How Speakers Work
Simply, speakers are air pistons that move back (on the negative cycle of an electrical signal) and forth (on the positive cycle) creating different degrees of air pressure. These movements translate into different frequencies that then translate into Mozart or Metallica at your eardrum.
To do this, an amplifier produces electrical impulses that alternate from positive to negative voltages and create an electromagnet when they reach the "voice coil" (a spool of wire) inside the speaker. This electromagnet will then either be repelled or attracted by the fixed magnet at the bottom of every speaker. The voice coil is attached to the speaker cone, and it moves the speaker cone back and forth as it's attracted or repelled. The surround (a rubbery circle that joins the top of the cone and the speaker's metal basket) and the spider (a circle of corrugated material that joins the bottom of cone to the speaker's magnet) make the cone returns to its original position.
All this moving around makes "sound."
Coaxial/Three-Way speakers (or "three-ways") are two (or more) speakers built into the same frame.
Advantages: They are inexpensive and easy to install, often fitting into vacated factory locations. Because coaxial woofers and tweeters are coordinated in one housing, you're assured good imaging.
Disadvantages: They lack the flexibility of separate components, and you can't aim them. Coaxials come with simple crossovers built-in, and for the most part you're stuck with them. You can't add your own external crossover to better customize and control your sound, although Polk's got a new line of dX coaxes that welcome an external crossover. Just thought I'd throw that in.
Separates, or components, are sets of separate woofers and tweeters with an external crossover.
Advantages: More placement/system flexibility, and a better crossover.
Disadvantages: Placement flexibility means you'll be drilling holes in your car to put separates in custom locations. You may be able to mount the woofer in the place vacated by your factory speaker, but you'll still need to make a separate hole for the tweeter. (The most common place to install tweeters is in the top front corner of the door panel.)
Subwoofers add the lower frequencies to a whole system. We could have a whole section just on subwoofers. In fact, here comes one now...
Subwoofers
Some things are just true. The sun rises and sets. Christmas comes every year. Subwoofers need power. They need more power than anything else in your system. This is simply because they are bigger, and need more power to move their cones farther than other types of speakers. If you have an amp supplying 50W to each of your four front and rear speakers, you can bet on needing at least 100W for your sub(s).
And you need to use a low-pass crossover to block the high frequencies from getting to your sub-woofer and messing up the mix.
Low frequency signals reproduced by subwoofers are "non-directional." That is, it's most difficult for humans to tell where lower frequencies are coming from. So you can (theoretically) put a subwoofer anywhere without worrying about a loss of sound quality due to poor aiming and direction. To make bass response more effective, subwoofers must be housed in a "subwoofer enclosure."
That would be a box.
With a subwoofer in it.
Because of this, subwoofers are most often placed in a car's boot. A subwoofer enclosure can be simple ("sealed"), or complex (bandpass, that's pretty complex). Each kind of enclosure has its own characteristics, and the "one size fits all" rule of stretch denim and baseball caps does not apply here. Ask us (go ahead, ask us), and we'll say that bigger is better. But- honestly-a good quality, well-enclosed 8-inch will outperform a cheap 12-inch sub any day. Big, cheap subs have slower responses and can sometimes be "boomy," while smaller subs tend toward a tighter sound. A bigger sub also needs a bigger box in which to enclose it. And if you fill up your boot with a giant sub box, how are you going to haul the bodies? If you're going to go "big," don't skimp. Do it right.
Free Air
Free air subwoofers use the whole boost as their enclosure. If you're driving a hatchback (stop this now) this won't work, since the trunk has to be sealed as tight as possible for the woofers to get any benefit. Free air subs are not very accurate and are unpredictable - sometimes they sound good, sometimes they sound really really bad- and they require a lot of power.
Sealed
Sealed enclosures are the most common kind of box, and the easiest to build. They deliver the flattest frequency response and best sound quality. Make sure you get the size right, since a box that's too small will deliver tighter bass but suck up more power from your amp, and a box that's too big will simply sound muddy. Use about 500g of loosely packed damping material per cubic foot of enclosure volume when you're building one, being sure to keep the fill out of the speaker's pole piece vent.
Ported
Ported enclosures are usually bigger than sealed enclosures and feature a "tube" or port that lets air out of the box. The theory is that a speaker port pushes and pulls the air at the same time as the woofer, thus reinforcing the bass. The box itself can often act like an amplifier, sometimes adding 3 - 4dB to bass levels. Again, about 500g of loosely packed damping material is recommended per cubic foot of enclosure volume (be sure to keep it away from the port).
Bandpass
Bandpasses enclose a woofer between a sealed and a ported box. Sometimes both parts of the box are ported. Bandpass enclosures yield more bass than sealed or ported boxes (especially at lower frequencies), but over a narrower frequency range. The enclosure acts to block upper and lower frequencies. Bandpasses are big, hard to build, and can be messy when precise volumes and port sizes are not followed.
A tip: When you're working in your boot, put duct tape over the boot latch so you won't accidentally get trapped inside. Couldn't happen to you, right? Just thought we'd mention it.
http://www.polkaudio.com/car/
Next week: how to listen to systems in the shop - and whether to install your own.
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