port dimensions and enclosure design help (UM-18)...

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HaLo6

HaLo6

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Joined 2008
#1
Looking for help with optimal port size 🙂 ... I'm building a down-firing vented enclosure for a UM-18 (18x24x58 -> 11.7ft3) (Optimal size according to PartsExpress/BassBox pro)

Tried using WinISD but it wont let me change any of the drop down sizes...

Any help would be appreciated... this is my first sub build.
 
#2
Parts Express recommends the following vented enclosures:

Car audio recommended alignments:
• Vented 4 cubic ft. (net internal, not including driver or vent ) tuned to 22 Hz, with 2 flared vents 4" diameter by 32" long, for an f3 of 24 Hz

Home theater/stereo recommended alignments
• Vented (home theater): 8 cubic ft. (net internal, not including driver or vent) tuned to 20 Hz, with 3 flared vents 4" diameter by 28.5" long, for an f3 of 18 Hz
• Vented (infra-sub): 12 cubic ft. (net internal, not including driver or vent) tuned to 15.5 Hz, with 3 flared vents 4" diameter by 32" long, for an f3 of 15 Hz


If you want to design it yourself, this tool can calculate the minimum port diameter (after the box volume and tuning frequency have been determined with WinISD or BassBox pro): Flare-it - Free Speaker Design Software

Then WinISD (maybe BassBox Pro too) can calculate the corresponding port length.
 
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#3
HaLo6 said:
Looking for help with optimal port size 🙂 ... I'm building a down-firing vented enclosure for a UM-18 (18x24x58 -> 11.7ft3) (Optimal size according to PartsExpress/BassBox pro)

Tried using WinISD but it wont let me change any of the drop down sizes...

Any help would be appreciated... this is my first sub build.
Click to expand...

You can manually type in any port diameter (or w x h) values you want, you don't have to stick with the defaults in the dropdown.
WinISD then calculates the port length that goes along with your given inputs.
 
#7
HaLo6 said:
I tried this calculator and if I use a single 6" flared port I can do one 29" port...

Products Menu

and use their 6" port?

6" Precision Ports

Thanks for the help all!
Click to expand...

Hmm, the more accurate Hornresp [AFAIK] calculates that at 1 kW/16.7 Hz, even a flared Sd/2 = ~609 cm^2 [~11" dia] is barely going to meet the ~5% max vent mach [~17 m/s] required to ensure minimal audible 'chuffing' [~3.5-4% max to ~'silence' it], so the [3] smaller 4" [~243.22 cm^2 combined] is much too small, flared or not unless the flare is much larger than in the kit [like small tractrix horn size on both ends], making the relatively tiny 6" [~182.42 cm^2] an acoustical choke, though some prefer this to use for a bit of driver protection and/or as an audible reminder to turn down the power.

All that said, if the sub is only required to need much power down this low for movie LFE, then a lot of folks perceive some audible vent noise as part of the 'action' or never actually need it if there's a 'rumble' [high pass] filter to protect it, in which case dual 6" is theoretically sufficient.

GM
 
#8
GM said:
Hmm, the more accurate Hornresp [AFAIK] calculates that at 1 kW/16.7 Hz, even a flared Sd/2 = ~609 cm^2 [~11" dia] is barely going to meet the ~5% max vent mach [~17 m/s] required to ensure minimal audible 'chuffing' [~3.5-4% max to ~'silence' it], so the [3] smaller 4" [~243.22 cm^2 combined] is much too small, flared or not unless the flare is much larger than in the kit [like small tractrix horn size on both ends], making the relatively tiny 6" [~182.42 cm^2] an acoustical choke, though some prefer this to use for a bit of driver protection and/or as an audible reminder to turn down the power.

All that said, if the sub is only required to need much power down this low for movie LFE, then a lot of folks perceive some audible vent noise as part of the 'action' or never actually need it if there's a 'rumble' [high pass] filter to protect it, in which case dual 6" is theoretically sufficient.

GM
Click to expand...

Ok... cool half that went flying over my head lol... what about my 1.75-2” x 22” rectangular vent idea? What occurs when a vent is too big?
 
#9
Try to keep the aspect ratio (port length / port width) close to square, otherwise the friction losses increase. As a rough guideline, up to 1:5 is good and do not go beyond 1:9 unless some loss is acceptable.


A large cross section results in a large length. If the vent is too long, the first pipe resonance lies within or close to the operating frequency band. The first resonance occurs when the port length is 1/2 wavelength. To get some feeling for it, a 1 meter long port has a resonance at 172 Hz, at 2 meter long port at 86 Hz. If this resonance is a problem, replace the port by a passive radiator (expensive solution) or opt for a transmission line enclosure.
 
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#11
ok... so If I pull in my braces a bit (I'm trying to use those as the side walls of the port) to 20" and bump up the height of the port to 3" (1:6.3 ratio) that gives me a 1st port res at 168Hz which is above the high pass filter I'd be targeting for the sub... so that should be good right?

It seems that MartyCubes use a 2" x 20" ish port...

The whole port engineering thing confuses the hell outta me lol
 
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