The Maintenance Files

Car Stereo Installation

(Spring/Summer 2007)

Tuning

Tuning Stage 2:

Previously: "Radio Shack handheld SPL meter & a CD of test tones (Stereophile CD #2).  Note that this is a test of the entire system in a 'real world' situation so the graph ain't flat.  They never are in the 'real' world.  I mounted the SPL meter on a tripod & positioned it roughly at a right driver's ear location pointing towards the front of the right door - so a bit more than 1 metre away.  I used it A-weighted.  Also note that meter positioning & direction will alter the results significantly.  I also tried car center above the console & got a different graph but then I decided that it was worthless because no one ever sits there.  Since it's a cab, I measured with top down & windows down in a single car garage, so these results will be a significantly different from confined-top car i.e. a coupe or a targa.  All coupe'd up are you?  Haha.  Couldn't resist.  The system volume was set at 0 dBA with pink noise at -20 dBFS L+R.  The reality of this volume is it's LOUD.  It's 27 out of 46 on the volume dial which I had previously enhanced via amplifier gain.  And as always, the CD volume is lower than the radio volume by a significant difference.   I HATE that.  Can't they equalize them?  Especially when it's built in?  Rant off.  But you can hear a CD quite well at 70 mph - top down, so it's LOUD."

You're probably wondering what happened to Stage 1.  You're not?  Then skip to the paragraph below.  I got all scientific & setup my handy SPL meter.  Using test tones from the Stereophile Test CD # 2, I plotted graphs of the dB readings at specific frequencies.  This gave me pretty graphs.  But they didn't correspond to what my ears were hearing.  The High & mid frequencies were similar, but the bass was way out of balance - on the graph!  So I scrapped it all & started again differently . . .

After a lot of trial & error, I switched my SPL meter weighting to C & then the graphs finally matched what I was hearing.  A weighting just didn't accurately accentuate the sub 200 Hz frequencies.  But C did & now I could dial it in.  I mean, really . . .  my listening environment was a open sports car in a garage.  Not a good laboratory.  So, I played with x-overs - both speaker & amplifiers, I adjusted gain, tweaked the sub-amp boost amplitude & frequency etc etc etc.  I'm surprised I didn't kill the battery from all the testing, interspersed with some listening.  I went in circles, of course.  Too much.  Too little.  Bass overhang.  And on & on.

And finally . . .

I decided to dive into the door again.  I was unsatisfied with the drivers firing back into a damped - but not stuffed - big metal enclosure.  So I had a brainwave.  Stuff a plastic bag with some polyester fill, jam in in the door & just tape the ends to the wood spacer.  You don't try to make it airtight.  You want some pressure release.  But make it watertight at the top & sides which would make a good water shield too.  I thought I was onto a nice 911 solution.