First I want to thank you guys for everything. I have been trying to get this formula I got from your website to work for me but doesnt seem to be working for me. the sub is a power acoustic pw1-12l the recomended air space is 2.17cu.ft. @ 34 hz they recomend a port of 3.3in. in diameter and 5.5 in. in length I tried the equation and keep getting 32 can someone help me?
Replies (1)
swez on 08/3/2007 00:50:39
Port tuning is a function of several factors. Most box making programs use the Theil/Small parameters of a given sub and we plug in these numbers and then select the enclosure parameters based on various plot alignments available. A Qtc alignment of 0.7 is preferred for HT sub applications. For mobile apps, we often look for a Qtc of 0.85 - 0.95. (Techie talk for advanced users)
When calculating the PTF, (Port Tuning Freq) we need to account for the Fb of the air chamber, minus the port and sub displacements. That's probably where the discrepencies are. These displacement values are very minimal as compared to the Vb box dimensions. My guess is that is where the numbers do not match. (A 2 Hz. deviation is pretty darn good and nothing to fret about)
FYI: Vb is the net internal airspace the box offers after sub and vent displacements are accounted for. A typical 12" sub displacement is in the range of 0.07 - 0.12 ft^3
How the box sounds, (In your vehicle) is far more important then what a therorectical plot may look like. Try the box as given by the Mfg. If you like the results, that's all that matters. If not, adjustments can be made later, but after the sub is well broken in.
I just ran a quickie set w/ your numbers. If the box has an Vb of 2.17 ft^3, the port is round and an I.D. of 3.3", to get to 32 Hz., the port needs to be 8.0" long. Using your numbers, the PTF come out to about 36.5 Hz with the port length at 5.5". That's not all bad either.
Tech Tip: The only thing I would be watchful of here are port noise issues. The greater the X-max of a given sub, the larger the port diameter must be to accomodate the air flow w/o port noise. When we increase the port diameter, air velosity is lower and less port noise too. But, we also have to increase port length as well to hit the desired tuning target. Low PTF's need longer port lengths.
Swez
Replies (1)
swez on 08/3/2007 00:50:39
Port tuning is a function of several factors. Most box making programs use the Theil/Small parameters of a given sub and we plug in these numbers and then select the enclosure parameters based on various plot alignments available. A Qtc alignment of 0.7 is preferred for HT sub applications. For mobile apps, we often look for a Qtc of 0.85 - 0.95. (Techie talk for advanced users)
When calculating the PTF, (Port Tuning Freq) we need to account for the Fb of the air chamber, minus the port and sub displacements. That's probably where the discrepencies are. These displacement values are very minimal as compared to the Vb box dimensions. My guess is that is where the numbers do not match. (A 2 Hz. deviation is pretty darn good and nothing to fret about)
FYI: Vb is the net internal airspace the box offers after sub and vent displacements are accounted for. A typical 12" sub displacement is in the range of 0.07 - 0.12 ft^3
How the box sounds, (In your vehicle) is far more important then what a therorectical plot may look like. Try the box as given by the Mfg. If you like the results, that's all that matters. If not, adjustments can be made later, but after the sub is well broken in.
I just ran a quickie set w/ your numbers. If the box has an Vb of 2.17 ft^3, the port is round and an I.D. of 3.3", to get to 32 Hz., the port needs to be 8.0" long. Using your numbers, the PTF come out to about 36.5 Hz with the port length at 5.5". That's not all bad either.
Tech Tip: The only thing I would be watchful of here are port noise issues. The greater the X-max of a given sub, the larger the port diameter must be to accomodate the air flow w/o port noise. When we increase the port diameter, air velosity is lower and less port noise too. But, we also have to increase port length as well to hit the desired tuning target. Low PTF's need longer port lengths.
Swez
Copyright ClubKnowledge 2009 * All Rights Reserved
