Aperiodic Enclosure Design
- Thread starter Skalfell
- Start date
I've never even heard of this type enclosure. What the heck is it..............???
audiozaz
rtfm!? - tl;dr
There is not that much information on this out there actually. Here's a thread somewhat detailing my experimentation with aperiodic.
http://www.audiokarma.org/forums/showthread.php?t=327319
Basically, aperiodic permits one to use a driver in an un-optimally small closed enclosure. Ideal candidates for this are drivers with a somewhat low Fs and mid-high Qts. The trade-off for a smaller box is reduced bass extension; at least the aperiodic vent reduces the bomminess you'd get in the same volume sealed box.
My procedure goes like this: design a sealed box for your driver that will give you a system Q (Qtc) around or slightly greater than 1. The exact value depends on what is the final Qtc you want to end-up with; I suggest between 0.707 and 0.8. The aperiodic vent will lower the Q, but the speaker system will have a higher f3 than a larger completely sealed box of same Qtc. The build detailed in the above linked thread was extremely lean on bass, due to the low final Qtc.
Executing the vent can be as simple as buying one or more Scan Speak Variovents or making you own, like I did with a fiberglass sandwich between guttermesh, or making holes in the back panel and inserting open-cell foam of proper density.
Being able to measure system impedance is a must if you want to know what you are doing, meaning measuring Fs/Qts and Fsc/Qtc. Doing it by ear with music and/or puretones is possible; afterall, you want it to sound go to you. I like to do it both ways, to get good sound and detail the procedure with numbers/values and hard data.
az
http://www.audiokarma.org/forums/showthread.php?t=327319
Basically, aperiodic permits one to use a driver in an un-optimally small closed enclosure. Ideal candidates for this are drivers with a somewhat low Fs and mid-high Qts. The trade-off for a smaller box is reduced bass extension; at least the aperiodic vent reduces the bomminess you'd get in the same volume sealed box.
My procedure goes like this: design a sealed box for your driver that will give you a system Q (Qtc) around or slightly greater than 1. The exact value depends on what is the final Qtc you want to end-up with; I suggest between 0.707 and 0.8. The aperiodic vent will lower the Q, but the speaker system will have a higher f3 than a larger completely sealed box of same Qtc. The build detailed in the above linked thread was extremely lean on bass, due to the low final Qtc.
Executing the vent can be as simple as buying one or more Scan Speak Variovents or making you own, like I did with a fiberglass sandwich between guttermesh, or making holes in the back panel and inserting open-cell foam of proper density.
Being able to measure system impedance is a must if you want to know what you are doing, meaning measuring Fs/Qts and Fsc/Qtc. Doing it by ear with music and/or puretones is possible; afterall, you want it to sound go to you. I like to do it both ways, to get good sound and detail the procedure with numbers/values and hard data.
az
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audiozaz
rtfm!? - tl;dr
It is also important to take into account the stuffing of the enclosure. This will make the box look bigger and reduce Fsc and Qtc. In a way, the aperiodic vent also makes it look like the box is bigger, but only for the purpose of lowering Qtc, not for Fsc; the latter is not altered within the normal, useable range of an aperiodic vent, i.e., if you go with not enough resistive material or too thin, you should start to see some ammount of helmholtz resonance (doulbe hump on the Z_curve).
Audiozaz,
Thanks for your response--it's quite detailed. I have a question though: I find that car audio buffs are using aperiodics for woofer enclosures, which need to be smaller in (most) cars than a home subwoofer. You say that the design gives reduced low-f response. Where do you find the low-f cutoff to begin?
I have a pair of Dynaco A35's which impressed me so much that I decided to find out about aperiodic design. In the A35's, however, the woofer port feeds into a closed chamber and not to the outside. Do you know how that works? It doesn't seem to have reduced the enclosure size.
By the way, those are pretty amazing-looking speakers!
Skalfell
Thanks for your response--it's quite detailed. I have a question though: I find that car audio buffs are using aperiodics for woofer enclosures, which need to be smaller in (most) cars than a home subwoofer. You say that the design gives reduced low-f response. Where do you find the low-f cutoff to begin?
I have a pair of Dynaco A35's which impressed me so much that I decided to find out about aperiodic design. In the A35's, however, the woofer port feeds into a closed chamber and not to the outside. Do you know how that works? It doesn't seem to have reduced the enclosure size.
By the way, those are pretty amazing-looking speakers!
Skalfell
Dynaco literature states that the A-50 & A-35 use a double compartment. The woofer(s) are in one compartment which is internally vented to the 2nd. "The result is a sealed system embodying the patented aperiodic design virtue of variable volume action for critical damping at resonance, plus the benefits of a larger sealed enclosure for minimum distortion at the lowest frequencies."
TerryO
Super Member
Aperiotic loading helps suppress resonance peaks and gives a smoother load at the lower frequencies of the driver. This is quite effective when using tube amplifiers, especially SETs, which are sensitive to the impedance of the speaker. This is one reason why Dynaco speakers quite often sound so nice with tubes.
Best Regards,
TerryO
Best Regards,
TerryO
westend
Audiopile
George Short has some design principles and the math: MAPD alignment.
I built a pair of small bookshelves using the Northcreek approach. Design criteria was to allow the driver to play lower and keep to a good power handling level. In the end, my then-teenage Son blew one trying to use them as PA boxes. The sound was very good, using a small (4.5") full-range driver. Mr. Short's math will get you right on .707 QTS and. if you have a worthy woofer, reproduction is very balanced.
Good luck with your speaker project.
I built a pair of small bookshelves using the Northcreek approach. Design criteria was to allow the driver to play lower and keep to a good power handling level. In the end, my then-teenage Son blew one trying to use them as PA boxes. The sound was very good, using a small (4.5") full-range driver. Mr. Short's math will get you right on .707 QTS and. if you have a worthy woofer, reproduction is very balanced.
Good luck with your speaker project.
Westend, thanks for that link--it looks good!
TerryO, thanks for mentioning tubes. I am going to break out my Fisher and see how the Dynacos sound with that! Fun for the weekend!
Gearhound, I'm not sure about the zorbel because I don't know anything about the differences between damping peaks electrically versus acoustically. Since the speaker eventually goes in a box, the two must have to interact.
Skalfell
TerryO, thanks for mentioning tubes. I am going to break out my Fisher and see how the Dynacos sound with that! Fun for the weekend!
Gearhound, I'm not sure about the zorbel because I don't know anything about the differences between damping peaks electrically versus acoustically. Since the speaker eventually goes in a box, the two must have to interact.
Skalfell
Can this method be used to get some usable output from a really high Qts woofer in a normal sized box?
I have some Morel made Macrom 59.21 car drivers i would like to use in 25-35L boxes in a HT setup, but sims show a System Q in the 1.5-1,7 area if i go with sealed. could a number of Variovents help me get enjoyable results with these drivers?
Since i already have the drivers and think they work very well crossed over to Audax 34mm domes with only a cap and some padding on the tweeters it would be great if i could find a way too solve the cabinet problem.
Ps using them in a HT setup means very little output from them below 80hz, since that area will be handled by 15" subs
I have some Morel made Macrom 59.21 car drivers i would like to use in 25-35L boxes in a HT setup, but sims show a System Q in the 1.5-1,7 area if i go with sealed. could a number of Variovents help me get enjoyable results with these drivers?
Since i already have the drivers and think they work very well crossed over to Audax 34mm domes with only a cap and some padding on the tweeters it would be great if i could find a way too solve the cabinet problem.
Ps using them in a HT setup means very little output from them below 80hz, since that area will be handled by 15" subs
audiozaz
rtfm!? - tl;dr
Since you ran some box simulation, I assume that you have the driver's T/S parameters; what is their Qts? System Q (Qtc) will never go lower than this figure.
az
audiozaz
rtfm!? - tl;dr
IME, 50% smaller is realistic; it's what I got away with in my Philips box. That is, going with measured Qtc and equating it to the required sealed volume; but as there is no free lunch, what you give up is bass extension. More might be pushing it a bit, but I'd have to actually try to know. In the same experiment, stuffing increased apparent volume close to 30%.
az
audiozaz
rtfm!? - tl;dr
Mineral wool sandwiched between guttermesh, here's the thread with pictures:
http://www.audiokarma.org/forums/showthread.php?t=327319
Quick version:
Simulated unstuffed 56L for Qtc=0.707;
I built 28L, which came out to ~36L once stuffed, Qtc=0.79;
de-Q'ed them to 0.66 with the aperiodic vent.
A straight sealed unstuffed Qtc=0.66 would have required 78L.
I guess if the stuffing is taken into account, I probably got less than 50% Vb reduction with the aperiodic vent alone, but, IMHO, I could have pushed it a bit further still. Maybe ~50% is a high, but doable target?
az
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