickness, and put the woofer back in. Seems easy, right?

Here's an angle shot so you can see the shape of the enclosure better.

12's in the doors are beautiful things! Yes, I skipped MANY parts of the total door panel construction, but this was mainly to show you guys the AP enclosure. In the S10, the windows do not go all the way down. I had to remove the window motors to get the room to do a 12 in there, and it had to be as flush as possible because we didnít want the 12 sticking way out into our leg area. I "can" use a 2" actuator in the future to operate the windows ? way. Just remember, if you want meat in your doors, you must sacrifice something.


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