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Thread: JBL 2245 : 10 or 12cft

  1. #1
    Senior Member Lee in Montreal's Avatar
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    JBL 2245 : 10 or 12cft

    I 've been calculating a 4th order bass reflex cabinet at 10,1cft using Vb=(15)(Vas)(Qt2.87)

    Then found F3 32,5Hz using F3=0.26(Fs)/(Qt1.4)

    And Fb 27,3Hz (basically the port tuning) with Fb = 0.42(Fs)/Qt 0.9

    What if I increased cabinet size to 12cft?
    How lower will F3 be? Any drawback?

    I want to build two cabinets to use up to 800Hz for home duty. No PA action here.

  2. #2
    Senior Member ivica's Avatar
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    Quote Originally Posted by Lee in Montreal View Post
    I 've been calculating a 4th order bass reflex cabinet at 10,1cft using Vb=(15)(Vas)(Qt2.87)

    Then found F3 32,5Hz using F3=0.26(Fs)/(Qt1.4)

    And Fb 27,3Hz (basically the port tuning) with Fb = 0.42(Fs)/Qt 0.9

    What if I increased cabinet size to 12cft?
    How lower will F3 be? Any drawback?

    I want to build two cabinets to use up to 800Hz for home duty. No PA action here.
    From some simulation program (BBox) minimal F3=32Hz, for 12cft (340 Lit) can be get for Fb=29Hz., while for 10cft box with Fb=27.3Hz, F3=35Hz.
    So the differences from the calculated and simulated values I expect is
    'coming' from the damping material influence.

    The only drawback that I can see is maximal Xmax (cone displacement) is about 2mm greater for the same frequency, so limitation of about 9.5mm with the smaller box is reached at 16.9Hz, while for larger that is about 17.4Hz, all under 100W power 'applied' to the driver.
    What would be the real solution depends mainly of the influence of the damping material applied in the box.

  3. #3
    Senior Member Lee in Montreal's Avatar
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    Hi Ivica

    Thanks for your insights. One subject I'd like to discuss with you as well as others is the port tuning. Some people will tune the ports to Fb (the cabinet's resonance), and others will tune to Fs (the driver's free air resonance in order to avoid phase cancellation at very low frequency. Myself, I would tune to Fs.

    It is also my understanding that if a woofer runs lower than the port tuned frequency, it will "unload" and go into high excursion. Which is why I think that if Fs and port tuning are the same (20Hz in this case), then the driver could handle more power before reaching Xmas.

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    Senior Member Eaulive's Avatar
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    Quote Originally Posted by Lee in Montreal View Post
    then the driver could handle more power before reaching Xmas.
    The advantage here is if it doesn't handle the power and burns out before Xmas, you'll still have time to ask Santa for a new one

    My avatar: 4520 loaded with 2225H on E140 frames,
    1x B&C 12PE32 on custom front loaded horn, 2x 2426 on 2370.

  5. #5
    Senior Member Lee in Montreal's Avatar
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    BTW The intent of the project is to be flat fro 30Hz to 800Hz.

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    Do a 6th order filter ...

    Quote Originally Posted by Lee in Montreal View Post
    BTW The intent of the project is to be flat fro 30Hz to 800Hz.
    ... and end up at 22 Hz. You need an electronic xover which can give You the missing two orders: 4 orders come from the box, and two from electronic filtering.

    See the Thiele papers.

    Ruediger

  7. #7
    Senior Member Lee in Montreal's Avatar
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    Quote Originally Posted by Ruediger View Post
    ... and end up at 22 Hz. You need an electronic xover which can give You the missing two orders: 4 orders come from the box, and two from electronic filtering.

    See the Thiele papers.

    Ruediger
    Hi Ruediger. You lost me here ;-) How will the 12db electronic crossover affect the lower end? Do you mean a small "bump" down there?

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    It's easier than You may think.

    Quote Originally Posted by Lee in Montreal View Post
    Hi Ruediger. You lost me here ;-) How will the 12db electronic crossover affect the lower end? Do you mean a small "bump" down there?
    General Audio Discussions -> Technical References Thread -> Thiele Papers Part I -> Page 6 resp. 186: "Sixth-Order Butterworth Response". See also Figure 6 on page 8 resp. 188.

    A filter of any order can be achieved by chaining filters of order one and two. Four orders will be supplied by the box, and two by the electronics.

    You even have three choices, which two of the six orders You handle by electronics, and which four by the box. Not all of these choices will always lead to a good design.

    There is a good design in the tables of that paper.

    May I say "EV DX-38" again?

    Ruediger

  9. #9
    Senior Member ivica's Avatar
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    Quote Originally Posted by Lee in Montreal View Post
    Hi Ivica

    Thanks for your insights. One subject I'd like to discuss with you as well as others is the port tuning. Some people will tune the ports to Fb (the cabinet's resonance), and others will tune to Fs (the driver's free air resonance in order to avoid phase cancellation at very low frequency. Myself, I would tune to Fs.

    It is also my understanding that if a woofer runs lower than the port tuned frequency, it will "unload" and go into high excursion. Which is why I think that if Fs and port tuning are the same (20Hz in this case), then the driver could handle more power before reaching Xmas.
    From my point of understanding Fb is the frequency at which box+port make "resonance", where most of the sound is reproduced by port(s), and the depending of expect-able response of the whole system (driver+box+port) Fb is desired. My suggestion of tuning the system is based on "almost maximal flat response". That is usual tuning for "standard home or studio monitoring us", but, of course, the other more complicated solutions can be arranged. In such case ( flat-tuning) using "much" lower frequency then Fb is not suggest-able, mainly because of large cone excursion, and all the problems that are in relation with that.
    You can find much more detail about in Thiele/Small papers or some presentation of their works.

    On one place :


    http://www.readresearch.co.uk/articles.php
    http://www.readresearch.co.uk/thiele..._article_1.pdf
    http://www.readresearch.co.uk/thiele..._article_2.pdf
    http://www.readresearch.co.uk/thiele..._article_3.pdf
    http://www.readresearch.co.uk/thiele..._article_1.pdf
    http://www.readresearch.co.uk/thiele..._article_2.pdf
    http://www.readresearch.co.uk/thiele..._article_1.pdf
    http://www.readresearch.co.uk/thiele..._article_2.pdf
    http://www.readresearch.co.uk/thiele..._article_3.pdf
    http://www.readresearch.co.uk/thiele..._article_4.pdf

    http://www.readresearch.co.uk/loudsp..._article_1.pdf
    http://www.readresearch.co.uk/loudsp..._article_2.pdf
    http://www.readresearch.co.uk/loudsp..._article_3.pdf

    http://www.readresearch.co.uk/measuring_ts_1.php

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    No need to guess

    Quote Originally Posted by ivica View Post
    From my point of understanding Fb is the frequency at which box+port make "cabinet resonance", and the depending of expect-able response of the whole system (driver+box+port) Fb is desired. My suggestion is based on "almost maximal flat response". In such case using lower frequency then Fn is not suggest-able.
    When using the Thiele paper You start with Your drivers Qt. Locate an entry with the same Qt as that of Your speaker. If there is no exact match You need to interpolate. Read how Qt is influenced by DC resistance of cables and xover coils. Often You can adjust Qt by making it larger.

    One column gives You f3/fs. Fs is that of Your speaker in air. F3 is the -3dB frequency of the whole system.

    Another column gives You f3/fb. Fb is the box resonant frequency.

    Yet another column gives You Cas/Cab which is equal to Vas/Vab. Vab is the box volume.

    Ruediger

  11. #11
    Super Moderator yggdrasil's Avatar
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    Quote Originally Posted by Lee in Montreal View Post
    BTW The intent of the project is to be flat fro 30Hz to 800Hz.
    800Hz? IMO that will be stretching it.
    Johnny Haugen Sørgård

  12. #12
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    JBL ran the 4430 and 4435 up to a 1K crossover with the 2235 and we all know how good they are. I have run the 2235 out to 1200hz and they sounded pretty good. Didn't do any real testing though, it was more of an experiment to compare them to a 2225 in the same cabinet. I also had a 2425 on top..

    Allan.

  13. #13
    Senior Member ivica's Avatar
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    After all, I think, for Lee_in_Montreal these links can be very use full:

    http://www.lansingheritage.org/html/.../1983-subs.htm

    http://www.audioheritage.org/vbullet...ox-12-Cubic-Ft


    Quote Originally Posted by Ruediger View Post

    When using the Thiele paper You start with Your drivers Qt. Locate an entry with the same Qt as that of Your speaker. If there is no exact match You need to interpolate. Read how Qt is influenced by DC resistance of cables and xover coils. Often You can adjust Qt by making it larger.

    One column gives You f3/fs. Fs is that of Your speaker in air. F3 is the -3dB frequency of the whole system.

    Another column gives You f3/fb. Fb is the box resonant frequency.

    Yet another column gives You Cas/Cab which is equal to Vas/Vab. Vab is the box volume.

    Ruediger
    I totally agree with you that all of the T/S are 'dependent' of other ambient things such, as: temperature, humidity, air pressure, amplifier output impedance, cable impedance, applied power, age of the driver, total operating hours etc, but keeping all of that in mind, T/S parameters are very good starting point for any loudspeaker project.

    Some theoretical presentation of the influence of the T/S parameters variations in short, and nice presented in Dr.Keele publication:

    http://www.xlrtechs.com/dbkeele.com/PDF/Keele%20(1973-05%20AES%20Published)Sensitivity%20of%20Alignments .pdf


    There are a lot off free calculating-predicting characteristics programs,
    some short form formulas can be find:

    http://www.audiogearreviews.com/tech...enclosures.asp


    examples:

    http://www.randteam.de/_Software/AkA...ad-AkAbak.html

    http://www.audiogrid.com/audio/avi(2).zip

    http://image.ajdesigner.com/vc/ajvented.exe

    and at the end, all of them suggest that, for 'bass-reflex' (Vented box) design make at list 30% LONGER port duct, and then, by "try-measure-and error" finally decide what would be acceptable duct(s) length.
    From my point of view all of such measurements has to be done in appropriated "acoustical surroundings" (I suggest 'easier solution' would be open air at the roof-top, as we can see JBL did in early days), and has to be repeated in the real 'listening' room.

    Concerning the project that would incorporate some kind of 'active" equalization Dr.Keele paper:

    http://www.xlrtechs.com/dbkeele.com/PDF/Keele%20(1975-07%20AES%20Published)%20-%20New%20Set%20of%20VB%20Alignments.pdf

    would be a good starting point, but as I remember there were some work "in that way" presented in upper mentioned:
    http://www.lansingheritage.org/html/.../1983-subs.htm

    May be some other use full links starting here:

    http://en.wikibooks.org/wiki/Enginee...n#Introduction

  14. #14
    Administrator Robh3606's Avatar
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    Why are you guys making this harder than it has to be?? Run a simulation in a decent box program, tune for the desired frequency response and power handling and you are done.

    Rob
    "I could be arguing in my spare time"

  15. #15
    Senior Member ivica's Avatar
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    Quote Originally Posted by Robh3606 View Post
    Why are you guys making this harder than it has to be?? Run a simulation in a decent box program, tune for the desired frequency response and power handling and you are done.

    Rob
    I can agree with You if the intention is only "to make and forget", but if some kind of deeper understanding is the "goal", then more time has to be spent, and much, much more work too.

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