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Sorry guys, but the Flared port isn't the Nirvana anymore, the Contoured port now sits on the throne as King!

There's been some activity in modern times with regards to speaker enclosure vents or ports. Engineers have been giving more attention to bass reflex ports. JBL patended the Slipstream flared port, Bose patented one too, and as i recall a Pevey Engineer did so for another one. Manufacturers increasing their thinking and time for this device.

The flared idea (ends similar to a conical horn shape) remains in the Contoured port but its maximized, as well as on other aspects, the latter is seemingly larger size. Advanced aerodynamics are part of the solution here i guess.

However, the flared port's career doesn't appear to be over yet, far from it, due to an inevitable reason. The Contoured seems to be application specific, said otherwise cabinet specific, custom made for premium high performance or high power systems. Haven't seen off the shelf contoured ports for sale, they're probably low volume dedicated production items. Maybe available from a speaker manufacturer as repair parts to replace a broken one on a system for example??

The HLA series port looks like a variation of this where Engineers were able to get almost 3 db more output with it.

Don't throw away your older ABS or cardboard tubes. The good news is that the straight flanged port isn't so bad after all, still delivering quite acceptable performance for most use, providing the vent area is large enough to avoid air turbulence (Eargle: "A simple cylindrical port, as shown at (1) in figure 4-15A, works very well when the port is large enough in diameter so that air particle velocity is small", P. 82).

I'm not a pro drafter or artist but in order to give members here an idea of what a Contoured port looks like i made a hand sketch with a pen (probably 95% accurate) based on what Eargle reports and shows in his Loudspeaker Handbook.

Note on pic the contoured is larger, the tube in the middle is curved, not straight like a regular flared one, and the rear "horn" end seems a little smaller than the front one, which would make sense based on Harman Engineers paper on maximizing vent performance.

Please don't repost my sketch on other forum(s) or site(s).

Richard

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DOUBLE WOOFER BOX VENTS

It has been mentioned many times in speaker litterature that a vent or port can be located almost anywhere on a cabinet, its not critical, as long as you don't put it too close to a speaker box panel, where undue interactions may happen. Generally such rule is pretty easy to abide by and to remember.

Nonetheless in some cases, though Engineers aren't too picky with port location, some of them appear to have kind of a preference when it comes to double woofer Sound Reinforcement cabs, as seen in the case of a few manufacturers. And i think they aren't dumb.

I know not all double woofer cabs comply with what follows, but designers also have a number of issues to deal with, such as box dimensions and baffle configuration.

Locating these port(s) somehow in between the woofers seems like the preferred way for conventional dual driver LF SR boxes. Among the few manufacturers, none of them explained that choice they made. However, i think i found the reason why its more or less in between woofers, naturally within the limits imposed by the enclosure.

In one of his many experiments Dickason (see note 1) has found that larger 4" and 6" vents perform better in close proximity to the woofers. I assume Engineers follow those findings (when possible).

The first pic, of a group to come, illustrates the above, its from British Fane Acoustics speaker building manual. On pic at the top the phrase starts with "Ports should be..."

Richard

Note 1: Vance Dickason, The Loudspeaker Design Cookbook, 5th ed, P. 55

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E-V is another well-known loudspeaker manufacturer that uses somehow in between woofers port location. Among the five E-V double woofer box plans on-hand for their DL Series woofers, 4 out of 5 use the in between woofers port configuration, a 5th one partially only due to box design (7 other plans are for single woofer boxes). The pics are from EV's Tech note Pro Sound Facts No. 7, and there's no copyright indication in sight.

On pics the cutout diameter gives away driver size from 12 to 18", and most having vents close to drivers and somehow in between them. In a couple of cases cab size constraints are obvious.

Fig. 10 cab is the black sheep here in terms of vent and center brace locations (compare with Fig. 12). In Fig 10 the center brace is not centered with the woofers but rather with the box middle, which makes sense based on the conventional bracing rule of dividing a panel haflway or in the middle of it. That layout allows limited options, but its a different box option by itself vs other models.

Fig. 8 style shown is special regarding port sizes (pair of 6" and a pair of 4"! The former in between drivers and the latter at box top and bottom, sort of "squeezed in"). I can't remember seeing another having two different port sizes on the same LF speaker baffle. I'm still debating this idea in the back of my mind with no definitive answer.

Each vent is part of the total vent area, that seems to be the thinking. Therefore equivalent to a single larger vent, simply divided in four (usually identical ones though). The obvious question coming to mind: at high drive level which of the 4" or 6" ports used will choke first, or none of them will? My initial guess is the 4" dia. might suffer from port compression, while the other side of the coin treats these multiple ports as a single larger one...

EV Engineers aren't dumb, remember that Engineers Doug Button and D.B. Keele, for example, came to JBL from EV, with Keele going back to EV later for a second term. Moreover, EV's Chief of Loudspeakers and systems Engineering, Ray Newman (RIP), was a quite smart Engineer, honored for his contribution by a bunch of his former colleagues in the pages of the JAES after he passed away.

Its interesting to see different LF cab configurations as this gives potential DIY ideas on baffle space use for example.

Richard

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With regards to JBL SR box ports its a little more difficult to identify a definite trend on port location since they use various vent shapes (round, rectangular, triangular, etc) which impacts port location for efficient use of space. The intended cab configuration has to go in hand with box requirements, such as size.

One thing is sure though, for the flagship Touring HLA Series bass cab (4897) the large vent is not only close to the drivers, but also between these, which appears to confirm the other guys (E-V, Fane Acoustics) were not wrong in their port placement on SR double woofer cab...

Pics are from the HLA Series Owner Manual. BTW the 4897 does have a Contoured port.

Richard

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COVERING OR STUFFING A BASS-REFLEX VENT, A GOOD IDEA???

I've seen this again some months ago where someone seems to have tightly packed the box port tube with yellow fiberglass wool. This somehow turns the vented box into a sealed one, though possibly imperfect since the original cab design is for vented. I can certainly understand a member wanting to enjoy, with such simple modification, some benefits of improved transient response a sealed cab provides. However there's another forgotten side to this coin. And i'm not convinced most audiophiles know what that box "conversion" means in practice.

This is not intended to criticize the persons who did it, each does his own things, that's fine its their own money after all, but do they really know the implications of stuffing a vent for example? These are relatively serious. Initially, lets put aside for now two particular cases: musicians and the case of the B6 LF alignment.

1- The appeal of a closed box remains strong for some because of its better LF transient response vs a vented box. This is well known and documented by Eargle, Dickason and others, where its shown the impulse response of a number of sealed and vented LF alignments. Moreover, flat alignments have better transient response than bumpy ones, and low Qts drivers also perform better in this regard than higher Qts drivers. Musical instrument drivers (e.g. E series) are good examples of this, take the E140 having a very low Qts 0.17, its not an accident. Musicians love good transients even if it means sacrificing VLF.

2- The case of the B6 LF alignment is where e.g. one of two ports is blocked with a plug or part of a single larger one is reduced in size with a half-circle or so plug to tune the box lower. Then apply a boost/cut filter, EV had a step-down mode kit for this. That's a very different situation than stuffing a vent, in the B6 case the vent is not totally blocked or cancelled its still a vented box with a working port, but of smaller area.

That being said, there's a trade-off involved between transient response, efficiency and distortion when going from a vented to a sealed box. At low level it may be acceptable but as the SPL goes UP and/or frequencies go down things change in the now sealed cab. The sealed box remains less efficient than a vented one, so that's a loss. Other things being equal lower efficiency systems tend to have higher distortion, another kick in the butt.

Moreover, each time you go an octave lower with a sealed cab the driver excursion is multiplied by FOUR, for flat bass output, that's not trivial. More excursion = more distortion. This is probably why people simply let the sealed woofer box roll-off naturally for the VLF side of things. Otherwise the driver may run out of excursion if the sealed bass is EQed to be flat, considering the four times excursion rule. So another loss.

In my book the only thing the sealed cab has in its favor is better transient response, and there's penalties to get this. Cab construction may be a little easier with no vent, but never had real problems making/installing a port.

In the 1979 JBL speaker building kit i have (explanations, plans, etc) they indicate having tested just about every type of cab (vented, sealed, horn, transmission line, etc) and the best compromise in their view was the vented box. And this is still what they mostly sell decades later. e.g. JBL bass horns have been out for a while...

In his Loudspeaker Handbook (2nd ed, P.66) Eargle shows a little experiment he did with the JBL 2240 18" woofer, and modeled it in a 3 cu.ft. sealed box! That type of woofer would typically be modeled in say a 8 cu.ft. box or so. Therefore Eargle's cab isn't a tough challenge nor a demanding VLF situation for that woofer. In fact he indicates the bass roll-off is on at 100 hz, and woofer excursion increasing markedly below 200 hz. On his graph its a little worst, driver excursion starts climbing from 300 hz and continues rapidly. Must be an effect of the more gentle bass roll-off.

Nonetheless, its stunning. Again the driver is an 18" and not even expected to reproduce flat 30hz, 50hz or 80hz due to LF rolloff. Little deep bass but sizeable excursion! Anyone could have bet money this should be a piece of cake for the 2240. Well, being a sealed box with no vent the cone travel increases steady, probably driver distortion too.

At this size woofer and say 100 to 300 hz signal (add some LF "leftovers" from driver rolloff) the excursion increase should be slower, even at high power, but it remains quite notable. The woofer wasn't overloaded though near the limit. This little experiment says a lot regarding one of the sealed box weaknesses, and i think that's what Eargle was trying to illustrate with the simulation.

A group of pics is on the way. Please don't repost pic(s) on other web site(s) or forum(s).

Richard

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One of John Hoge's landmark papers. Here distortion and efficiency.

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The FOUR times the excursion in a sealed box explained. First from EV Pro Sound facts document, followed by JBL Eargle's Loudspeaker Handbook details.

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Another group of pics explaining all this will be posted tomorrow, gotta go now.

Richard

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From Alden (mathematician)/D'Appolito in Advanced Speaker Systems

An example of the Musicians' exception with regards to a sealed box having a pretty low Qtc for utmost transients

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From Eargle's Loudspeaker Handbook

His description of his little experiment with a 2240 in a 3 cu.ft. sealed box

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As reported by Dickason in The Loudspeaker Design Cookbook, Thiele's take on vent stuffing etc., nicely and swiftly discarded idea

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The partial word at top right of pic is: resistive

Yeah, why bother? lol

Amen.