Mapping the Transfer Function
by Eddie Runner

Putting a speaker in a vehicle (or room for that matter) will make the speaker sound different. This is caused by reflections, absorptions and resonance's that exist in that car. This change is called the transfer function. 

All cars have a transfer function, typically the most noticeable change to speakers when placed in a car is MORE BASS.  More bass is inherent because the car is like a box, an enclosed space that promotes reinforcing reflections of the bass sounds (because the bass waves are long enough to be reflected and still be close enough to in phase and reinforce).... 

But the transfer function is more than just a change in bass, it also can cause major changes in the midrange speakers because of the major glass surfaces for the sound to bounce off of! Or the high frequency sounds from the tweeters can be absorbed into the headliner or car upholstery (intentionally sound absorbent by the car manufacturer to absorb road noise)...

So, we cant easily change the transfer function, but we can map it, and then build our system to take advantage of our cars transfer function.

To map the transfer function you will need a Speaker box, a CD with test tones or an audio test generator or a pink noise generator and a decibel meter.  There are several CDs available with test tones, I prefer the tone generator but they are fairly expensive and hard to find an install shop with one, pink noise is OK but pink noise generators are probably just as hard to find as audio generators. And for a decibel meter Radio shack sells one for about $30 that will work. Oh, and a piece of graph paper to write down your results.

First, lets map the speaker box.  Take your speaker box outdoors in the back yard and aim it away from the house to minimize reflections, hang the SPL meter or microphone for it a few meters in front of the speaker box. Fire up your test tones and set the volume level where the SPL meter gets a reading on most of your test tones (if the low ones don't register don't worry), but not too loud, we don't want to damage the test speaker!  Now without touching the volume control, run your test tones and write down on your graph paper the SPL at each tone. You can even make a graph on the graph paper if you want to.

Your results is the frequency response of your test speaker!

Lets put this box in the car and do the test again and see what changes!

Set the test speaker in a nice location, preferably close to where the real speakers will go later. Hang the microphone in the center of the car near where your ears would be if you were driving, and run through the test tones again, reading the SPL meter and writing down the results on the graph paper.

When you done, subtract the back yard numbers from the in car numbers for each frequency and write down your results. For instance, if you had 90dB at 120Hz in the back yard and 95dB in the car then (95-90=5) write down 120Hz=+5dB, and do this for every frequency you mapped. Some frequencies may have negative numbers, this is OK...  When your done, you should have a series of frequencies=numbers something like this:  20=+4, 40=+6, 80=+12, 120=+3, 200=0, 400=-3, 600=-4, and so on...

This is your transfer function, you can now look at any frequency and tell what 
your car is going to do to ANY SPEAKER you install.