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Author: Wayne Harris Click HERE to view more articles by Wayne Harris. Originally appeared in the May/June 1990 issue of Car Stereo Review magazine. © All rights reserved. |
Do you ever get the feeling that there's a hole in the music generated by your sound system – right about mid-dash? Does most of the sound originate from the door and side of the dash closest to you? Do vocals sound diluted or distant instead of up front? Is the sound generally muddy or lacking in definition? If any of these symptoms rings a bell, it's time to visit the car stereo doctor. Your system may be one of the millions that suffer from a common disease known as Black-Hole Syndrome. But don't be alarmed – this ailment isn't terminal, and auto sound researchers have discovered several ways to cure it.
Before we can address the treatments, however, we have to take a closer look at the problem. Close your eyes and imagine that you've been transported to New York City's Madison Square Garden. You're there to see your favorite band, and the seats are perfect: tenth row, smack in the middle. Then the opening chords ring out, and you're engulfed by music. Since you're a stone's throw from the stage, the sound is immediate and well defined; the drums are big, but they don't overpower the vocals or the other instruments.
Now, go sit in your car. Pop in a favorite CD or cassette and close your eyes. Let me guess: The sound radiates from the sides of the vehicle and – if you're sitting in the driver's seat – mostly from the left side. Little or no sound is coming from the dash area. And it's difficult, if not impossible, to pick out the band members on an imaginary stage. These are typical manifestations of Black-Hole Syndrome.
Okay, so you want to close that sonic gap and inject a dose of realism into your system. But how? As any experienced installer will tell you, the cure for Black-Hole Syndrome is a center-channel speaker. Retrofitting your system with one involves one of two basic surgical procedures: the simple "wired" approach or the more complex "signal-processing" method.
When executed properly, either method will deliver a fuller, more accurate stereo image – one that adds depth and life to music in general and vocals in particular.
There are two ways to wire a center channel: the "bridged stereo" method and the "summed mono" method. The bridged-stereo approach is the simpler of the two, since it doesn't require an extra amplifier. You create the center channel simply by tying a third speaker into your existing left and right front speakers; it will be mono, since it is the sum of the left and right channels.
The procedure is as follows: Splice the positive lead of the center speaker into the positive wire running to the front-left speaker and its negative lead into the negative wire running to the front-right speaker (see Figure 1). The link-up is passive, since it's made after the amplifier. To control the volume of the center speaker, you install an L-pad (available for about $10 at Radio Shack) in line with the center speaker. This device is simply a high-power volume control that allows you to vary the power delivered to the speakers without changing the impedance "seen" by the amplifier.
Another volume-control option, though a less flexible one, is to install a high-power resistor in line with the speaker. Resistors don't have volume knobs, however, so power to the center speaker will drop a fixed amount according to the value of the resistor you've installed. For example, you can halve the output level of a 4-ohm speaker by wiring a 4-ohm resistor in series with it.
The summed-mono method calls for a dedicated amplifier and a passive summing network, which combines left and right-channel information before it reaches the amplifier (see Figure 2). You can buy a summing network from your local auto sound shop for $10 or so, or you can make your own by inserting a 10-Kilohm resistor in series with each of the left and right amplifier inputs; the resistors effectively mix the left and right signals together and match the input impedance of the amplifier. The combined signal is then amplified by a mono or bridged-stereo amplifier and sent to a centrally located speaker; the volume of the center channel is set using the amplifier's input-sensitivity control.
One cautionary note: Before wiring in a center speaker using either the bridged-stereo or summed-mono method, make sure the amplifier you're tapping into is capable of driving low-impedance loads. Wiring in a third speaker will lower impedance, and if it drops too low, the amplifier may fry.
If you want your center channel to be the best it can be, the doctor's prescription requires the purchase of a signal processor with center-channel provisions. Such standalone components are "active," which means that they process the audio signal before amplification; typically, they're installed between the source unit and the amplifier.
Center-channel processors produce true center-channel information – not just the mono signals delivered by wired center channels. By "true" I mean that complex algorithms may be used to achieve the best approximation of accurate center-channel information. One of these processors even includes the kind of surround-sound processing circuitry that's used to decode movie soundtracks; the others provide for some kind of synthesized surround effect.
The Gavotte from Fosgate ($400) is one of the most acclaimed and widely used signal processors in this category. Folks like it because it adds spaciousness and depth to the sound of a car stereo system. In addition to two center-channel outputs, the Gavotte features two subwoofer outputs, a surround-sound processor, onboard bass equalization, and an active high and low-pass crossover that's factory set at 160 Hz. Typically, it's used only in complex, multi-channel installs.
A newcomer to the signal-processing arena is Audio Control's ESP-3 ($299), a dedicated, analog center-channel processor. The heart of this device is a patented processing circuit. In addition to creating a center channel, the ESP-3 is said to improve ambience. It comes with a dash-mountable control and includes a built-in active crossover. As Figure 3 indicates, the ESP-3 is right at home in simple hi-fi systems.
The Eclipse EQS-1000 ($1,000) is the first car stereo signal processor that performs its key processing in the digital domain. In addition to center-channel provisions, it features sophisticated circuitry designed to simulate the acoustical characteristics of various non-mobile listening environments, including those of a small club, a concert hall, a church, and a stadium. The unit also has controls that allow you to "create" your own listening space by adjusting the delay times of early and late reflections. Simulating the acoustical characteristics of a church, for example, would require an emphasis – and hence lengthier delays – on late sound reflections; this would mimic the effect of sound waves bouncing off a distant rear wall and a high ceiling. The EQS1000 also features a built-in 10-watt amplifier to drive the center channel, a subwoofer crossover, and controls that can be used to adjust the level of the processing that's mixed into the output signal. Outputs are provided for front, rear, center, and subwoofer channels. The sophisticated nature of the EQS-1000 is a mixed blessing. On one hand, its fourteen controls make it difficult to properly set up; on the other, it offers an unparalleled degree of flexibility. (See Figure 4.)
Installing a center-channel speaker is a relatively straightforward process once you decide which center-channel scheme you're going to follow. But as always I recommend that you map out the procedure before you break out a screwdriver. I suggest that you follow this procedure:
When it comes right down to it, there are only three practical mounting locations to choose from: in the middle of the dash, on the back of the rearview mirror, or in a centrally located air-conditioning duct. (If you're a skilled installer and have a vivid imagination, you can come up with an alternate mounting scheme – putting a speaker in the ash tray or in place of the clock, for example.) In-dash mounting is by far the most desirable option, however – provided your vehicle has a factory-speaker cutout in the center of the dash.
If there is no factory hole and you're not quite up to the task of dash reconstruction, consider attaching a speaker to your rearview mirror. Although it sounds bizarre, it's actually a viable alternative given the speaker's proximity to your ears. Rearview mirror installs aren't problem-free, of course. They must be tastefully executed or the speaker will be obtrusive and ripe for a rip-off, it could even present a hazard if the driver's view is obstructed. In addition, it's likely that only tiny 1/2 or 3/4-inch tweeters will fit, which means that you'll have to settle for a center channel with a less-than-ideal bandwidth.
The third option – putting a speaker in a central air-conditioning duct – is a tricky proposition, and I suggest tackling it only as a last resort. Yes, the end result is a cleverly hidden speaker, but there are two problems to consider: one, the speaker will obstruct air flow, and two, your choice of drivers will be limited due to space restrictions.
Once you've nailed down the mounting location, it's time to select a speaker. The big factor is the amount of room you have to work with. I recommend using the largest driver that will fit – the larger its diameter, the wider the speaker's bandwidth, and this translates into better sound. Ideally, you want a speaker that is capable of reproducing the practical frequency range of the human voice, about 300 to 3,000 Hz. Once you decide on size, you can consider speaker type: Should it be a tweeter, a full-range, or a coaxial?
My first choice is a coaxial, which combines a midrange and tweeter on a single frame. Co-axials tend to cost more than full-range drivers or tweeters, but they can deliver the best sound quality: A coaxial combines the low-end response of a full-range driver with the extended high-frequency response of a tweeter. The only drawback is the limited selection you'll have to choose from in the 3 to 4-inch range, which is the size commonly used in center-channel installs. You may have to do some hunting to find a quality coax that fits, but the extra time will be well spent.
Full-range speakers are widely available and tend to be relatively inexpensive. As the name implies, they're designed to reproduce the entire musical spectrum, which is a difficult task – especially when very low or very high frequencies are involved. In reality, most full-range drivers don't reproduce frequencies above 10,000 Hz accurately, but they should do okay with those between 300 and 3,000 Hz. In general, you can expect good – rather than excellent – performance from a full-range speaker.
The big advantage to a tweeter is its small size and low cost. Of course, its limited bandwidth is a major drawback – most tweeters are designed to reproduce only those frequencies above 5,000 Hz, which is well above the region where most vocal tracks lie. My advice is to use a tweeter only if you can't find a good coaxial or full-range that fits.
Once you determine the size and type of speaker you need, audition as many different brands and models as possible before making your purchase. Look for an auto sound shop that's set up to do A/B comparisons, and start by listening to products made by reputable companies. Remember, sound quality is highly subjective, so you should narrow your selection down to speakers that sound good to your ears – not the salesman's. Even though your center channel won't be a primary sound source, its quality is still critical. The last thing you need is a speaker that sounds raspy or dull.
One last thing to consider when selecting a speaker is its impedance. If you've opted for the bridged-stereo configuration, make sure your center-channel speaker has a rated impedance that's at least twice as high as that of the front-left and front-right speakers; this will help to offset the additional power that's delivered to the center speaker. Say you're tapping into an amp that delivers 25 watts x 2 and is being used to drive a pair of 4-ohm speakers. 1f you wired in a 4-ohm center speaker, it would receive 100 watts, or four times the power sent to the primary speakers, due to the voltage swing it sees from the left and right channels. Wiring in an 8-ohm center speaker would cut this power differential in half – to 50 watts, in this example. To further balance the front stage, you'd need to install a power resistor in-line with the center speaker's positive lead. As discussed earlier, the resistor restricts the power going to the speaker.
Once you've settled on the driver you're going to use, start thinking about a crossover point. In general, you'll need a passive high-pass crossover network to filter out potentially damaging low frequencies. For full-range and coaxial drivers, I recommend a point somewhere between 100 and 500 Hz and a slope of 6 or 12 dB per octave; for tweeters, I recommend using a point of 3,500 Hz or higher and a slope of 12 or 18 dB per octave.
Two things determine the amount of power you need to drive a center-channel speaker: the power-handling capability of the speaker (both continuous and maximum ratings apply) and the "electrical value" of the passive crossover employed, which is determined by the type of capacitors and inductors used in the network.
In general, systems that employ an active crossover require less power than those using passive networks, since active devices filter out large-amplitude bass frequencies before they even get to the amplifier. Passive devices filter out frequencies after amplification, which means that you may need an amplifier with a higher output and more headroom to prevent clipping during bass-heavy musical passages.
In most installations, 20 watts or so should be more than enough to power a center channel. Remember, you don't want the center channel to call attention to itself; rather, it should complement and fill in the front sound stage produced by the front-left and -right speakers.
Once you've mapped out your center-channel strategy, it's time to either hire an installer or dust off your installation gear. If you're going to do the work yourself, take a few moments to organize your work space and make sure you have all the necessary tools – a screwdriver, wire snips, and a crimping tool or soldering gun. If everything checks out, it's time to roll up the ol' sleeves and get down to work.
Replacing a factory dash speaker should be relatively uncomplicated – as long as you use a speaker that's the same size as the factory driver. First, snap off or unscrew the grille cover and remove the stock speaker – if there is one, that is, and not just a hole.
Next, snake an appropriate length of speaker wire through the dash and out to where the crossover network and amplifier (if you're using them) are located. Use rubber grommets wherever the wire passes through sheet metal; to secure the wire under the dash, use tie-wraps.
Then connect the wire to the positive and negative speaker terminals. It can't hurt to wrap a piece of electrical tape around the terminals to help prevent an electrical short.
Finally, screw the new driver in place and then reattach the stock grille cover.
If you choose a speaker that's smaller or larger than the factory cutout or decide to create your own cutout, you're in for a time-consuming fabrication job that's beyond the scope of this article – one that requires a custom mounting plate and an aftermarket grille assembly.
The unfortunate reality of dashboards is that many don't have a centrally located factory speaker, which brings the rearview-mirror option into play. Step one is to construct a small, lightweight speaker enclosure that can be mounted above, below, or behind the rearview mirror. I'd make it out of ABS plastic, since this material is relatively easy to work with and you can paint it to match the mirror.
The basic procedure: Cut out the pieces for a little box; cut a hole whose diameter matches that of the tweeter's rim in the panel that will serve as the box's faceplate.
Mount the speaker, with the wires attached, to the box using glue or screws. Specifically, you need to secure the speaker's rim to the outside ' of the faceplate; this will allow you to pop it out any time.
Then, drill an exit hole for the wire in one of the side-panels; pull the wire through the hole so that it's taut inside the box and dangling loosely outside it.
Next, glue the box together, and secure it to the mirror with Velcro, silicone glue, or a homemade clip.
Then, carefully push the wire under the trim bordering the headliner; the trim is fixed, but the upholstery gives. Run the wire over to the nearest windshield pillar. Unscrew the pillar's trim molding and snake the speaker cable down the pillar. Then, run the wire to the amplifier as you would in any install. Take your time, so you don't pinch the speaker wire or tear the headliner.
If the headliner in your vehicle is made of hard plastic, you might have to remove the trim, which is usually held on by plastic clips. Be very careful when removing the trim or the clips will break off.
Regardless of the exact installation method you use, the important thing is to make sure that the enclosure is firmly secured to the mirror and does nothing to obstruct the driver's field of view.
If you're not up to the task of fabricating a mini-enclosure, here's an option to consider: Custom Acoustics of Tampa, Florida (800-533-5487) makes the Ambience Tweeter Mirror ($130) – a unique rearview mirror that houses a pair of 1/2-inch hard-dome tweeters and a 6-dB-per-octave passive crossover set at 10,000 Hz. (That crossover point seems pretty high to me.) I haven't heard the Tweeter Mirror yet, so you're on your own here.
If you decide on this approach as a last resort, I suggest that you consult an experienced installer – doing this type of job correctly is an art, and it's a pain in the butt, too. But if you're a brave soul, tackle the beast by first removing any panels or assemblies that provide access to the air-conditioning duct you've selected.
Once the duct is exposed, you'll need to fabricate a bracket to secure the speaker inside the duct; the driver should face the interior, of course. There is no set way of doing this, however, so you'll have to use your imagination here.
One option is to fabricate a wood "donut" that will fit snugly in the duct; its hole must be large enough to accept your speaker's rear assembly and its diameter must match that of the speaker's rim. Drill a small speaker-wire hole in the duct several inches behind the mounting location of the driver; this should be 2 or 3 inches away from the dash. Then mount the driver on the donut, attach the speaker leads, and secure the donut in the duct with silicone glue. (This type of glue is flexible and won't dry out.) Finally, reattach the duct cover.
Once the speaker is installed, it's time to fine-tune its output. The key point here is not to overdo it. Remember, the job of a center-channel speaker is to fill in that black hole – not to stand on its own as a separate, identifiable music source.
Adjusting the volume is a straightforward process when you've chosen the summed-mono or signal-processor route. First, turn the center-channel amplifier's input-sensitivity control all the way down. Then, while playing your system, slowly turn the center channel up until it just fills the void between the left and right speakers; some signal processors, including the Gavotte, have a separate level control for the center channel. (IASCA's new test CD is great for this, by the way, since it has special tracks that were specifically chosen for testing staging and imaging.)
If you traveled the bridged-stereo path and installed an L-pad in-line with the speaker, simply turn up the L-pad's potentiometer until the center channel's output blends in appropriately. Adjustment is more involved without an L-pad: First, you have to gauge how much louder the center channel is (twice as loud, four times as loud, and so on). Then consult the resistor chart (Figure 5) to determine the proper resistor value. Insert the resistor in series with the positive speaker lead. The resistor must be wired in prior to any passive crossover networks; otherwise, the additional resistance it creates will electrically alter the crossover frequency you set.
| Relative Power | Speaker Impedance | Resistor Value | Resistor Power Handling |
| 2X | 4 Ohm | 4 Ohm | 15 Watts |
| 2X | 8 Ohm | 8 Ohm | 10 Watts |
| 4X | 4 Ohm | 8 Ohm | 15 Watts |
| 4X | 8 Ohm | 16 Ohm | 10 Watts |
Note: It takes 10X the power to sound twice as loud.
Once the resistor is installed, play the system at medium-high levels for a while and touch the resistor to see if it has grown hot. If it has, you'll have to replace it with one that has a larger power-handling capacity.
OKAY. Crank up the volume and sit back. Remember how your system sounded before it underwent therapy? Your fix should allow you to "see" things on stage a little more clearly, and that means you now have the best seat in the house.