Professor.Ham.Slap Posted June 4, 2004 Share Posted June 4, 2004 Alright, the time has come and I'm finally building my sub enclosure. I got the 4" diameter precision flared ports from Parts Express... My question is, how do I measure them properly? Is it from tip to tip, or just the tube that connects the flared ends? Quote Link to comment Share on other sites More sharing options...
John Warren Posted June 4, 2004 Share Posted June 4, 2004 ---------------- On 6/4/2004 2:37:42 PM Professor.Ham.Slap wrote: Alright, the time has come and I'm finally building my sub enclosure. I got the 4" diameter precision flared ports from Parts Express... My question is, how do I measure them properly? Is it from tip to tip, or just the tube that connects the flared ends?---------------- Actually, it's the volume of air and hence the mass of air contained in the port that is important. Consider how it operates, the air mass in the port and the compliance of the air in the enclosure form a "spring-mass" system that will have a resonance frequency. Changing the port length changes the mass (larger port = lower resonance frequency). The flange provides correction for end effects which can make a straight port "look" longer than the length you cut it. In other words, if your software package says you need a 4" diameter port that is 6" long and you know that end effect compensation is not considered, the 6" port will need to be reduced by a factor of about .3 X D where D is the port diameter to compensate for end effects. Since you have a flared port you can cut to the exact length (tip to tip) without any correction. Quote Link to comment Share on other sites More sharing options...
formica Posted June 4, 2004 Share Posted June 4, 2004 ---------------- On 6/4/2004 5:18:58 PM John Warren wrote: Actually, it's the volume of air and hence the mass of air contained in the port that is important.... In other words, if your software package says you need a 4" diameter port that is 6" long and you know that end effect compensation is not considered... ---------------- As usual John is correct... but I just wanted to bring to your attention that it depends on what the software assumes (as he mentioned). The simulation software I've tried (WinISD and Bassbox pro) give slightly different theoretical results... neither of which matched the actual port length I needed for my measured tuning point. I trimmed my "lightly flared port" using test tones and a multi-meter... and it turned out shorter than I expected. What driver you using? Rob Quote Link to comment Share on other sites More sharing options...
Professor.Ham.Slap Posted June 4, 2004 Author Share Posted June 4, 2004 ---------------- On 6/4/2004 7:02:06 PM formica wrote: As usual John is correct... but I just wanted to bring to your attention that it depends on what the software assumes (as he mentioned). The simulation software I've tried (WinISD and Bassbox pro) give slightly different theoretical results... neither of which matched the actual port length I needed for my measured tuning point. I trimmed my "lightly flared port" using test tones and a multi-meter... and it turned out shorter than I expected. What driver you using? Rob ---------------- I'm using a Stryke AV-15 in each tube... I used WinISD to model the box and I have it set for 2 4" ports in each tube. I'd give you exactly the numbers it gave me, but I have all the plans on another computer. Also, I'm tuning it to 21 hz with a some punch... Basically, I'm trying to come out with a sound that is similar to the RSW series. Quote Link to comment Share on other sites More sharing options...
formica Posted June 6, 2004 Share Posted June 6, 2004 ---------------- On 6/4/2004 11:50:40 PM Professor.Ham.Slap wrote: I'm using a Stryke AV-15 in each tube... I used WinISD to model the box and I have it set for 2 4" ports in each tube. I'd give you exactly the numbers it gave me, but I have all the plans on another computer. Also, I'm tuning it to 21 hz with a some punch... Basically, I'm trying to come out with a sound that is similar to the RSW series. ---------------- Ironically, my original design objective was to use a Stryke AV15 (original version) in a lightly padded 6ft3 box with a 20 21Hz tune too. I used a single 5.5" dia port and flared the inner and outer edges with a 3/4" round-over bit on my router. Modeling this in winISD gave me a ±20" port... so I left it a little long to fine-tune it manually. This table shows the results of my tests (bold numbers are the tuning points but the voltages are arbitrary)... As you can see... the port needed to be substantially shorter than I expected even if I add a flare compensation. Given that my rear brace was placed at 17-1/2" from the front... but I would have needed a 15" to 16" port to get the 20-21Hz tune... I decided to lower my tuning to about 19.5Hz and reduce the risk of turbulence from the brace, It performs quite well in the series of in-room RTA tests I ran... but I have yet to take it outdoors to get a better idea. A pair will surely get you vibrating to the beat... Rob Quote Link to comment Share on other sites More sharing options...
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