Using Impedance Measurements to Inform Loudspeaker Filter Design


An impedance measurement gives useful information about how a loudspeaker driver behaves. It shows resonances (including box and driver), the port tuning frequency and apparent low-frequency extension. This can be very useful information to view when designing loudspeaker processing.

For example, knowing the LF driver port tuning frequency and adjacent resonances may aid in setting the high-pass filter frequency used to protect against over-excursion. At higher frequencies where the impedance may show driver or cabinet resonances, filtering (IIR or FIR) can be set to attenuate these frequencies so as not excite the resonances. These problematic frequencies could also be avoided by choosing crossover frequencies that effectively hand these frequencies to another loudspeaker driver.

In the most recent versions of FIR Designer and FIR Designer M, an impedance measurement can be loaded from file and viewed on almost any response plot in the filtering workflow. In FIR Designer M, a unique impedance measurement can be loaded for each channel.

Load the impedance data as a simple comma, space or tab separated two-column (frequency and impedance magnitude) or three-column (frequency, impedance magnitude and impedance phase) text (TXT) file. (Lines starting with non-numeric characters are ignored as comments or metadata. The impedance phase is not used nor displayed.)

On the right is a portion of an example impedance measurement file exported from DATS.

The screenshot below shows the new impedance file loading window in FIR Designer and FIR Designer M.


* This data was exported from the Dayton Audio Test System: DATS
* Manufacturer: 
* Model: 
D 0.0 [mm]
Re 0.00 [Ohms]
Fs 0.00 [Hz]
Zm 0.00 [Ohms]
BL 0.00 [N/A]
Qms 0.000
Qes 0.000
Qts 0.000
Vas 0.000 [liters]
L10k 0.00 [mH]
n0 0.00 [%]
dBSPL 0.00 [1W/1m]
Ms 0.00 [grams]
Cms 0.00 [mm/N]
Freq Impedance Phase
1.029 6.856 -2.745 
1.059 6.856 -2.745 
1.091 6.856 -2.745 
1.122 6.856 -2.745 
1.155 6.856 -2.745 
1.189 6.856 -2.745 
1.224 6.856 -2.745 
1.260 6.856 -2.745

This quick video shows how to load an impedance measurement and how the impedance appears behind other responses in the filter design workflow.


FIR Designer M Release Notes

FIR Designer Release Notes