Tech Talk
'A' is for amperes, a measure of electric current. How much is enough? (We like to say that having too much amperage is like having too much money.) Your car's stock alternator probably provides enough amperage to power a basic car audio system. But if you want to add multiple high power amplifiers, you might require some additional current, and the installation of a new alternator or stiffening capacitors. Ampere is commonly abbreviated as "amp" (not to be confused with amplifiers), but sometimes it's "I." Electricity is like that.
A crossover is an electronic circuit that directs which frequencies go to which speakers. For example, since subwoofers are designed to best reproduce only the lowest frequencies, a subwoofer crossover (a low-pass crossover) allows only the low frequencies to pass through to the subwoofer. Freed from the task of reproducing heavy bass, your other speakers will rejoice by performing better and playing louder. You need crossovers so that you don't send unnecessary signals to a speaker (which could damage it).
'dB' is for decibel, a measurement of power ratios and volume. All you really need to know is that to gain 3dB in volume (just enough to hear the difference), you must double your power. There. That's it.
'DC' is direct current, a type of circuit. In a DC circuit, the current always flows in one direction. In your car, you're dealing with a 12 volt DC system (twelve volts of direct current). In a car, it's important to keep track of which wires are attached to the ground (or "negative") lead of the battery.
'Imaging' is a description of the illusion of being able to locate certain sounds as "coming from" certain places. If you have a system with good imaging, the sound should seem to come from different distinct instruments and voices, not from speakers. A singer would generally be in front of you (centre stage), and the band would be arranged around them. See also "staging."
'Staging', like "imaging," is a description of your system's ability to "fool you" into thinking that everything (including bass) is right in front of you. Like on a stage (hence the term "staging"), the singer should (in general) be in the centre, and the band should be located to the left and right. Good staging (and good imaging) are not easy to achieve in a car audio situation. One of the hardest aspects of staging is getting the illusion that the bass is coming from the front of the car, even though the woofers are in the back. You may have to experiment with speaker locations, directions, and crossover roll-off points. Cheat the bass by overlapping the frequencies played by your mids and subs so that your semi-directional mids actually "pull" the bass to the front. To do this, use a high-pass crossover to roll off your midbass drivers as low as you can (without getting distortion). Then set your sub's low-pass crossovers at a slightly higher frequency. This will mix the bass coming from the front and rear, giving the illusion that the bass comes from the front. Adding a centre channel improves staging as well, but that's another lesson.
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Can Your Impedance Be Too Low?
Yes it can. It all depends on how well your amplifier can handle the increase in current flow that comes with lower impedance speakers. The more current, the hotter your amp will get. An overheating amp is trouble. A good amp will simply shut down when overwhelmed with current. A poor quality amp will burn. Make sure your amp can handle the impedance of your speakers.
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'THD' is total harmonic distortion, or how much a device distorts a signal. These figures are usually given as percentages. THD figures below approximately 0.1% are inaudible, but-like bad karma-distortion adds up.
'V' is for volts, the measure of "electric potential." Voltages don't do anything, they simple measure stuff. 'W' stands for watts, another measurement of electrical power. One watt is equal to one volt times one amp. Don't be misled by wattage specifications. All things being equal, a good, expensive 50W amplifier will outperform a cheap, marginal 75W amp. Here's why: In order to play even 3 decibels louder, an amp must double its power output. The difference between 50 and 75W output is so small, maybe a dB or so, that you probably won't even be able to tell. The human ear just doesn't pick that up. To actually double the apparent volume, you'd have to have a 10dB increase in level. Basically, you're better off with a more expensive, more efficient, better built lower wattage amp than with that "200 Watt" amp you picked up for $39.95 at the flea market.
'Hz' is for hertz, a measure of frequency. One hertz is equal to "one cycle" per second. A cycle of sound is the duration between similar portions of a sound wave (between two peaks, for instance). Frequency can describe both electrical circuits and sound waves, and sometimes both. For example, if an electrical signal in a speaker circuit is going through one thousand cycles per second (1000Hz, or 1kHz), the speaker will vibrate at 1kHz, producing a 1kHz sound wave. Got all that?
An Ohm is a measure of resistance and impedance that tells you how much a device (like a speaker) will resist the flow of current in a circuit. If the same exact signal is sent into two speakers, one rated at 4 ohms of impedance, the other at 8 ohms of impedance, twice as much current will flow through the 4-ohm speaker as the 8-ohm speaker. All things being equal, the 8-ohm speaker requires twice as much power to achieve the same volume level, since power is proportional to current. (See "dB.")
Speaker Sensitivity, measured in dB, is how loud a speaker plays with a given amount of power going into it. Conveniently, the usual measuring stick is 1 watt at 1 metre. A higher sensitivity rating means that the speaker will play louder using the same power as a speaker with a lower rating. So, should you always buy the speaker with the higher sensitivity rating? Not necessarily, because you'll usually end up trading off some other aspect of system performance like bass response or power handling. Sometimes a lower sensitivity rating gives a speaker a better (flatter) balance of frequency responses without sacrificing efficiency or value. How you announce your intention to spend your paycheque building a car audio system is your sensitivity rating!
Crossovers
Chose your crossover points and crossover slopes by consulting the frequency response measurement on your speaker specs. The frequency response is the range of frequencies that the speaker can successfully reproduce. The frequency response of two separate speakers (woofer and midrange, for example) must overlap a little, or you will hear a "gap" in the music. The crossover point appears within this overlap.
The crossover slope is a measurement of how abruptly the crossover cuts off the speaker's sound beyond that crossover point. If your speaker frequencies overlap just a little, use a steeper slope. The steeper the slope, the narrower the range within which two speakers are producing the same signal, and the smoother the transition from one speaker to the next. The opposite is also generally true.
You can severely damage your speakers if you don't pay attention to the way your amps and crossovers work together. For example, if you're forcing too much high volume bass out of a smaller midrange or tweeter speaker, you're going to force the speaker to its "excursion limit," or its limit of movement. The voice coil in the stressed out speaker bangs around, gets bent out of shape, and destroys your speaker.
Not surprisingly, this is a common problem with 4-inch and smaller full-range and coaxial speakers. And equally unsurprising, there's an easy way to prevent it. Simply "roll off" speakers with an in-line capacitor- or "bass blocker"-to keep the lowest bass frequencies from getting to that driver. (You won't be "missing" anything, since you'll be filtering out frequencies the speaker can't reproduce anyway, and your sub-woofer will easily pick up the slack.) Any good car stereo dealer can help you choose a capacitor value that's right for your speakers. They probably have them in stock, too.
There are two types of crossovers: active and passive. A passive crossover appears in the circuit after your amplifiers, and divides the signal that then goes to your speakers. A passive crossover has no power, ground, or turn-on leads and is rather inexpensive. But, they tend to be inefficient and can even add some distortion.
An active, or electronic, crossover does its job pre-amp (taking the signal directly from your head unit before it gets to the amplifier) and needs an external power source. Active crossovers give you control over which frequencies you want to use as the crossover points for bass and treble. Some active crossovers allow you to customize the crossover slope as well as the crossover point. Because they filter frequencies before the signal is amplified, active crossovers ensure that the amp gives its full attention to the filtered signal, which is very efficient.
What's That Noise?
Hiss. Hum. Noise. Every system is going to have some noise. Unless you're going to enter a sound competition, you may not even have to worry about it. Usually, you won't hear that small hiss when you're driving. But if you're concerned, you can test your system. Make sure your amplifier is isolated from your car's chassis. Then, try this:
- Check your RCA patch cords. Disconnect the cords from you amp and reinsert just one strand into the right and left input jacks of your amp. Now start your engine and turn on your system. If you get noise, try re-routing your cords, separating them from other connectors (like your power cable) by at least 45cm. Also, try a better quality of patch cord, consider trading in coaxial cable for "twisted pair" cables.
- Check your antenna. Unplug it and see if the noise goes away. If it does, you need an antenna noise filter.
- Check your head unit. If you stop hearing the noise when you jiggle or pull out your head unit while it's playing a tape, the noise may be radiating from an ungrounded component into your tape or CD system. Try shielding the back of your head unit with metal foil (available at retailers) or filtering the unit's power leads.
- Check the electrical system. Is your battery filled? Have a mechanic check your alternator and give your car a tune up. A tune up will stop the noise at its source: the spark plugs, wires, distributor cap, and coil. Install resistor-type spark plugs with shielded carbon-core wires. Worse case, have a mechanic check the grounding of your ignition, charging, and injection systems. All of these things can radiate noise into your system.
Defeating Distortion
Distortion can destroy your speakers. Distortion is the audible expression of a speaker's limitations or an amplifier running out of clean power. Distortion happens when speakers are forced to play frequencies that they were not meant to play, which happens when speakers lack the proper crossovers (frequency blockers). If you don't have the crossovers, and you jam low frequencies through your little tweeters, you will hear distortion.
Since beauty, as it were, is in the ear of the beholder, there's lots of argument over what distortion actually sounds like. But it's safe to say that when the stuff you're listening to gets "muddy," when it starts to lose the definition of individual elements, when it all starts to blend together into a loud jumble... That, my friend, is distortion.
If you start hearing distortion, turn it down!
Distortion is bad. Power, on the other hand, is good...
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Trust Your Ears, Not The Specs
Specs can tell you how a speaker will sound, but they can't tell you what a speaker sounds like. Trust your ears. And if you don't know what to listen for, trust someone with listening experience, like the professional in the showroom.
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Power Up!
How much power do you really need? A lot. But all car audio components come with their own power handling specifications, and you should start there when determining how much power you'll need.
On amps, for instance, you'll see two types of power specs: Continuous, and Peak (or Max) power output. The continuous power output rating is determined using a constant test tone. The peak power output rating describes how much power the amp produces in short bursts. This is more comparable to the nature of music, which tends to go up and down a lot.
For a speaker rated at 100 watts peak, you should get a 100 watts per channel amplifier to safely get the greatest amount of volume from that speaker. If all you know is the continuous power of a speaker, use "The 3/4 Rule": divide the continuous rating by 0.75 to calculate the maximum amplifier size. (For example, a speaker with a 50 watt continuous rating can be safely used with an amplifier of 70 watts/channel [50รท0.75 = 66.7, round up to 70 watts].)
For most systems, 30 to 50 Watts (per channel) should be fine for primary speakers. Apply more (two to three times more, or 100-150 Watts) to your subwoofers. If you're powering your tweeters independently, they can get away with less power (20 - 40 Watts). A caveat: Speakers can be harmed when you push an amp beyond its power capabilities. It'll "clip" the signal, which produces both mechanical and thermal stresses on a speaker's voice coil. The speaker's voice coil gets banged around, overheats, and ultimately breaks. But you're actually less likely to blow a speaker by using too much power than you are by using too little power. If you like to play it loud, get a bigger amplifier.
http://www.polkaudio.com/car/
Next week: car sound FAQs
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