A Transmission Line Subwoofer

Designed & Built by Yves Mattei

This design is a single fold transmission line using a 330mm (12") Audax driver (HD 33 S 66). The port loads out the back. Very little stuffing is used.

It as a SubWoofer in a Home Theater System with an active 4th order low pass at 70 Hz. Since the cross-over is low, upper resonnances ( > 150 Hz ) are not a big problem. This line is not optimized for higher crossover frequencies.

Results:

  • From 50 Hz to 100 Hz driver and terminus are in phase (+/-10°)
  • The - 3dB point is near 45 Hz
  • At the Fr of the line (40 Hz) driver and terminus are 90° out of phase
  • Below 40 Hz they are out of phase

The stuffing could have be better optimised, reducing the effect of the null at 140 Hz.

If I were to do it again I'd make the terminus a bit larger. (or use the line for a 10"/ed)

Line Configuration
  Physical length 1.6 m
  Physical fs at 1/4 wl 54 Hz
  Acoustic 1/4 wl (measured) 2.13 m/40 Hz
  Measured c' 0.75c
  (speed of sound in the line)
  Begining cross section 838 cm^2
  Terminus section 262 cm^2
  Taper 1.6 : 0.5 Sd
  First odd harmonic 140 Hz
  (null of driver, max of terminus)
  Max of terminus 90 Hz
  (level of the terminus is 3 dB below the driver )
  Stuffing 50 mm Rockwool

Plans

Audax PR 330 M0*
  Sd 538 cm^2
  Vas 264 liters
  Cms 6.5 e-4 N/m
  Cas 1.9 e-6m^5/N
  Mms 52 g
  Fs 27 Hz
  Bl 13.6 N/A
  Re 5.8 Ohms
  Qms 6.7
  Qes 0.28
  Qts 0.27
  Pmax 150 Watts
  Xmax +/-4 mm
* Audax HD 33 S 66 replaced by PR 330 M0

Notes:

  1. Testing: Monacor MCE2000 electret capsule mounted in an amplifer g=100 that I built. I used a sinewave generator and a oscilloscope.
  2. The data on the phase was made using comparative measures with the dual trace scope

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