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Author: Wayne Harris (Click HERE to view more articles by Wayne Harris.)
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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 factor.
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.
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).
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 sufficient.
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 accomplished.
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 connections.
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 wiring.
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?