How Do Speakers Work?
Speakers are air pistons that move back (on the negative cycle of the signal) and forth (on the positive cycle), creating different degrees of air pressure at different frequencies. The amplifier (either separate or built-in your radio), produces electrical impulses that alternate from positive and negative voltages (AC). This current reaches the voice coil inside the speaker, creating an electro-magnet that will either be repelled, or attracted by the fixed magnet at the bottom of the speaker. The voice coil is attached to the cone, moving it back and forth, creating sound. The surround (rubbery circle that joins top of the cone and metal basket) and the spider (usually yellow corrugated circle joining bottom of cone to magnet) make the cone return to its original position.
Speaker Sensitivity, measured in dB, is how loud a speaker plays (usually 1 Watt, 1 meter). A higher Sensitivity rating means that the speaker will play louder using the same power as a speaker with a lower rating.
The back and front parts of the speaker should be isolated from each other. When the front of the cone is pushing air, the bottom is pulling air, creating a canceling effect. Ideally every speaker should be in an enclosure. If you are mounting a speaker in a big hole, make sure you build a panel to isolate the front and back of the speaker (baffle).
Imaging And Staging
Imaging is being able to pick certain sounds from certain places. The singer would normally be located towards the middle of the car, in a car audio installation, guitars, trumpets, and other instruments towards the sides of the car, in a car audio installation. If you scatter speakers all around the car, in a car audio installation, your imaging would be very poor, since you would be producing the same sound at different places. If you have a system with good imaging, the sound should seem to come from different instruments and voices, not speakers.
Staging is the ability of a system to "fool you" into thinking that everything (including bass) is in front of you. The sound should be similar to a stage in a concert, where the singer would be in the front center, and the rest of the instruments and background vocalists would be located to the left and right (but always on the front). Good staging and imaging are not so easy to implement in a car audio installation. It takes a lot experimenting with speaker location and direction.
Directivity of sound is related to frequency. At higher frequencies it is easier to pinpoint where the sound is coming from, than lower frequencies. This can be used to our advantage in car audio installation. Tweeters are the most important part of getting good staging. They should be aimed towards the middle of the car, in a car audio installation. A way to "bring" the bass to the front of the car is to fool our ears by overlapping frequencies played by midbass and subs, so that your midbass actually "pull" the bass to the front, since lower bass in not too directional. You should crossover your midbass as low as you can (without getting distortion). Then cut your subs at a bit higher frequency. This will mix the bass coming from the front and rear, making the bass seem to come from the front. Adding a center channel also improves staging, if it is set up correctly, in your car audio installation.
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