View Full Version : ARTICLE: How to Add Rear Fill to your System

Randy Savage
10-06-2005, 11:27 PM
[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
live performance.

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
money later.
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.

Randy Savage
10-06-2005, 11:28 PM
If you elected to use the factory speaker locations in the vehicle, now
would be a good time to remove the grills covering these locations. Also,
if you're retrofitting the stock speakers with aftermarket drivers, you
should remove the old speakers at this time. Store these speakers in some
out-of-the-way place as you may want to re-install them in your vehicle if
you ever decide to sell it or trade it in.

Another word of advice here; don't be tempted into using the factory's
speaker wiring harness in the vehicle. It is usually of inferior quality,
you don't know what it's connected to, and you have no way of knowing what
type of noise-inducing sources it is running alongside. By spending an
extra 15 minutes running your own speaker cable, you'll alleviate a lot of
unknowns that could come back to haunt you in the future.


With the speaker mounting locations prepared, you can begin the wiring
process. You'll need enough speaker wire to reach from each speaker to the
amplifier or cassette receiver, depending upon how you will be powering the
speakers. For rear-fill drivers, 16 or 18 gauge speaker wire should be

You may elect to snake the speaker wire through the vehicle from either the
power amp end or speaker end of the installation. Just be careful that you
don't cut or snag the wire's insulation on sharp objects while doing this.
Also, pay particular attention to the area where the wire will be located.
It must be situated so that it will not get pinched or damaged during the
normal operation of the vehicle. Typically, speaker wire is run in-between
the carpeting and floorboard, but the actual installation location may vary
from vehicle to vehicle. Be sure to leave enough wire at each end so that
connecting the speaker and amplifier to the wire can be readily

Once you've gotten the speaker wire in place, use the hand crimpers to
crimp terminals onto the speaker end of the wire. These terminals must be
selected so that they mate properly with the "spade-lug" mounting terminals
on the speaker. Radio Shack is probably your most convenient source for
crimp-on terminals.

To connect the speaker wire to the speaker, insert the positive spade-lug
of the speaker (usually denoted by a "+" sign or red dot on the speaker's
frame) into the corresponding crimp terminal attached to the positive
conductor of the speaker wire. (The positive conductor is denoted by "+ +
+" stenciled along it's insulation or by the gold color of the wire strands
within.) Connect the negative conductor to the negative speaker terminal in
a similar fashion and then repeat the entire process for the other
rear-fill speaker.

When both speakers are wired, go ahead and mount them in their respective
mounting locations. Use extreme care when securing the mounting screws as
screwdrivers are notorious for inadvertently poking holes in speaker cones.
If this unfortunate event happens to you, all may not be lost, a small
amount of clear silicone glue can usually repair the damage.

After mounting the speakers, I immediately install the speaker grills in
order to insure that no harm will come to the drivers. If you're using
stock speaker locations, simply replace the factory grills in the reverse
order that they were removed. For custom installations, use the grill that
was included with the aftermarket driver. (These usually just snap over the
mounting flange of the speaker.)

Before wiring the speakers to the amplifier or cassette receiver, I always
like to test the installed speaker wires for shorts and continuity. The
simplest way of doing this involves the use of a single 9 volt battery.
Take the battery and simultaneously touch both conductors of a speaker wire
to it's two terminals. There should be an audible "pop" or "click" over the
speaker under test. Repeat this process for the other speaker as well.
(Important - Don't connect the battery for more than a few seconds or you
might damage the driver.) If each speaker made a pop during it's respective
test, this is a good indication that everything is okay. If you didn't hear
anything, then recheck the wiring to the speaker that remained silent.

Now that we know there aren't any shorts in our speaker cables, it's time
to connect these cables to either an amplifier or cassette receiver,
depending upon the particular installation we have undertaken. In either
case, it is imperative that you consult the proper owner's manual before
actually doing any wiring or you could end up toasting your system.

From a generic point of view, we simply want to connect the left speaker to
the left amplifier output and the right speaker to the right amp output.
Again, pay close attention to polarity. The gold conductor goes to positive
and the silver to negative. Use butt crimp terminals to make these

Now comes the moment of truth - There'll either be music, or, a small
thermo-nuclear fireball and mushroom cloud coming from you rear-fill
speakers. Just joking. When you turn on your system for the first time,
remember to KEEP THE VOLUME LOW! This could prevent damage to the system if
something in the wiring is amiss. Place one ear close to each speaker and
listen. If sound can be heard coming from each speaker, you can proceed
with the remaining tests. If not, turn the system off and recheck you're

This next test verifies the functionality of the balance control. First,
increase the output level of the system by using the source unit's volume
control. Also, check to make sure the unit's fader control is centered.
Next, adjust the unit's BALANCE control fully to the left. Listen to each
speaker again. Sound should only be heard from the front left and back left
speakers. Adjust the balance control fully to the right. Sound should only
be heard from the front right and back right speakers. If everything tests
out okay, proceed with the next test.

The purpose of this test is to verify the operation of the source unit
fader. If your rear-fill speakers are wired in parallel with your primary
speakers, you can skip this test, otherwise, start by repositioning the
source unit's balance control to it's center detent position. Next, adjust
the fader control fully to the front. Sound should only be heard from the
front left and front right speakers. Now, adjust the fader to the fully
back position. At this point, sound should only be heard from the back left
and back right speakers. Before proceeding, verify that both the balance
and fader controls are reset to their center detent positions.

If you've gotten this far, it's a safe bet that everything in your system
is working properly and that only a few more level adjustments are all that
are required in order to complete the installation process.

As you'll recall, the purpose of rear-fill loudspeakers is to add depth and
ambience to your system. Your goal should be to integrate the sound from
these speakers into the overall sound of the system without the sound from
these speakers becoming distinctly audible.

Fortunately, this task is quite easily accomplished. If you're source
unit's fader works, simply start with the fader in it's full frontal
position and then slowly fade backwards until the system's sound is rich
and full. That's all there is to it.

If you chose to wire your rear-fill speakers in parallel with your primary
drivers, however, your level controlling options are practically nil unless
you incorporate an L-pad in-between the amplifier and rear-fill speakers.
(For more on adjusting signal levels, refer to Wire Service - Car Stereo
Review - Jul/Aug 1990). In this type of installation, adjust the L-pad for
the same full, rich sound mentioned previously.

As a final thought, I'd like to discuss crossovers for a moment. Rear-fill
loudspeakers do not need to play below about 150 Hz. If you're using
full-range drivers for rear-fill, you'll want to use either an active or
passive crossover adjusted to somewhere around this frequency. For
installations incorporating co-axials, tri-axials, or separates, use the
passive networks supplied with these drivers. Finally, if you want to
experiment with your own passive crossover networks, I'd recommend reading
two articles on crossovers in the Wire Service column of Car Stereo Review
- Jan/Feb 1989 and Mar/Apr 1991.

This completes the installation process. Now it's time for the road test.
With the rear-fill speakers playing, you should immediately notice a
tremendous improvement in sound quality. The music should sound rich and
spacious and should exhibit a sense of dimensionality and realism that was
previously missing. And, after all, isn't realism what we're really trying
to accomplish?

[Wayne's World] Wayne's World

This page, and all contents, are Copyright © 1997 by Wayne Harris
Enterprises, Mesa Arizona, USA.

10-06-2005, 11:30 PM
dang i read all of that too that was hard!

10-06-2005, 11:30 PM
wow thats long ill read it in the morning because im interested

10-07-2005, 10:49 AM
Where's the synopsis?

10-07-2005, 10:53 AM
1. Rear fill can be good.

2. I like cheese.

Seriously, if you want to read the article, read it :) It's not that long...and if you aren't concerned with rear fill, don't read it :)

10-07-2005, 11:03 AM
It's actually a retarded artical. All it says is do you want rear fill? Then here is what to do. Put speakers in the rear of your car...

Ok waste of time.

10-07-2005, 11:11 AM
It's actually a retarded artical. All it says is do you want rear fill? Then here is what to do. Put speakers in the rear of your car...

Ok waste of time.

Yeah, it really doesn't say anything that we don't already know...just a very descriptive way of saying that :D

10-07-2005, 11:28 AM
uh...its common knowledge that a proper way of integrating real fill is by fading the shyt out out it so its not localizeable...

But as he makes the point that in a hall the sound waves that reflect off the back walls add to fill, so they do in the car...as the car has many more, stronger reflections, and in a smaller sized environment- thats why a proper fronstage in a vehicle does not yield for the need of rearfill, it would sound full to begin with....

In any case, great read for those that think rear fill means fully powering a pair of Pioneer 4ways full blast, no fading...

10-07-2005, 11:35 AM
If you get lucky and/or put lots of work into aiming your frontstage, it can have that effect without adding satellite speakers. When my dad sat in my car and I told him all the sound was coming from the front speakers, I had to pop the trunk and let him look inside for him to believe that I didnt have speakers back there.

Randy Savage
10-07-2005, 03:46 PM
Anyone notice who wrote the article?....;)

Johnny Drama
10-07-2005, 03:55 PM
Anyone notice who wrote the article?....;)

I did :D

Very nice guy.