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Thread: Vent tuning on M2

  1. #1
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    Vent tuning on M2

    I ordered a pair of the 2216nd woofers for my new build (2way vented cabinet with a horn tweeter). They finally arrived.

    For the bass bin, two obvious candidates to model are the M2 and s-4700, since they are both vented designs and use the same 2216nd woofer.

    So far my reading has uncovered the s-4700 is a 4.8 cu ft (135 liter) cabinet with a vent tuned at 32 Hz (shown in the EDS sheet posted elsewhere).

    The specifics on the M2 are more elusive. It looks like the cabinet is about 10% larger (perhaps 5.2 cu ft). Folks have suggested the vent tuning might be in the mid-20 Hz region. This seems low since the free resonance of the woofer is either about 36 Hz or 39 Hz (I have seen both specs on the EDS sheet). However the comments about the vent tuning seem to be more speculative or second hand.

    So my question is: has anyone measured the impedance in the region around the vent tuning (say 20-40Hz) with a RMS meter, power resistor and tone generator? Of course the DSP filters would need to set on bypass for the the measure to be meaningful.

    It is actually fairly easy to measure, but requires access to an M2 cabinet which I do not have. Maybe someone has already made the measure.

    Thanks for any assistance or reference.
    -Tom

  2. #2
    Senior Member baldrick's Avatar
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    Its not uncommon with lower tuning than FS, org DSP settings also have a HP filter of 21hz that might indivate that tuning is aprox 21

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    Quote Originally Posted by baldrick View Post
    Its not uncommon with lower tuning than FS, org DSP settings also have a HP filter of 21hz that might indivate that tuning is aprox 21
    Thanks for your input.

    The rule of thumb that I had heard was that the vent tuning should not be less than about 75% of the free air resonance (IOW, about 25 Hz). Going below this, one would certainly want a high pass filter to protect the cone from a wild excursion. I also assume that tuning the vent "lower than necessary" has a penalty of decreased efficiency.

    However, it is not clear what you think the vent tuning might be. Do you think it is in the low-20's? Have you seen this measure?
    Thanks,
    -Tom

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    Senior Member baldrick's Avatar
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    According to the Attached document it is in fact 27hz and 130l Box, but i've also read that it is 21, so I'm not sure, but my Guess is that the document is pretty accurate.
    Attached Images Attached Images

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    Quote Originally Posted by baldrick View Post
    According to the Attached document it is in fact 27hz and 130l Box, but i've also read that it is 21, so I'm not sure, but my Guess is that the document is pretty accurate.
    Thanks for the attachment.

    I may have come across it before when I was first researching this. I am not trying to pick a fight with whoever created it, but there are some things that don't make sense. For one thing I am fairly confident that the internal 130 liter volume of an M2 is actually larger. Even with the larger horn, the M2 should have a larger internal volume than the s-4700 cabinet (the M2 is a bit wider and about 5 inches taller).

    Like you, I have also seen numbers on the vent tuning that range from the low 20's to the high 20's.

    I already have mostly completed the winISD modeling of the s-4700 cabinet since I have the EDS sheets on the s-4700 (including the vent tuning and geometry as well as the cabinet's net volume) and the 2216Nd woofer.

    Now that I think about it, maybe I am mistaken about the net internal volume of the M2 (5.2 cu ft or 145+ liters)

    Thanks,
    -Tom

    edit: No, the internal volume on the M2 should be about 5.2 cu ft and clearly larger than the s-4700

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    Senior Member Ian Mackenzie's Avatar
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    I agree it is odd.

    I did a simulation in bass box .

    After you subtract the bracing , drivers and horn then allow for fibreglass the net value is around 130 L.

    Just start off bigger and reduce if you feel you need to.

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    [QUOTE=Ian Mackenzie;414360]I agree it is odd.

    I did a simulation in bass box .

    After you subtract the bracing , drivers and horn then allow for fibreglass the net value is around 130 L.

    Just start off bigger and reduce if you feel you need to.[/QUOTE
    ----------------------
    Thanks for your comments. So I guess you are advocating that I assume the s-4700 and the M2 are both "about" 130 liter. In one case (s-4700) the vent is tuned to 32Hz and the other ( M2) tuned to mid 20's.

    This may partially explain why the s-4700 has about 2 dB more sensitivity, but I am only guessing. I need to check against a winISD simulation to see how much the sensitivity could change went the vent is tuned lower.

    I re-read the massive M2 threads and it looks like no one has taken a volt meter and power resistor and actually measured the vent tuning.

    Thanks,
    -Tom

  8. #8
    Senior Member Ian Mackenzie's Avatar
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    Hi Tom,

    When Jbl designs and builds these things there are several inputs not just the designer. For example marketing who always want a small footprint and that means a smaller volume.

    I would make it deeper and not as tall to suit a home situation assuming you plan to use it at home.

    That said if you can find the DSP EQ setting and look at that simulation l am of the view it was a 27 hertz tuning.

    My reasoning is that anything from 27-34 hertz gives a range of viable tunings and offers enough useful output from the port to give the woofer subjectively more headroom.

    A tuning like 21-25 hertz range such a Low tuning that the port would not offer appreciable headroom in the 27-40 hertz range where its most needed most of the time. (Jbl does offer a sub for the M2)

    They then did a global dsp EQ on the whole thing and increased the bass incrementally and the crossover and horn EQ.

    My quick calculation of the volume was 44 x 30 x 110 cm =145 L gross volume.
    I then allowed 6 L for the woofer, 10 L for the Horn - driver, 10 L for bracing.

    So that gives enclosure vol 145L - internal deducted space 26L = 119L.
    The effect of fibre on 5 internal walls is to add approximately 10% volume.

    You end up with a net volume of about 130L as seen by the woofer.
    A few Litres either way isn’t going to make a scrap of difference. What matters is manual adjustment of the ports to get the real port tuning frequency (Fb).

    This is my educated guess from the coffee shop and l have no doubt others will weigh in with their opinions which is fine.

    As l said earlier just make it a bit bigger (like the 4700) and you have the option of a simple un assisted tuning if you prefer.

    It’s diy so you are allowed to be flexible unlike the JBL offer to the market.

  9. #9
    Senior Member Ian Mackenzie's Avatar
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    [QUOTE=withTarragon;414365]
    Quote Originally Posted by Ian Mackenzie View Post
    I agree it is odd.

    I did a simulation in bass box .

    After you subtract the bracing , drivers and horn then allow for fibreglass the net value is around 130 L.

    Just start off bigger and reduce if you feel you need to.[/QUOTE
    ----------------------
    Thanks for your comments. So I guess you are advocating that I assume the s-4700 and the M2 are both "about" 130 liter. In one case (s-4700) the vent is tuned to 32Hz and the other ( M2) tuned to mid 20's.

    This may partially explain why the s-4700 has about 2 dB more sensitivity, but I am only guessing. I need to check against a winISD simulation to see how much the sensitivity could change went the vent is tuned lower.

    I re-read the massive M2 threads and it looks like no one has taken a volt meter and power resistor and actually measured the vent tuning.

    Thanks,
    -Tom
    I have not looked closely at the 4700 for a while so l am not sure on that.

    On the tuning the inductance of the woofer will often through out the voltage reading.

    Flared ports of various types are all different so simulation is not always accurate

    Run a sine wave and carefully check for the minimum real motion of the woofer . That is the real tuning frequency

    Edit

    2216Nd in S4700 enclosure:
    Vb = 135 liters
    Fb = 32 Hz

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