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Since the box acts as a filter, mechanically blocking lower and upper frequencies, a crossover is not needed in most cases. These enclosures are usually big, and very unforgiving when precise volumes and port sizes are not followed. Bandpass boxes also tend to mask distortion. Both the membrane and cone can not be in the same exterior space. Either the membrane part has to be isolated by cutting a hole in the car so that it is outside, or the subwoofer has to be isolated from the rest of the trunk in a similar fashion to free air woofers.
Aperiodic membrane configurations are very hard to design and tune, but give good frequency response and respond faster to transients, giving accurate and tight bass as opposed to boomy sound. They are not ruled by Thiele-Small parameters like other designs, so any woofer would work with the membrane. Free Air — subwoofers are either mounted under the rear deck or behind the rear seat of a car. This configuration will not work very well for hatchbacks.
Holes have to be cut where the woofers are to be mounted. Since the woofers use the whole trunk as a box, the trunk has to be as sealed as possible from the cabin. Trunk can be isolated usually by putting particle board under the deck and behind the seat. The drawback of free air subwoofers is that bass will not be very accurate especially at lower frequencies , and more amplifier power will be required than with a regular box, but then again, you still have a full trunk.
A good choice for small cars and ideal for hatchbacks and pickup trucks. They usually take up very little room, putting out to fairly good bass. Their design is a ported box.
The woofer has to be close to a wall or, better yet, to a corner. To fine-tune, the bass tube is moved either closer, or farther form the wall or corner. It is convenient to get an amplified tube, since amplifier, crossover and subwoofer are all integrated in a small package. Forums Blogs Articles Groups. Anyone remember aperiodic membranes? Previous 1 2 template Next. Going back to my car audio days in the mid 90's aperiodic membranes were the trendy thing for a short while. I think the were made famous by Richard Clark? Not sure if I can find it or not though. Is there any relevance to useing something like this in an IB setup.
Downsides, upsides, not worth the effort? Anyone have any experience with these things. Have to say I've never heard of such a thing. Of course, I didn't even have car until 99' so maybe that's why. I actually have a pair of Dynaco A25 XL's similar to photo below that are still going strong after about 30 years of use. There were more units of this speaker manufactured in Denmark with Seas drivers sold than any other speaker in history.
Decent ones still fetch a pretty good price on e-bay. The A25XL Aperiodic Vent is pretty primitive - about an inch of medium density fiberglass and a vinyl coated screen that is stapled in place to hold the fiberglass in the port. But it works very well. It would be interesting to see how a scaled-up version of this vent might work in an IB setup.
If a driver is placed into a sealed box that is too small, the result will be a high Q, which will cause a peak in the lower frequency response and a high impedance peak. Because the sound quality of the Aperiodic Membrane system is so different, it is often difficult to hear when the woofers suspension limits are reached. Previous 1 2 template Next. Custom Bass Boxes Many manufacturers such as JL and MTX are making custom boxes with subs included to fit in center consoles, under seats, or in other small spaces. Forums Blogs Articles Groups.
I'm guessing that your goal might be to have lower tuning from a smaller volume enclosure but with IB that's all relative. Good luck on your quest. I remember the article that your are thinking of. There was also a little black box used with the membranes. I can't really remember the function of the black box. I think that he concluded that the membrane just reduced the sensitivity of the drivers and was useful for limiting excursion or over-excurson. For my round membrane, I will cut out a 6" round hole. This gives you a recess to mount your membrane inside of.
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 stuffpaper backing on one side, itchy 'glass on the other, about 3" thick uncompressed, etc. 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. 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 handya CD with unfiltered pink noise tracks on it and an RTA meter. 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 As you look at your freq response curve, remember back to what an AP does YOu should see 25 and 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?
AP Enclosures-The Aperiodic Cookbook. An Aperiodic enclosure is one in which the woofer breathes through a resistive membrane to control it's cone. Aperiodic Membrane is one of the latest successful methods in car audio. The membrane material is a resistive element that the woofers play through.
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.
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! If it is 2 or more steps higher, then the membrane must be thicker. 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 Hz 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. You obviously have a woofer that is not exhibiting a huge resonant peak which is great! 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. 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. Sorry I forgot to mention it there. Oh well, give me a break man! 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 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.