The problem with vented systems is that they require a larger cabinet than most designers are willing to give them. Run simulations and you will see that, once a woofer is chosen, the ideal vented box is roughly twice the volume of the ideal sealed box.
For commercially available systems that usually means that the vented alignments are typically undersized and boomy as a result. Years ago when I was at JBL they were selling a lot of products with the cabinet about right for sealed box, yet the systems were vented. The alignments were always on the boomy side (L150a excepted, which had wonderfully extended response).
If you have the right cabinet volume then you shouldn't need port loss, such as from a high aspect ratio slot vent. I would be concerned that such designs (including the Onken types) would be more prone to turbulence and noise. Resistive ports (including most "transmission line" types) are a very mixed bag, quickly losing the power handling and extension benefits that a well designed vented system should always have.
Regards,
David
For commercially available systems that usually means that the vented alignments are typically undersized and boomy as a result. Years ago when I was at JBL they were selling a lot of products with the cabinet about right for sealed box, yet the systems were vented. The alignments were always on the boomy side (L150a excepted, which had wonderfully extended response).
If you have the right cabinet volume then you shouldn't need port loss, such as from a high aspect ratio slot vent. I would be concerned that such designs (including the Onken types) would be more prone to turbulence and noise. Resistive ports (including most "transmission line" types) are a very mixed bag, quickly losing the power handling and extension benefits that a well designed vented system should always have.
Regards,
David
Of all the reflex style enclosures (not including TL's and the like) I have heard, the Onken is the best at doing what a port is supposed to. I hear them described as resistive ports but with as many as there are, perhaps that aids in the lack of port chuffing. In these here, I see 8 ports that if combined seems like a large area.
http://www.diyaudio.com/forums/full-range/65061-full-range-speaker-photo-gallery-73.html#post3900043
http://www.diyaudio.com/forums/full-range/65061-full-range-speaker-photo-gallery-73.html#post3900043
If your area is great enough then port turbulence may not be a problem. If you like the performance of an Onken (or aperiodic, or whatever cabinet type), then great. Still, every port configuration defines a ratio of cross sectional area to surface area. If your aspect ratio is extreme (as it is with all slot shaped ports) then there is a lot of wall surface for the effective area. If your goal is high port Q (low losses) then that is the wrong way to go.
If you need high port losses, then you should question your basic design. Certainly the benefits of a ported box, reduced excursion from the vent resonance and good response extension will be watered down by lower port Q. The physics is very clear on this.
I would also worry about the linearity of port resistance. I'm not sure that port losses are constant for all levels if friction is involved. And clearly air turbulence is much more likely when narrow ports are used.
Regards
David S.
If you need high port losses, then you should question your basic design. Certainly the benefits of a ported box, reduced excursion from the vent resonance and good response extension will be watered down by lower port Q. The physics is very clear on this.
I would also worry about the linearity of port resistance. I'm not sure that port losses are constant for all levels if friction is involved. And clearly air turbulence is much more likely when narrow ports are used.
Regards
David S.
A good question. Of course a vented box doesn't make a woofer any more efficient but if you juggle all of its parameters then you can have the same response extension with more efficiency in a given size box. Once vented design became straightforward (computers and well worked out TS theory) most manufacturers jumped on the vented box band wagon. Its hard to give up a relatively free increase in power handling (or extension or box size) that venting offers.
Class D is certainly more efficient but that still doesn't mean that Watts are free, so vented boxes are going to remain popular. The DIYer doesn't have to compete in the marketplace so choosing sealed box for its simplicity or slower roll off is a valid personal choice.
David
There are a couple of other aspects of vented designs that I would like to bring up. Vented systems IMHO are poorly understood and much maligned.
The first point is how the change of the driver parameters with excursion influence the response. In general when you start to use a woofer at higher excursions the following things tend to happen:
In case it is not obvious to the reader, if you design a "maximally flat" vented system, the response will develop a "hump" at somewhere around 2Fs or so. This is in the vicinity of 100Hz or slightly higher, and when this part of the frequency response is elevated and rings, you get the well known "boomy" or "flabby" sound.
It's my opinion that this fact is too often overlooked, but it anecdotally explains why the "EBS" type vented alignments were popular and more accepted than others. By designing in a "droop" in the small signal response of a vented system, you are anticipating the large signal behavior such that it will "fill in" the dip.
This does not seem to be such a problem with sealed systems.
Another problem is the use of too small of a vent. This leads to noise and additional dropping due to loss of efficiency of the vent at higher SPLs. Larger diameter vents must be longer for the same port resonance frequency, and once the D/L ratio is more than about 3-4 you run the risk of port self-resonance becoming a problem. As Dave pointed mentioned, this is especially a problem with smaller boxes, and this supports the idea that larger (in terms of Vas) boxes make better vented systems.
Also, the issue of unloading of the cone below the tuning frequency is often simply ignored. This can cause the cone to have lots of excursion (all for nothing), and this influences the response above Fs where the system can actually produce SPL because the driver is now operating far away from its linear region.
There are a number of design issues that need to be addressed in order to create a good sounding vented system. Unfortunately, it is all too easy to write off all vented systems as "bad sounding" based on the many poorly designed ones out there, and this is what I observe for instance in the opinions presented in the initial post of this thread.
The first point is how the change of the driver parameters with excursion influence the response. In general when you start to use a woofer at higher excursions the following things tend to happen:
- Qts increases
- Fs increases
In case it is not obvious to the reader, if you design a "maximally flat" vented system, the response will develop a "hump" at somewhere around 2Fs or so. This is in the vicinity of 100Hz or slightly higher, and when this part of the frequency response is elevated and rings, you get the well known "boomy" or "flabby" sound.
It's my opinion that this fact is too often overlooked, but it anecdotally explains why the "EBS" type vented alignments were popular and more accepted than others. By designing in a "droop" in the small signal response of a vented system, you are anticipating the large signal behavior such that it will "fill in" the dip.
This does not seem to be such a problem with sealed systems.
Another problem is the use of too small of a vent. This leads to noise and additional dropping due to loss of efficiency of the vent at higher SPLs. Larger diameter vents must be longer for the same port resonance frequency, and once the D/L ratio is more than about 3-4 you run the risk of port self-resonance becoming a problem. As Dave pointed mentioned, this is especially a problem with smaller boxes, and this supports the idea that larger (in terms of Vas) boxes make better vented systems.
Also, the issue of unloading of the cone below the tuning frequency is often simply ignored. This can cause the cone to have lots of excursion (all for nothing), and this influences the response above Fs where the system can actually produce SPL because the driver is now operating far away from its linear region.
There are a number of design issues that need to be addressed in order to create a good sounding vented system. Unfortunately, it is all too easy to write off all vented systems as "bad sounding" based on the many poorly designed ones out there, and this is what I observe for instance in the opinions presented in the initial post of this thread.
Yes, the total vent area/volume will equal that of a larger single, but I think what Dave is saying is that the high aspect ratio (width/height) of each slot adds resistance that can approach aperiodic characteristics. Is that a bad thing?
Several years back we conducted several "subjective tests" of both construction materials, and port shapes. For the latter we made two builds of same enclosure volume & vent tuning - one with conventional round port, the other with narrow slot vent. Even with drivers as small as the Fostex FR in our builds ( FE127E and FE167E, IIRC), the slot vents exhibited more articulated mid bass and smoother extension to the system's lower limit - in the vernacular, I found them "cleaner".
Not that i know of. I aim for a specific curve shape in the sims that is then changed by the high R vents.
dave
May not work in an "official" Onken alignment, but with a quick look, using the WT2 numbers, looks well suited to our miniOnken alignment. F10 in the low 40s. 13 Litre. emailmeif you want more detail.
dave
Our miniOnken alignment usually ends up with a volume about the same or a bit smaller than a sealed bessel alignment, the extra R makes them more tolerent of dynamic changes in T/S, and the vent are is usually much larger than the minimum needed to avoid chuffing.
The criteria used has yielded boxes for dozens of different drivers where the bass is very finessed (as opposed to maximum extension), quality over quantitiy.
dave
Onkens usually aren't resistive vents; they're descended from the original BR alignments via the Jensen Ultraflex, & a major goal is often to use the massive vent system to provide maximum efficiency at Fb. Whether you actually want maximum efficiency at Fb is another matter entirely of course.
Trouble is, you can sometimes end up with fairly long vents with their own harmonic modes / structures. Same as anything else, depends on how well the individual design has been realised. The kind of designs Dave Dlugos for e.g. uses look similar to Onkens / Ultraflex / whatever but are functionally almost the opposite & approaching damped-vent alignments, albeit using the friction of very high vent aspect ratios rather than stuffing.
This is a timely thread. I'm looking at designing a pair of subs using some 8" Vifa drivers originally used in Mackie monitors that made an appearance on the surplus market a few years ago. I was debating using a pair of 10" instead (some Goldwood polypropylenes), but the increase in bass extension was minimal, and I aleady have the Vifas. The Vifa woofers have a 0.27 Qts. With a pair of drivers per side, The response looks decent, but I need three ports. I was thinking of using a slot port instead. ?? I'll post a screen shot of the design tonight if some wants to look and comment.
I was thinking of active crossover with a HPF to limit ELF excursion. They would cross over into some sealed speakers.
I was thinking of active crossover with a HPF to limit ELF excursion. They would cross over into some sealed speakers.
Hi,
http://www.scan-speak.dk/datasheet/pdf/18w-8542-00.pdf
A good tuning is 15L tuned to 37Hz, rather
than the classic tuning of 10L tuned to 50Hz.
(Sealed is about 7L - good for AV and a sub.)
With such a low Qts driver there is no mileage
in resistive or semi-resistive ports in any sense.
The modified tuning has the same -3dB point
as the classic tuning (60Hz) but less bass
above that point and more below, - 6dB
being 42Hz versus 51Hz.
Free space placement is best to properly
utilise room gain. Done right the vented
version will not sound remotely one note,
and do bass very well, sealed for this driver
simply doesn't, Qts is far too low for that.
rgds, sreten.
Numbers are approximate as here the series
resistance of the needed bassmid inductor
will lift the effective low Qts of the driver.
http://www.scan-speak.dk/datasheet/pdf/18w-8542-00.pdf
A good tuning is 15L tuned to 37Hz, rather
than the classic tuning of 10L tuned to 50Hz.
(Sealed is about 7L - good for AV and a sub.)
With such a low Qts driver there is no mileage
in resistive or semi-resistive ports in any sense.
The modified tuning has the same -3dB point
as the classic tuning (60Hz) but less bass
above that point and more below, - 6dB
being 42Hz versus 51Hz.
Free space placement is best to properly
utilise room gain. Done right the vented
version will not sound remotely one note,
and do bass very well, sealed for this driver
simply doesn't, Qts is far too low for that.
rgds, sreten.
Numbers are approximate as here the series
resistance of the needed bassmid inductor
will lift the effective low Qts of the driver.
Last edited:
Note that your Vas is exactly as I predicted, and the box response in the one I designed will be the same as factory specs. Compare in whatever program you desire with factory specs vs your measured. Often the difference from a strictly Cms change such as this one is minimal, but it depends on the details
BTW, Charlie says Fs increases with excursion, when the opposite is true, and Q usually decreases somewhat. Q can increase but typically that is due to voice coil heating, not excursion. see here in the notes on large signal behavior: Thiele/Small - Wikipedia, the free encyclopedia
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