Mapping the Transfer Function - By Eddie
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
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
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
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.