Frequency response curve

This testing was done on my A4.  I measured with a digital sound meter using the Stereophile Test CD2 1/3 octave warble tones (tracks 16,17,18).  This is hardly a scientifically accurate measurement but it does give a "ball park" idea of what's happening.  For the volume, I chose 76dB @ 250Hz as it's a comfortable volume for "normal" listening. I did not touch the volume until finished with all measurments.  I did this to satisfy my own curiousity... and maybe yours. 

The stock system has a definite hump in the frequency response around 50Hz that was very obvious before I even started measuring.  The system is surprisingly smooth (for a car) except for the dip in response at 400Hz which tends to make male voices sound a bit lean.  It drops off quickly after 10kHz.  Don't let the bass curve fool ya... road noise masks a lot of it. 

The sub reproduces the dynamics and transient response that the stock system is missing.  Don't be deceived by what appears to be a small gain in the graph... it makes a BIG difference.  Remember that it takes double the power to realize a 3dB increase in sound pressure level and 6dB is a doubling of the sound pressure level.   Most people perceive something as being "twice as loud" when the volume is increased by 10dB.  The frequency curve only tells a small part of the story.   If I had the means to plot "energy vs. time" you'd see what I'm talking about.  The factory system has "slow" bass meaning it does not faithfully reproduce bass that has a sharp attack (like a drum kick).  This is pretty common... it takes a lot of power and good drivers to reproduce bass correctly.