I've been tinkering with my DIY speakers a bit to see if I can improve certain aspects of their performance (see this thread for more info on them). Previously, the drivers were connected directly to the amplifier outputs with all filtering done digitally, but recently I decided to experiment with adding passive filters after the amplifier. Some people here seem to believe that doing this can only be detrimental to system performance. However, as explained in this Purifi application note, passive components can significantly reduce drive unit distortion if used judiciously. An additional benefit in my setup is a reduction in hiss from the compression driver (~8dB perceived reduction) without needing to build/buy exceptionally low noise amps.
The primary goals I had in mind when designing the passive filter networks were:

Driving (a.k.a. lookback) impedance seen by each driver (solid is magnitude, dashed is phase):

Frequency responses (both passive and digital filters):

The woofer has a rising response characteristic, which is convenient in this case—lots of output in the upper midrange that I can throw away. Final sensitivity is around 96dB/2.83V, save for the bottom end of the woofer's range.
I measured the distortion with the drivers connected directly to the amp outputs, then again with the passive filters (and modified digital filters, of course). The frequency responses are within ±0.2dB except for a narrow band difference of 0.5dB around 1.9kHz. Multiple sweeps were used to suppress noise. Repeatability was very good above ~40Hz.
Direct, 86dB@1m:

With passive filters:

Direct, 96dB@1m:

With passive filters:

Direct, 103dB@1m:

With passive filters:

The primary goals I had in mind when designing the passive filter networks were:
- Low driving impedance at low frequencies and high driving impedance at high frequencies.
- Flat-ish frequency response from both the woofer and tweeter over their intended passbands before applying digital filters.
- Similar overall sensitivity for both sections.

Driving (a.k.a. lookback) impedance seen by each driver (solid is magnitude, dashed is phase):

Frequency responses (both passive and digital filters):

The woofer has a rising response characteristic, which is convenient in this case—lots of output in the upper midrange that I can throw away. Final sensitivity is around 96dB/2.83V, save for the bottom end of the woofer's range.
I measured the distortion with the drivers connected directly to the amp outputs, then again with the passive filters (and modified digital filters, of course). The frequency responses are within ±0.2dB except for a narrow band difference of 0.5dB around 1.9kHz. Multiple sweeps were used to suppress noise. Repeatability was very good above ~40Hz.
Direct, 86dB@1m:

With passive filters:

Direct, 96dB@1m:

With passive filters:

Direct, 103dB@1m:

With passive filters:

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