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Frequently Asked Questions
Questions

1. Definitions
1.1 What is meant by "frequency response?"
1.2 What is meant by "anechoic?"

2. Electrical
2.1 My speakers make this high-pitched whine which matches the engine's RPMs. What is it, and how can I get rid of it?
2.1.1 Level 1: Check out the Amplifier(s)
2.1.2 Level 2: Reduce the System
2.1.3 Level 3: Move the Head Unit
2.1.4 Level 4: Testing the Car
2.1.5 Level 5: Adding Signal Processors
2.1.6 Level 6: Processor Isolation Tests
2.2 My system "pops" when I turn it off. What is happening and how can I get rid of it?
2.3 What is the best power wire to use?
2.4 What is the best speaker wire to use?
2.5 I heard that I should run my power wire directly to my car's battery. Why should I bother, and how do I do it?
2.6 Should I do the same thing with my ground wire, then?
2.7 Sometimes when I step out of my car, I get a really bad shock. What is wrong with my system? 2.8 When my car is running and I have the music turned up loud, my headlights dim with the music. Do I need a new battery or a new alternator?
2.9 What is a "stiffening capacitor", and how does it work?
2.9.1 Do I need a capacitor?
2.9.2 Can I just upgrade my headlight wiring instead?
2.9.3 Will the dimming go away if I upgrade the amplifier power/ground wiring?
2.9.4 What do I look for when buying a capacitor?
2.9.5 How do I install a capacitor?
2.9.6 I have more than one amp in my audio system. Which one should I have the capacitor run? 2.9.7 Will my bass response improve by adding a capacitor?
2.10 When should I upgrade my battery or add a second battery?

3. Components
3.1 What do all of those specifications on speakers mean?
3.2 Are component/separates any better than fullrange or coaxials?
3.3 What are some good (and bad) brands of speakers?
3.4 What do all of those specifications on amplifiers mean?
3.5 What does "bridging an amp" mean?
3.5.1 Why should I bridge my amp?
3.5.2 Why shouldn't I bridge my amp?
3.5.3 What happens when an amp is bridged?
3.5.4 Does bridging an amp would halve the impedance of the speakers?
3.5.5 Can I bridge my 4 channel head unit?
3.6 What is "mixed-mono?" Can my amp do it?
3.7 What does "two ohm stable" mean? What is a "high-current" amplifier?
3.8 Should I buy a two or four (or more) channel amplifier?
3.9 What are some good (and bad) brands of amplifiers?
3.10 What is a crossover? Why would I need one?
3.11 Should I get an active or a passive crossover?
3.12 Should I buy an equalizer?
3.13 What are some good (and bad) brands of equalizers?
3.14 Should I buy a detachable faceplate or pullout CD player?
3.15 What are some good (and bad) brands of CD head units?
3.16 Can I use my portable CD player in my car? Won't it skip a lot?
3.17 What's that weird motor noise I get with my portable CD player?
3.18 What's in store for car audio with respect to MD, DAT and DCC?
3.19 Are those FM modulator CD changers any good? What are my other options?
3.20 What kind of changer will work with my factory head unit?
3.21 What are some good (and bad) brands of CD changers?
3.22 Why do I need a center channel in my car, and how do I do it?
3.23 Should I buy a sound field processor?
3.24 What are some good (and bad) brands of signal processors?
3.25 I keep hearing that speakers for Company X are made by Company Y. What's the deal?
3.26 What is a Line Driver? Do I need one?
3.27 Can I play MP3 files in my car?

4. Subwoofers
4.1 What are "Thiele/Small parameters?"
4.2 How does speaker sensitivity affect real world SPL? Will a higher sensitivity give me a larger SPL?
4.3 What are the enclosure types available?
4.4 Which enclosure type is right for me?
4.4.1 Infinite Baffle ("free-air")
4.4.2 Sealed Box
4.4.3 Ported Box
4.4.4 Bandpass Box
4.5 How do I build an enclosure?
4.6 MDF for Dummies
4.6.1 What is MDF?
4.6.2 Where can I get MDF?
4.6.3 What type of saw blade works best when cutting MDF?
4.6.4 What type of router bits work well with MDF?
4.7 What is an "aperiodic membrane?"
4.8 Can I use my subs in the winter?


5. Installation

5.1 Where should I buy the components I want?
5.2 What is "rear fill", and how do I effectively use it?
5.3 How many devices can I attach to my remote turn-on lead?

6. Competition

6.1 What is IASCA, and how do I get involved?
6.2 What is USAC, and how do I get involved?

7. Literature

7.1 Can I contact any manufacturers on-line?
  Answers

  1. Definitions
 
1.1 What is meant by "frequency response?"
The frequency response of a device is the range of frequencies over which that device can perform in some fashion. The action is specific to the device in question. For example, the frequency response of the human ear is around 20Hz-20kHz, which is the range of frequencies which can be resolved by the eardrum. The frequency response of an amplifier may be 50Hz-40kHz, and that of a certain speaker may be 120Hz-17kHz. In the car audio world, frequency responses should usually be given with a power ratio range as well, such as (in the case of the speaker) 120Hz-17kHz +/-3dB. What this means is that given an input signal anywhere from 120Hz to 17kHz, the output signal is guaranteed to be within an "envelope" that is 6dB tall. Typically the extreme ends of the frequency range are the hardest to reproduce, so in this example, the 120Hz and 17kHz points may be referred to as the "-3dB points" of the amplifier. When no dB range is given with a frequency response specification, it can sometimes be assumed to be +/-3dB.

1.2 What is meant by "anechoic?"
"Anechoic" means not echoing. It usually refers to a style of measuring a speaker's output which attempts to eliminate echoes (or "reflections") of the speaker's output back to the measurement area, which could alter the measurement (positively or negatively).

2. Electrical

2.1 My speakers make this high-pitched whine which matches the engine's RPMs. What is it, and how can I get rid of it? [IDB]
This is a set of instructions to debug a stereo installation if there is any noise present after it is completed. Follow each step carefully! If you have more than one amplifier, repeat level one for each amp to be sure that none of them are responsible for the noise

2.1.1 Level 1: Check out the Amplifier(s)
After you have determined that there is noise in the system, determine if the amplifier is causing the noise. To do this, mute the signal at the inputs to the amp by using shorting plugs. If there is no noise, then the amp is fine, and you can proceed to level 2. However, if there is noise, then use a test speaker at the amp's output. If this stops the noise, then the problem is originating in the speaker wiring, or the passive crossovers. Check to make sure that none of these are shorting with the body of the car, and start again at level 1. If noise is still present when using the test speaker, then there may be a problem with the power supply on the amp. Try connecting an isolated power supply - if this does not get rid of the noise, then there is something seriously wrong with the amp, and it should be replaced. If the noise goes away, then there may be a problem with power supply filtering or isolation. This can be fixed by changing the amp's ground point or b adding external supply filtering

2.1.2 Level 2: Reduce the System
The amps have been determined to be noise free. If you have any processors between the head unit and the amps, disconnect them and connect the head unit directly to the amp. If this gets rid of the noise, then one (or more) of the processors must be at fault, so proceed to level 5. Otherwise, try running the signal cables over a number of different routes. If you are able to find one that does not produce any noise, permanently route the cables in the same manner, and proceed to level 5. If not, then you must isolate the head unit from the car's chassis (except for its ground!) - don't forget to disconnect the antenna, since it is also grounded to the car. If isolating the head unit does not solve the problem, the move the grounding point of the head unit. Hopefully the noise will be gone, and you can install the head unit with a quiet ground and proceed to level 5, otherwise go on to level 3.

2.1.3 Level 3: Move the Head Unit
The amplifiers are fine, but moving both the ground for the head unit and the signal cables does not solve the noise problem. Take the unit completely out of the dash, and put it on either the seat or carpet, and run new signal cables to the input of the amp. If this solves the problem, re-install the head unit, one step at a time and skip to level 5. But if the noise persists, then move the head unit as close to the amp as possible and use the shortest possible signal cables. This will verify that the original signal cables are not causing the problem - assuming the noise is gone, reinstall the head unit one step at a time and go to level 5. Otherwise, there may be a problem with the power filtering for the head unit. As with the amps, power the head unit with an isolated power supply (again making sure that the head unit isn't touching the car's chassis at all). If the noise goes away, you can add power supply filtering or an isolated power supply; go to level 2. But if the isolated power supply does not solve the problem, then you can either replace the head unit and go to level 2, or check the car's electrical system in level 4.

2.1.4 Level 4: Testing the Car
There does not seem to be a problem with either the head unit or the amplifier, and the car's charging system is suspect. To see if this is the case, we can use a system in a car that is already known to be "quiet." Bring both cars together as if you were going to jump one, and use jumper cables to connect the two batteries. Start the engine of the car with the noise problem, and listen to the "quiet" car's system. If the noise does not go away, there is a SERIOUS problem with your car's electrical system (possibly a bad alternator). Have a qualified mechanic check the charging system out. If there is no noise in the "quiet" car, then the "noisy" car's charging system is definitely quiet, so continue with level 5

2.1.5 Level 5: Adding Signal Processors
We have proven that the amplifiers are good, the head unit is good, and the car's electrical system is good. Now we need to reconnect each signal processor. Repeat this level for each signal processor used in your system; if you have added all of your signal processors, and there is no longer any noise, CONGRATULATIONS! You've removed the noise from your system! Connect the signal processor. If there isn't any noise, then go on to the next signal processor. Otherwise, try re-routing the signal cables. If this cures the problem, the route them permanently over the quiet path, and install the next processor. If not, then isolate the processor from the car's chassis except for a single grounding point. If this works, then permanently isolate the processor, and move on to the next processor. If isolation does not help, then advance to level 6

2.1.6 Level 6: Processor Isolation Tests
Now, noise enters the system when one particular processor is installed, but regrounding it does not help. Move the processor very close to the amp, and check for noise again. If there isn't any, then re-install the processor, carefully routing the cables to ensure no noise, and continue at level 5 with the next processor. Otherwise, use an isolated power supply to power the processor, making sure that no part of the processor is touching the car's chassis. If this solves the problem, the consider permanently installing an isolated power supply or possibly a 1:1 transformer, and go to level 5 with the next processor. Otherwise, separate the processor and isolated power supply from the car by many feet and re- test. If there is still noise, then there is a serious problem with the processor's design. Get a different processor, and continue at level 5 with it. If separating the power supply and processor from the car does solve the noise problem, then either the processor is damaged, or your tests were inaccurate. Repeat level 5...

2.2 My system "pops" when I turn it off. What is happening and how can I get rid of it?

This kind of problem is often caused by transients in the signal processor as it powers down finding their way into the signal path, which the amplifier then transmits to the speakers. Usually this can be solved by adding a little turn-off delay to the processor. This allows the processor to stay powered on for a short time after the amplifiers have powered down, thus preventing the pop. Many components sold today (such as crossovers, equalizers, etc) have delays built-in. Read your manual to see if it is possible to set this delay on your piece of equipment or be sure to look for this feature during your next car audio purchase. If your processor does not have this feature, you can build your own delay circuit with a diode and a capacitor. Add a 1N4004 diode in series with the processor's turn-on lead, striped side towards the EQ. Then add a capacitor in parallel, the (+) side of the cap connects to the striped (processor) side of the diode, the (-) side of the cap goes to ground (not the radio or EQ chassis - connect to the car chassis). Experimenting with the cap value will give you the right amount of delay before the EQ shuts off. You don't want it too long, just long enough to make sure the amp is off before the EQ powers down. 220 - 1000 uF is about right, and make sure the cap is a polarized electrolytic, 16V or higher. Also keep in mind that the diode will introduce a 0.7V drop on the remote wire, which can cause the processor to power down before the rest of the system.

2.3What is the best power wire to use? There is much debate over the benefit of certain wiring schemes (oxygen-free, multistranded, braided, twisted, air core, you name it). However, most people do agree that the most important factor in selecting power wire is to use the proper size. Wire is generally rated in size by American Wire Gauge, abbreviated AWG, or commonly just "gauge". To determine the correct wire size for your application, you should first determine the maximum current flow through the cable (looking at the amplifier's fuse is a relatively simple and conservative way to do this). 2.4 What is the best speaker wire to use? Again, there is much debate over the benefit of the various schemes that are being used by different manufacturers. In general, however, you will probably want to upgrade your speaker wire from the factory ~20 gauge to something bigger when you upgrade your amplifiers and speakers. In most cases, 16 or 18 gauge should be sufficient, with the possible exception of high-power subwoofers. According to an example by Jerry Williamson, using 18 gauge instead of 12 gauge would only result in a power loss of 0.1dB, which is essentially undetectable by humans. Thus, other factors play more important roles in the selection of speaker wire. One issue is that different wires will have different line capacitances, which could cause the wire to act as a low pass filter. Generally, however, the capacitances involved are so small that this is not a significant problem. Be sure to heed the warnings above regarding cable flexibility and insulation, especially when running wire into doors and other areas with an abundance of sharp metal.

2.5 I heard that I should run my power wire directly to my car's battery. Why should I bother, and how do I do it? [JSC]
For some components, like head units and equalizers, it's acceptable to use the stock wiring for power. However, amplifiers generally require large amounts of power, and accordingly will draw large amounts of current. The factory wiring in most cars is not designed to handle large amounts of current, and most wires have 10-20A fuses on them. Thus, you will almost always want to run the power line for your amplifier directly to the positive terminal of the battery. This could require drilling a hole through the car's firewall, or at least spending time hunting for an existing hole (the steering column is a good place to start looking). Always remember to place a fuse on your wire as near to the battery as possible! For various reasons, such as an accident or simple wear and tear, your wire's insulation may eventually crack, which could allow the conducting wire to make contact with the chassis of the car and short the battery through this wire, which could lead to a serious fire. The closer you place a fuse to the battery, the more protected you are. Also, when running wire through areas with sharp metal corners, it is a good idea to use rubber grommets to provide extra protection against tearing through your wire's insulation.

2.6 Should I do the same thing with my ground wire, then?
No. In almost every case, the best thing to do is to ground your amplifier to a point that is attached to the chassis of the car and is as close to the amplifier as possible. The ground wire should not need to be more than about eighteen inches long, and should be at least as large as the power wire. The point to which you make your ground connection should be an unpainted piece of bare metal. Some cars (Audi, Porsche) have galvanized bodies, and in these cars, you must find one of the manufacturers' grounding points or else some noise can result.
2.7 Sometimes when I step out of my car, I get a really bad shock. What is wrong with my system?
Nothing. This is caused by static buildup by rubbing against the seats, floor mats, etc., just like walking across a carpet in a home. You can avoid this shock by touching something metal on your car _before_ you put your foot on the ground.

2.8 When my car is running and I have the music turned up loud, my headlights dim with the music. Do I need a new battery or a new alternator?
The headlights will dim because of a momentary drop in the voltage level that is available to power the vehicle's accessories, including the headlights, amplifiers, the engine, etc. This voltage drop can be caused by a very large current demand by an accessory, such as an amplifier trying to reproduce a loud bass note. The first thing to do is to get your battery and alternator checked for proper functioning. A failing battery can place undesirable loads on the alternator, leaving less power for your system. If the power system appears to be working correctly, an improved alternator may be required for the large current demands of the audio system. When upgrading an alternator, be careful in your purchase, for there are some potential problems. An alternator which advertises a certain output level may only achieve that output at very high engine RPM ranges, for instance. Also, the new alternator must be adjusted to provide an output voltage within a reasonable range in terms of the voltage regulator. If you find your car will not start after playing the stereo for long periods of time with the engine off, and the present battery is in good working order, then another, paralleled battery could prevent this embarrassing problem. 2.9 What is a "stiffening capacitor", and how does it work? "Stiffening Capacitor" (note capitals) is a trademark of Autosound 2000. However, "stiffening capacitor" (note lowercase), as a generic term, refers to a large capacitor (several thousand microfarads or greater) placed in parallel with an amplifier. The purpose of doing so is to provide a sort of reserve power source from which the amplifier can rapidly draw power when it needs it (such as during a deep bass note). The electrical theory is that when the amplifier attempts to draw a large amount of current, not only will the battery be relatively slow to respond, but the voltage at the amplifier will be a little lower than the voltage at the battery itself (this is called "line drop"). A capacitor at the amplifier which is charged to the battery voltage will try to stabilize the voltage level at the amplifier, dumping current into the amplifier. Another way to think about it is that a capacitor in parallel with a load acts as a low pass filter (see Section 3.10), and the voltage level dropping at the amplifier will appear as an AC waveform superimposed upon a DC "wave". The capacitor, then, will try to filter out this AC wave, leaving the pure DC which the amplifier requires.

2.9.1 Do I need a capacitor?
Before installation, it's often difficult to predict whether or not a capacitor will be beneficial to you. It's generally best to install the audio equipment prior to making the determination, so that you can address which symptoms need to be remedied and assess the severity of the symptoms. This will not only help you decide whether or not you need a capacitor, but also how much capacitance would be beneficial. The most common symptom in need of added capacitance is headlight dimming (and sometimes dimming of the interior/dash lights). It's caused by a drop in system voltage associated with excessive current draw. While there may indeed be several loads drawing substantial amounts of current from the electrical system (eg. heat, AC, and so forth), it's usually the transient draws that best manifest themselves in noticeable dimming. This is partly because our visual systems are most sensitive to detecting rapidly changing intensity levels rather than steady absolute differences. Once you've assessed whether or not the dimming is noticeable (and sufficiently annoying), you must decide whether a capacitor is warranted or if you'd be better served by upgrading the alternator. After initially having your alternator and battery checked out (some places will do this for free), the choice should be based on the severity of the dimming. A commonly-used estimate for determining the appropriate size capacitor is 1F/kW (one farad per kilowatt). For example, a system running at 300W would need a 0.3F (or 300,000uF) capacitor. However, there are several variables at play here, including the capabilities of the vehicle's electrical system (which generally varies from idle to higher RPMs), the efficiency of the amplifiers, and the listening habits of the user (ie. the tone controls and the type of music). These factors should all be considered when making the determination. Moreover, the voltage drop can be so severe that added capacitance is nothing more than a band-aid. That is, even several Farads of capacitance would not be able to sustain the voltage for as long as the drop persists. This is when an alternator upgrade may be in order.

2.9.2 Can I just upgrade my headlight wiring instead?
Although headlight wiring upgrades can often be beneficial for achieving a higher steady-state illumination, it will not improve the dimming situation. Since the headlights are not the cause of the voltage fluctuations that are producing the dimming, upgrading the wiring will not fix the problem. The voltage fluctuation is present at the battery terminals, so it will be transmitted to the headlights regardless of how the headlights are wired. If you think of the fluctuation as an AC signal, then it becomes readily apparent that this circuit can be represented by an AC signal in a voltage divider. Decreasing the resistance in series with the load by upgrading the headlight wiring actually serves to slightly enhance the AC signal at the headlight's terminals. In other words, the dimming effect could become even worse by upgrading the headlight wiring!

2.9.3 Will the dimming go away if I upgrade the amplifier power/ground wiring?
A common myth in the car audio community is that upgrading the power or ground wire to the amplifier will result in the amplifier drawing less current and therefore decreasing the voltage fluctuation. While the logic is sound, the premise is not. Most amplifiers on the market have semi-regulated supplies which don't maintain a steady power output at a range of supply voltages. This is reflected in the power ratings provided by many manufacturers; some provide ratings for their amplifiers at two different voltages, and the lower voltage almost always causes the amp to deliver less power. In general, the difference in power output tends to correspond well with the supply voltage such that the current draw remains roughly constant (assuming somewhat similar efficiency). Consequently, upgrading the power/ground wiring, which serves to increase the voltage at the amplifier's terminals, will not reduce headlight dimming.

2.9.4 What do I look for when buying a capacitor?
The single most important attribute is the capacitance value (expressed in Farads). Put simply, more is better. Another important consideration is to make sure the maximum voltage rating of the capacitor safely exceeds the operating voltage of your vehicle's electrical system. In addition, ESR and ESL values may be provided with some capacitors to essentially indicate the amount of voltage drop that occurs when a capacitor is delivering current. Smaller values are better in this regard.

2.9.5 How do I install a capacitor?
If you conclude that your best course of action is to install a capacitor, it should be installed in parallel with the amplifier and, generally speaking, should be wired with approximately the same gauge wire used for a single amplifier (usually 8 ga. is sufficient even for rather large capacitors).

2.9.6 I have more than one amp in my audio system. Which one should I have the capacitor run?
The amplifiers are all connected in one way or another to the battery. In fact, unless you're running separate power wires to each amplifier all the way from the battery, they're usually connected at a more proximal site (a distribution block, for example). The effects of the capacitor are felt by the entire electrical system, including the amplifiers. Therefore, you cannot selectively dedicate a capacitor to a specific amplifier.

2.9.7 Will my bass response improve by adding a capacitor?
A capacitor serves to smooth the voltage fluctuations associated with transient current draw. As a result, the supply voltage presented to the amp during peak demands tends to be slightly higher than without the capacitor. For most amplifiers, this will increase the power output of the amplifier during transients. The degree to which it increases, however, typically leads to an inaudible improvement. To illustrate, if you consider an amplifier that delivers 100 watts at 14v and 80 watts at 12v (these numbers are somewhat typical), the difference in output from the speaker will be at best 1 dB when the supply voltage fluctuates from 14v to 12v. However, when you take into account the fact that no practical amount of capacitance can completely eliminate this voltage drop during transients, the difference in output becomes even less pronounced. Further, if you take into account other factors such as loudspeaker power compression (discussed elsewhere in the FAQ), the equivalent series impedance of the capacitor, the length of the transient, and the human's decreased ability to perceive differences in intensity for shorter intervals, this difference in output becomes negligible.

2.10 When should I upgrade my battery or add a second battery? The battery is most important when the engine is turned off, because it supplies all of power to the audio system. The stock battery in your car may not be up to the task of running a stereo with multiple (or large) amplifiers if it can't supply enough current to the amplifiers. Upgrading your current battery to a larger model may help solve the problem because batteries like the Optima 800 offer a larger number of cold cranking amps. Generally, adding a second battery is great if you want to listen to your stereo with the car turned off (and be able to start the car again later!). This is accomplished using a dual-battery isolator: a device which allows the second battery to be charged by the alternator, but prevents the amplifiers that are connected to the second battery from drawing any power from the main battery. Installing a second battery may be done instead of upgrading the main battery.
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