Tuning points and vent speed

Discussion in 'Home Theater Projects' started by KyleGS, Mar 24, 2004.

  1. KyleGS

    KyleGS Second Unit

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    I'm putting together a little Tempest project that will allow multiple tunning points. I plan on tuning with three 3in flared ports. With three open the tuning will be ~24.5hz , two open = 20hz, one open =16.4hz.

    Initially I though I would never use the 16hz tuning just due to lack of port (lots o' noise) with 350w.


    I plugged and chugged into LspCAD and was a little surprised. These generic results are with 350w and 214 L net.


    Tuning pt:
    25hz with #3 3in ports open- 22 m/s @ 25hz
    20hz with #2 3in ports open- 20 m/s @ 25hz
    16hz with #1 3in port open- 22 m/s @ 25hz

    I'm using 25hz as a reference because this set-up will be 75% music and most DVDs don't extend much lower.

    How can the speeds of these be similar with decreasing port area?

    I know the port air output increase around tuning freq but
    I expected that a Tempest w 350w and only a single 3in flared port would really be puffing at 25hz.

    Will the 16.4hz tuning and a single port be usable with moderate levels?

    Any thoughts?
     
  2. Patrick Sun

    Patrick Sun Moderator
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    Double-check your input parameters in LspCAD for the single port tuning performance?
     
  3. KyleGS

    KyleGS Second Unit

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    I went back through the program- I still get the above results. I'm using one of the first versions of LspCAD. Would that matter? I'm using the "yellow" version.

    I get similar results (although different values) with WinISD PRO.
     
  4. Eric Eash

    Eric Eash Second Unit

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    if 25Hz is going to be your reference, why bother with other tuning points, just tune it so everything to 25Hz is great. also, i show 30m/s with one port, and that is only at 25Hz, play a good bass heavy dvd like fight club, and that thing might be whistling. also, of all the tuning points, i liked 20Hz the best, but i would go with 4" or 5". good luck and keep us posted.

    eric
     
  5. KyleGS

    KyleGS Second Unit

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    I'm not after bass down to 25hz. I simply used 25hz as an arbitrary reference point to compare the different port speeds. I'm going with the multi-tuning for a number of reasons. I like the 25hz option for loud listening levels when quality isn't an absolute must- eg parties, loud movie night with friends and wifey out of town etc. I would prob use the 20hz for normal movie and music where a good blend of SPL and SQ is achieved. Finally the 16hz tuning should be great for low level, high attention critical stereo listening. (hopefully if port noise isn't a problem)

    I've had many bass reflex setups and there are good points to most all of them. With this design it gives me freedom to choose how I want my low end given my mood, company, etc.

    Can someone plug my numbers into a newer verson of LspCAD and see if similar results are produced?
    ----------------------------------------------
    Could this be the reason for LspCAD's numbers:
    At a 25hz tuning the ports will be doing a lot more of the "work" per se (@25hz) and therefore will be carrying alot more air. As you tune lower and lower the woofer is responsible for a lot of the work in the 25hz region and the port only really kicks in around the tuning freq. eg 16hz.
    This is all I could come up with.

    Do you guys think I would run into a tremendous amount of port noise with one 3" port for low level stereo reproduction.

    If I do hear a bit of noise with one 3" port its not a big deal. I think I will be plenty happy with having the option of 20hz and 25hz.

    Sorry for the length- and thanks for any help and opinions.
     
  6. Mike Keith

    Mike Keith Second Unit

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    I played Sine Wave Test Tones at the enclosures Fb (20Hz) in 2 different mockup Sonosubs (245L & 320L) with a Maelstrom driver and used about 8 different ports, 2 different diameters and various lengths both flush and partially sticking out of the Sonosub. I found that WinISD Pro plotted the audible Port Noise at right around 19m/s. Each time I changed the mockup port out with a different length or size I recalculated the Net Volume and plugged in the new numbers and plotted Port Velocity. Each time I heard a port "Cuffing" I noticed the corresponding numbers between 20 and 35m/s, anything in the 30’s was extremely bad port noise at Fb.

    Personally I don’t ever want to hear Cuffing during loud playback, it’s a very disturbing sound, so I choose to go with the larger than normal cross-section port then take any chance. Just don't make it so big that port's own resonance is in the audible band.
     
  7. GeoffM

    GeoffM Agent

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    [​IMG] that's exactly what I was thinking.
     
  8. Eric Eash

    Eric Eash Second Unit

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    okay, first thing i see is at 214L/350W/16Hz tuning, at full power you would exceed Xmax between 23-30Hz and also from 14Hz and below exponentially. at 25Hz tuning, Xmax at 20Hz. at the 16Hz tuning, one port open, 25Hz is 30m/s, 20Hz is 48m/s, and 16Hz is 71m/s. bottom line is if you are going to be playing movies loud, you will hear chuffing, and if you tune it to 25Hz for movies or music, with 3 ports open, i show 35m/s at 25 Hz. also of note, if all 3 ports are the same length, you won't be able to get 25Hz,20Hz, and 16Hz, because the margin between 16Hz (7.5") and 25Hz (9.85") is a big difference. approx tuning using 3 3" ports, 9.75" in length would be 25Hz, 21Hz, and 14.5Hz. this again affects port speed, but not by enough to make any difference. also, all of this is relative to your enclosure size and driver choice. my suggestion for a nice sounding tempest ported box is as follows;

    250L
    tuned to 18Hz
    1-6" aeroport 22" long
    300W

    the 300W keeps the driver under Xmax until 15.5Hz at full power, the big aeroport increases SPL and keeps port speed under 24m/s at all freqs, the slightly larger box extends alot lower for movies while staying flat to 20Hz. calculated max anechoic SPL is 110dB @ 20Hz, and 115dB @ 70Hz. i think SVS might be on to something with their variable tuning, but i think it is more like a novelty. more than a few people have stated that the B4+ is under ported with all 4 4" ports open, let alone porting all but one. anyways, that's my .02

    eric
     
  9. KyleGS

    KyleGS Second Unit

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    I appreciate it Eric. That's what I was expecting to see but I got a little different results with my version of LspCAD. In general I will be listening at low levels. I'll be surprised if I hear much port noise with 2 ports open with my granny listening levels. It will only be cranked on it when there is lots of background noise and the tuning will be 25hz anyway.

    I still think I may go for the variable tuning thing. It's just MDF! One day I'll settle my curiosities and just build a simple bass reflex Tempest like Eric suggested. I think it may be fun to play around with the tuning until then. Besides I always have my 0.7 Q sealed box I could use if I dislike my current project.

    I noticed SVS used a similar design in their PB2-ISD. I wonder how much noise they get with the 16hz tuning?
     

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