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Thread: Box volume and accoustic treatment

  1. #1
    Senior Member Eaulive's Avatar
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    Box volume and accoustic treatment

    Hello.
    According to the litterature, adding insulation (or mineral wood or fiberglass wool or stuffing or pillows or whatever you call it) inside a cabinet actually increases the effective volume of that cabinet.
    Is there any way to calculate that? Any data?

    By the simulations I made in WinISD, I built a 254l cabinet tuned to 27Hz with ports of 32cm.
    After measuring, the cabinet actually has it's impedance dip at 24-25Hz which would suggest a volume of 321l with that port.

    Question: Can the adding of "stuffing" increase the effective volume from 254l to 321l?
    I don't feel like stripping clean the interior of the cab just to check, is it a good idea to assume my cab has now a volume of 321l and base my port calculation on that?

    Thanks for any insight.
    My avatar: 4520 loaded with 2225H on E140 frames,
    1x 2202H on custom front loaded horn, 2x 2426 on 2370.

  2. #2
    Senior Member grumpy's Avatar
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    I’d be more inclined to think the port length just needs adjusting, but if you’re not in a hurry, I can put this into bb6 with zero damping and then overstuffed to ballpark check the range one might expect (?)

    Edit:

    Using 3x 9.2"L 4" dia ports and just changing the stuffing from none/typical/heavy, moved the minima from 29->26Hz or so (using the same vents, typical stuffing, but substituting 321l, does result in a 24Hz tuned box... perhaps re-check volume calculations?):

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    if you're able to shorten the ports a few inches (to 5-6"), you might get back to your target... do double-check me before cutting

  3. #3
    Senior Member Eaulive's Avatar
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    Thanks Grumpy, very useful.

    I first checked if we have the same T/S parameters for the simulation and we have.
    However in WinISD, for a 254l box with heavy filling and a 29.1Hz tuning frequency I have a port length of 10.4" and you get 9.2".
    In your simulation of "no filling", with the same volume, does the port length returned by the program is the same? I would be curious to know that.
    My guess is that it changes, but by how much?

    I definitely know my ports are too long, I don't know why I ended up with 12.5" ports but I must have had a brain freeze at some point, I made so many simulations.

    To endup with the 320l of effective volume I think I have, I measured the impedance dip with a generator, a 0.5R resistor and a voltmeter across.
    Then I enter the frequency where the box resonate (24Hz) and played with the box volume in WinISD until the port length returned is the same as what I have now, 12.5".
    I ended up with 321l.
    I say 24Hz, but it can be 23.5-24.5, my generator has 1Hz increments.

    Anyways, I still have a lot of fun, building them was only the beginning
    My avatar: 4520 loaded with 2225H on E140 frames,
    1x 2202H on custom front loaded horn, 2x 2426 on 2370.

  4. #4
    Senior Member Eaulive's Avatar
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    Ok Grumpy, I just realized that in your program you enter a port length and the programs returns the tuned frequency.
    WinISD is the other way around, you choose a tuning freq. and it returns the port length.

    You said from 26 to 29Hz with no filling to heavy filling and a 9.2" port.
    I'm gonna crunch some numbers...

    Thanks!!
    My avatar: 4520 loaded with 2225H on E140 frames,
    1x 2202H on custom front loaded horn, 2x 2426 on 2370.

  5. #5
    Senior Member grumpy's Avatar
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    "I just realized that in your program you enter a port length and the programs returns the tuned frequency.
    WinISD is the other way around, you choose a tuning freq. and it returns the port length."

    Right. I had to try it a few times to hold the change down to just having the stuffing amount affect the tuning,
    but I think I got it right. BB6 and WinISD were pretty close when the same constraints were set
    (including the 'typical' stuffing level, which I don't know how to set in WinISD)

    Also, I might have used 28Hz instead of 29.1 for the nominal/typical case, so don't worry about a sub-inch difference in port length.

    Unofficial rule of thumb/WAG: normal amounts of damping material mostly compensate for the loss of volume for the driver and bracing.

  6. #6
    Senior Member RMC's Avatar
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    Hi Eaulive and Grumpy,

    RE "I have a port length of 10.4" and you get 9.2"."

    Linear team, the makers of Win ISD, issued an interesting paper some years ago, which i have somewhere, about the different port lengths that various software programs yield for the same parameters. They compared 8-10 known programs, including ISD and Bass Box as i recall, on Lv and explained why/where the differences come from.

    The port math formula used and the speed of sound plus temperature assumptions were part of the explanation.

    Therefore i would't lose sleep over small differences. I use ISD and Winspeakerz, then start with the longer of the two port lengths i get and adjust Lv on the box for the tuning frequency i want.

    As i remember, Bass Box was not necessarily the most accurate every time for Lv, if you take for granted that a majority of programs with similar results must be right. But they could all be "wrong" too... Regards,

    Richard

  7. #7
    Senior Member Eaulive's Avatar
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    I'm gonna tell you something weird.
    I have three ports, before cutting them I made some impedance tests.
    With the three ports open, dip was at 24Hz, with one port blocked (2 open) dip was at 21Hz, with two ports blocked (one open) the dip was at 16Hz
    Now this appears to be perfectly normal but after removing 4" from each port which is supposed to tune the box to 32Hz, the dip is still at 27Hz.... this is why I think that something in the box makes it appear larger.

    My first tuning was intended to be at 28Hz and it ended up at 24Hz, now this new tuning is intended to be 32Hz and it really is at 27Hz.

    Anyways, I am now more satisfied with the response of the enclosure, the curve appears to be smooth, except for that darn cancellation around 45Hz in my room

    Thank you all for your input!
    My avatar: 4520 loaded with 2225H on E140 frames,
    1x 2202H on custom front loaded horn, 2x 2426 on 2370.

  8. #8
    Senior Member Ian Mackenzie's Avatar
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    What l would do is a physical determination of the minimum diaphragm motion to get the true port resonance.

    Use a simple oscillator down load for this.

    According to Greg Timber’s woofers with large inductance are difficult to model.

    Greg said the measured impedance dip will always be a bit lower than the actual port resonance.

    If you are using three ports depending on the port location(S) the practical results can vary.

  9. #9
    Senior Member RMC's Avatar
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    Eaulive,

    RE "this is why I think that something in the box makes it appear larger."

    You need a fair amount of damping material in a box to make it appear larger (virtual volume) and my guess is i doubt you are there if you used a normal amount of insulation (e.g. 1" on most panels as JBL used).

    Your persistent 27 hz Fb may possibly be explained by the following (maybe not that weird):

    A) You are using multiple ports. And these have an impact on Fb caused by air drag on tube walls. They may require an Lv adjustment. See "Notes on Porting" below from JBL's Drew Daniels (excerpt) (Note Daniels mentions a 10% to 20% Lv adjustment, another JBL doc mentions 5% to 10%. Which is better, i don't know).

    B) As i recall your 3 ports are close to a side wall of the cabinet. There's a possible interaction between the vents and wall/insulation?:

    "Overall, it's safest to locate the port somewhere on the baffle with the woofer(s) far enough away from side walls to avoid interaction between port and enclosure wall or the fiberglass insulation on the wall." (JBL)

    C) Another item that may reduce vent effectiveness is nearby obstacles (e.g. bracing) impeding vent free air flow?

    I'm assuming you calculated correctly box volume for all that goes in the box (drivers, bracing, vents, etc).

    Since you're using Win ISD this must be reminded: You may (or may not) be aware that it uses for default box loss assumption QL10 (more optimistic, not standard), the norm being QL 7 for modeling (as on most other softwares). This has an impact on proper box volume, and many people forget to change in ISD the QL10 to QL 7. For large boxes more prone to losses, using QL 5 is even more appropriate, creating a larger box volume which compensates for those losses. Is the use of QL 10 a part of the explanation regarding the volume discrepancy between you and Grumpy? I assume Bass Box default he used is the standard QL 7, and can be changed like in ISD and Winspeakerz.

    Finally, for small Vb deviations don't worry both EV and Eargle mention that up to 5% error on box volume has no material effect on sound.

    Richard

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  10. #10
    Senior Member Ian Mackenzie's Avatar
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    I am not familiar with your enclosure plans

    One way of checking your in the ball park enclosure size is this:

    Your enclosure internal dimensions give you a gross enclosure vol in cu ft.

    If you allow 1/10 of the gross volume for all solid objects inside the enclosure including the bracing, the driver and port and lay one inch fibreglass wadding on all surfaces you will end up with a net enclosure volume about the same as the internal dimensions.

    This is a rule of thumb. If you used other than fibreglass you will loose 10% of the gross enclosure volume. In that case your going to re think your tuning frequency anyway.

    Typically a large enclosure unless it has very rigid panels will have a QL less than 7, about 5.
    So l would use QL= 5 in your simulator as a starting point.

    As mentioned by other(s) the port is not an ideal resonator and will interact with other surfaces and other ports and boundaries inside the enclosure. The effect is frictional losses on the air load lowering the effective port frequency.

    So your in the land of empirical tuning which means cut an inch off the ports at a time. Listen and check the tuning frequency visually. It’s relatively obvious when the tuning a right. The port will provide aggressive bass to compliment the woofer without boom.

    If you feel the bass is still lacking the box could be overall a bit too large.
    So reduce it one cu ft with a solid object(s). Re tune the ports and listen.

    If you feel the bass lacks definition or has a boomy under damped quality then your enclosure is too small. Yes it means a new enclosure. Adding additional fibreglass is NOT the answer as this will absorb Bass energy.

    BTW if your not to using an active crossover the dcr of the choke in series with the woofer effects the correct tuning frequency. Typically you will need to make the enclosure bigger and tube to a lower frequency to avoid a boomy response in thus situation.

    If you want to get really scientific you need to make a test box check the TL parameters and then calculate your prototype enclosure. It’s trial and error. A simulation is only as good as the assumptions it’s based on.

    A simulation is really only the beginning to obtaining an optimal tuning in practice. The port length advised by a simulation is not a given. You still need to asses the tuning correctly.

    If you want to contemplate some tuning make the box 10-15% larger than the suggested tunung for your Woofers Qt

    So if your woofer has a QT of .2XX. Add 5 % to the QT value. Is will compensate for cable and power amp output impedance. Your TL tables will suggest a bit bigger box by about 10%.

    So if your gross volume was 254LT make it 279LT.

    Check you are using a QL = 5 for this calculation.
    The difference is physical enclosure size is minor.

  11. #11
    Senior Member grumpy's Avatar
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    I used QL=5 for both.
    the woofer minimum motion test seems worth the effort.
    Other than that, the box is what it is. Tune to taste.

  12. #12
    Senior Member Eaulive's Avatar
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    Hello all.

    Interesting thread... Well, honestly I'm quite satisfied with the response now, I did some sweeps with a generator/anaylzer between 20Hz and 150Hz at different points in the room to avoid the modes ans overall it's very clear that I have a flat response (+/- 1dB) down to 32Hz. at 32 it starts rolling off smootly. (that is, with the EQ I simulated in Win ISD which is +3dB @ 28Hz Q1)

    So, to answer your questions, after retaking the measurements my enclosure has 253l, the bracing takes up 7l so 245l.
    I did not calculate the volume taken by the 2370/2426, I guess around 3-4l.

    My ports are setup 1.5" from the side of the internal wall and the top one is 4.5" from the top wall. They are spaced 5.5" apart, measured from the center, just like the 4345.

    There is no stuffing behind or around the ports.
    However my box is not like the 4345 because the midbass enclosure takes the whole depth so there is this space above it where the 2370/2425 lies that is kind of "isolated" from the rest.

    Take a look at that picture:


    I don't know if this configuration may have a negative effect....

    Anyways, like I said, I'm quite satisfied now, I listened to different materials and the bass sounds smooth and linear across the band.
    They don't have the punch of my 4520s obviously, but that was not the intent, they are smoother and run deeper... a lot deeper

    I'm just gonna post the link of the other thread for future references.
    http://www.audioheritage.org/vbullet...finement-blues
    My avatar: 4520 loaded with 2225H on E140 frames,
    1x 2202H on custom front loaded horn, 2x 2426 on 2370.

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