[Wayne's World Articles]
How To Add Rear Fill To Your System
Author: Wayne Harris
Originally appeared in Car Stereo Review magazine (1991).
© All rights reserved.
One of the more difficult obstacles to overcome in auto sound installations
is the simulation of a natural listening environment. The reason for this,
perhaps, is that most vehicles don't even come close to exhibiting real
world acoustical characteristics. Some vehicles are so bad, in fact, that
one has to wonder if it's manufacturer has deliberately tried to break
every rule with regard to proper acoustical design.
Hard, reflective, multi-faceted surfaces are combined with soft, spongy
seats and upholstery to create a hodge-podge of standing waves,
reflections, reverberations, peaks, dips, and just about anything else one
could think of and then maybe more. On top of that, we've got speakers
installed in some of the most mundane locations ever contrived by man. I
mean, when was the last time you heard a concert with the highs coming from
in front of you, the mids from below, and the bass from behind?
Seriously, we know automobiles make for a poor listening environment, but
isn't there anything that can be done to improve the situation?
Fortunately, the answer to that questions is "yes". First, you could try
and smooth out the acoustical anomalies of the vehicle with equalization
and crossovers. (Wire Service - Car Stereo Review - Sept/Oct 1990). Next,
you might try adding a center channel loudspeaker to improve staging. (Wire
Service - Car Stereo Review - May/June 1990). Additional improvements might
be garnered with the addition of "rear-fill" loudspeakers to your system.
This is the approach we're going to take in this issue.
For those readers just getting into auto sound, "rear-fill" is the term we
use to describe the sound that emanates from a secondary or "satellite" set
of speakers located behind the listener. The purpose of this sound is to
add "depth" and "realism" to the overall sound of the system.
To understand how this is accomplished, refer to figure 1. When you attend
a concert, the majority of the sound you hear comes directly from the stage
in front of you. Some of the music you hear, however, has been reflected
off of the walls of the auditorium. Since these reflected sounds are
usually much lower in amplitude than the direct sounds radiating from the
stage, their presence is usually very subtle. Nonetheless, they contribute
greatly to the full, rich sound one experiences at a concert.
In an automotive installation, we'd like to achieve the same results. There
should be a good, solid, frontal-image, and just enough rear-fill to make
the system sound natural. As you may have guessed, this type of system
design is called the "front-stage/rear fill" approach. It is very popular
with professional installers and sound-off competitors because systems
designed in this fashion are quite capable of creating the illusion of a
Since almost all vehicles have some type of speakers mounted in front of
the listener, I'm going to assume that you already have adequate
front-stage, so I'll only be focusing on adding rear-fill in this issue. (
For more information on front-stage and adding a center channel
loudspeaker, see Wire Service - Car Stereo Review - May/June 1990 ).
As with almost any type of speaker upgrade, the first major concern an
installer has to contend with is the potential mounting locations available
in the particular vehicle he is working with. Furthermore, the installer
must decide if he wants to limit his selection to those locations that have
pre-existing factory cut-outs, or if he wants the flexibility to install
the rear-fill drivers in any location that will accommodate them. Some
potential mounting locations for various vehicles is given in figure 2.
Another decision that must be made is what size driver you should use. The
answer to this question will be determined, in part, by the decision to use
the vehicle's stock cut-outs (if any) or not. If these stock locations are
to be utilized, then the driver you select must be able to fit into the
existing factory location. On the other hand, if you'll be making your own
cut-outs, the size driver you use will be strictly up to you. (Typical
rear-fill drivers are usually less than 6 inches in diameter.)
The type of driver you select is, to a lesser extent, another factor that
must be taken into consideration. Full-range drivers will give adequate
results, but co-axials, tri-axials, and separates will almost certainly
outperform them. You can also bet that just about any after-market driver
will outperform the stock speakers in the vehicle (if any). Just remember,
the purpose of rear-fill loudspeakers is to create ambience by "filling-in"
the area behind the listener. And while quality is always an issue, it can
be sacrificed somewhat during rear-fill installations if price is a major
Finally, you must decide how you are going to power this new set of
speakers. Adding another amplifier will give you better performance, more
flexibility, and the ability to control the amount of rear-fill by simply
adjusting the source unit's fader control. (See figure 3.) In addition, you
can use active rather than passive crossovers for each set of speakers.
(For more information on crossovers, see Wire Service - Car Stereo Review -
Jan/Feb 1989 and Mar/Apr 1991).
If you choose to add a rear-fill amplifier to your system, you'll need to
select one with the right amount of power. Too much power and the amp won't
be utilized to it's full potential. Too little power and the rear-fill
output may not be able to keep pace with the output of the primary
loudspeakers and distortion could result. As a general rule-of-thumb, I
usually try to select an amplifier with about 1/10 to 1/2 the rated power
of the amp that's driving my primary speakers.
An alternate method for driving your rear-fill drivers would be to use the
same amplifier that is used for the primary frontal loudspeakers. (See
figure 4.) Although this approach will work, it will not provide the
dynamic fading capability found in the multi-amp installation above unless
a high level fader is incorporated into the system. (For more information
on high level faders, see Wire Service - Car Stereo Review - Jul/Aug 1990.)
Load impedance must also be considered, as amplifiers are limited in their
ability to reproduce power into low impedance loads. (For more information
on series and parallel speaker wiring, see Wire Service - Car Stereo Review
- Jan/Feb 1991.)
The final approach to powering your rear-fill satellites would be to use
the built-in power amplifier in your cassette receiver. (See figure 5.)
This is a viable, low-cost solution that will work quite well if don't
anticipate playing your system at moderate to high listening levels.
Once the above decisions have been made, all you lack is a little
preparation and you'll be ready to start the actual installation process.
First, gather up all the materials and supplies you'll be needing,
including the speakers, amplifier, speaker wire, grommets, crimp lugs,
screws, etc. In addition, you'll also need some general purpose household
tools including screwdrivers, wire strippers, crimpers, etc.
As always, you should plan your installation thoroughly before you begin. A
block diagram of your system including all vital components should suffice
nicely. You might also arrange your installation area so that everything is
orderly and clean before you begin. This will make the installation process
all the more enjoyable.
Cutting the Hole
Now we're ready. The first thing I like to do when installing speakers is
to get all of the drilling and cutting out of the way. This may or may not
be necessary depending upon whether you are using existing factory cut-outs
or not. If cutting is not required, then you can skip these steps,
otherwise follow them closely in order to minimize the risk of damage to
the vehicle (and yourself).
1. Safety first: If you don't know how to use power tools, learn - or let
an experienced person operate them. Secondly, always wear eye
protection when using power tools.
2. Make a template of the speaker you decide to use: Measure the diameter
of the speaker at its widest point - excluding the flange or lip -
and, using a compass, draw a circle of corresponding size on a piece
of cardboard. Then cut out the hole with scissors or an Exacto knife.
3. Measure twice, then cut; it could save you a whole lot of grief and
4. Double check everything: Are you absolutely, positively sure this is
where you want the hole to be?
5. Exercise extreme caution when making the cut: A hole saw, saber (jig)
saw, or air chisel will do the job, but if you're an amateur, please
don't use an air chisel. This device is like a miniature jack hammer,
and it can do incredible damage in the wrong hands.
6. Use a rat-tail file to smooth out ragged edges; rough metal can slice
your skin to ribbons.
7. Place the speaker in the new hole and mark the locations of its screw
8. Remove the speaker and drill the screw holes.