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Thread: JBL 2245 : 10 or 12cft

  1. #31
    Senior Member ivica's Avatar
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    Quote Originally Posted by Lee in Montreal View Post
    Actually, it gave me some clues, but didn't answer all my interrogations. Per exemple, the ports is de facto tuned to Fb. I can't find how to tune the port to the driver's Fs 20Hz, or somewhere between Fs and Fb.

    So far it goes like that. 10cft vs 12 cft tuned at Fb 27,3Hz

    Response

    Attachment 53670

    Excursion at 350w

    Attachment 53671
    Changing the size (typically the length) of the port-tube you can change "tuning of the WHOLE SYSTEM", (box+port resonant frequency Fb) and what would be the influence of such (tuning or changing) to the WHOLE SYSTEM RESPONSE, you can easily see using mentioned simulation programs.
    Here are some examples:
    it seems that "what-ever-you-do" you will get -6dB around 30Hz....
    so it seems to me that optimum in maximal cone displacement "in band", and "out of band" would be the 'leading rule'.
    Attached Images Attached Images   

  2. #32
    Senior Member Lee in Montreal's Avatar
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    Quote Originally Posted by ivica View Post
    Changing the size (typically the length) of the port-tube you can change "tuning of the WHOLE SYSTEM", (box+port resonant frequency Fb) and what would be the influence of such (tuning or changing) to the WHOLE SYSTEM RESPONSE, you can easily see using mentioned simulation programs.
    Yes. That I know. Only I haven't yet been able to find how to do it with the Excel simulator I have.

  3. #33
    Senior Member ivica's Avatar
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    Quote Originally Posted by Lee in Montreal View Post
    Yes. That I know. Only I haven't yet been able to find how to do it with the Excel simulator I have.
    that depends on what are the INPUT and what are the OUTPUT variables to the program, and what kind of "constrains" are applied.

  4. #34
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    You should read a text read about well-defined filters ...

    Quote Originally Posted by Robh3606 View Post
    That's the point I was making. It took you what 5 minutes if that to find the answer you were looking for.

    Rob
    ... such as Bessel, Butterworth, Cauer, Chebyshew and others.

    These filters do have well defined properties and thus a predictable behaviour.

    If You build *any* filter it will have *any* properties and *any* behaviour.

    It is the merit of Mr. Thiele that he has made the design process rational. And You guys go back to the days when the punchcards were made from wood

    Once You are used to the table in the Thiele paper it will take You less than a minute to understand what is possible with a certain drver, and what not.

    Ruediger

  5. #35
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    Quote Originally Posted by Ruediger View Post
    Once You are used to the table in the Thiele paper it will take You less than a minute to understand what is possible with a certain drver, and what not.
    I personally use Bass Box Pro these days.

    The tables were all we had back in the 70's and 80's. While simulation software has made using them all but obsolete I do think it's important to know that those tables are the basis for the simulation software.

    An interesting side bar, one rather famous JBL engineer was an expert in theory but couldn't design a good sounding loudspeaker if his life depended on it. Another JBL engineer wasn't all that high on theory but he sure could design some great sounding systems. Both still work for Harman after all these years.

  6. #36
    Junior Member jdelange's Avatar
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    Subwoofer port tuning

    Quote Originally Posted by ivica View Post
    Changing the size (typically the length) of the port-tube you can change "tuning of the WHOLE SYSTEM", (box+port resonant frequency Fb) and what would be the influence of such (tuning or changing) to the WHOLE SYSTEM RESPONSE, you can easily see using mentioned simulation programs.
    I followed the subwoofer article by Kramer and Timbers (http://www.lansingheritage.org/html/.../1983-subs.htm). Installed a 12" (29 cm tube + part of baffle) port in an 8 foot cabinet which gave following response (Woofer Tester 2).

    Name:  WT2 subwoof 29 cm poort lr.jpg
Views: 598
Size:  104.2 KB

    This puts the system frequency (fm) at about 26 Hz which is as per the article.

    Yet the fact that the impedance peaks of port and speaker do not match up as they should in a proper tuned system got me confused initially. Therefore I reduced the port length stepwise to get them aligned. It turned out the system requires a port length of about 9 cm + baffle (4"), resulting in a system frequency of 33.5 Hz, again as per the article.

    Name:  WT2 subwoof 9 cm poort lr.jpg
Views: 540
Size:  105.3 KB

    Re-reading the article I now understand that the BX63 filter, electronically compensates for the "error" of the longer port. Though I do not exactly grasp how the bass boost makes the impedance curves of port and speaker match.

  7. #37
    Senior Member ivica's Avatar
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    Quote Originally Posted by jdelange View Post
    I followed the subwoofer article by Kramer and Timbers (http://www.lansingheritage.org/html/.../1983-subs.htm). Installed a 12" (29 cm tube + part of baffle) port in an 8 foot cabinet which gave following response (Woofer Tester 2).

    ...............
    Re-reading the article I now understand that the BX63 filter, electronically compensates for the "error" of the longer port. Though I do not exactly grasp how the bass boost makes the impedance curves of port and speaker match.
    BX63 compensate (boost) lower frequency region taking care of mechanical driver limitations ( Xmax). It does not have any influences of the diver(in the box) impedance.
    Attached Images Attached Images  

  8. #38
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    Since you guys are so good at measuring woofer boxes......
    How about getting busy and measuring the acoustic impedance
    (and timing) mismatches a mid horn .. creates... in a flat baffle cone woof box

  9. #39
    Obviously... not a golfer grumpy's Avatar
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    I'm not sure what this has to do with a 2245 or about
    a suitable cabinet... nor the purpose of the question
    that appears to be more of a statement. Perhaps start
    a new, direct thread.
    It's uh... uh... it's down there somewhere, let me take another look...

  10. #40
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    It does not have any influences of the diver(in the box) impedance.

    grumpy
    I'm not sure what this has to do with a 2245 or about
    a suitable cabinet... nor the purpose of the question
    that appears to be more of a statement. Perhaps start
    a new, direct thread.
    I was just reading the last entry about "diver (in the box) impedance"
    and that horn/cone imped. diff. thing ...popped .up in my head...?!!

  11. #41
    Senior Member ivica's Avatar
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    Quote Originally Posted by ivica View Post
    BX63 compensate (boost) lower frequency region taking care of mechanical driver limitations ( Xmax). It does not have any influences of the diver(in the box) impedance.
    More technical info about BX63 (a):
    http://www.jblproservice.com/pdf/Vin...BX63A%20ts.pdf

  12. #42
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    Bx63

    Just buy one of these things and forget about it.

    http://www.parts-express.com/pe/show...number=300-811

    It has a built-in 25 Hz bump filter (similar to the BX63 but with a lower Q) as well as an extra EQ feature, plus a bunch of power.

    10. Bass Boost

    Selects a bass boost filter with +3dB @ 25Hz, Q=1.4. Allows

    the user to add boost to the low end response without using

    the built-in parametric EQ. This leaves the parametric EQ

    function to address room modes to cut out an excessively

    loud room resonance.

    11. Subsonic Filter

    Selects a subsonic filter with -3dB @ 18Hz, Q=.8. Allows

    the user to remove the subsonic information from the

    signal and tighten the low end response of some

    subwoofer systems.

  13. #43
    Senior Member pathfindermwd's Avatar
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    Quote Originally Posted by 4313B View Post
    Just buy one of these things and forget about it.

    http://www.parts-express.com/pe/show...number=300-811

    I too had been contemplating a 2245 project. Thanks very much for posting this!

  14. #44
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    Quote Originally Posted by 4313B View Post
    Just buy one of these things and forget about it.

    http://www.parts-express.com/pe/show...number=300-811


    Do you have first hand experience with this amplifier? Is the high pass passive? Does it sound as good as the BX63-A?

    Thanks

  15. #45
    Senior Member Ian Mackenzie's Avatar
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    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.
    <What if I increased cabinet size to 12cft?

    Asbolutely no advantage going from 10 to 12 cu ft with an un assisted alignment in a normal domestic situation.

    >Any drawback?
    Yes. Size of enclosure and what is known as group delay in the base response. JBL do not recommend this size enclosure operater above 80 hertz

    Actually, it gave me some clues, but didn't answer all my interrogations. Per exemple, the ports is de facto tuned to Fb. I can't find how to tune the port to the driver's Fs 20Hz, or somewhere between Fs and Fb.

    So far it goes like that. 10cft vs 12 cft tuned at Fb 27,3Hz

    Response. The thin green line is the gain/loss between 10 and 12 cft
    Your simuation is incomplete.

    You need to know where the enclosures will be located and thw resulting effect of room gain in your room.

    The effect of room gain is going to have a much greater impact on the overall reponse than incremental tuning differences.

    When this is accounted for add the room gain to the driver/enclosure simulated response.

    To give you a few clues if you are considering using the 2245 above 80 hertz you will want the smoothest bass in the pass bandband response from 35 - 300 hertz.

    Otherwise use the assisted alignment as advised above for sub woofer applications below 80 hertz.

    The effect of room gain is to create a hump that decends from 120 hertz to 40 hertz with the 2245 in 8 cu ft tuned to 28-29 hertz ie maximally flat response.

    According to Witold Waldman a scientist in Aust who created Calsod the 1st system optimisor software written in DOS, the human ear is particularly sensitive to distortion and response flatness below 100 hertz.

    If your amp has a bass tone control this is quite obvious.

    The size of the hump will vary according to the enclosure location.

    You can triangulate the Q and magnitude of the hump according the height of the woofer centre above the floor and distance from the wall boundary.

    The 2245 has strong boundary reinforcement so it is desirable to have the woofer centre at least 15 inches above the floor level

    Given the size of this enclosure you are not going to have too many placement options so we will assume its located against a floor / wall boundary. Corner placement will exagerate the size of the bass hump.

    In other words it will sound more woolly and possibly boom.

    The idea is to use the room gain to your advantage to extend the system response with minimal ripple or bass hump.

    So how do we fix this without applying active equalization?

    By moving to a larger box and a lower tuning frequency we can see the tilt in the response below 100 hertz which is advantagous to compensate for room gain.

    To tune the system correctly you cannot however pick any combination of the enclosure size and FB and we refer the the formula below.

    Sub optimal tuning will response in ripple in the passband. The effect of this is poor transient performance.

    We also need to consider the dymanic performance and the displacement demands on the woofer in the 35-50 hertz range where the port is going to actively contribute the systems overall dynamic range.

    If you tune too low you will loose some of the benefit of the port in the useful range of 35-50 hertz.

    The un assisted tuning below is a very good balance of box size, bass extension in room and in room response flatness.

    I have used this alignment with the 2245 and in practise ands its better for the above reasons than the 4345 box tuning.

    The beauty of a diy project is you are free of marketing and manufacturing contraints and can design the system to suit you room and your needs.

    Enjoy and have fun.

    10,1cft using Vb=(15)(Vas)(Qt2.87)

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

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

    Remember the Vb is the net volume allowing for bracing, displacement volume of the woofer and effect fill which increases the virutal enclosure Volume due to frictional losses QL.

    In an enclosure of this size the effect of normal fill ie 1 inch fibre glass will cancel out the volume subracted for the bracing and the woofer.

    I recommend 3 four in ports and trim the length so fb is 27 hertz.

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