Dynamic Power Rating
Low Level Inputs/Outputs
High Level Inputs/Outputs
Db per Octave
Power Ratings at Different Ohm Loads
Calculating Cubic Air-Space
RMS (Root Mean Square) Power Rating - RMS power is the reading of how much power a head unit or amplifier produces on a continual basis at 12.6 volts. This is is also known as the Competition Standard. Good amps are almost always rated in RMS power, and will usually give you the best power and sound quality.
Dynamic Power Rating - Dynamic power is the power that an amplifier produces for instantaneous peak musical instances. It is also known as the IHF-202 Standard, and is measured at 14.4 volts. The peak power is usually double what the corresponding RMS value is.
Stereo Imaging - Perfect stereo imaging is pretty difficult to get in an automobile, because of the environment. Stereo imaging is how the music presented. For example, when you go to a concert, listen to live music, etc. Certain instruments, music is coming from certain areas in front of you. A system with good stereo imaging will present different musical sounds across the sound stage. You'll hear drums off to the side, the symbol closer to the middle, the synthesizers closer yet, and then hear the vocals in the middle. This gives you a greater presence or feeling that you are actually there, live, listening to the music.
Crossover - A crossover is a device that helps block or allow certain frequencies to reach the speaker. This is especially useful if you have mid-range speakers that keep distorting because of too much bass. It allows for cleaner sound at a higher volume level.
Db (Decibels) per Octave - Db per octave is a term usually heard in crossovers. I'll try to explain this without getting too much into how crossovers work and the different roll off curves they produce. When a crossover has a higher db per octave number, it cuts the frequencies off better. A 24 db octave lowpass crossover will cut out higher frequencies better than a 12 db octave lowpass crossover. With a 12 db 100 Hz lowpass crossover you will still have some low frequencies around 100 Hz get through to the speaker. An ideal crossover would have a perfectly vertical roll off curve.
Gain - The gain adjustment on most amplifiers is an adjustment of the sensitivity of the amplifier. A common misconception is that if you want more power, turn up the gain. All that is doing is causing the amplifier to clip, peak out sooner. The gain should be set this way. Turn all the amplifier gains down to minimum. Put a fairly dynamic musical track in, turn the volume 3/4 of the way up on the head unit. Slowly turn the gain up on the amplifier. Listen very carefully for distortion. Stop turning the gain adjustment as soon as you hear distortion. Turn the gain down just a hair from the point of distortion. That's it. Now the head unit and the amplifier are set in sync with each other. The head unit and amplifier are reaching their peak around the same time, rather than having the head unit at a low volume and the music is cranked. This also gives you a greater dynamic musical range.
THD - Total Harmonic Distortion is a good number to help determining a good amplifier from the cheapos. For starters, a THD of 0.1% or greater is audible to the human ear, anything less you won't notice. Head units usually have a very low THD. THD usually comes into play in amplifiers, especially when running at a lower ohm rating.
Power Ratings at Different Ohm Loads - An Ohm is a measurement of resistance in wiring. Every type of wire has resistance, if you have no resistance you have a short, and lots of sparks. Manipulating Ohm loads with speakers and amplifiers can be very beneficial. When you decrease the ohm load by half, you will increase power by a factor of two. Lets say you have an amplifier that puts out 50 watts/channel at a 4 W load. If you have a 4 W speaker, you get 50 watts. If you have a 2 W speaker you will get 100 watts, but there is one caution. When you decrease ohms, you increase THD.
Engine Noise - Engine noise, or static noise in generally caused by bad wiring, connections, and placement of RCA patch cables. Number one rule when wiring is that your ground wire from any amplifier, head unit, etc should not be longer than 2 feet, and should be connected to a direct chassis connection. I have seen some weird problems that have been caused by bad grounds. I have also seen people fry amps and watch the wiring melt to the inside of the car, not a pretty sight or smell! Next, when you run power wire for amps, run it directly from the batter terminal, and use a connector, don't just wrap the wire around and clamp it in. Also, make sure you fuse the power wire no more than 2.5 ft from the battery, you'll thank yourself when you blow the fuse instead of your amp. Those three things will save you a lot of headache and problems. The big culprit for engine noise is RCA cables. When you run them you want to run them on the opposite side of the car as the power cables, or at least 2-3 ft away from each other. If you have to cross the two sets of wires, cross them at a 90o. This will help minimize RF (Radio Frequency) noise. Also, do yourself a favor and buy a good set of RCA cables.
Speaker Wiring - There is two main alternative wiring types for automotive speakers. The first is called Series wiring. When you connect two speakers in series the main thing this will do is add the ohm load of the two speakers. If you have two 4 W speakers you will have a combined ohm load of 8 W's. To wire two speakers like this, you take an extra piece of speaker wire, connect one end to the negative lead of speaker one, then connect the other end to the positive lead of speaker two. Connect the positive speaker wire end, from the amp, to speaker one's positive terminal. Connect the negative wire end to speaker two's negative terminal. This is really only useful, since you decrease the power to the speaker, if you want a certain ohm load before connecting the leads to the amp.
Parallel wiring is more useful because it drops your ohm load and increases power. To wire two speakers in parallel, take two extra pieces of speaker wire, connect one end to speaker one's positive terminal. Connect the other end to speaker two's positive terminal. Now do the same for the negative terminals. To connect the speakers to the amp, connect the positive end to either speaker's positive terminal, and vice-versa with the negative end and terminal. Here is a basic diagram to help illustrate the concept.
Relay - A relay is a device to help in high amperage draw situations. They are fairly simple. When the smaller amperage device sends an electric current through a magnetic solenoid it pulls down on a metal shaft that connects the higher amperage power source and device. Here is a wiring diagram of a common relay.
Pin 30 is the high current power source. Pin 87 is the high current device. Pin 85 and 86 are the low current power source and device.
Subwoofer Enclosures - There are three main different types of enclosures. The first one, and most simple, is a sealed enclosure. The subwoofer is usually mounted inside the enclosure. This design is very simple, and usually has a pretty smooth response curve, i.e. the bass is louder at higher frequencies and slowly decreases in volume as frequencies drop. The enclosure also is very good if you have a high power amp, the air helps cushion the speaker from bottoming out. The only big drawback is this enclosure is not very efficient, it will use a lot of power.
The second type is a ported enclosure. This is similar to a sealed, except it has a port. Contrary to common knowledge, the port is tuned to a specific frequency. Different port sizes with the same speaker and enclosure will produce different results. This can be useful if you have more than one subwoofer. The larger subwoofer can be tuned to lower frequencies, while the smaller can be tuned for higher bass. This enclosure does not have quite as nice of a frequency curve. The porting throws it off a little. This enclosure however is more efficient, and will be louder then a similar sealed enclosure, with the same amount of power. Just by porting an enclosure a 3 db gain in loudness is not uncommon.
The third type of enclosure is a band-pass enclosure. The name basically refers to what it does. The enclosure only allows frequencies between a certain range to be heard. This box is definitely more difficult to design and build. It's not something you want to do if you don't have specified dimensions or cubic air-space. This type of enclosure usually has a sealed chamber and a ported chamber. It varies to whether the front or back of the speaker is sealed or ported. This enclosure will give you the most bang for amp power. The major drawbacks are that it's more complicated, and if it's not designed correctly you will not have flat response curve for the frequency range you want.
Calculating Cubic Air-Space - This is actually pretty simple. All you have to do is calculate the volume, and then convert it into cubic feet. For a rectangular enclosure the formula is Length x Width x Height. The volume of a port can be calculated by taking (r=radius) (r2) x P x Length. Assuming you take your measurements in inches you will need to convert the volume to cubic feet. To do this take Volume / 1728. This will give you a number that can commonly be worked with. When you calculate the air-space take into account the thickness of material. If you are using 3/4 inch material you will need to deduct 1.5 inches from your length, width, and height measurements. In your design also take in account the air-space the back of the subwoofer takes up.
Here's a little program to help you in your subwoofer enclosure design. (Note: This program takes in account for the material thickness.)