OK, I did the install today. Another satisfied customer. Because I'm a car nut but also an audiophile, I took some measurements before and after the passive high pass filter was installed.

Test Conditions

• Engine Off - so the SPL meter was hearing the audio system and not engine noise.
• Windows Up - to eliminate wind and other environmental noise.
• Radio Shack Analog SPL Meter Set To C Weighting Scale With Meter Set For Fast Response
• 0dB Level Set For 90dB @ Volume Level 33 on Crossfire Nav Audio Unit With Microphone Placed Where Driver's Head Is Located.
• Stereophile Test Disk...12 one 3rd octave warble tones centered at 1,000Hz; 200; 160; 125; 100; 80; 63; 50; 40; 31.5; 25; 20

Before Modification
Red Line = Baseline OEM System with Bass Control = 0 and Treble Control = 0
Yellow Line = OEM System With Tone Correction with Bass Control = -3 and Treble Control = +3

After Modification
Blue Line = High Pass Filter with Bass Control = 0 and Treble Control = 0
Yellow Line = High Pass Filter With Tone Correction with Bass Control = -2 and Treble Control = +2



You can clearly see in the RED line what everyone is noticing. Bass response is elevated from about 140Hz down to about 40Hz. The red line was generated with tone controls set flat. The one anomaly here is the FR bump at 200Hz that disappears from the other curves which have very consistent FR curves.

Next up is the YELLOW line where I measured the response with the tone control settings (Bass -3 / Treble +3) I had been using before the filter was installed. Note that while the overall curve shifted downward, it still maintained the same overall shape as the RED curve as you would expect. The Mid/Upper Bass was still exaggerated but the hump at 200Hz magically swung the opposite direction. No explanation here.

Next up, the BLUE line which is after the high pass filter (capacitor) was installed but with the tone controls set flat. Note that with the high pass filter in place the absolute levels in the front speakers dropped significantly at some frequencies causing me to recalibrate the 0dB level by raising the volume setting. That's why the blue curve has the highest overall level. As you can see, the curve is now much smoother but still rising to a 50Hz bass peak. Speaking of the 50Hz peak, it must be a system resonance reflecting the overall speaker/ car interior interaction. It's present in all the curves to varying extents and this high pass filter had little effect on it.

Lastly, the GREEN line. I played around with measurements of a bunch of tone control settings but I settled on (Bass -2 / Treble +2). That's what you see in the chart. The end result is a much smoother frequency response from about 140Hz down to about 60Hz followed by that damned bump at 50Hz and then a steep rolloff down to about 30Hz. The high pass filter combined with these tone control settings yielded a result I can live with. Note also that the tone control now has a great impact. i.e. -2 setting on the bass control produces a much bigger drop from the blur curve to the green curve than a -3 setting dropped the yellow curve from the red curve before the capacitor was installed. It is MUUUUCH better!

I used the following recordings before and after to assess progress.....

• Norah Jones - Come Away With Me - Female Voice
• Mark Knopfler - Golden Heart - Male Voice
• Santana - Supernatural - Low Bass & Male Voice

The OEM system now sounds pretty good and much better than before the filter was installed. Voices are more natural, the bass has much more definition, and the high frequencies are much more balanced/ audible. FM radio announcer's voices are especially improved. It now sounds like an Infinity OEM sound system.