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  1. #436
    Senior Member RMC's Avatar
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    Cabinet stacking and box downtuning: Two unwanted side effects...



    Post somewhat late vs what was mentioned before, still better late than never...

    JBL Engineers Mark Gander and John Eargle did a study on loudspeaker arrays performance. An interesting aspect of this study is summarized in Eargle's Loudspeaker Handbook 2nd Ed. They noted a downward shift of vented boxes tuning frequency when increasing the number of LF cabs together. The box group down tunes (self-downtuning!) when they're multiplied. Also worth noting, Eargle added in his book the mention of a possible woofer displacement overload, not seen in the original article, caused by an unusual cone excursion limitation.

    The experiment involved ground plane measurements of the same LF enclosure but the number of them varied. One cab Fb 31 hz to start with, always doubling their number, going to two of them, then four, finally eight boxes (in Eargle's book its limited to 4 boxes). The tuning frequency, as measured by the impedance null for system resonance, was recorded for each test. Result: for each doubling of cabs the group's tuning frequency went down by 2 hz, so it would be a 6 hz shift in Fb for 8 cabs in a stack.

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    Initially doesn't seem like a big tuning shift but it could end up being problematic, and knowing that down tuning puts more strain on a woofer. Consider a user having down tuned 2 or more pairs of boxes to achieve extended LF reach, then stacked them... Might be like running towards a cliff, not knowing its there. The unsuspected issue is driver Xmax may not be reached first where one would expect (VLF) but instead somewhat higher in the spectrum (LF). With low power this may not appear as an issue, whereas at higher drive level cone travel limit might be reached unexpectedly, specially for older and limited woofer models (with the increased distortion included).

    The stacking process for LF boxes leads most of the time to mutual coupling of these (drivers closely spaced), its well known and purposely used in order to increase bass output, in addition to saving floor space on stage or at home. Here its not so much the frequency response we're concerned about, its rather the driver's acoustic output capability which may be somewhat compromised. Its another, but less known, aspect of mutual coupling.

    Little explanation is given as to why this box tuning shift of stacked vented LF cabs takes place, except for this mention in the original paper: "The downward shift is caused by mutual coupling increasing the radiation reactance." (Gander & Eargle, JAES, 1990, P.214)

    The risk of using enclosures actually tuned lower than driver Fs may exist too.

    * Have to reorganize the rest of this post (already written) due to pic files size. Next part to follow shortly.

    Richard

  2. #437
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    Part two of cabinet stacking and box downtuning...



    Apart from box tuning shift lower in the 1rst part, there's an interesting parallel that comes to mind regarding the other aspect Eargle raised (cone displacement overload risk) which leads to a reduction of output at some frequencies. A similar situation occurs with EV's own step down tuning (often used to mimic an assisted B6 alignment with boost/cut EQ filter). Picture is from EV's Pro Sound Facts # 7, P. 3

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    Some comments on the above pic text.

    RE reduces output slightly. Its not a huge reduction but not trivial either when it represents a 3 db loss for example

    RE smoothly falling response. Yes as long as one down tunes reasonably, Keele/EV rule is about half an octave lower than normal tuning. I've seen some going for more than this. If you approach the octave down tuning instead of half the curve isn't so smooth falling anymore, plus the loss in output capability at certain frequencies may be larger.

    RE 6db boost close to the new Fb. That boost is not always exactly at Fb when you look at how EV does it, may vary a bit up or down, they model the cab different ways in software and decide on which gives the best outcome for a particular driver/box combo in view of intended application.

    So the VLF gain has a counterpart, this time in the region of the original box tuning where there's a partial loss of woofer output capability. Its not clearer why exactly driver/box react this way when subject to box down tuning, but we know its caused by the lower than normal box tuning.

    EV mentions its "B6 EQ" has no effect on max output, however stepping down tuning does impact, since the driver LF extended reach usually comes at a lower level (hence the required EQ; more power used instead of larger box).

    Looking at the graph below i posted in another thread, there's not much bass left there after down tuning and a B6 filter probably won't suffice to bring back the LF in line with the rest since its 10 db down near 30hz (in that example normal tuning would be 40hz, half octave is 30hz but i took someone's number at 25hz). Driver could take 200W for Xmax in that case, but in other LF examples with less extreme tuning it could take 300W for Xmax.

    Part three with actual EV examples to follow.

    Richard

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  3. #438
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    Part three of cab stacking and box downtuning...



    Note for the three EV examples to be shown in this part, the down tuning they used is the recommended one (half octave or so). Going for even lower than this might be like walking on a ice rink...

    EV TL 3512 (18")

    Max acoustic output, left graph normal mode tuning and right graph step-down tuned. Normal mode box Fb about 40 hz, step down box Fb 30hz or so. Taking the +10 db lines as reference level here. To compare "apples with apples" lets take the 50hz vertical lines for output comparison purpose.

    On left graph output is +1 db above the +10 db reference line. On the right graph with the same conditions the output level is now -2 db below the +10 db line. So, from +1 to -2 db that's a 3 db difference (maybe a bit more) which translates to a 3 db loss of max output. The loss doesn't apply to a single frequency, like i used here, but rather to a range of them with varying DBs as can be seen when looking at both curves.

    The output dip on both graphs, just above each tuning frequency (40 and 30hz respectively), is likely due to cone travel limit (Xmax). However, the loss of output at Fb on the right side graph vs left one, is notable and curious since radiation there is mostly done by the vent, normally not mechanically limited like a cone driver is. As i recall this Fb output issue with downtuning has not been mentioned or explained by Keele or EV.

    The reason why that issue exists. As with many enclosures the TL 3512 has two front ports, the EV step-down kit provides a plug to cover one of the two ports. The smaller vent area that results tunes the enclosure lower. But this is a 50% reduction of port area, having lower air flow capability, and its not trivial. As drive increases to high level such vent gets overwhelmed limiting VLF cab output at Fb. I think on the right side graph Fb output level EV engineers stopped pumping watts just before things get out of hand for the remaining port, the other is blocked.

    Recently a member mentioned that, long ago as a student summer job, he was hired to check cabs coming back from touring. If i remember well, he said for Ted Nugent's tour 35% of woofers were blown? This is crazy, more than a third of them. Makes me wonder if it was due to overpowering alone ("fried" voice coil)? Or if the box tuning shift and woofer displacement overload issues were not actually part of the explanation... (known or not known at the time).

    Two other EV examples to follow.

    Richard

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  4. #439
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    Like for the first example in previous post, these here illustrate what Eargle mentioned about displacement overload.

    The first EV example was an 18", the second and third ones are 15" and 12" drivers. There appears to be no correlation between driver size and the presence of cone displacement overload. Though there MAY BE one with cone travel capability. The larger the Xmax the more leeway the woofer has before travel overloading?

    Looking at the knee region of the LF max output curves on the second graph for each driver shown, the 2-3 db output loss stated is visible on the dotted curves, which cannot match the speaker's normal mode output seen on the solid curves (due to downtuning/cone travel limitation). Moreover, this happens in a region of usually rather high energy (LF). Had it occured say 2-3khz who cares, crossed over at 500, 800 or 1000hz it could be irrelevant.

    There's a tradeoff involved here, for something that you do lower in the spectrum (down tuning) there's a repercussion higher up with the risk of earlier displacement overload. Excursion may "hit the wall" sooner than expected and where it was not really anticipated.

    EV TL 606 (15")
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    EV TL 806 (12")
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    If we take a 3 db output loss for e.g., seems often about what it is, this means in that higher region of the spectrum the driver can only handle half the power it would normally take (-3db being half power) at the frequencies involved and Xmax could likely be reached there first (instead of VLF). This is probably why EV mentions that if maximum SPL is required from driver (e.g. PA/SR work), it is recommended to do without Step Down/EQ (EV, SEQ, Step Down Kit, p.2). Sort of making a 200W woofer, for example, now acting as a 100W driver when downtuned...

    It is possible drivers with cone travel to spare (e.g. 2269H) might be less affected by the cone displacement problem but i don't think they're totally exempt from the issue, might be just delayed further vs power applied.

    BTW i'm not against step down/EQ, simply showing here the other side of the coin. As with many things pros and cons come with it. In a house setup with normal home listening levels issues may never arise. Still a worthwhile way to go as long as the lower tuning and the power input applied aren't excessive.

    Richard

  5. #440
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    Driver EBP guidance with a grain of salt...



    The Efficiency Bandwidth Product was intended to help in deciding which type of enclosure would be more suitable for a driver.

    In its EBP definition Eminence speaker manufacturer indicates that driver Qts should also be considered, which makes sense.

    A driver's Fs divided by its Qes parameter provides the magic number: around 50 sealed box, 100 or so vented box and about 200 horn loading. Its a good rule of thumb, a tool, but certainly not something strictly followed now.

    Design Engineers and users violate rules frequently, and things still work. From a user one may wonder how well. As for Engineers there seems to be a trend of extensive modifications or corrections to drivers in the crossover that might not have been used in the past. In other words using the unthinkable of yesterday and make it work more recently.

    Having a look in recent times at seven 8-12" units (random), most were not used as EBP suggests, while the others seem to. Could be that users and Engineers are more daring, or that crossover softwares make daring easier (tweaking).

    JBL 2202H: EBP 294, Qts 0.16, two arrows pointing in the direction of horn loading, nevertheless used in sealed sub chamber in 4355

    JBL 2204H: EBP 102, Qts 0.35, this one seems to be used generally as intended, vented box

    JBL 2206H: EBP 153, Qts 0.32, also usually loaded in the intended vented box, though some have views towards horn loading it

    JBL 2251J: EBP 277, Qts 0.20, another one begging for horn loading as in HLA, some user(s) have it in vented box, and as i understand maybe eyeing for sealed box too?

    Looking at a few from another manufacturer (Fane Acoustics) to see if the situation is similar or different.

    Fane 12MB: EBP 159, Qts 0.295, Manufacturer says intended for use in ported enclosure, seems to make sense

    Fane 8M: EBP 268, Qts 0.284, designed solely for midrange for use in sealed box, whereas EBP is for horn loading

    Fane 10M: EBP 190, Qts 0.301, same note for mid only in sealed box, and again EBP indicating a horn cab

    Those two Fanes have the Qts for a vented box, the EBP for horn loading, while being specified for sealed box!

    The rule of thumb regarding EBP isn't absolute and the enclosure type loading doesn't seem to matter as much anymore, even when Qts is considered. It appears that EBP has lost some of its meaning over time, getting more difficult to get accurate guidance from that number...

    Richard

  6. #441
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    Found in my archives that i still have paper version of catalog and brochure of older McCauley Sound with Specs/TS for drivers from their 1000 series which was discontinued in 2001. That series is made of two groups of cone drivers, the M series with M1020, 1220 and 1520, as well as the B series with B1240, 1540, 1560, 1840 and 1860. Moreover this includes 7 different cab volumes and tunings suggested PER DRIVER.

    Their more recent 6000 series is pretty much on their web site, whereas the above mentioned seem to have vanished and didn't find the Specs either reading about 30 results of each google search. However i do see some used McCauley gear for sale, including old ones.

    They also made some compression drivers (with two coaxial) and a bunch of various horns: exponential, constant directivity and lens horns. The latter similar to JBL's own, might be a substitute in case one doesn't find the JBLs. Have Specs for those too. Saw on another forum someone was looking for specs of their 411 horn, have it but not a member there and its unlikely i'll be. Also saw that Speaker Exchange has listed a number of parts for McCauley.

    In addition to USA i see in the old catalog McCauley was distributed in Europe and Australia too for example. So there might be some hanging aroud in different places. In case you're looking for specs on a specific older McCauley model let me know here. Even have their full 1996 price list, giving an idea what it was worth back then.

    Richard

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  7. #442
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    This is an interlude while i'm working on something else to be posted in the near future.

    My Pocket Tool Mania

    What will get you out of trouble time and time again on small jobs, in absence of toolbox? Swiss knife or Pocket Tool naturally...

    Bought a bunch of these over the years, however seem to have mostly used the same two (Gerber pliers and Victorinox Swiss Officer knife with blade open) during the last 25-30 years. The Swiss logo on that knife has long faded away but it keeps on going. The larger one at the top of pic, model Champion if i recall correctly, is pretty loaded on both sides and a bit heavy.

    Gave my son-in-law a new one same as the packaged one bottom left on pic, he lost it. Gave him another identical one, he doesn't know where it is , but he claims its not lost! Pretty much the same to me. I never lost one, period.

    Did a job recently with him, had my Swiss knife and Gerber tool with me, and he confessed been eyeing the Gerber multi tool on the Web. Recently i saw on their site around $75.US but out of stock. On the Gerber the pliers slide in/out, not on the others i have.

    Well it was his lucky day, gave him a third identical Swiss knife and the Gerber tool with belt pouch, my preferred one, both bottom left on pic. He's happy. The belt pouch shows some mileage but the tool looks almost new.

    I'm now using the Stanley, don't like it as much, but it has the advantage of providing popular screwdriver bits usable with the tool. Have yet another multi plier tool that stays in the car in case i need one, but this one is China stuff.

    The yellow and red ones strapped with rubber bands are from Wenger, the other Swiss made knife Company also claiming to be the official Swiss army provider. On the side of the red one you can see the Swiss logo. Eventually the official supplier matter was settled when Victorinox purchased Wenger, case closed i assume.

    I'm not a collector, simply have the bad habit of buying a number over time. Added two more pics since the more i look the more of them i find.

    I guess most of what is seen might end up in my estate one day, hoping son-in-law has enough for a while...

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  8. #443
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    Simply passing along a driver info.

    In case you're looking for a pair of new and affordable 5" midrange drivers for a 3-way project, i noted that Solen has one on sale $49.CA each (reg. $86.) (one pair in stock, 4 ohm) from Fane Acoustics model Studio 5 HPM (similar to JBL LE5 or 104H). This one has cloth edge.

    They also have another 5" pair, not on sale and more expensive, less interesting in my view and with rubber roll edge, model Studio 5 FRK.

    The first one has higher sensitivity, flatter response in the intended band, takes more power, larger voice coil, more excursion capability, good for 300 to 3khz or so, cast frame, etc. Their Pro series is usually well made.

    Have no affiliation with Solen, i simply check from time to time on their web site what they have to offer. Since i already have a nice pair of 104H-2 mids waiting for me i don't need the Fanes here, otherwise would have bought them.

    Check the spec sheet and decide if those mids meet the requirements for your project. It is indicated as a full range driver, its not really, and in the previous version of the spec sheet i also have it said "performance is optimized for midrange", its the same driver, and response curve shows midrange use is best.

    Richard

    FANE STUDIO 5HPM-DS141117.pdf

  9. #444
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    Having my first problem with the Mackie mixer i use as preamp, "patch bay", routing signals. First issue in 25 years isn't that bad, but this one was made in the US not China like the more recent ones. Intermittent signal drop in one or two of the channels. So will have to drop it at repair shop in the near future for diagnosis and estimate. Need to confirm a couple more things before.

    Haven't seen yet dealer's 2021 repair price schedule for mixers, its based on the number of channels on console. Late 2020 it was $75. - $300. per hour for labour, plus parts and taxes. As i recall over 48 channels or so, its $300/hr! Glad mine doesn't have 56 channels, rather only 12 of them on this one (4 mic+ 4 stereo), otherwise it could break the bank quickly, lol. The $75. rate (or new higher one) applies to 1-12 channels units. These rates are based on mixer complexity, and the repair techs skills and number involved. Particularly on large units, they have to know and understand the mixer's logic, signal routing system, its architecture. The repair techs usually like to work on my older gear, its different than the newer cheap parts stuff from Asia, plus many prefer working on analog gear.

    Present signal path is turntable signal on Ultralinks goes to Bellari phono pre, from there Monster Cable RCAs to Aphex interface (-10 to +4db) and this output now balanced on XLR connectors/1/4"TRS, out to Mackie mixer. While the Mackie will be out for service i need an alternative to continue spinning some LPs.

    In theory i could use the Bellari phono preamp to drive the amp directly since it has up to +4db gain but this is unbalanced RCA connector stuff. Have the cables and adaptors needed if i want to do it, that's one option.

    Could also use the other Allen & Heath console however its larger, won't fit in the same space and would have to move things around which i don't really want to do for a month or so. The NAD integrated amp, having both preamp and amp sections joined or separated by U shape metal jumpers at the back, is used in another system.

    Got another idea in making myself a clean temporary preamp. The closest thing to what i have now, without the mixer, would be to connect the Aphex outputs directly in amp inputs. Here the advantage of balanced lines remains, no need for different cables, +4db level is kept and gives more flexibility gain adjustment wise, as this will be part of my "volume control".

    The Aphex has two recessed gain controls +/- 6db, being recessed (sort of tamper proof) and having two of them isn't practical to manipulate each time between LP's. However, the Bellari has a large single gain control for L/R, covering -10 to +4db range, and not recessed, that's an easy one to use. Therefore it will be my main "volume control", though not a real one since it won't go down to zero level, though it should be pretty low.

    Just need to test a few gain splitting scenarios between the devices for best outcome. Won't have the 3-band EQ, channel Pan, signal routing, etc. as with the Mackie but i don't normally use a lot of EQ anyway.

    The A&H console btw has 14 channels, therefore will fall in the next hourly rate for repairs, when time comes, so about $100./hr (or more in the future), the pain might increase a little...lol

    Richard

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  10. #445
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    Unless you're driving the output of the phono pre-amp a few hundred feet thru a power plant there is no real need for a true balanced interface. Connect the tip of the phone plug output to pin 3 of the XLR in on the Mackie and the shield of the RCA out to pin 2 and you'll be in business. BTW the round up the usual suspects version of repairing those old Mackies is to exercise / clean any ribbon cable headers and to clean any of the Schadow style push button switches. Simply exercising the switches ( pushing the suspect switches on and off a few dozen times ) will often bring back a bad or intermittent channel.


    Quote Originally Posted by RMC View Post
    ...

    Present signal path is turntable signal on Ultralinks goes to Bellari phono pre, from there Monster Cable RCAs to Aphex interface (-10 to +4db) and this output now balanced on XLR connectors/1/4"TRS, out to Mackie mixer. While the Mackie will be out for service i need an alternative to continue spinning some LPs.

    In theory i could use the Bellari phono preamp to drive the amp directly since it has up to +4db gain but this is unbalanced RCA connector stuff. Have the cables and adaptors needed if i want to do it, that's one option.

    ...
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  11. #446
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    Hi Riley,

    Thanks for the input, i appreciate. I don't balance my phono line due to distance. Rather because with one or two of my phono cartridges (8) i perceive a tiny little noise (hum, rumble, direct drive motor noise?) which gives me pimples. Fortunately its not with the best carts, but still. They're all installed in new premiun headshells, following every manufacturer's connections sheme plus re-checked each and everyone and still that tiny sound depending on cartridge.

    The Bellari has a 20hz rumble filter, always on, so rumble not likely. Probably not floor resonance either TT is mounted to be decoupled from the floor also with absorption material.

    Balancing the line was a no brainer for me and don't regret it a second. It got rid of the small issue and the signal is ultra clean, pimples gone, lol

    I got a great deal on the two used Aphex, both in very good condition, so the solution wasn't costly ($55. each). Could be my best spent 55 bucks in audio! If need be i'd get a 3rd one, works for me.

    I exercise the mixer controls regularly, more so for those i mostly use, sometimes cleaned a few pots with Caig's Deoxit Fader F5 for a little static noise. But eventually it comes back, guess can't deny the mixer's age and use (25 y.o.) . That's why i bought the A&H console knowing the Mackie won't last forever. If i can extend it a few more years at reasonable cost i'll do it, so much useful to me.

    Yesterday spinned records for a while, using channels 3&4, and 4-5 times got level drop and not long after it goes back up to normal. (i mostly use chan 1-4 also associated to the mic preamps, panned hard left and right, as i find these to have cleaner sound than the four stereo channels).

    Tonight spinned LPs again for 3-4 hours using channels 3&4 like yesterday, and also channels 1&2 to check those too, well not a single level drop for the whole night! lol, Yup electronics. Keeping an eye on this and next level drop it will go to repair shop. I don't like but don't really mind having to pay say $200. or so if it prolongs its life. These guys are usually magicians fixing the big ones: Soundcraft, Yamaha, Digico, Midas, Solid State Logic, etc. Regards,

    Richard

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