| # Speaker Project # Testing the Woofer (1)

> > > Just as in designing the subwoofer, to start the design for the main speakers we need to test the woofer. By finding the impedance in free air and the impedance in a sealed box (or with added mass) we can find all of the T/S parameters required to calculate the necessary box volume for the main speakers. The tweeter is always sealed and does not need to be analyzed in this fashion.

> > > Before beginning, make sure that your jig (or series resistor) is working well by measuring a passive component or two. Do this by creating a generator then selecting Measure / Passive Component. This will also verify that your volume is set appropriately.

> > > The first step in testing a woofer is to find the impedance of the driver. This will tell you the resonant frequency (FS) and all of the T/S parameters (in particular QTS). The first impedance we find is the Impedance in Free Air. To find it,

> > > 1) Create a new driver resource and give it an appropriate name (mine is Vifa PL18). I built a new folder (named Vifa) to put all of this data in - keeping my database in an orderly fashion.

> > > 2) Hook the woofer up to the impedance jig (or series resistor)

> > > > a) The woofer frame should be as stable as possible while keeping air resistance to a minimum. The best solution is to have two tables - one on each side of the woofer and clamp the frame to the two tables (two clamps, therefore). This keeps the woofer in free air, clamped, and reduces any outside interference.

> > > > b) If that is too tough for you, have someone click the Impedance in Free Air menu option while you carefully hold the driver firmly by the magnet.

> > > 3) Go to Options / Preferences and set the Sample Rate to 48000 and the sample size to 131070. You can use a smaller sample size if you have a slower CPU (I use a PIII-600) but keep the sample rate low so that the low frequency accuracy is maximized.

> > > 4) Open the driver resource (or click to it) and select the Measure / Impedance in free air command. This will run a short MLS signal (a 128K sample signal at 48000 will take about 3 seconds) and then it will analyze the resultant signal.

> > > After doing this, if I just open my measured signal (named VifaPL18.Free Air) it looks like:

> > > Note that the phase fluctuates wildly below 10Hz so make sure the frequency range of interest when you estimate parameters is above 8 Hz (where the zero phase gets incorrectly hit).

> > > To get a first estimate of the driver,

> > > 1) pull up the driver properties dialog (open the driver resource - which will be a blank display for now - right click in the window and select Properties, or just select the Edit / Properties command).

> > > 2) Go to the Data tab and enter a DC resistance (here it is 5.8 Ohms) and click the Use this DC Res button. I used a good quality voltmeter to measure the DC resistance, but most factory specs are pretty accurate. Speaker Workshop can not accurately estimate the DC resistance so this step is crucial for good results.

> > > 3) Click OK to close the driver properties dialog.

> > > 4) Select the Calculate / Estimate Parameters... command and use an appropriate frequency range. For me that was 10 Hz to 10000 Hz (clipping the high frequency noise out of the estimate). Make sure you make the upper cutoff high enough to show plenty of impedance or the estimates of the inductances could be pretty far off (which will not affect things much when it comes to designing a box).

> > > 5) When the process completes you can do a few things to check your estimate. They are enumerated on the next page.

> > > last updated March 12, 2001