Bass Reflex Box Calculator (max. flat amplitude)
with the possibility to customize Vb


In the early seventies, A.N.Thiele and Richard H. Small devised a set of formulas to predict how a driver would perform in any given box. Thiele was the senior engineer of design and development for the Australian Broadcasting Commission and was responsible at the time for the Federal Engineering Laboratory, as well as for analyzing the design of equipment and systems for sound and vision broadcasting. Small was, at the time, a Commonwealth Post-graduate Research Student in the School of Electrical Engineering at the University of Sydney.

Thiele and Small devoted considerable effort to show how the following parameters define the relationship between a speaker and a particular enclosure. However, they can be invaluable in making choices because they tell you far more about the transducer's real performance than the basic benchmarks of size, maximum power rating or average sensitivity.



Calculate Vb , f3 , fb , Lp , Pmin and SPL


Thiele-Small parameters:

Calculation Results:

Vas :   liters    ** Vb :   liters
Qts :   unitless F3 :   hertz
Qes :   unitless Fb :   hertz
Fs :   hertz Lp :   cm
Sd :   cm2 Pmin :   cm
Xmax :   mm SPL :   dB (1W/1m)
Vent parameters ** Change Vb
# of ports:   unitless
Port diameter:   cm
Step 1: Step 2:

  • Vas - Volume of air equal to the speaker's spring characteristics
  • Qts - Total Q of driver
  • Qes - Electrical Q factor
  • Fs - Free air resonance of speaker
  • Sd - Effective piston area
  • Xmax - Driver excursion

  • Vb - The airspace volume of the box. The volume of the driver and any building material needs to be added to this value.
  • F3 - lower -3db frequency
  • Fb = Box frequency
  • Lp - Port length
  • Pmin - minimal Port diameter
  • SPL - Speaker efficiency @ 1w/1m


Frequency Response