[Folsom]

Just curious if anyone has tried it. These are the things.

[nik.d]

At Dynaudio same thing (could say much better) is used for decades already. Dynaudio call it 'Variovent'. It is built in such way that you can adjust stiffness of damping material (several steps) thus adjusting air flow capacity. Clever product used in highest range of Dynaudio loudspeakers. Example would be Dynaudio Consequence Ultimate Edition. 

[dB Cooper]

Dynaco was doing this in 1968. Not sure why it didn't catch on more widely. It makes for a flatter impedance curve which presents the amplifier with a much easier load, and tighter bass due to better electrical and mechanical damping.

[Folsom]

The bass is different. It isn't necessarily as impactful with a lot of music.

[LesterSleepsIn]

I thought the aperiodic vents were to be used in non-ported (unported?) sealed enclosures? Weren’t the Dynacos ported?

[dB Cooper]

Most were, including the very popular A25 and less expensive models, but 'upscale' models like the A35 and A50 used two-chamber sealed cabinets with the damping created by a port between the two chambers to control the woofer at resonance and behave as a sealed enclosure below that, as explained on Greg Dunn's Dynaco page:

http://home.indy.net/~gregdunn/dynaco/components/speakers/

Later on, David Hafler produced speakers with frequency-dependent loading under the Hafler brand, North Creek made aperiodic-tuned speakers, and Morrison audio omnis have a multi-chamber cabinet designed with the same goals: Tight driver control and flat electrical impedance.

I know of no inherent reason why an aperiodic design can't produce impactful bass unless one is addicted to high-Q bass at resonance. The A50, with two 10" woofers, could produce plenty of 'slam' when called for. I suspect that with modern CAD tools for driver and enclosure design and testing/measuring, this approach could yield even better results than the classic models of the past, even though the higher power amps of today aren't challenged as much by high impedance spikes as the amps of the Dynaco era (Dynaco's best-selling amp when the A25 came out was the 35WPC tube Stereo 70).

As far as the Scanspeak items depicted, I don't know how well they'd work with a one-size-fits-all approach. With the A25, Dynaco tuned each individual system by feeding a low frequency square wave into the speaker and watching the driver behavior on an oscilloscope while adjusting the vent resistance by adding/removing stuffing. (And worth mentioning that they did it for eighty 1968 dollars MSRP; my, how the business has changed....)

[Folsom]

You can actually port and use an aperiodic vent at the same time. But generally it seems most speaker makers do one or the other, or only the bass driver is ported if the mid is aperiodic vented.

If you do both, you will get reduced output compared to regular porting. Porting works because a resonance is created between the driver and the port. The aperiodic vent basically bleeds off the resonance so that it won't be as strong.

[poseidonsvoice]

Thanks dbCooper.

This is an excellent read by George Short of the now retired North Creek Music Systems of his development of the North Creek Catamount MTM loudspeaker which used an aperiodic vent and was designed to be “tube SET friendly.” Of note, the use of the aperiodic vent dropped the impedance peak at system resonance by 50% which is significant:

http://www.northcreekmusic.com/Catamount/CatamountInfo.htm

Furthermore, George wrote his Master’s thesis on the Aperiodic vent while at Virginia Tech in 1988.

The enclosure design is here:

http://www.northcreekmusic.com/Catamount/CatamountInfo.htm

Input impedance measurements demonstrating the differences are included in the 1st link along with loads more info.

George also used the concept in a more budget system named the CM7 but unfortunately that link is nonexistent.

George now works for Wisdom Audio.

Best,
Anand.

[roscoe65]

To expand on DBCooper's excellent post, I have owned a pair of Dynaco A25XL's which used an aperiodic vent.  They sounded excellent.

Two other factory instances I know of resistive vents being used are Goodmans and Proac.  Goodmans offered an "ARU" (Acoustic Resistance Unit), a 10" square felt covered screen that was used for its 12" and 15" high efficiency drivers.  The vents were used similarly to cabinet stuffing to reduce the required cabinet volume.  Goodmans factory designs specified a vented enclosures of 12,500 cu in (!) for the 12" drivers or 7,800 cu in with the ARU installed.  Proac took a different appoach with the Response 2, providing a conventional tubular vent but filling it with what looked like plastic drinking straws.  This, longer resistive vent also provided aperiodic loading.

[dB Cooper]

Porting works because a resonance is created between the driver and the port. The aperiodic vent basically bleeds off the resonance so that it won't be as strong.

Exactly.They are each ways of approaching resonance. The ported or 'bass reflex' (technically, 'phase inversion') enclosure works as a resonator, like blowing air into the mouth of an empty bottle. This increases bass output but also raises 'Q' and produces the sharp impedance spike often seen in designs of this type (8:1 is not unheard of). The aperiodic approach is about taming the system 'Q' and making its behavior more linear (the A25 had only about a 2x bump at resonance and the Morrison speakers claim to hold impedance to within a 1.5:1 ratio).

But to repeat myself, just randomly plopping those scanspeak vents into some cabinet may not necessarily produce an 'aperiodic' result. That requires careful design and tuning.

[Folsom]

All of the GR box sizes for bookshelves are approximately the correct size for the aperiodic vent sold by Madisound, using just 1 per box.

There's a chance you might have to play with stuffing. Just at a quick look it seems like it is pretty simple conversion. The box sizes are small enough that bass doesn't seem to change much in output.