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Wednesday, November 22, 2017
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    Thiele's Alignment based on QT

Thiele's method is to choose an alignment based on QT.
Alignment Box design
Order No Type Ripple(dB) f3/fs fB/fs VB/VAS QT
Quasi Third Order 1 QB3 --- 2.68 2.000 0.0954 0.180
2 QB3 --- 2.28 1.730 0.1337 0.209
3 QB3 --- 1.77 1.420 0.2242 0.259
4 QB3 --- 1.45 1.230 0.3390 0.303
Fourth Order 5 B4 --- 1.000 1.000 0.7072 0.383 optimally flat
6 C4 --- 0.867 0.927 0.9479 0.415
7 C4 0.13 0.729 0.829 1.372 0.446
8 C4 0.25 0.641 0.757 1.790 0.518
9 C4 0.55 0.600 0.716 2.062 0.557
9.5 C4 1.52 0.520 0.638 2.60 0.625




Example:
  • fs = 30.4 Hz

  • Vas = 125.9 liters

  • Qe = 0.249

  • Qm = 3.9934

  • Rs = 7.4 ohm
EBP = fs/Qe = 122 --> vented

With 0.5 Rext external, QT = 0.249

Looking at the table, this QT suggests
alignment No3 ( QB3 ), giving:

Vb = 28.23 liters
fb = 43.2 Hz
f3 = 53.8 Hz
 
 
fs [Hz] :
Vas [liters] : Vas [cu.ft.] :
Qe [] :
Qm [] :
Re [ohms] :
Rext [ohms] :
 
EBP [] :
QT [] :
Vb [liters] : Vb [cu.ft] :
fb [Hz] :
f3 [Hz] :
 
Port calculations

  Dv : [cm] Lv : [cm] Pa : [cm2]
  Dv : [inches] Lv : [inches] Pa : [sq.inches]
 
If you change the pipe diameter [Dv], click on 'Port' to recalculate Port length.



GLOSSARY OF SYMBOLS
  • fs - resonance frequency of driver in free air
  • Vas - volume of air having same acoustic compliance as driver suspension
  • Qe - Q of driver at Fs considering system electrical resistance only
  • Qm - Q of driver at Fs considering system nonelec-trical resistances only
  • Re - dc resistance of driver voice coil
  • Rext - dc resistance of crossover-filter-coil and speakercable

  • EBP - efficiency bandwidth product
  • QT - total Q of driver at fs including all system resistances
  • Vb - net internal volume of enclosure
  • fb - Helmholtz resonance frequency of vented box
  • f3 - alignment (-3dB) cutoff frequency

  • Dv - Diameter of the bass reflex pipe
  • Lv - Length of the bass reflex pipe
  • Pa - Area of the bass reflex pipe