DEEPER BASS: D. B. KEELE/E-V's STEP-DOWN MODE

[RMC]

B2) KEELE

The "vent modification assembly" Keele refers to in his paper (P. 356) isn't really a "modification", instead it's a simple vent blocking device, as executed by E-V in the TL series and other boxes. More on this further.

The trick of making two vents and blocking one for step-down, mentioned in a post here, has been around for at least 25 years and the fatherhood of that idea doesn't appear to be Keele's, plus its not in his sixth-order alignment paper. For example, In previous posts in this Thread links were made to E-V's 1993 TL 3512 low frequency enclosure data sheet. On page one, Description section of that data sheet, E-V indicates: "The low-frequency limit of 38hz (3db down) may be extended to 28hz BY COVERING ONE VENT and applying appropriate low-frequency equalization (see Step-Down section)." That system is built with two vents, and a supplied port cover for one as indicated in the spec sheet's step-down section. Because Keele wasn't working for E-V In 1993, being elsewhere between 1976 and 1996 as per his own career path on the home page of his Web site, that idea of having two vents but covering one for step-down mode, seems to originate more from E-V. Ray Newman (RIP), then Chief Loudspeaker Engineer at E-V could be the father of that idea, when reading the acknowledgment at the end of Keele's paper.

More important though, blocking one port MAY not be the "Nirvana" that some think it is, at least at first sight. Initially, for me this raises a big red flag, that members, Keele nor E-V never mentioned in their writings, as far as I know: the possibility of a significant increase in vent air speed or velocity (Mach), with a reduced vent surface area by 50%!, that may well be accompanied by port noise. A non-issue? Why then? Or is it simply ignored/skipped by Keele and E-V? I haven't seen yet any mention or explanation about this.

In the References section of his Sixth-order Vented-box article, Keele did mention as Ref. # 2 and 10, R.H. Small's Vented-Box Loudspeaker Systems, Parts 1 (small-signal analysis) & 3 (synthesis). Keele didn't mention Part 2 (Large-Signal Analysis, vents are part of this one), which is surprising since in Part 3 Small says "The vent design is carried out in accordance with Section 8 of Part 2."(P.334) That section 8 of Part 2 titled "Vent Requirements" (starting P. 329) could be one of the easiest to understand among all Small's stuff. Published 1973, 1-2 years before Keele's paper it's impossible in my view he didn't see it, same applies to E-V I think (i.e. if one reads parts 1 and 3 why not part 2?). With a simple look at Small's Part 2 article summary, right on the first page, one can read:

"The vent area must be made large enough to prevent noise generation or excessive losses; the required area is shown to be quantitatively related to enclosure tuning and to driver displacement volume." (P. 326)

[BTW much later, A. Salvatti, A. Devantier and D.J. Button of Harman have also insisted in the conclusions of their extensive vent study on the importance of largest vent area: "Vast historical data and the results presented in this paper suggest that the largest port area allowable by a given design should be employed to keep the air velocity down if low port compression and low distortion are desired." (Maximizing Performance from Loudspeaker Ports, JAES, Jan./Feb. 2002, P. 43)]. This shows the requirement has not changed and that it's not a trivial detail.

Considering woofer air displacement volume increases with power input, at low level power this may not be a problem, but as the input level increases, so does cone travel and air moving. Then it could become an issue, particularly since we are in deeper bass territory here...

A possible way out of this vent noise I can think of, assuming its a non-issue, is the somewhat reduced LF SPL output compared to normal mode SPL and/or Faux applied higher than normal in some cases, specially when boosting at Fb or close where maximum air passage in the vent occurs... In other words, more of deeper bass output than normal mode, but this at a lower level than normal mode permits a little higher in the spectrum (e.g. 113 db @ 40 hz VS 110 db @ 30 hz; equivalent to half power).

By analogy, if you model a 300 watt driver/box combo in speaker design software with a vent surface area that's smaller than ideal, the program should raise a vent Mach red flag. However, if you reduce in the software the driver's 300 w rating to 150 watts or so (i.e. half power), the red flag often turns to green light! Minimum required vent surface area IS also related to power input, and Fb, Vd. Lower level = less air flow = less noise.

The boost and cut filter may also be part of the explanation when Keele mentions its use to "... return the response back to a roughly flat condition as shown in Fig. 2, curve b." (P.356) It can be seen in that figure 2 response curve, and also on some of the response curves in Fig. 1 and 7, that ROUGHLY flat often entails a VLF response level (boost aspect) that is somewhat lower in the step-down range than what the normal mode range shows. Naturally, things don't get better with power input increases, as shown by the LF maximum acoustic output curves (Fig. 4, 6, 8). The cut aspect of that filter, generating a much steeper slope below passband, would also be a contributor for less air flow and noise in that tentative explanation.

This COULD explain why a single operating vent representing only half the normal mode vent surface area, MAY still manage to take the amount of air passing in the vent without whistling.

More to come.

Richard

[RMC]

B3) KEELE

Another way to try to make sense of this possible vent noise issue, would be if Keele/ E-V oversized their vent diameters to begin with so that the remaining unblocked vent would still be large enough for free air flow? Though one can get into space trouble with larger vent area also requiring longer duct for same Fb. In any case, neither Keele nor E-V mention or discuss vent dimensions/noise matters (only Fb).

BTW In their B6 2245H boxes Kramer/Timbers indicated (P. 5 & 6) a 5 7/8" dia. vent 12 in. long (8 cu.ft. assisted, Fb 26hz) and a 9" dia. vent (12 cu. ft. assisted and unassisted, only duct length varies: 30 in. and 20 in. respectively, Fb 20hz and 25hz respectively). Now, compare these very long duct lengths (20 in. and specially 30 in.!) with what R.H. Small says about this in his Vented-box... Part II, P. 330, top right, second paragraph. Looks like some bending the rules occured here, from knowledgeable fellows (Kramer/Timbers).

More on the way.

Richard

[RMC]

B4) KRAMER/TIMBERS LONG VENTS CLARIFICATION

Since someone asked me what R.H. Small wrote about long vents (re post # 77), here is the quote for the benefit of all:

"Also, tubular vents for which the length is much greater than the diameter tend to act as half-wave resonant pipes, and any noise generated at the edge is selectively amplified. In these cases it is better to use a drone cone or passive radiator in place of the vent [2], [23]."

Small's references # 2 and 23 here are from B.N. Locanthi, and Olson, Preston and E.G. May respectively, both from the 1950's. If my memory is correct, both Locanthi and May were former JBL Engineers... If so, then Company memory might have forgotten something. Maybe Kramer/Timbers did test their boxes for the resonant pipe issue raised by Small. But I sure don't recall seeing that in their article I read many times, and where a simple phrase saying "we tested for that and no problems occured" would have buried the issue. Or they might have just ignored the bugger, similar to Keele/E-V on another vent issue? (see post # 76, 3rd paragraph, last lines).

Kramer/Timbers also mentioned in their references, R.H. Small, Vented-box..., Part III:Synthesis, not Part II where vent requirements are outlined however. But as I mentioned before, Small did write in Part III "The vent design is carried out in accordance with Section 8 of Part 2."(P.334) Doesn't that sound familiar? That alone would make me rush to go check Part II, as I did (see post # 76, 4th paragraph). Small being in 1973, long before Kramer/Timbers in 1983, they presumably saw that.

Some aspects of vent parameters/performance appear to be left on the sidelines by some experts, even though these are far from being negligible aspects. I suppose they wanted to avoid opening a "can of worms" type of discussion? Or the associated complicated explanations? My only point here though is simply that product end users may not be informed about all the relevant stuff there is to know... So, when I see it I raise it.

Some interesting vent blocking stuff in the next post, coming soon.

Richard

[RMC]

C1) VENT BLOCKING SUGGESTIONS

"The vent area may be either all-in-one or split into two or more separate vents, as long as the total vent area remains the same." (E-V, DL18W, data sheet, 1984, P.2)

Blocking one of two vents or half of a single vent is basically the same issue (i.e. a 50% vent surface reduction). MAYBE a one remaining complete vent could have better air circulation than a single vent partially blocked, both having the same surface area? On the other hand, experts have mentioned many times (e.g. JBL Enclosure guide) that vent shape (round, rectangular, square) doesn't matter. I've also seen some triangular or so shaped vents, though inside angles were rounded which makes sense, near the bottom front baffle's corners on commercially available speakers.

Multiple vents take more space than a single one of same area, in my view, simply because of repeated vent tube thickness and spacing required between each one. In a large box that is not really a problem, as multiple smaller diameter vents can be placed and spaced almost anywhere on the baffle. However, as box size decreases, which is the main appeal for step-down use (smaller box with deeper bass), then it may become an issue. e.g. For space reasons most of my boxes are rather small (< 3 cu.ft.Vb), so a single vent makes more efficient use of space available on the front baffle, specially for 2-3 way systems, though I know the vent could be placed on the back, top or side panel (which I never do because of extensive panel bracing).

Surprisingly, E-V had no suggestion or option for Home Speaker Builders, in their Pro Sound Facts no. 7 "Step-Down Bulletin", to block a vent or part of it, contrary to their commercially available TL series and others sold with a supplied port cover in numerous cases. I imagine they figured people would build the normal version or the step-down version (both in the Bulletin for each box size). Why not have it both ways, i.e. same box that can do normal and step-down modes like the TL series? [BTW in the Bulletin I counted 13 boxes with single vent (2 in normal mode and 11 in step-down mode), others having mostly two, some three or four vents]. My own vent blocking suggestions follow for speaker builders, since none are given by E-V (you either built normal or step-down version). Members may well have quite a few more of their own to add...

For double round vent home made systems, blocking one vent is relatively easy even in the absence of a commercially available port blocker. For example, thefoamfactory.com sells closed cell cylinder shape pieces of foam in 1.5, 2, 2.5, 3, 4, 5 and 6 inch diameter which should tightly fit in most common port sizes. Failing that, you can tightly pack or stuff a vent with similar type of removable closed cell foam. Open cell foam is less desirable though "...due to the high air-flow its structure allows". Remember, here you're not looking for a working resistive vent (like many in the old days), but rather for a blocked vent with no air going through. Doing it this way you can easily revert back to normal mode if required (e.g. normal mode if the highest possible low frequency SPL is needed for a specific application, as E-V said in an earlier post).

Rectangular or square vents are also relatively easy to block completely (1 of 2) or partially for single vent, in a way you can also revert back if need be as above. Cut an appropriate dimension wood block that will tightly fit inside the vent (all or half the internal volume of it, as the case may be) and use removable caulking to seal air tight around the wood block's perimeter. Example of such product: Mulco's, Zip, Seal' N Peel, removable clear sealant. In the case of half vent blocking, use sandpaper on the wood block's side exposed to vent air flow for smoothness.

Single round vent partial blocking for step-down can be more challenging. Making a half foam cylinder from one of The Foam Factory above MAY not be ideal (though removable sealant may help to hold it and seal it). First, since it might not fit and hold tight in place, will it stay there during loud speaker use without glueing (for later removal if need be)? Second, I'm not sure the foam's soft material would have as smooth surface as required exposed to air flow, once cut lengthwise with a sharp blade for a ducted port. Cutting this way tends to be more messy than precise (smooth & straight) which may impede vent air flow.

You'll have to make a half-moon shape out of something fitting tightly in the vent for the full length of it, preferably smooth & straight on exposed surface, like a rigid plastic or wood piece, and fasten it temporarily at each end of the vent, for usage and later removal as required. Removable sealant can be of help here too, not only to hold the half vent blocker in place, but at the same time to seal air tight on the device's rounded perimeter at each vent end. In a pinch, tape could also be used to do the same as sealant, even for testing purposes like checking the new Fb.

Still more to come.

Richard

EDIT: RE E-V "you either built the normal or step-down version" and "why not have it both ways in the same box" above. Naturally, when building the normal mode version its easier to convert the box to step-down mode by simply blocking half of the vent surface area. However, when building the step-down mode version its a lot more (vent) work to convert it to normal mode because the vent dimension(s) are not the same for the two versions as per E-V's Bulletin (diameter and/or length). Lower tuning (step-down) usually implies smaller diameter and/or increased length vent, whereas higher tuning (normal mode) usually implies larger diameter and/or shorter length vent. So it makes more sense (less work) to start with and build a normal version and step-down it as required, than the other way around.

Richard

[RMC]

D1) JBL

The JBL modelings of B6 or so alignments were in very limited number compared to E-V, maybe because many (not all) JBL's often have higher Xmax number which makes Step-Down mode less necessary or appealing for deeper bass achievement alone. However, still quite interesting for smaller box size, with deep bass. Kramer/Timbers understood the size reduction opportunity quite well: "For this project, we chose the latter system [RMC: 8 cu.ft. assisted box] because of its more moderate size. The 26-hz cutoff was deemed low enough..." (...) "... Why equalize? In order to keep the efficiency up and the box size down,..." (P. 4 of their 1983 article).

Their priority towards smaller box size is justified by a good reason. A similar performing (F3) unassisted system has a 12 cu.ft. net box in their article, and such a large size isn't an easy sell to the public. Even more so when considering that about 15 cu.ft. gross volume would be needed normally to get that net volume when everything is accounted (driver, bracing, vent, losses). Naturally, this was also the opportunity for JBL to present and launch their B460 and B380 subwoofers/BX 63.

Kramer/Timbers have shown for the 2245H in 8 cu.ft. assisted box VS same driver in 12 cu.ft. unassisted box a noteworthy 4 cu.ft. size reduction and "The 12-cubic-foot unassisted design gives essentially the same response curve as our 8-cubic-foot project box... Again, the trade-off is enclosure volume versus amplifier power." (P. 6) This means the F3 of the unassisted 12 cu.ft. box would also be at 26hz or so (F3 mentioned for 8 cu.ft. box).

Since the F3 of the assisted 12 cu.ft. box is said to be "about 21hz", that leads to a small 5 hz F3 difference between 12 cu.ft. assisted VS unassisted (same box size and driver). Is that minor gain here really worth the trouble/money of going the assisted way? Which they considered here overkill for most program material.

Moreover, in post # 29 here JBL's 2269H was modeled in a unassisted vented box, QL 5.4, Vb 8.9 cu.ft., Fb 26 hz, F3 32.47 hz. Then same box/driver with 26 hz EQ boost/cut filter assistance, filter Q 1.5 (not the usual Q 2), and the new F3 stands at 24.63 hz. The difference between the two F3 (pre and post EQ) is 7.84 hz. Again not a major VLF assisted gain for a super driver, compared to what some moderate ones benefited in LF.

The above JBL examples "unbolt" the myth that VLF capable drivers would be better candidates for deeper bass with assisted alignments. They probably have more to gain from assistance's usual box size reduction capability, than on F3. These two woofers can achieve very deep bass on their own, with no EQ, but at the cost of very large boxes. Hence the appeal of assisted alignment for a smaller box size with comparable F3.

Also, contrary to another myth, there ARE a number of older JBL or other drivers with limited cone travel capabilities that could benefit more of deeper bass this time, than box size reduction, from step-down. That is patently shown by Keele in his paper, even when comparing vented QB3 VS vented pseudo B6 (not only sealed VS vented box). See post # 75.

BTW In their article Kramer/Timbers did build and listen to a sealed box, both sealed and vented equalized for similar response and using 2245H, where they indicated "... the dynamic performance of the vented system was judged to be significantly better." (P. 3)

More will follow.

Richard

[Ian Mackenzie]

Hi Richard,

FYl

If you refer to DB Keeles formulae the Jbl 2269H will have an assisted step down Fb of 23.3 hertz
The box volume allowing for 20% box losses is 150 litres internal volume.

I quick online bass reflex calculator while l am walking the dog suggests an unassisted tuning of 191 litres, Fb is 30 hertz nand F3 is 31 hertz (W.J.J Hoge)
http://www.mh-audio.nl/reflexboxcalculator.asp

Jbl 2269H
Fs 28 hertz
Qt 0.36
Vas 237 litres

I don’t have time to re think your last post.

Okay l have posted the Bassbox simulation to illustrate what the Jbl 2269 does under 3 box scenarios

People can make their own mind up on preferences.

The Magenta curve is maximally flat alignment 137 L 4.8 cuft3
The Cyan curve tuned to 30 hertz 197L 6.9 ft3
The Yellow curve is the Keele assisted alignment f3 about 23.7 hertz , Fpk 25 hertz 150L 5.3 cuft3


A few points

Obviously the enclosure for the Keele alignment only a bit larger than the maximally flat alignment.
The larger 197 L unassisted alignment offers more extension with the penalty of box size.

The maximally flat F3 is 35.5 hertz versus 23.7 hertz on the assisted Keele alignment. That’s huge but under what conditions will that be subjectively important?

Comparing the Keele assisted alignment maximum displacement limited output to the unassisted alignments it drops 9 dB in output @29.5 hertz, the Fb of the maximally. That’s huge. The trade of is extension to 23 hertz.

The Keele assisted alignment is a compromise on the maximum displacement limited output for bass extension.

What the subjective result is is a matter of setting up an enclosure and auditioning these scenarios.

[Ian Mackenzie]

“The above JBL examples "unbolt" the myth that VLF capable drivers would be better candidates for deeper bass with assisted alignments. They probably have more to gain from assistance's usual box size reduction capability, than on F3. These two woofers can achieve very deep bass on their own, with no EQ, but at the cost of very large boxes. Hence the appeal of assisted alignment for a smaller box size with comparable F3.

Also, contrary to another myth, there ARE a number of older JBL or other drivers with limited cone travel capabilities that could benefit more of deeper bass this time, than box size reduction, from step-down. That is patently shown by Keele in his paper, even when comparing vented QB3 VS vented pseudo B6 (not only sealed VS vented box). See post # 75.”

Based on my simulation and unless l am having a “seniors moment “ your first paragraph is a generalisation based on less than appropriate evidence.

My base line examples clearly illustrate the capabilities of an appropriate hi powered hi Xmax driver under 3 operating standard operating conditions.

The strengths and caveats are clear.
The user can make informed enclosure/ alignment decisions to suit their specific application.

My suggestion is set up an Excel spreed sheet and do a comparison of a dozen Jbl woofers and compare them with 3 alignments as l have done. You can then talk about trends.

You might then be in a position to make a recommendation.

[RMC]

D2) "SUPER WOOFERS", AND OTHERS

As I mentioned in an earlier post RE E-V's huge 30W woofer (and comparison with JBL's 2245H) there isn't a whole lot to gain in terms of assisted deeper bass, when dealing with already low F3 numbers these drivers can achieve by themselves. That amount of additional benefit is less evident or important for super drivers.

In recent posts here I showed the following lower F3 gains from assisted alignment VS unassisted for the JBL 2245H, 2269H and E-V 30W, with same box size, to be 5hz, 7.84hz and 8.5hz respectively. This is less than what "ordinary" woofers have shown, which is usually a good 10 hz. Keele's own pseudo B6 modelings of three quite "ordinary" woofers showed 12hz, 13hz and 17hz lower F3 improvements compared to standard vented box (these were respectively 15", 12" and 8" woofers).

Even though the 2235H isn't considered a super woofer by today's standards, I'm including it here for comparison purposes, considering JBL's previous mentions about it, such as: "... drivers for demanding professional applications, in which very high sound pressure levels at very low frequencies are a requirement." Reference is also made to 25-50hz octave at levels in excess of 110 db (Brochure LFS/5-83).

In post # 11, a 2235H modeling was done: Vb 4.5 cu.ft, Fb 26hz, 150W, for both assisted and unassisted versions. "Response" with EQ flatter and 6 db higher at 30hz than with no EQ, plus around 24hz still about 4-5 db above the no EQ level. The "F3" is at 40hz (no EQ) compared to "F3" of about 26hz with EQ ("response" and "F3" since not at usual 1W, but at 150W input). The 150W output level represents a healthy 116 db @ 30hz and 113 db around 26hz which beats the above 110 db. In terms of lower F3 improvement, the assisted box shows here a sizable "F3" gain of 14hz. Another older driver (35 years old) showing a fairly large VLF extension gain from assisted alignment.

For super drivers, I see more practical potential on box size reduction, with similar F3, than on chasing the minor or ultimate lower F3 using assisted LF. These can pump huge amounts of VLF, if required, in the case of JBL's due to their extra long cone travel capability, and in the case of E-V's due to its enormous cone size. As indicated by E-V in many documents, the "penalty" for not using step-down is much larger box size required: "To achieve a similar response extension without equalization would require an enclosure at least twice the size, ..." (E-V, TL 3512, data sheet, P.3). Links to this one were made in a previous post.

With regards to drivers with more limited Xmax capabilities, the major gain from step-down use is deeper low frequencies at a higher output level, as shown in Keele's article examples.

As for the appropriate woofer size myth, Keele's modeling of a CTS 12" driver (and 8" woofer) speaks by itself: for the 12" woofer, in QB3 box, F3 39hz, thermally limited > 27hz, but in the "B6" box F3 26hz, thermally limited down to 21hz! Plus, I have another such example from a reputable source which we'll see in an up-coming post.

Richard

[Ian Mackenzie]

Hi Richard

I can see what you trying to say.

However, I think the apples for apples comparison are a bit off.

Firstly, when referring to an unassisted alignment it needs to be understood which alignment, QB3, SBB, maximally flat or extended shelf alignment is being discussed for comparison purposes.

Without making a selecting a standard un assisted alignment any comparison for discussion purposes is too random when comparing to B6 alignments.

My suggesting is use the Hoge alignment.

http://www.mh-audio.nl/reflexboxcalculator.asp

I have not looked at it too deeply but attempting to gain a lower f3 in the case of an un assisted alignment for the JBL 2269 beyond the 6.9 cu ft3 Hoge alignment results in a sagging response below 100 hertz by about 2 db or so and then a corner F3 of around 25 hertz. ie 9 cu ft3 tuned to 25 hertz or below down to 21 hertz as seen elsewhere.

In an application as a sub crossed over any say 65 hertz you could live with that but if it was a 2 way system the woofer response would warrant a LF shelf EQ lift below 100 hertz to balance the overall system.

I am not sure why you are singling out the 2269 in such a large box when in 197 Litres (6.9 cu ft3) it does an admirable job with an f3 of around 30 hertz without being too big and the response below 100 hertz is maintained at reference sensitivity.

In my comparison the actual B6 is 150 litres and it gets down to 23 hertz. The moment of truth is the caveat of the significantly reduced LF output at 30 hertz.

I am not sure what you are referring to as a super woofer or in your mind what its meant be constitute?

As far as the B6 F3 is concerned its simply the ratio of F3/Qts.

The lower the Fs and higher the Qt the lower the F3 will be where F3= Fs/Qt x 0.3

The B6 Vb is governed by the formula Vb = (Qt )2 x 4.1 x Vas. In this formula the lower the Qt and the lower the Vas will yield a smaller Vb.

In practise it will often be a lower Vas and higher Qt as in the case of the JBL 2269 or a lower Qt and a large Vas like the JBL 2245. (237 L versus 821 L respectively)

In the case of the JBL 2269 you end up with a lower F3 and a smaller enclosure while the JBL 2245 you end up with a larger enclosure for B6 purposes than un assisted QB3 alignments.

[RMC]

"Vented Box Calculator

according to W.J.J. Hoge

This set-up provides the highest precision and a flat frequency response"


A word of caution and explanations regarding the often mentioned Mh-Audio box calculator, for the benefit of all readers. The Mh-Audio calculator is using an APPROXIMATE vented box design procedure (not the exact real thing, Mh Audio doesn't say that), as I mentioned in a post in another thread sometime ago.

I know since I used the method a number of times many years ago before computer design software became widely available to the public. It was the next best thing to Thiele and Small writings, also not widely available to the public in the old days, unless one subscribed to JAES from the early 70's and on. I've had this method since It was published by John Hoge in "Confessions of a Loudspeaker Engineer", Audio, August, 1978, P. 47. As a Loudspeaker Egineer, Hoge described here ONE of the methods he used (the easiest Re public use) for vented box design, along with some speaker related physics. Hoge himself indicates on page 50 "...developed by Keele and represents an approximation of the charts given by Small for systems with an enclosure loss factor of QL = 7."

Contrary to some belief, Keele never published that, not even on his Web site, but he did circulate his method in private communications to experts in the audio industry (John Hoge, John Eargle, David B. Weems, Ray Alden, etc. have indicated getting this procedure through private communication from Keele). For my part, I got after Hoge's article, an "improved copy" of Keele's original procedure "Vented Box Design Using Pocket Calculator, February 1976", made by Bolt, Beranek and Newman, the famous Acoustical Consultants firm, and that one is dated June 1, 1977.

My edition of this is an 8 page compilation of Thiele, Small and Keele design and measurements equations/formulas for vented box. After a couple of pages of parameters definitions and measurements techniques/calculations, it says: "Now use Vented Box Design and Vent Design calculations by D. Keele." and Keele's approximate method follows on 3 pages. Pretty loaded but concise document, just enough text but lots of math to do with a pocket calculator...

So, Keele's method is also outlined here with corrections/improvements from the folks at Bolt, Beranek and Newman (BBN). Its a typed document on 8 1/2" X 11" but a number of alternative equations/formulas are hand-written next to some of the original ones. Its in this document I was able to confirm a typo existed in Hoge's equation # 19 (vent length formula) where the exponent is negative, instead of being positive as in the BBN version. The vent Lv numbers calculated finally made sense...

John Eargle (JBL): "The approximate design equations, as given by Keele, are shortcuts to estimating certain aspects of system response." He adds they have an accuracy of about + or - 10% (1 db). (Handbook of Sound System Design, ELAR, 1989, P. 104).

However, Robert Bullock, Mathematician and Professor of Applied Mathematics at University, has shown things can get worst than that: "Formulas also exist for Small alignments; for example, Hoge provides formulas for QL = 7. These formulas can be off by as much as 25 percent for some alignments. (...) If you like to play with formulas, I have included some of my own construction for QL = 5, 7, 10 in table IV. I make no claims about their accuracy except to say that they should be as accurate as those Hoge used." (Bullock on Boxes, 1991, P. 8).

Bullock further mentions about his own formulas in Table IV (P. 6 ) " The value of H [RMC: re Fb] is usually within 2%, the value of F3/Fs [RMC: re F3] within 6%, and the value of Alpha [RMC: re Vb) between - 17% and + 25%." The latter is the major bugger I see here with Alpha's tolerance (Re: Vb), considering Hoge indicates: "Remember, cabinet volume, Vb, is probably the single most important specification of a loudspeaker system." (P.47)

So, "Highest precision" as claimed by Mh-Audio??? No, not at all in my view, as one can see from both Eargle and specially Bullock, even more so when compared to today's numerous speaker design software programs available. It seems Mh-Audio took the easiest way, period. "OK" for some "trial run" re woofer potential , quick test or to get some idea, but not enough accuracy for the final product (box) by today's standards.

There's nothing wrong with using an approximate box design method, I did LONG ago, but one has to know what he's using, know its shortcomings (e.g. for QL 7 only; accuracy tolerance) and be willing to accept these facts. It's also ok if one has no access to anything better. Unfortunately, Mh-Audio isn't straightforward about all this.

BTW another known approximate design method exists: Gary Margolis (JBL) and R.H. Small, Personal Calculator Programs for Approximate Vented-box and Closed-box Loudspeaker System Design, JAES, Vol. 29, 1981, P. 421-441. Some of their equations/formulas were reproduced in article(s) of Speaker Builder Magazine.

Richard

[Ian Mackenzie]

The point is the benefit of looking at several different types of alignments to compare enables the user to make the most informed choice to suit a particular application with a particular driver.

I am not aware of any evidence in your posts that supports the above.

I have numerous program and that online calculator was convenient at the time

The actual process can be as complex or as simple as a user requires and that’s not your call.

[ivica]

The point is the benefit of looking at several different types of alignments to compare enables the user to make the most informed choice to suit a particular application with a particular driver.

I am not aware of any evidence in your posts that supports the above.

I have numerous program and that online calculator was convenient at the time

The actual process can be as complex or as simple as a user requires and that’s not your call.

Hi Ian,

I have to agree with You.
Here, from the theoretical point of view, it can be written more then a book, but we have to be aware that in usual home listening surroundings, bass response in our rooms is so 'un -predictable' and its variations is much, much more then +/- 3dB, so EQ, here talking about would be fare from the calculated prediction.
Even putting bass driver in the middle of the wall would not help enough , if very low frequency is of interest, as driver(s) mutual coupling (if stereo is applied) or drivers 'image' (as a reflection from the large surfaces such as walls, floor, ceiling ).
I think forum members with the good experience about the topic would give us some good experience with the bass response in standard home listening conditions, but that would be CONFIRMED with some measurements data (not only such as:..."move 1m from the walls, etc.."

regards
ivica

[Robh3606]

Here, from the theoretical point of view, it can be written more then a book, but we have to be aware that in usual home listening surroundings, bass response in our rooms is so 'un -predictable' and its variations is much, much more then +/- 3dB, so EQ, here talking about would be fare from the calculated prediction.

Hello Ivica

Not sure I can agree with you on this. You most certainly get the extended bass response as promised in the simulations. The overall response will be tailored by room placement just like any other system but make no mistake you would have to be deaf not to hear it the difference. Room response be damned you cannot miss the extra half octave.

Rob

[ivica]

Hello Ivica

Not sure I can agree with you on this. You most certainly get the extended bass response as promised in the simulations. The overall response will be tailored by room placement just like any other system but make no mistake you would have to be deaf not to hear it the difference. Room response be damned you cannot miss the extra half octave.

Rob

Hi Rob,

I want to say that here it is talking about several dB of improvement , but in the room, it can be get the variation over 10dB from the 'open space' prediction (2pi), especially if we have two (stereo) drivers working in LF frequency domain. But I agree that any speaker response would be tailored by room placement.

regards
ivica

[Ian Mackenzie]

Room modes are a can of worms.

Richard has not to the best of my knowledge mentioned room modes despite sifting through numerous papers.

Back to the fun stuff has anyone heard a Sunfire sub. You know the 2700 Watt 1 ft square cube?