The Subwoofer DIY Page
Measurements
last updated: 14 April 2011
The Subwoofer DIY Page There are many different ways of measuring the Thiele/Small parameters of a driver. The methods outlined below provide a way of for the beginning DIYer to measure these parameters with the minimum of equipment.

Measuring Re, Fs, Qes, Qms and Qts
To measure these parameters using the method outlined below, you'll need to have the following items:

  • One (1) amplifier
  • One (1) audiofrequency generator
  • One (1) digital multimeter
  • One (1) 5 watt resistor (approximately 4 or 8 ohms)
  • Two (2) pairs of alligator clip leads

Preferably, the multimeter should be capable of measuring frequency, as well as voltage, resistance and current. The amplifier should be capable of reproducing 20 Hz to 200 Hz with no variation in output and it should be insensitive to the loads above 4 ohms. The frequency generator should also produce a signal whose voltage doesn't vary as the frequency is adjusted.

Method:

  • Measure the resistance, Re, across the driver terminals.
  • Measure the resistance, Rs, across the resistor.
  • Connect the audiofrequency generator to the input terminals for the amplifier
  • Connect the multimeter to the speaker terminals of the amplifier
  • Set the audiofrequency generator to approximately 100 Hz
  • Set the output of the amplifier to Vs, where Vs~0.5V to 1.0V. You may have to experiment with different voltages, depending on the accuracy of your reading for Im.
  • Calculate Is, where Is = Vs/(Re+Rs)
  • Connect the following circuit (using the alligator clip leads where necessary)
    • Attach one leg of the resistor to the positive speaker terminal on the amplifier
    • Attach the other leg of the resistor to the positive terminal on the driver
    • Attach the negative terminal of the driver to the negative terminal on the amplifier
    • Attach the multimeter's leads to each side of the resistor
  • Adjust the frequency until the voltage across the resistor reaches a minimum level.
  • Measure the frequency, Fs
  • Measure the voltage across the resistor, Vm
  • Calculate the current, Im = Vm/Rs, flowing through the circuit
  • Calculate the driver's impedance at resonance, Rm = (Vs-Vm)/Im
  • Calculate the -3db curent, Ir = (Im*Is)^0.5
  • Calculate r0=Is/Im
  • Calculate the -3dB voltage, Vr = Ir*Rs
  • Measure the frequencies, Fl and Fh, for which the voltage across the resistor is equal to Vr
  • Check that (Fl*Fh)^0.5 = Fs
  • If the above check out, then Qes, Qms and Qts can be calculated as follows:
    1. Qms = Fs*(r0^0.5)/(Fh-Fl)
      Qes = (Qms/(r0-1))*(Re/(Rs+Re))
      Qts = Qms*Qes/(Qms+Qes)

    You can use the following spreadsheet to perform the calculations automatically for you: TSPARAM.ZIP


    Measuring Vas (equivalent air compliance)
    To measure Vas, use a good solid enclosure of known volume that's about a cube of the nominal speaker size. Determine the total volume, including the speaker cut-out and that trapped by the cone with the speaker mounted on the outside of the box for easy access. Measure the resonant frequency in this situation as well as the free air space resonant frequency.

      Vas = Vb((Fb/Fs)^2 - 1)

    where

      Vb is the volume trapped by the speaker and box
      Fb is the resonance frequency of speaker and box

    my thanks to John Straede of the BASS and DIY Loudspeakers List for providing this method of measuring the Vas of a driver