Announcement

Collapse
No announcement yet.

Seeking advice for down firing port length/tuning in a subwoofer enclosure

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • unclejunebug
    replied
    I could see how a slight different in the height could make a big difference for a slot port, especially at 52" long. I believe the modeled port length I got was somewhere around 25" and I would certainly use some kind of spacer to ensure correct slot height. Though I think I've convinced myself to go with the Precision Port. It will take up a little less space in the cabinet and will be easier to terminate alongside the veneer.

    Leave a comment:


  • Chris Roemer
    replied
    10 y.o. build, already! HiVi SP-10. SA250 amp (w/NO boost).
    12"w x 17-1/2"h x 18-1/4" long (deep). Driver chamber ended up at 0.88cf i THINK (after amp & GIANT MAGNET). LOOKs like about a 2 cu.ft. sub (size-wise).
    Slot is 10-1/2" wide by 1" high, x 52" long. Target Fb was 24Hz. If it didn't turn out exact, it wasn't off by more than 1 Hz or so.

    The slot "height" ends up to be a critical dimension, as a slight variation in that short dim. (times the slot's width) can change the tuning quite a bit.
    For the same tuning at only 7/8" high, the slot length would have to change by about 8".
    I used 1" spacers between the 3/4" slot "fins" when doing the glue-up. I THINK the white material might have been a pre-finished shelving board (but I did not purposely sand/paint it - except maybe the end "turns")? It IS pretty smooth, though. Exit is rounded-over.

    This (used - door prize from meniscus) driver did not match HiVi's T/S specs. The Qts was pretty close (0.56), the Fs not much higher (40Hz), but the Vas was quite small (0.26 cu.ft.). The surround on this thing was like an over-inflated bicycle tube. You could lay the (heavy) driver face-down on the surround and it would barely budge it. These specs/box/tuning look pretty funky in WinISD - until you throw a 50Hz low pass on it. It's only used for the bottom octave (with 8" mains). Output peaks right at the tuning freq.

    Leave a comment:


  • unclejunebug
    replied
    Originally posted by Chris Roemer View Post
    This slot isn't down-firing, but no reason why one can't be (with clearance).


    What was the expected tuning and the slot height you used? Looks like maybe 1" to 1.5"?

    Leave a comment:


  • Chris Roemer
    replied
    This slot isn't down-firing, but no reason why one can't be (with clearance).



    Leave a comment:


  • unclejunebug
    replied
    Originally posted by billfitzmaurice View Post
    Your 1.5 x 12 port will probably give a lower Fb and higher velocity than predicted. There's a zone of high friction that extend perhaps 1/4 inch from the walls, effectively removing that zone from the port area. That's not a problem when the smallest dimension is 3 inches or more, but it makes that 1.5 inches act more like 1 inch. This, BTW, is the reason why all ports tend to give a lower than predicted Fb. As for the precision port, whatever chuffing that it may create will be filtered by the down firing configuration.
    Thanks for the confirmation, Bill! I will go forward with the 4" Precision Port then. That was my hope since returning the darn thing would have cost about the same, if not more, than what I paid for it.

    Leave a comment:


  • billfitzmaurice
    replied
    Your 1.5 x 12 port will probably give a lower Fb and higher velocity than predicted. There's a zone of high friction that extend perhaps 1/4 inch from the walls, effectively removing that zone from the port area. That's not a problem when the smallest dimension is 3 inches or more, but it makes that 1.5 inches act more like 1 inch. This, BTW, is the reason why all ports tend to give a lower than predicted Fb. As for the precision port, whatever chuffing that it may create will be filtered by the down firing configuration.

    Leave a comment:


  • unclejunebug
    replied
    I recalled the other day why I originally decided against a slot port. The reason being that I was wanting to flare any port that I used so if I went with a slot port I'd add a round-over to the port opening. Since I'm planning to cover the enclosure with a walnut veneer the round-over didn't seem to be so compatible with that type of finish. After thinking about it a little more I don't think this would pose as big of an issue as I originally thought. I could even make the round-over deep enough to where the bit could cut a nice 90 degree edge at the start of the round-over which would make for a good transition for the veneer.

    I originally ordered a 4" Precision Port kit and was planning to use that but the the thing is massive and the air port velocity I'm getting in WinISD is a little higher than I'd like (max at just over 24 m/s) while the slot port provides better results (right at 16 m/s). Is the port velocity of the 4" Precision Port a concern at all?

    Click image for larger version

Name:	flared_port_vs_slot_port.png
Views:	119
Size:	61.8 KB
ID:	1481867

    Leave a comment:


  • unclejunebug
    replied
    Originally posted by Chris Roemer View Post
    The Frequency of resonance (Fs, Radio Shack used to call it Fo <-this being a zero), measured in "free air" (meaning no box, not even a baffle) does not change with (a "reasonable amount of) drive voltage. Many modern "systems" (like DATS, or the WooferTesters) use something below 2.83v (I THINK?). Maybe more like 1 volt? They run a 20-20kHz sweep and it's not very loud.
    That's good to know, thanks, Chris. I'm currently still in prep phases so it will be some time before I have an enclosure to actually test with but when I get there I will make sure to keep the volume low. I can't imagine a 20-20kHz sweep being very pleasing to the ear when blasted haha.

    Leave a comment:


  • Chris Roemer
    replied
    The Frequency of resonance (Fs, Radio Shack used to call it Fo <-this being a zero), measured in "free air" (meaning no box, not even a baffle) does not change with (a "reasonable amount of) drive voltage. Many modern "systems" (like DATS, or the WooferTesters) use something below 2.83v (I THINK?). Maybe more like 1 volt? They run a 20-20kHz sweep and it's not very loud.

    Leave a comment:


  • unclejunebug
    replied
    Originally posted by billfitzmaurice View Post
    No, but tread very carefully with series resistors. It takes a surprisingly low power level to toast them. The same applies to using a DMM in current measuring mode.
    Thanks for the heads up, Bill! So if that's the case then, I assume volume also doesn't have an effect on resonant frequency? So I should keep the volume level fairly low to avoid toasting my resistor and DMM but still be able to measure?

    Leave a comment:


  • billfitzmaurice
    replied
    No, but tread very carefully with series resistors. It takes a surprisingly low power level to toast them. The same applies to using a DMM in current measuring mode.

    Leave a comment:


  • unclejunebug
    replied
    Perhaps a silly question but is resonance frequency affected by either the amount of wattage driving the woofer or the ohm load of the woofer itself? I assume that it is not since the steps listed at that link suggest connecting a resistor in series with the driver increasing the load which I would then expect to reduce the amp's power output.

    Leave a comment:


  • unclejunebug
    replied
    Originally posted by tktran View Post
    With a multimeter and a website like audiocheck.net

    Have a read or Brian Steele’s excellent

    http://diysubwoofers.org/faq.html

    Thanks for that link! A single 4 ohm resistor is a level of test equipment I can currently get on board with

    Leave a comment:


  • tktran
    replied
    With a multimeter and a website like audiocheck.net

    Have a read of Brian Steele’s excellent website
    on how to check the tuning frequency of a box:

    http://diysubwoofers.org/faq.html
    Last edited by tktran; 01-24-2022, 08:13 AM.

    Leave a comment:


  • unclejunebug
    replied
    Originally posted by tktran View Post
    DATS or an impedance jig to use with REW or ARTA is one of the first and most useful things to building your own speakers.
    Or a least a multimeter and some test tones online

    Modelling only takes you so far; in fact I always find that the suggested port Length is often longer than is necessary.

    Here’s a speaker with a downfiring port. The tuning frequency was anticipated to be 24Hz; but ended up being 20Hz.

    Was it my drivers, was it my damping, was it my down-firing port?

    too many variables and the only way to know is through measurements
    I completely understand the benefit of being able to take measurements. As far as my own builds go, I've only ventured as far as building a subwoofer and haven't justified the cost of the equipment. If I were more serious about designing my own speakers and crossovers I'd be more inclined to do so, and maybe someday I'll get there.

    You mentioned test tones and a multimeter. Is this some form of testing that can be done without a microphone?

    Leave a comment:

Working...
X