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Thread: Do dual-driver midbass horns have lobing issues?

  1. #1
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    Question Do dual-driver midbass horns have lobing issues?

    Hi,

    I'm curious to know if anyone has experienced any lobing issues with dual driver midbass horns like Altec 817 or JBL 4550, in relation to the midrange horn they cross over to.

    I see lots of DIY guys also building dual stacked midbass horns. I guess the reason for this is to get the efficiency high enough to avoid padding down the midhorn in a passive system.

    My own experience tells me that lobing is a huge issue when using dual 15" drivers on an open baffle, I just did this with 2 x Faital 15PR400 per side.
    The problem is the distance between the lower woofer and the midhorn, hence the use of MTM/D'appolito configuration to remedy this issue.
    Another way around this is 2.5/3.5-way systems, where both woofers are used for bass frequencies but only the upper woofer is used to cross over to the midhorn.

    So I don't understand why this wouldn't be an issue with the dual driver bass horns as well, or maybe it is?
    Or do the two drivers 'blend' together to act like one driver at the mouth of the horn?

    I have been thinking about this for a while, so hoping to get lots of responses here

  2. #2
    Senior Member RMC's Avatar
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    Hi rubjel,

    Unfortunately your question has not attracted the amount of replies you were expecting so i'll try to fill in the gap.

    First, dual woofers in a proper cab or dual single woofer cabs on top of each other is pretty much equivalent. This is Sound Reinforcement, where efficiency and output are king. Wired in parallel you can get +3db for efficiency and another +3db for increased power handling, for a total +6db which is a sizeable amount. You don't necessarily get deeper bass, but you sure get more of what a single can provide (SPL).

    Second, i've never seen a polar response graph for the front horn loaded 4550/4560, nor for the folded rear loaded horn 4520/4530. Hence the difficulty of assessing precisely lobing or the extent of it.

    I've looked quickly at JBL's Professional Series Low Frequency Enclosures 5/80 document which covers their LF horns, as well as JBL's John Eargle Loudspeaker Handbook, to try to confirm the issue. The lobing matter for these cabs is not directly mentioned there, however looking at peripheral info on 4550/4560 my impression is that you might have a point.

    In the low frequency enclosures document, comparing similar design cabs info is interesting, more so since the SAME woofer was used in both. The 4560 has single driver, the 4550 has double drivers PLUS a phasing plug between the woofers, as indicated by Eargle. Hard to see it in the LF enclosures doc above since the pic is dark, but easier seen in the 6/1978 Pro Catalogue. Looks like an angled separation for the two drivers' initial output.

    Dispersion pattern for 4560 is 90°H X 60°V and for 4550 75°H X 30°V. So the vertical pattern reduction rule, with two vertically aligned woofers close to each other, is maintained in the horn.

    On-axis response for both cabs is about the same shape, however for the 4550 there's a notable dip at 200hz and a smaller one at 350hz, the 4560 doesn't have, which is kind of a little "red flag".

    Simply stated, Eargle defines lobing errors as departures from smooth response, off-axis, which meets the concept of power response. JBL's LF enclosures document above doesn't give the power response (off-axis) for the 4550 with drivers, however Eargle does show it in is Handbook for that specific cab. In absence of real polar plots, i think power response can help to infer some things.

    On the power response graph the main dip mentioned extends little further down and to 300hz, stabilizes at 300-400hz, then the system starts high end roll-off near 500hz (the graph for this is very small 1"X1.5", precision not at its best, so frequencies mentioned here are approximate).

    So based on this it seems possible there's some lobing in that region. But on the other hand, knowing exactly what causes the response irregularities (lobing) is difficult to pin point for sure, could be the interactions between the two sound waves and/or a phasing plug side effect? Hopefully this helps you somehow. Regards,

    Richard

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    Hi Richard,

    thank you very much for your elaborate response, very much appreciated!

    I'm not too concerned with lobing between the the two midbass drivers, as I think they are generally spaced close enough to avoid any negative interference within their passband.
    But the transition to the midhorn is where problems arise with the dual driver arrangement.
    I think this article explains lobing well;

    http://audiojudgement.com/speaker-lo...olar-response/

    Excerpt from the article;

    "Speaker lobing is a phenomenon which appears when there are 2 or more speakers, that play the same frequency, and are of certain distance apart from each other. This is almost always the case in multi-way speakers. Let’s say, for example, you have a 2-way system with a mid-bass driver and a tweeter. These drivers will play the same frequencies at the crossover region. Also, they have some distance between them, because they are not coaxial drivers. Depending on the size of the drivers, the crossover frequency, and the distance between the drivers, speaker lobing effects will happen to a certain degree."

    As long as the distance between drivers that cross over to eachother is less than one wavelength at the crossover frequency, it should be fine.
    Say for instance we want to cross our dual midbass horn (15" drivers) over to a midhorn at 600hz. One wavelength at 600hz is 1.88 feet. If the midhorn isn't too large (tall) we can manage a center to center distance of less than 1.88 feet between the mid driver and the upper 15" driver. However the distance to the lower 15" driver would not be able to accommodate this requirement.
    Hence we will get destructive interference, lobing, between the midhorn and the lower 15" driver.
    And this is exactly what I heard when I used both 15" drivers in my open baffle up to the crossover frequency to the midhorn. The sound was very much out of focus, generally a very 'muddy' sound. Everything snapped back in focus when disabling the lower woofer.

    Quote Originally Posted by RMC View Post

    In the low frequency enclosures document, comparing similar design cabs info is interesting, more so since the SAME woofer was used in both. The 4560 has single driver, the 4550 has double drivers PLUS a phasing plug between the woofers, as indicated by Eargle. Hard to see it in the LF enclosures doc above since the pic is dark, but easier seen in the 6/1978 Pro Catalogue. Looks like an angled separation for the two drivers' initial output.
    That was actually my next question, what is the function of the horizontal divider between the two 15" drivers in some dual driver midbass horns. The JBL 4550 has it but the Altec 817 doesn't.
    Is this really a phase plug? A phase plug is generally used in compression drivers to ensure that all frequencies arrive at the same time at the aperture.
    I thought it had something to do with providing better loading at the throat..

    Best Regards,
    Rune

  4. #4
    Administrator Robh3606's Avatar
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    This is a cinema box but it's not all that different.

    Rob

    https://jblpro.com/en-US/site_elements/4648a-spec-sheet
    "I could be arguing in my spare time"

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    Thanks for the link Rob,
    however as mentioned I am more concerned with the response in relation to the midrange horn one would use above this sub.
    I don't think lobing will be an isssue with this box (between the two 15" drivers) if crossed over at 500hz, also higher than this and the drivers will start to beam as can be seen on the off-axis response charts. But crossing over to the midhorn at 500hz will still be a problem since the distance between the lower woofer and the midrange driver will probably be close to 3 feet
    The wavelength at 500hz is approx 2.2 feet so there will be some degree of lobing.

    I should probably say that the reason for this focus on lobing is that I'm considering building my own midbass horns. And since I would like to use passive crossovers, it would be very useful to have 2 x 15" a side for increased efficiency. However based on my aforementioned experience with my 2 x 15" open baffles, I am very reluctant about this.

    So the big question is; will a 2 x 15" midbass horn behave differently compared to direct radiators? Is there some sort of frequency summing occuring at the mouth that avoids the lobing issues?
    I mean alot of high end horn systems use this configuration, on the other hand direct radiator systems used for HIFI application are more likely to be 2.5/3.5 way or a D'Appolito configuration to avoid lobing.

  6. #6
    Administrator Mr. Widget's Avatar
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    Look at the way JBL and others have handled their line arrays. I have no experience in this area, but there are numerous examples out there where they have used multiple mid bass horns in vertical arrangements.

    Realize that vertical and D'Appolito arrangements also have lobing, but it is in the vertical plane and not the horizontal plane.


    Widget

  7. #7
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    My understanding is that on-axis lobing is not an issue so long as the drivers are located within 1/4 wavelength of each other. A 1kHz tone is ~1 foot, so 1/4 wavelength is ~3 inches. So as long as you place the centers of the drivers within 3 inches of each other, you are good to go..., at least up to 1kHz.

    BTW, the recommended lower and upper crossover frequencies for the 2251J, used in the mid-base horn, are 295Hz and 1.4kHz, though the driver goes well beyond this. I use crossover contouring to take it up to ~2,600-2,800Hz where it meets the Great Heil.

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    Senior Member RMC's Avatar
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    Rune,

    Have some other stuff for you. No time now, will be back later tonight, gotta go...

    Richard

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    Senior Member RMC's Avatar
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    Hi Rune,

    RE "Is this really a phase plug?"

    Have a look at the pic, Eargle/JBL do call it a phasing plug between the two woofers of the 4550. Though "unconventional" it seems reasonable to assume they know what they're talking about.

    Next to it you have cab response AND power response, the latter i've never seen elsewhere. As a bonus you get similar data for the rear-loaded 4530.

    At last found the other more recent illustration i had in mind of another application of the "horizontal divider" (phasing plug) between cone drivers, this time for mid-frequencies: Screen Array 4632. Similar principle used between each pair of vertical MF drivers. Read quickly the Tech Sheet and seen no added info on this either. In relation to Eargle's pic of 4632 he presents it as a manifold arrangement of drivers.

    Can't recall seeing yet from JBL/Eargle some explanation of the reason(s) or science behind the use of such phasing plug between two cone drivers, LF or MF.

    As you imply it might have something to do with throat loading, or preventing the early merging of sound waves so that they add properly ? As far as i remember EV might be the first to have used Manifolding drivers in a box, but executed differently than here.

    I'm adding the 4632 tech sheet since the picture of it is much better than in Eargle's book.

    JBL 4632 Cinema.pdf

    Regards,

    Richard

    IMPORTANT NOTE TO All: The pics shown here are JBL info posted on a JBL related Web site, which is acceptable. Eargle's comments and explanations are not shown since his expertise belonged to him (RIP) and his expression of that is covered by Copyright, which i respect. Please do NOT repost the data pics on any other site, since copyright issues may then come into play. Meaning would have to stop providing any such info here...

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    Thanks for the references Richard!

    Would be interesting to know why they call this a phasing plug though..

    Not much response wrt the dual driver midbass horns and lobing issues

  11. #11
    Administrator Robh3606's Avatar
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    Not much response wrt the dual driver midbass horns and lobing issues
    Did you look at the polar measurements in the 4648 cinema box?? If the spacing is similar it won't be too far off especially vertical.

    Rob
    "I could be arguing in my spare time"

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    Hi Rob,

    yes I did, please see post #5
    I'm not worried about lobing between the two woofers, but the lower woofer/midrange horn will be a problem.

  13. #13
    Administrator Robh3606's Avatar
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    I'm not worried about lobing between the two woofers, but the lower woofer/midrange horn will be a problem.
    Oh I got you! Did you look at any of the smaller cinema systems with a 15 and Horn?? They might be able to help you. Same with SR systems

    Rob
    "I could be arguing in my spare time"

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    Senior Member ivica's Avatar
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    Speaker Lobing

    Quote Originally Posted by rubjel View Post
    Hi,

    I'm curious to know if anyone has experienced any lobing issues with dual driver midbass horns like Altec 817 or JBL 4550, in relation to the midrange horn they cross over to.

    I see lots of DIY guys also building dual stacked midbass horns. I guess the reason for this is to get the efficiency high enough to avoid padding down the midhorn in a passive system.

    .........
    Or do the two drivers 'blend' together to act like one driver at the mouth of the horn?

    I have been thinking about this for a while, so hoping to get lots of responses here

    Hi rubjel,

    As it has been shown in the
    http://audiojudgement.com/speaker-lo...olar-response/
    lobbing in the speaker systems are mainly influenced of the speaker acoustic centers spacial differences relative to the line between the speaker system and the listener. (listning axis). Usually it can be seen in the vertical plane, but if we are talking about large box (height of the box larger then listener listening spot, or listener listens ON-AXIS) talking about the vertical dispersion speaker characteristics is juts theoretical discussion. And using proper frequency network, and proper driver phasing, lobbing can be controlled (as it has been shown on the fig.4. of the mentioned paper). It has to be emphasis that the crossover networks would introduce some value of the delays depending of the network design( not only drivers positions - spacial dissociation).

    May be can help

    https://www.google.com/url?sa=t&rct=...M_E55IaPiUyeQG
    tn_v3n01.pdf
    (Technical Note Volume 3, Number 1)

    regards
    ivica

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