# I want to build a Transmission Line/ T-Line Subwoofer Enclosure Answered

I have been wanting to build a Transmission Line subwoofer enclosure to house 2 10 inch subwoofers. I'm not exactly sure what subwoofers I will be using yet, or the specifications on them yet either. The only thing that I am worried about while constructing this box, is the calculations. I am good in math, and geometry and all of that stuff, but I don't know where to start. If someone could take a moment and explain to me what the calculations and the steps that would be needed to construct such an enclosure, it would be much appreciated.

Thank you.

Tags:

## Discussions

Transmission Line speakers deal with the speaker diameter (stated size) and Fo (free air resonant frequency). I have William R. Hoffman's QBasic design program (one of several) that were published in Popular Electronics in the mid 90's. If you would like a copy, contact me through here. I think that would be the best way to follow how to calculate them, as it is a very short program.

How to calculate the Qts and Vas using more than 1 driver depends on how they are used. If they are coupled in an isobaric configuration (face to face, sealed as a physical unit), the Vas stays the same, but the Qts is divided by the number of drivers. If you mount the drivers side by side and they are not coupled, the Vas is multiplied by the number of drivers and the Qts stays the same. I'll have to look back in my notes to verify that, but that is the way I remember it.

Qa

Okay, here's the short code in QBasic:

*****************************************
PRINT : PRINT : PRINT : PRINT : PRINT
PRINT "                   WHAT IS THE FREE AIR RESONANCE FREQUENCY OF THE"
INPUT "                   DRIVER (Fo) IN Hz"; FO
PRINT : PRINT
INPUT "                   WHAT IS THE DIAMETER OF THE DRIVER IN INCHES"; D
LET A = (1130 / FO) / 4
LET F = 1 + (.5 / .074)
LET F = SQR(F)
LET F = 1130 / F
LET B = (F / FO) / 4
LET XS = (D / 2) ^ 2 * 3.14159
LET XF = XS / 144
LET BV = XF * B
LET WS = BV * .5
CLS : PRINT : PRINT : PRINT : PRINT : PRINT
PRINT "              FOR A DRIVER OF"; D; "INCHES IN DIAMETER,WITH A FREE"
PRINT "              AIR RESONANCE FREQUENCY OF"; FO; "Hz,AND AN ENCLOSURE PACKED WITH"
PRINT "              DAMPING MATERIAL AT 1/2 LB. PER CU.FT.,THE BASIC"
PRINT "              SPECIFICATIONS FOR A TRANSMISSION LINE ENCLOSURE ARE:
PRINT
PRINT "              LINE LENGTH ="; B; "FEET"
PRINT
PRINT "              LINE CROSSECTIONAL AREA ="; XS; "SQ.IN."
PRINT
PRINT "              APPROXIMATE BOX VOLUME ="; BV; "CU.FT."
PRINT
PRINT "              WEIGHT OF DAMPING MATERIAL ="; WS; "LBS."
PRINT
PRINT "              FOR A HARD COPY PRESS THE <PRN SCRN> KEY"
*****************************************
If you need to convert the Diameter from metric to imperial (mm or cm to inches), use one of the many online unit converters.

Enjoy!

Qa