Aperiodic enclosure for TAD 1603

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Greets!

What can I say? GA has literally ‘written the book’ on various types of resistively venting a cab and his TL simulating software mirrors MJK’s proven version that was derived a bit differently, so if anybody should know how to accurately design a vent system it would be him, so all I can say is that there’s apparently some typo or misunderstanding on your part because well proven math shows that a single large vent with a pipe area [Av] equal to six of these vents will be much longer than ~5.31” and since this is the most acoustically efficient vent, then the more it’s divided up into smaller Av pipes, the longer they each must be due to the increasing friction losses of all this extra pipe wall area with increasing number of pipes.

All this assumes of course that they will be in close enough proximity to sum as one [<1/4 WL away of the vent’s HF corner]. If the cab was large enough to allow each vent to work individually, then theoretically they could be short, but compression would be so high that it would be better to build it as a sealed alignment.

FWIW, for 10.73 ft^3/27.86 Hz/4.04882” dia., I calc Lv = ~0.67”, so not even a typical baffle thickness long.

Bottom line, AFAIK, tuning of this type alignment will need to be found empirically, so at least one prototype will be required, though not sure which way to make it adjustable will be the easiest to make.

GM
 
At the request of WRLCO, my only comment is that resistive-vent enclosures have to built and measured before you can determine the box frequency. The box frequency is the minimum impedance between the two impedance peaks, or more accurately, the frequency where the phase angle between voltage and current is zero degrees (if using an XY scope, the trace is a straight line at 45 degrees with no circularity).

Another cross-check is to use a probe microphone and examine the frequency response of the vent output, which should be much broader than the single narrow peak of a conventional vented box. Once you determine the effective box frequency (the volume is already known), you can see how close the enclosure comes to the desired alignment. (The most accurate you can measure the box frequency is about 1~3%, so there's no reason to calculate T/S parameters to more than 3 decimal points.)

Filling the vent with damping material changes the vent tuning, as well as adding resistance (which is the goal of a resistive-vent enclosure). Since damping materials such as wool felt, long-fiber wool, or Bonded Logic UltraTouch are not well-characterized by closed-form equations, you have to measure and see what it's really doing.

I agree with GM that damping the vent drops the velocity substantially ... in the limit case, the filling absorbs all the sound, and nothing would come out of the vent at all. In the real world, no such materials exist, particularly in the 30~50 Hz frequency range we're interested in. Real-world acoustical absorbers, such as the materials mentioned above, are somewhat nonlinear with level and can even move around if the sound levels are high enough (which is why I prefer UltraTouch as filling).

One simple method for an adjustable vent is an enclosure that floats above a solid plywood base-plate, with the height above the base-plate adjusted by adjustable bolts, and a circular vent that opens on to the center of the base-plate. Adjust the height of the bolts (say from 1/4" to 2" high), and you can adjust the vent tuning. The space between the base-plate and the bottom of the enclosure can be filled with wool felt or UltraTouch, so the damping is adjustable as well.

I should also mention that vents that open directly on to the floor behave differently than vents that are 4 inches or more above the floor, possibly because of more efficient coupling to the floor image. I don't really know why this happens, but floor vents appear to produce 2~3 dB more deep bass than vents placed at the mid-level of the cabinet.

P.S. The base-plate is there for a reason; if the vent fires directly into the carpet (downward), results are not generally good (in my experience). By forcing the vent sound into a 90-degree turn (from the circular opening in the cabinet into the space between the bottom of the cabinet and the base-plate), there's also a moderate low-pass filter that decreases colorations from inside the cabinet exiting through the vent.
 
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Thanks, Lynn, for your comments. It is a variation of the center vent feeding to a space under the enclosure that I want to develop using multiple or a single vent, whichever gives the most musically authentic sound.
Hi GM, I finally found the Augspurger article from the 02/1991 Speaker Builder that if have been referring to in my posts. I will get it scanned and post it soon. In the meantime, I will add a few quotes here for your reading.
"In practice, separated multiple vents do not behave as if they were isolated from one another, nor do they behave as a single vent of equivalent area. There is mutual coupling between vents, but it is not easy to predict."
"Fortunately, Fb does not wander too far whether the vents are separated by one diameter or many. This allows you to derive an end correction multiplier based solely on the number of vents. My experiments suggest that you should first calculate the appropriate end correction for a single vent, then multiply it by the fourth root of the number of vents."
"A good approximation for ducts of any length including 0 is 1.0times the diameter. In other words, the effective length of a cylindrical duct in a typical loudspeaker enclosure is its true length plus its diameter. The reason this end correction differs from theoretical values is that it compensales for a number of effects, including internal padding and the volume displaced by ducts, bracing and loudspeaker."
"My empirical formula for rectangular vents says that the required end correction is equal to 1.13 times the smaller dimension (height or width)multiplied by the fourth root of the aspect ratio [larger number divided by smaller number]. This seems to be accurate within 5% for aspect ratios between one and ten."
"If you assume that padding thinkness should be 5 - 10% of the longest dimension of the box, it usually turns out that about a third of the box's volume is filled with padding."
"Assuming a typical vented loudspeaker enclosure with about 30% of its internal volume occupied by porous absorptive padding, I modified the Helmholtz formula as follows: Fb=Ku*SQRT (A/V*L); where Fb is vent resonance in hertz, A is the vent area[ in sq. in.], V is the true box volume [ in cu. ins.] divided by the number of vents, and L is the true duct length plus end correction [in inches]. Ku is a lumped conversion factor [in this case it is 2131.00]."
Near the end of the article, Mr. Augspurger offers the following suggestions:
1. Use ducted ports rather than simple vent openings. A ducted port located well away from boundaries or padding is predictable, stable and efficient.
2. Do not subtract driver, vent(s), or bracing displacement from the box volume.
3. Beware of air leaks. In a medium to large enclosure small air leaks show up as resistive losses in the system. In a small box one or two air leaks may effectively add to the port area.
4. If Fb is critical, allow for adjustment. If your design uses multiple ducts, then Fb can be adjusted by making just one of the slightly longer or shorter.
5. ...you might wish to ...stagger-tune multiple ducts to suppress the organ pipe effects.
After re-reading this article GM, and Lynn's comments above, I think I now understand better the use of one larger vent.
I want to get started building, but because I am committed to using a special custom-made, high pressure laminated Baltic plywood made from two 1/2" sheets of 5'x5', I cannot afford to make errors; one laminated sheet costs several hundred dollars. One last round of calculations this week and I will submit the results for your comments. My thanks for all you have done so far.
Best regards,
Bill
 
Hi GM
Sorry to have been out of touch for so long. My wife has been fighting cancer for the past four years and took a turn for the worse in late 2013. All my time has been taken up with her care and have had to set the hobby aside for a time. She has come thru two more rounds of chemo and is currently much improved so I can start to pick up the project again. Am currently trying to catch up on recent trends and then will decide on next steps.

Thanks so much for your contributions and continuing interest in this long postponed project. I'll be back in touch soon to let you know of progress.
Best regards,
Bill Lyon
 
Greets!

Bummer! Sorry to hear this, though glad she's had the perseverance/force of will to ' beat' it for now :up:. Having been down this 'road' with a long time GF and later a third wife, both long gone now from the big 'C', I know how tough it can be for the both of you and considering your age, glad you've been up for the task and still able to consider building some big speaker cabs! For sure, you're doing better than me.

GM
 
Hi GM,
I took my wife to Denmark for her mothers 80th birthday and since gotten back I've been playing catch-up on diyaudio for a few weeks. I saw an item from you regarding an appropriate size enclosure for a Altec 416 suggesting 13 cu. ft. enclosure with a 6" dia. vent, around 4" long. Is that tuned to Fs or to around 38 Hz?

Thanks for your kind words in your previous post. Wife is great and I'm pleased to be back in the game.
Best regards to you.
 
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