Aperiodic enclosures

[Geoff H]

Hi GM. I was under the impression "critically damped" referred to placing a tightly stretched piece(s) of material over the port of a full sized BR (ie not ducted) in order to dampen residual oscillations or overshoot.

Does that term now cover damping in lieu of bv, etc.

Geoff.

 

[GM]

Greets!

It's a way to achieve it and works on ducted ports also. Another way is to do it globally by not damping the box, instead covering the back of the driver with a damping blanket. Works great on OBs.

'bv'??

GM

 

[Geoff H]

Hi GM,

bv? Had me a bit. Re-reading the post, I must have meant box volume.

Will probably try the blanket on the weekend. OBs underway, just for fun.

Cheers,
Geoff

 

[wednesdaysouth]

Hi Geoff,

Its been some time since you posted on this. I was just wondering about the outcome of your experiments as I have come by a pair of Super 10 RS/DD, and as a callow beginner in all of this am hoping to gain by the experience of others. It sounds like you were quite happy with the 27 liter aperiodic cabinet you made, or did you prefer the larger one? Did you make any further refinements?

PS. I don't suppose you (or anyone else) has T/S parameters for these drivers? I was also wondering about building some form of TL variant for them. Would anyone care to comment on such an ambition?

Regards,
Jem

 

[Geoff H]

Hi Jem, I haven't done much more other than bracing the baffle between the driver and the port area. These enclosures are a space saving compromise. The best I've heard the Super 10s in wide range mode is in the large rear vented enclosure as described above driven by a Pioneer SMB161 - PP 6BM8s ~ about 10w per channel.

I still have them hooked up as the "B" speakers, however prefer my OBs.

Note that the drivers I used are the 1966 version with the ceramic magnet and not the earlier AlNiCo, and shorter 8 ohm voice coils. I would suggest treating the roll surround with bituminous "paint" and avoid MDF.

You might want to follow this:

http://www.diyaudio.com/forums/showthread/t-102144.html

Regards,
Geoff

 

[wednesdaysouth]

Hi Geoff,

Thanks for your thoughts. So many options. I might have a go at a larger aperiodic enclosure then, although I don't think I can manage the dimensions of the rear vented box you describe.

I also have the ceramic magnet versions, and will get onto those surrounds with something sticky.

Thanks again,
Jem

 

[freddi]

Hi SoundRight - I like the idea of the ~75L airspace 18" Kilomax sub (convertible to cardioid back to back operation) - are drain pipe 10cm diameter cross-hatch grates suitable for the vents? How does one choose the vents vs speaker displacement?

Is there a straight forwards recipe for the two chamber aperiodic enclosure? (only seen that in the little Dynaco SEAS boxes of the 1970s)

 

[SoundRight]

Hello Freddie...here are some rules of thumb. Drivers with a qts between .4 and .6 (high) work well. Cabinet volumes are around 1/3 of VAS for single chamber designs. Dual chamber is trickier and requires a large cabinet.

The first chamber volume should be around 1/3 of the VAS figure. This is vented into a chamber between 3/4 and 1/1 of the VAS. More extended than the single chamber and rolls at 12dB/octave, but a big cabinet. Better power handling of course.

As for number of vents, up to around 40l, one vent, around 70l, 2 ( ish!). For dual chamber this figure applies to the smaller chamber. Final tweaking can still be done with damping material in the first chamber. Don't block the vents though! Usually no damping material is required in the second chamber of a dual chamber design.

You could fabricate them from rain pipe section. To copy the scan speak one you would need a fibreglass wadding disc fitted tight to the edge and preferably compressed in someway. I believe Madison Sound still sell the scan speak ones?

For final tuning you could measure the basic parameters of the design with the DATs software and tweak accordingly, usually aiming for a qts around .7?

Hope this helps....

 

[scottjoplin]

Hi, do you find this type of enclosure relieves the pressure effect of a sealed box in room? One of the reasons I prefer dipoles is I find pressure uncomfortable on my ears

 

[freddi]

Hi SoundRight - that's very helpful - thank you for such detail and your time.

 

[twinter]

Post #7 in the thread below discusses Multichamber Aperiodic Progressive Damping (MAPD), by George Short III of North Creek Audio, which is no longer in business.

Dual aperiodic box idea

 

[Ron E]

The first chamber volume should be around 1/3 of the VAS figure. This is vented into a chamber between 3/4 and 1/1 of the VAS. More extended than the single chamber and rolls at 12dB/octave, but a big cabinet. Better power handling of course.

If you have so much to teach, let's hear your theory about how this dual chamber works, and why it has "better power handling". Also what's so special about 1/3 of Vas, other than it leads to Fc=2x Fs and Qtc=2x Qts for the system without the variovents? Why wouldn't Vas/Vb vary with Qts? What are the parameters that vary the size of the second chamber?

 

[keilau]

Hi,

On this topic, does anyone have any good resources for Aperiodic enclosures?
<snip>

The aperiodic bass loading is a compromised design to get non-resonant bass at moderate enclosure size.

The majority of today's loudspeaker are in a bass reflex enclosure which can go fairly deep with a smallish box and has good efficiency. But the twin impedance peaks in such an enclosure is impossible to completely damped out. It impairs the transient response of the bass and give it a "ringing" type character. The bass sounds "lossy" or "boomy" to some listeners, but good, deep bass to others.

The aperiodic enclosure gives the kind of tight (rapid transient and decay) bass some listeners consider "more accurate" in a moderate sized box.

It is a personal choice that which type of bass one prefers. But they are very different to listen to.

The design of aperiodic enclosure was first published by Ted (E. J.) Jordan in the Feburary 1956 issue of Wireless World, pp 75-79. Interestingly, Ted Jordan called it a "absorbing Labyrinth" enclosure and named it "Axiom". I am not sure when the term aperiodic came into being. It could mean "non-resonant" or not tuned to a single frequency. And this type of enclosure was made famous to the US audiophile by the Dynaco A25 speaker. The attached image is the last page from Jordan's Wireless article. The picture showed the different impedance curves and how his Axiom (aperiodic) design differs from others. In the last 2 paragraphs, Jordan described the benefits of his aperiodic design. This work was done around the time when Edgar Villchur invented the air suspension enclosure. The acoustic suspension design seemed to fall out of favor for deep bass design because the woofer is expensive to make. The closed box design is still the favorite in bookshelf enclosure for many loudspeaker designers due to the bass transient performance mentioned above.

The Dynaco aperiodic design uses fiberglass stuffing but many others use wool or foam in aperiodic designs. The Madisound Dynaudio kits used virgin wool in their aperiodic designs. I found the long fiber wool used by A. R. Bailey in his transmission line enclosure gave the best result stuffing the aperiodic enclosure. Fiber Glass comes in as second. The Polyfill batting is pretty useless.

 

[planet10]

The Dynaco aperiodic design uses fiberglass stuffing

The dozen or more i have parted out all used rock-wool damping.

dave

 

[Waboo]

SoundRight,
My current music listening room has a slim tower 3-way I built in 2002 or so, consisting of a SEAS Millennium tweeter, ScanSpeak 18W/8545K and a SS 25W/8565(side mount). The 8565 is aperiodically loaded with 2 ScanVents in around 60-70 liters(can't remember exactly). The bass is very nice, tight and accurate sounding, and still impresses me daily. It just sounds effortless. I'd do it all over again.

 

[Octavia]

Personally I think it's great to see some new life in this rather old thread. I don't claim to know much about aperiodic enclosures but according to scan speak their various vents increase the apparent enclosure size and lower the impedance peak. Besides that I would assume that a rear mounted vent would also eliminate or at least minimize reflected rear wave energy coming back through the cone. Seems like a good thing to me, sort of like attenuating the back wave on a dipole.

 

[Octavia]

One question about the Vas being 1/3. Just thinking out loud here but wouldn't a butterworth box Q of .707 still apply whether sealed or aperiodic in order to give flattest response with good transients? From what I have read the Scan Speak vent increases the apparent box size by 10-20%. Which for example for the Alpair 7.3 would mean that with a sealed box for a Q or .707 which requires a box of 6.385 liters (thank you Dave for your chart on that) to keep the same Q with vent would mean a box of between 5.108 and 5.745 liters both of which are larger than the driver Vas of 4.58 liters. Any comments would be most welcome

 

[Cal Weldon]

Need I remind the members of the rules around here?
Personal posts and collateral removed.

 

[Octavia]

Has anyone listened to the same driver in sealed vs aperiodic vs bass reflex?

 

[Sonce]

For one particular midwoofer with moderately high Qts, put in a bass reflex box, the bass was meaty and deep but overinflated and "slow". Staffing the box and the tube (aperiodic) brings tight and precise bass but not so deep. With complete sealed tube (closed box) it was somewhere in between.
On the end, everything depends on the values of Fs, Qts and Vas and the volume of the box Vb. Aperiodic is not a panacea. You can get excellent results with all three concepts, but only with three different woofers (different TS parameters). Or, for a particular woofer, only one of the three concepts is the best overall within the chosen compromise.