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Path: bloom-beacon.mit.edu!hookup!usc!howland.reston.ans.net!vixen.cso.uiuc.edu!uxa.cso.uiuc.edu!jsc52962 From: stealth@uiuc.edu (Jeffrey S. Curtis) Newsgroups: rec.audio.car,rec.answers,news.answers Subject: rec.audio.car FAQ (part 1/3) Supersedes: <rac-faq-p1-052805Apr61994@uxa.cso.uiuc.edu> Followup-To: rec.audio.car Date: 6 Apr 1994 05:47:14 GMT Organization: University of Illinois at Urbana Lines: 500 Approved: news-answers-request@mit.edu Distribution: world Expires: 6 May 1994 05:00:00 UT Message-ID: <rac-faq-p1-054711Apr61994@uxa.cso.uiuc.edu> Reply-To: stealth@uiuc.edu (Jeffrey S. Curtis) NNTP-Posting-Host: uxa.cso.uiuc.edu Summary: This article describes the answers to the frequently asked questions on the rec.audio.car newsgroup. This article is posted once per month. Originator: jsc52962@uxa.cso.uiuc.edu Xref: bloom-beacon.mit.edu rec.audio.car:14903 rec.answers:4772 news.answers:17720 Archive-name: car-audio/part1 Rec-audio-car-archive-name: FAQ/part1 Version: 2.1 Last-modified: 5 Apr 94 *+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+* + Frequently Asked Questions with answers for rec.audio.car + *+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+*+* 0 Introduction and Table of Contents This is the FAQ list for the Usenet newsgroup rec.audio.car, maintained by Jeffrey S. Curtis (stealth@uiuc.edu), with contributions from many other people (see the credits section). This document may be freely distributed and reproduced as long as it remains wholly unaltered and includes this notice. If you have suggestions for improvements to this document, or if you fail to understand any part of it, please feel free to send a note to the FAQ maintainer or to the author of the relevant section. The initials of the author(s) of each section can be found in brackets following each question. This document is posted once per month to the Usenet newsgroups rec.audio.car, rec.answers, and news.answers, and is available via anonymous FTP from rtfm.mit.edu in the directory "/pub/usenet/rec.audio.car". Table of Contents ===== == ======== 1 Definitions 1.1 What do all of those acronyms mean (A, V, DC, AC, W, Hz, dB, SPL, THD, ohm)? 1.2 What is meant by "frequency response"? 1.3 What is a "sound stage"? What is an "image"? 1.4 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? 2.2 What is the best power wire to use? 2.3 What is the best speaker wire to use? 2.4 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? 2.5 Should I do the same thing with my ground wire, then? 2.6 Sometimes when I step out of my car, I get a really bad shock. What is wrong with my system? 2.7 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? 2.8 What is a "stiffening capacitor", and how does it work? 2.9 Should I install one in my car? If so, how big should it be, and where do I get one? 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 is "bridging"? Can my amp do it? 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 How do I build my own passive crossovers? 3.13 Should I buy an equalizer? 3.14 What are some good (and bad) brands of equalizers? 3.15 What do all of those specifications on tape deck head units mean? 3.16 What are features to look for in a tape deck? 3.17 What are some good (and bad) brands of tape decks? 3.18 What are features to look for in a CD head unit? 3.19 Should I buy a detachable faceplate or pullout CD player? 3.20 What are some good (and bad) brands of CD head units? 3.21 Can I use my portable CD player in my car? Won't it skip a lot? 3.22 What's that weird motor noise I get with my portable CD player? 3.23 What are some good (and bad) brands of portable CD players? 3.24 What's in store for car audio with respect to MD, DAT and DCC? 3.25 Are those FM modulator CD changers any good? What are my other options? 3.26 What are some good (and bad) brands of CD changers? 3.27 Why do I need a center channel in my car, and how do I do it? 3.28 Should I buy a sound field processor? 3.29 What are some good (and bad) brands of signal processors? 4 Subwoofers 4.1 What are "Thiele/Small parameters"? 4.2 What are the enclosure types available, and which one is right for me? 4.3 How do I build an enclosure? 4.4 What driver should I use? 4.5 Is there any computer software available to help me choose an enclosure and a driver? 4.6 What is an "aperiodic membrane"? 5 Installation 5.1 Where should I buy the components I want? 5.2 What mail-order companies are out there? 5.3 What tools should I have in order to do a good installation? 5.4 Where should I mount my speakers? 5.5 What is "rear fill", and how do I effectively use it? 5.6 How do I set the gains on my amp? 5.7 How do I select proper crossover points and slopes? 5.8 How do I flatten my system's frequency response curve? 6 Competition 6.1 What is IASCA, and how do I get involved? 6.2 What is USCA, and how do I get involved? 6.3 What are the competitions like? 6.4 Should I compete? 6.5 What class am I in? 7 Literature 7.1 What magazines are good for car audio enthusiasts? 7.2 Are there any newsletters I can read? 7.3 What books can I read? 8 Credits 1 Definitions This section contains background information which defines some of the acronyms and terminology commonly used in the car audio world. Understanding these definitions is important in order to understand the other sections of this document. 1.1 What do all of those acronyms mean (A, V, DC, AC, W, Hz, dB, SPL, THD, ohm)? [JSC] "A" is for "amperes", which is a measurement of current equal to one coulomb of charge per second. You usually speak of positive current - current which flows from the more positive potential to the more negative potential, with respect to some reference point (usually ground, which is designated as zero potential). The electrons in a circuit flow in the opposite direction as the current itself. Ampere is commonly abbreviated as "amp", not to be confused with amplifiers, of course, which are also commonly abbreviated "amp". In computation, the abbreviation for amps is commonly "I". "V" is for "volts", which is a measurement of electric potential. Voltages don't "go" or "move", they simply exist as a measurement (like saying that there is one mile between you and some other point). "DC" is for "direct current", which is a type of circuit. In a DC circuit, all of the current always flows in one direction, and so it is important to understand which points are at a high potential and which points are at a low potential. For example, cars are typically 12VDC (twelve volts direct current) systems, and it is important to keep track of which wires in a circuit are attached to the +12V (positive twelve volts) lead of the battery, and which wires are attached to the ground (or "negative") lead of the battery. In reality, car batteries tend to be about 13-14VDC. "AC" is for "alternating current", which is a type of circuit in which the voltage potential fluctuates so that current can flow in either direction through the circuit. In an AC circuit, it is typically not as important to keep track of which lead is which, which is why you can plug household appliances into an outlet the "wrong way" and still have a functioning device. The speaker portions of an audio system comprise an AC circuit. In certain situations, it is indeed important to understand which lead is "positive" and which lead is "negative" (although these are just reference terms and not technically correct). See below for examples. The voltage of an AC circuit is usually given as the RMS (root mean square) voltage, which, for sinusoidal waves, is simply the peak voltage divided by the square root of two. "W" is for "watts", a measurement of electrical power. One watt is equal to one volt times one amp, or one joule of energy per second. In a DC circuit, the power is calculated as the voltage times the current (P=V x I). In an AC circuit, the RMS power is calculated as the RMS voltage times the RMS current (Prms=Vrms x Irms). "Hz" is for "hertz", a measurement of frequency. One hertz is equal to one inverse second (1/s); that is, one cycle per second, where a cycle is the duration between similar portions of a 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 resonate at 1kHz, producing a 1kHz sound wave. The standard range of human hearing is "twenty to twenty", or 20Hz-20kHz, which is three decades (three tenfold changes in frequency) or a little under ten octaves (ten twofold changes in frequency). "dB" is for "decibel", and is a measurement for power ratios. To measure dB, you must always measure with respect to something else. The formula for determining these ratios is P=10^(dB/10), which can be rewritten as dB=10log(P). For example, to gain 3dB of output compared to your current output, you must change your current power by a factor of 10^(3/10) = 10^0.3 = 2.00 (that is, double your power). The other way around, if you triple your power (say, from 20W to 60W) and want to know the corresponding change in dB, it is dB=10log(60/20)=4.77 (that is, an increase of 4.77dB). If you know your logarithms, you know that a negative number simply inverts your answer, so that 3dB corresponding to double power is the same as -3dB corresponding to half power. There are several other dB formulas; for instance, the voltage measurement is dB=20log(V). For example, a doubling of voltage produces 20log2 = 6.0dB more output, which makes sense since power is proportional to the square of voltage, so a doubling in voltage produces a quadrupling in power. "SPL" is for "sound pressure level" and is similar to dB. SPL measurements are also ratios, but are always measured relative to a constant. This constant is 0dB which is defined as the smallest level of sound pressure that the human ear can detect. 0dB is equal to 10^-12 (ten to the negative twelfth power) W/m^2 (watts per square meter). As such, when a speaker is rated to produce 92dB at 1m when given 1W (92dB/Wm), you know that they mean that it is 92dB louder than 10^-12W/m^2. You also know than if you double the power (from 1W to 2W), you add 3dB, so it will produce 95dB at 1m with 2W, 98dB at 1m with 4W, 101dB at 1m with 8W, etc. "THD" is for "total harmonic distortion", and is a measure of the how much a certain device may distort a signal. These figures are usually given as percentages. It is believed that THD figures below approximately 0.1% are inaudible. However, it should be realized that distortion adds, so that if a head unit, equalizer, signal processor, crossover, amplifier and speaker are all rated at "no greater than 0.1%THD", together, they could produce 0.6%THD, which could be noticeable in the output. "ohm" is a measure of resistance and impedance, which tells you how much a device will resist the flow of current in a circuit. For example, if the same signal at the same voltage is sent into two speakers - one of which is nominally rated at 4 ohms of impedance, the other at 8 ohms impedance - twice as much current will flow through the 4 ohm speaker as the 8 ohm speaker, which requires twice as much power, since power is proportional to current. 1.2 What is meant by "frequency response"? [JSC] 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. 1.3 What is a "soundstage"? What is an "image"? [CD] The "soundstage" is the position (front/back and high/low) that the music appears to be coming from, as well as the depth of the stage. A car with speakers only in the front will likely have a forward soundstage, but may not have enough rear fill to make the music seem live. A car with both front and rear speakers may have anything from a forward to a rear soundstage, with an accompanying fill from the softer drivers depending on the relative power levels and the frequencies reproduced. The high/low position of the soundstage is generally only obvious in a car with a forward soundstage. The music may seem to be originating in the footwells, the dash, or out on the hood, depending on how the drivers interact with the environment. The "stereo image" is the width and definition of the "soundstage". Instruments should appear to be coming from their correct positions, relative to the recording. The position of the instruments should be solid and easily identifiable, not changing with varying frequencies. A car can image perfectly with only a center-mounted mono speaker, but the stereo placement of the music will be absent. 1.4 What is meant by "anechoic"? [JSC] 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 This section describes various problems and concepts which are closely related to electronics. 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? This is perhaps the single most frequently asked question on rec.audio.car, and is coincidentally enough the most difficult to answer. But there will indeed be a lengthy answer soon - stay tuned. 2.2 What is the best power wire to use? [JSC] 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). Then determine the length of the cable that your will use, and consult the following chart, taken from the IASCA handbook (see 6.1): Length of run (in feet) Current 0-4 4-7 7-10 10-13 13-16 16-19 19-22 22-28 0-20A 14 12 12 10 10 8 8 8 20-35A 12 10 8 8 6 6 6 4 35-50A 10 8 8 6 6 4 4 4 50-65A 8 8 6 4 4 4 4 2 65-85A 6 6 4 4 2 2 2 0 85-105A 6 6 4 2 2 2 2 0 105-125A 4 4 4 2 2 0 0 0 125-150A 2 2 2 2 0 0 0 00 If aluminum wire is used instead of copper wire, the next larger size (smaller number) should be used. You should also consider the installation demands: will you need to run the wire around corners or through doors or into the engine compartment? These sorts of problems in the car audio application require some special care in cable selection. You will want to have cable that is flexible; it should have thick insulation as well, and not melt at low temperatures. You don't want to install wire that is rigid and prone to cracks and cuts, or else the results could literally be explosive. 2.3 What is the best speaker wire to use? [JSC,JW] 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. 2.4 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. 2.5 Should I do the same thing with my ground wire, then? [JSC] 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. 2.6 Sometimes when I step out of my car, I get a really bad shock. What is wrong with my system? [JSC] Probably nothing. This is usually caused by static buildup by rubbing against the seats, floormats, etc., just like walking across a carpet in a home. Shocks which can be felt are usually in the kilovolt range, so touching a 12VDC wire isn't going to do much to you. 2.7 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? [CD,MO] 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. 2.8 What is a "stiffening capacitor", and how does it work? [JSC] "Stiffening Capacitor" (note capitals) is a trademark of Autosound 2000 (see 7.2). 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 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 Should I install one in my car? If so, how big should it be, and where do I get one? [JSC] If you have a problem with dimming headlights when you have your music turned up and the bass starts to hit and the engine is running and you don't want to upgrade your alternator, or if the transient response of your amplifier is unacceptable to you, a stiffening capacitor could help you out. The commonly accepted "formula" for determining the proper size capacitor to use is 1F/kW (one farad per kilowatt). For example, a system running at 300W would need a 0.3F (or 300,000uF) capacitor. To install the capacitor, you should not simply attach it to your power and ground wires near your amplifier, as it will draw very large amounts of current from your battery and could blow fuses (or overcharge). Instead, you should insert a small-value power resistor (perhaps one ohm) or a 12VDC test lamp in between the power lead and the capacitor, and then charge it. If you use a lamp in series with the cap, when the lamp goes out, the capacitor is done charging. When it is done charging, carefully remove the capacitor's leads from the charging circuit, being certain not to touch the two leads together. You may then permanently install the capacitor by wiring it in parallel with your amplifier (be careful not to short the leads!). Large caps are currently available from some audio dealers, such as Phoenix Gold. You could also try electronics shops or mail-order houses. -- Jeffrey S. Curtis - stealth@uiuc.edu <> "You say these days are made of rust: Network Coordinator - UI Housing Div <> ``Counted out! Counted out in loss!'' Proton < Dodge > Pioneer <> I've got plans to prove them wrong.." Phase Linear < Stealth > StreetWires <> -- INXS _Full Moon Dirty Hearts_ 1993