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Thread: What to do with 2241H's

  1. #31
    Senior Member RMC's Avatar
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    Hi DT,

    RE: "I agree that cubic foot out of 5 is not going to be much difference either way." But it does make a big difference in favor of the vented box re better effiency and 15 hz lower bass with no sweat here.

    That someone has a preference towards the sealed box low-frequency sound I accept that, all tastes are in nature and that's fine. Its 12 db/oct LF roll-off is a plus and it doesn't "unload" driver like vented box does below Fb.

    I prefer the higher efficiency, lower F3 and reduced distortion of the vented box. I see these aspects as being more important in high level sound reproduction though.

    RE: " Added thought; 12dB boost at 35Hz for the sealed enclosure is flat down to 35Hz, not shabby."

    I'm not sure you realize what this implies.

    According to Eargle's nomograph for determining power ratios in decibels, a 12 db boost requires 15-16 times the power vs one watt. If one had planned on using or uses say 100 watts before EQ (well within the limits of 2241H), then with a 12 db boost applied the power requirement becomes 1,500 - 1,600 W! (10 db 10 X).

    Assuming one has an amp with 1.5-1.6 kw capability, the use of such power at 35 hz on this driver in a sealed box may result in seeing the cone just "flying off" the driver's frame?

    This is just to show that a 12 db boost is a tall order for an amp and more so for the driver. It gets worst faster on sealed box side because of constantly increasing LF distortion as power increases.

    In a vented box, because of the significant cone travel reduction at Fb (35 hz here), the driver would accept much more power with 35 hz EQ, than a sealed box would, also with much less LF distortion. Vent's bonus.

    Maybe this compromise for you: Vent plugged at low levels for sealed box sound and unplugged at high levels sound reproduction for vented box benefits? Regards,

    Richard

  2. #32
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    Hello,

    Yes 1 watt doubled 4 times is 16 watts or plus 12dBs.

    Yes 100 watts times 16 is 1600 watts; however we are not living on that planet. Remember the premise here. This JBL 2241 is operating in a living room in a home, by definition a small space. This JBL 2241 will never see 100 watts input even if boosted 12dB at 35Hz. Worth repeating, try the 2241 with the reflex port open you may like it better. Some folks do not like the sound of phase delay caused by reflex loading. It is more about sound quality preference rather than efficiency.

    For a two way system add a 2432 compression driver and 2384 horn.

    If you find the sub bass lacking add subwoofers or subwoofers under the seating, the house will be rockin. Multiple subs help smooth room modes.

    Thanks DT

  3. #33
    Senior Member Ian Mackenzie's Avatar
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    Attached is s simulation to assist in the discussion.

    On the top left is the bass reflex simulations of 5, 6, 7 and 8 cu ft3 enclosures( Red, green, purple and blue curves respectively). The other lower blue and white curves are the sealed enclosure response.

    The bass reflex tuning frequency Fb was chosen by the simulator for a maximal flat response. The Fb ranged from 39 hertz for the 5 cu ft3 enclosure to 33 hertz for the 8 cu ft3 enclosure. The fb for the 7 cuft3 enclosure is 35 hertz

    Below in the left hand screen is the amplitude response based on sensitivity versus frequency ref 2.83 volts.
    On the bottom right hand screen are the impedance simulations.
    The top right screen demonstrates a comparison of the maximum acoustic output versus frequency for the bass reflex and sealed enclosure simulations.

    My interpretations are as follows:

    Factually we want to know what the system options will do before the build in real terms.

    The trade offs for the bass reflex options are enclosure size versus bass extension and displacement limited output. The equalised sealed enclosure trade offs are enclosure size versus bass extension, system sensitivity and displacement limited output.

    Starting with the sealed enclosure option refer to the maximum acoustic output screen. Note the relative difference in acoustics output of the sealed versus bass reflex options. This translates into dynamic capability and distortion as mentioned by RMC.

    The system resonance for a sealed bass system is very high at 67.9 hertz for the recommended enclosure volume of 3.7 cu ft3. Typically a sealed system would be in the range of 30-40 hertz for a system Q of 0.707. If an equalised response was attempted by shaving the response with a low pass shelf filter at about 40 hertz the system would have a sensitivity of around 90dB for an f3 of 35 hertz (ref 2.83volts) That 9 db down on the nominal sensitivity of 99 dB ref 2.83 volts. Nearly 10 times the power would be required for the sealed enclosure to produce the same sound level.
    But power output is falling dramatically with descending frequency. Comparing the sealed and 7 cu ft3 bass reflex system the difference is 17 dB in displacement limited output (117 dB versus 134 dB).

    By comparison there are far more suitable woofers with could give a sensitivity of 90 dB and an F3 of 35 hertz in an enclosure of less than 3 cu ft3. So this is really not the right application for this woofer.

    Next the bass reflex simulations. Looking at the enclosure size the 5 cu ft3 is already a bulky enclosure with golden ratio dimensions compared to the OP other system.

    So why compromise the performance when significant improvement can be made with a 7 cu ft3 enclosure.
    The 7 cu ft 3 is a bit wider (37Hx23Wx14D versus 33Hx20.5Wx12.7D) This is typical of many Altec and JBL systems of the 50s and 60s.

    Looking at the bottom left screen the 7 cu ft3 enclosure is about 3 dB more sensitive than the 5 cu ft3 enclosure in the bass (91dB versus 94 dB at 35 hertz). Interpret that as power in versus bass output. Putting this driver in an enclosure at the bottom end of the recommended enclosure size sacrifices low frequency extension in the context of a domestic application. Even if 3 dB of bass boost was applied at 35 hertz is double the amplifier power against an additional 2 cu ft3 of enclosure volume viable?

    Increasing the enclosure volume an additional 1.25 cu ft 3 from 7.00 cu ft3 to 8.25 cu ft3 yields an additional 1 dB increase in sensitivity at 35 hertz and 1.5 dB at 30 hertz.

    In summary the drivers manufacturered application cannot to overlooked when considering the correct enclosure type and enclosure volume. Given the desire for a two way system and the drivers sensitivity a sealed system is not recommended.

    Selection of the optimal bass reflex volume is about finding an acceptable balance in the frequency extremes of this relatively sensitive system.

    My recommendation is the 7 cu ft 3 system which gives an acceptable bass response in an un assisted alignment. An assisted alignment may be considered if additional bass extension is required.
    Attached Images Attached Images  

  4. #34
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    Hello and happy New Year,

    We are back from the northern California coast.

    I agree with you all the way down to the fact that the 2241 is not a subwoofer. When it gets to compromises and performance improvements that is where we take different paths today.

    I am assuming that there will be a pair of 2245s added later and that I would like to avoid reflex loaded mid woofers. In the meantime the experiment of boosting the bottom octave of the 2241s in sealed enclosures sounds interesting to me. This may be a perfectly good compromise in a small listening space. I sit with a sound pressure meter in my lap. If I am bumping 90dBs it is too loud.

    It is not always about bigger and louder.

    Thanks DT

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