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Subject: rec.audio.car FAQ (Part 4/5)

This article was archived around: Tue, 9 Aug 2005 06:30:40 +0000 (UTC)

All FAQs in Directory: car-audio
All FAQs posted in: rec.audio.car, uk.rec.audio.car
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Archive-name: car-audio/part4 Rec-audio-car-archive-name: FAQ/part4 Version: 4.55 Last-modified: 08 August 2005
5 Installation ********************* This section describes how to do what you want once you know what it is you want to do. 5.1 Where should I buy the components I want? [JSC] ========================================================== Most of the time, you will either buy from a local dealer, or from a mail-order house. Buying from a local dealer can be good because you get to deal directly with a person: you can show them your car, ask specific questions, haggle prices, get quick service when there are problems, get deals on installation, etc. But there can also be advantages to buying mail-order: generally cheaper prices, sometimes better service, etc. In either case, you should always check prices before you buy, inquire about warranty service, and ask about trial periods. 5.2 What mail-order companies are out there? [JSC, JM, MM, IDB] ====================================================================== *Crutchfield* - 800/955-3000 1 Crutchfield Park, Charlottesville, VA 22906 USA * Advantages: - Great customer service - Generally knowledgeable sales and tech support personnel - Custom mounting kits, wiring harnesses, etc. free of charge. * Disadvantages: - limited product line - generally higher prices than local shops *J.C. Whitney* - 312/431-6102 1917-19 Archer Avenue, P.O. Box 8410, Chicago, IL 60680 USA * Advantages: - Lots of "miscellaneous" items - 10kW amps for $19.99 * Disadvantages: - 10kW amps that really only put out 1mW and break after first 10 minutes of use. *Parts Express* - 800/338-0531 340 E. First St., Dayton, OH 45402 USA * Advantages: - Large selection of electronics supplies at respectable prices. - Showroom prices said to be better than catalog prices. * Disadvantages: - Also carries some of the same quality-level components as J.C. Whitney. *Classic Research/Z-Box* - 520/571-0171 5070 E. 22nd St., Tucson, AZ 85711 USA * Advantages: - Creates custom door panels with car audio in mind. * Disadvantages: - Only services expensive sports and luxury cars. *MCM Electronics* - 800/543-4330 650 Congress Park Drive, Centerville, OH 45459-4072 USA * Advantages: - Sells lots of decently priced trinkets (fuses, fuse holders, wire, etc.) - Has excellent service and available technical support. * Disadvantages: - ? There are many other mail-order houses that can be found in the back of magazines, such as S.B.H. Enterprises, Speed and Sound and Smile Electronics, but people seem to have mixed feelings about these companies. The prices are very low, often due to the fact that these companies are not factory authorized - this means that there could be problems getting the unit serviced by the manufacturer should it break. To get around this, these mail order houses will often provide their own service departments, to repair defective units. Generally, it is advised to be careful when dealing with any mail order companies, to protect yourself. There are also a number advertisements for mail order houses (such as Apex Audio & Electronics or Insider's Audio) that offer special deals or free equipment if you buy their expensive catalog or pay a membership fee. While these ads may be enticing, keep in mind that they are often *too good to be true*! Usually you have to buy a large amount of equipment before you qualify for the "bonus" or the prices are very high for most equipment. 5.3 What tools should I have in order to do a good installation? [JSC] ======================================================================= *Electrical tape* Make sure you get some that can withstand extreme temperature ranges. *Wire cutters/strippers and crimpers* Get a big pair with stripper holes precut for individual wire sizes. *Angled screwdrivers* Makes taking dash and rear deck speakers out a lot easier. *Multiple size screwdrivers, both flathead and Phillips.* Magnetic screwdrivers can be a big help when trying to get screws into (or out of) tight spaces. *Various wrenches, pliers, and socket sets* The specific sizes you need will depend on your vehicle. *Metal drill and saw* You'll need these if you need to modify your vehicle for new speaker cutouts or to accommodate a new head unit. *Hot glue gun* Good for putting carpeting or door panel trim back in place after modifications. *Razor knife* Helps for detailed modifications of door panels or carpeting, especially when installing new speakers. *Wire* (see Section 2.3). *Soldering Iron* Makes excellent connections, but can be messy if not careful. *Shrink wrap or flex tubing* Good for protecting wire, especially in the engine compartment. *Multimeter* Helps to diagnose installations. *Extra hardware* Screws, nuts, bolts, connectors, etc. *Fuse puller and extra fuses.* In addition to the fuses for your stereo system, check your car's fusebox to find the various sizes you'll need. Also, you can use needle-nosed pliers to pull fuses. *Wire ties* Helps to tuck wire away in otherwise exposed areas. *Small light source* A flashlight will do - you just want something that you can poke around the innards of your car with. *Tape measure* 5.4 Where should I mount my speakers? ============================================ 5.5 What is "rear fill", and how do I effectively use it? [HK, JSC] ===================================================================== "Rear fill" refers to the presence of depth and ambiance in music. A properly designed system using two channels will reproduce original rear fill on the source without rear high frequency drivers. Since recordings are made in two channels, that is all you will need to reproduce it. What is captured at the recording session (coincident pair mics, Blumlein mic patterns, etc.) by a two channel mic array will capture the so called rear fill or ambiance. Many of the winning IASCA vehicles have no rear high frequency drivers. Also a lot of this has to do with system tuning. If rear high frequency drivers are added, however, the power level of the rear fill speakers should be lower than that of the front speakers, or else you will lose your front-primary staging, which is not what you want (when was the last time you went to a concert and stood backwards?). The proper amount of amplification for rear fill speakers is the point where you can just barely detect their presence while sitting in the front seat. Separates are not a requirement for rear fill; in fact, you may be better of with a pair of coaxial speakers, as separates may throw off your staging. 5.6 How do I set the gains on my amp? [JSC] ================================================== The best way to do this is with a test tone and an oscilloscope (*Note Oscilloscope::.) Since most people have neither item, the following will work approximately as well. 1. Disconnect all signal inputs to the amp 2. Turn all sensitivity adjustments as low as possible 3. Turn head unit on to around 90% volume (not 100% or else you'll have head unit distortion in there - unless you've got a good head unit) with some music with which you're familiar, and with EQ controls set to normal listening positions 4. Plug in one channel's input to the amp 5. Slowly turn that channel's gain up until you just start to notice distortion on the output 6. Turn it down just a wee little bit 7. Disconnect current input 8. Repeat steps 4-7 with each input on your amp 9. Turn off head unit 10. Plug in all amp inputs, and you're done If by some chance you do have an oscilloscope (and preferably a test disc), you do essentially the same thing as above, except that you stop turning the gains up when you see clipping on the outputs of the amplifier. Note that if you are paralleling multiple speakers on a single amp output, you need to set the gains with all of the speakers in place, since they will be affecting the power and distortion characteristics of the channel as a whole. 5.7 How do I select proper crossover points and slopes? [DK] =================================================================== Basically, this requires a degree of patience. The subwoofer should be started off at about 100Hz and adjusted until you are happy with the sound. Keep in mind that the higher the crossover point, the more power the driver on the high-pass will be able to handle but raising excessively may cause the low-pass driver to sound raspy or unnatural. The idea here is to first make rough selections to protect the drivers and then to fine tune crossover point selections to achieve optimum fidelity. It's all a matter of what sounds good to you after that, but remember that even *minute* changes in crossover frequency can make dramatic differences in the way your system sounds and images. As a rule, subs should be crossed over no higher than 120Hz, a 6 1/2 mid should be able to handle about 90 Hz, a 5 1/4" should be okay with about 100Hz, a 4" - about 500Hz, and tweeters vary from about 3500-5000Hz. These points all assume the use of a 12dB/octave crossover ... if you have a steeper roll-off a lower crossover point may be chosen. Remember, these are not hard and fast rules but rather a rule of thumb to help you get started (and so you don't blow up all your speakers when you are setting your gains!). 5.8 How do I flatten my system's frequency response curve? [IDB, DK] ======================================================================= First, you'll need a good quality equalizer - either a 2/3 octave (15-band) or 1/3 ocatve (30 band) equalizer or a quasi- parametric equalizer such as PPI's PAR 224 that allows you to choose the center frequency and bandwidth (Q) of each knob on the EQ. This will allow adjustments to very specific frequency ranges. Next, you'll need to get a hold of an RTA (Real Time Analyzer), which is an expensive piece of equipment that good shops will usually have. The shops can then equalize the system by making a measurement with the RTA, and varying the levels on the equalizer in order to make the overall response curve flat. Unfortunately, most shops will not do this for free, since proper equalization can take anywhere from a half hour to many many hours. Another method involves buying an SPL meter (available from Radio Shack for between $32 and $60) and a test disc (Autosound 2000 makes one that runs about $25) that plays discreet frequency ranges - in 1/3 octave groups. Then, moving through the range of frequencies, SPL measurements can be taken at each range, and you can plot out a "response" curve. You'll be able to see what frequency ranges need to be boosted and which need to be cut. This process will be time consuming (more so than an RTA, which can analyze the entire frequency spectrum simultaneously), but should be much less expensive than having it professionally done. One last note: While a smooth curve will get the most points at an auto sound competition, you must NOT rely only on the RTA to tell you what sounds good. Use the RTA to get a good start, and then use your (better, use someone experienced in tuning systems) ears to do the fine-tuning. 5.9 How do I wire speakers "in series" and "in parallel?" [IDB] ======================================================================= Wiring speakers in series involves connecting at least two speakers so that the first speaker's positive lead is connected to the amplifier's positive terminal, and the negative lead is connected to the positive lead of the second speaker. If there is a third speaker, its positive lead will be connected to the second speaker's negative lead ... and so on. The last speaker in the chain will have its negative lead connected to the amplifier's negative terminal. Speakers that are wired in parallel are all connected to the positive and negative terminals of the amplifier. So, when two speakers are wired in parallel, you'll connect each speaker's positive lead to the amplifier's positive terminal, and you'll connect each speaker's negative lead to the amplifier's negative terminal. Be careful when wiring multiple speakers in parallel or series so that you do not exceed your amplifier's rating. To calculate the effective impedance of a number of speakers, use the following formulas: Series Connections: Z(t) = Z(1) + Z(2) + Z(3) + ... + Z(n) That is, add up all of the impedances for each speaker to get the total impedance. For example, with 3 4-ohm speaker in series, the total impedance is 4 + 4 + 4 = 12 ohms. Parallel Connections: 1/Z(t) = 1/Z(1) + 1/Z(2) + 1/Z(3) + ... + 1/Z(n) That is, add up the inverse of the impedance of each speaker and invert the sum to get the total impedance. For example, with 3 4-ohm speakers in parallel, the total impedance is 1 / ( 1/4 + 1/4 + 1/4) = 1 / (3/4) = 1.33 ohms. 5.10 Are there any alternatives for Dynamat? It's too expensive! [MM, PK] ======================================================================= In this question, "Dynamat" refers to all commercial products that are marketed expressly for reducing ambient noise in the car. Dynamat, Stinger RoadKill, _et al._ all have similar pricing, so this question is intended to give non-standard options. There is a material known as "Ice Guard," which is used by roofing contractors. It is similar to Dynamat, both in thickness and density. It is self-adhesive on one side, and seems to work very well. Unfortunately, it is sold only in large quantities (225 ft^2 rolls), and runs about $70 for this much. Perhaps a few people could get together for a roll, or it might be possible to get scraps from a roofing contractor. MCM Electronics (see Section 5.2), sells a product called "Sound Deadening Pads" (part #60-2010) which cost $0.90 for each 10" x 10" square. 5.11 How many devices can I attach to my remote turn-on lead? [IDB] ======================================================================= The remote turn-on lead that most head units will not provide very much current (usually 250-300mA), so there is a limit to the number of components you can activate with it. Generally, it is safe to hook up two devices to the lead without having to worry about problems. However, if you'll be activating more components, then you should probably use a relay. 5.12 How do I wire a relay in my system? [IDB] ===================================================== There are two types of relays that are commonly used in 12-volt automotive applications: Single-pole Double Throw (SPDT) relays, which have 5 pins, and Single Pole, Single Throw (SPST) relays, which have 4 pins. Depending on the application, you can use either of these; for remote turn-on leads (see Section 5.11), an SPST relay is fine, SPDT relays are often used in alarm installations. Make sure that you get a 12-volt relay - this specifies the voltage required to make the relay "switch." The connections on the two types of relays look like this: SPST SPDT ===================== ===================== (87) (87) +---------+ +---------+ | --- | | --- | | | | | (86) | | | | (85) (86) | | --- | | (87a - center) | | | | (85 - right) | | | | | | +---------+ +---------+ (30) (30) Pins 85 and 86 connect to the coil which causes the relay to switch. On both relays, pins 30 and 87 are normally disconnected. When the relay is activated (switched) pin 30 and 87 are then in contact. The difference with the SPDT relay is that in the "normal" state, pins 30 and 87a are in contact. To hook up a relay (either kind) for a remote turn on, make the following connections: *Pin 30* +12 Volts (Battery +) *Pin 87* Amplifiers' remote turn-on terminal *Pin 86* Head unit remote turn-on lead *Pin 85* Ground *Pin 87a* No connection (SPDT only) 5.13 How do I design my own passive crossovers? [JSC, JR] ================================================================ A "first order high pass crossover" is simply a capacitor placed inline with the driver. A "first order low pass crossover" is an inductor inline with the driver. These roles can be reversed under certain circumstances: a capacitor in parallel with a driver will act as a low pass filter, while an inductor in parallel with a driver will act as a high pass filter. However, a parallel device should not be the first element in a set; for example, using only a capacitor in parallel to a driver will cause the amplifier to see a short circuit above the cutoff frequency. Thus, a series device should always be the first element in a crossover. When like combinations are used, the order increases: a capacitor in series followed by an inductor in parallel is a "second order high pass crossover". An inductor in series followed by a capacitor in parallel is a "second order low pass crossover". To calculate the correct values of capacitors and inductors to use, you need to know the nominal impedance Z of the circuit in ohms and the desired crossover point F in hertz. The needed capacitance in farads is then 1/(2 * pi * f * Z). The needed inductance in henries is Z/(2 * pi * f). For example, if the desired crossover point is 200Hz for a 4 ohm driver, you need a 198.9 x 10^-6 F (or 199uF) capacitor for a high pass first order filter, or a 3.18 x 10^-3 H (or 3.18mH) inductor for a low pass first order filter. To build a second order passive crossover, calculate the same initial values for the capacitance and inductance, and then decide whether you want a Linkwitz-Riley, Butterworth, or Bessel filter. An L-R filter matches the attenuation slopes so that both -3dB points are at the same frequency, so that the system response is flat at the crossover frequency. A Butterworth filter matches the slopes so that there is a peak at the crossover frequency, and a Bessel filter is in between the two. For an L-R filter, halve the capacitance and double the inductance. For a Butterworth filter, multiply the capacitance by 1/sqrt(2) and the inductance by sqrt(2). For a Bessel filter, multiply the capacitance by 1/sqrt(3) and the inductance by sqrt(3). You should realize, too, that crossovers induce a phase shift in the signal of 90 degrees per order. In a second order filter, then, this can be corrected by simply reversing the polarity of one of the drivers, since they would otherwise be 180 degrees out of phase with respect to each other. In any case with any crossover, though, you should always experiment with the polarity of the drivers to achieve the best total system response. One other thing to consider when designing passive crossovers is the fact that most passive crossovers are designed based on the speakers' nominal impedance. This value is NOT constant, as it varies with frequency. Therefore, the crossover will not work as it has been designed. To combat this problem, a Zobel circuit (also known as an "Impedance Stabilization Network") should be used. This consists of a capacitor and resistor in series with one another, in parallel with the speaker, e.g., ________ __ + o----| |----o-----o + | | / INPUT | Xover | R1 | |/ | | C1 | |\ - o----|________|----o-----o - |__| \ To calculate these values, R1 = Re (in ohms) x 1.25, and C1 = (Lces in henries / Re^2) * 10^6. See 4.1 for definitions of Re and Lces. R1 will be in ohms, and C1 will be in uF (micro- farads). As an example, an Orion XTR10 single voice coil woofer has Re = 3.67 ohms and Lces = 0.78 mH. So, R1 = 3.67 * 1.25 = 4.6 ohms. C1 = ( 7.8E-4 / 3.67^2 ) * 10^6 = 57.9 uF (be careful with units - 0.78 mH = 7.8E-4 H) As with the definition of crossover slopes, the above definition of the phase shift associated with a crossover is also an approximation. This will be addressed in future revisions of this document. 5.14 How do I build my own passive crossovers? [JSC] =========================================================== This section assumes that you have a basic understanding of how to solder, so the actual assembly of the crossover is not discussed. Rather, tips on choosing the proper types of capacitors and inductors are given here. To obtain low insertion losses, the inductors should have very low resistance, perhaps as low as 0.1 to 0.2 ohms. Also, be sure to select capacitors with proper voltage ratings. The maximum voltage in the circuit will be less than the square root of the product of the maximum power in the circuit and the nominal impedance of the driver. For example, a 4 ohm woofer being given 100W peak will see a maximum voltage of sqrt(100*4) = sqrt(400) = 20V. Make sure that the capacitors are bipolar, too, since speaker signals are AC signals. If you cannot find bipolar capacitors, you can use two polar capacitors in parallel and in opposite polarity (+ to - and - to +). However, there are some possible problems with this approach: the forward voltage rating will probably not be equal to the reverse voltage rating, and there could be a reverse capacitance as well. Both problems could adversely affect your circuit if you decide to use opposite polarity capacitors in parallel. 5.15 Can I split the single pre-amp output from my head unit to drive two amplifiers with a Y-cable? [IDB] =================================================================== [This section was written by someone who wishes to remain anonymous, but I will field any questions on the subject -IDB] Yes. When two loads are connected in parallel (such as with a Y-cable) they get the same voltage as each other. They do NOT get the same voltage as if only one load was connected because the head-unit has an internal resistance (typically around 600 ohms). So, given that the amp has a typical input impedance of around 10k ohms then we get something like this: ----------------------------- ---------------------------- HEAD UNIT ________ | | AMP | ______| |_________Vamp___________ | | | R(head)| | | | | _ | __|__ |________| | | __|___ |__| - _ | / \ | | | | | -___|__ | Vi | | | |R(amp)| | _- | \_____/ | | |______| __| _- | |_______________________________|________| - | | | | ----------------------------- ---------------------------- for the single amp situation. Please realize that the R(head) and R(amp) are internal to the head unit and amplifier and in fact are not deliberately added resistors but are characteristic of the real world circuits (non-ideal) in the head-unit and amplifier (and eq's, etc.). These numbers are typical, check your specific equipment for its particular specs. the worst case situation is a high source output impedance and low load input impedance. So, assuming a typical head unit and single amp the voltage seen at the amp (Vamp) is given by (Ohms law/Kirkov's law/1st year EE/high school electronics technology class/etc.): R(amp) Vamp1 = Vi * ------------------ R(amp) + R(head) Vamp1 = Vi * 0.94 Now, putting two amps in parallel from the original signal, R(amp) is effectively halved while R(head) is unchanged. Using the same voltage divider formula we get: 10000/2 Vamp2 = Vi * --------------------- 10000/2 + 600 Vamp2 = Vi * 0.89 So, for an Alpine 4V preout, Vi in the diagram (the open circuit head unit line level output) is 4V. Thus Vamp1 = 3.76V and Vamp2 = 3.56V. With two amplifiers' inputs connected in parallel, the voltage is reduced from 3.76V to 3.56V or approximately 5%, not a big deal. If you had a more typical 1V preout you would get Vamp1 = 0.95V and Vamp2 = .89V, also not a noticeable drop. This is also why this is slightly more susceptible to noise than a direct one-to-one connection. If the noise level inserted due to cabling was 0.1V per cable then the noise level in the signal reaching each of the two amps would be a slightly higher percent of the signal level but not doubled. (this is also why the 4V head unit is favored over the 1V unit for noise immunity: 0.1V noise / 3.76V or 3% is much less than 0.1V noise / 0.95V or 10% even in a one to one connection). 5.16 How do I turn a stereo signal into a mono signal [BW] ================================================================= Creating a mono signal is often necessary when you are powering a subwoofer by bridging the amplifier. Many people do not realize that bridging an amplifier does not always provide a mono signal - many amplifiers will simply use only one input channel, which means that the subwoofer won't be receiving the full signal. Some amplifiers have a switch that will allow you to combine the left and right channels into a mono signal. Some signal processors and head units provide a subwoofer-out channel that can be switched between stereo and mono. If you don't have this feature on any of your equipment, you will need to provide a mono signal to the amplifier. The common thought is to use a Y-adapter to "combine" the left and right channels. However, by using a Y-adapter, you are actually summing the line voltages and directly shorting the left and right channels at the head unit, which could cause problems. The correct way to create a mono signal is to cut off the ends of the RCA cables, combine the signal grounds (the outer shield), and then use a 1 kOhm (1/4 watt, 5% tolerance) resistor to each of the center conductors. Solder and insulate the resistors so that you don't short them prematurely, and then connect the two resistors together. Connect the summed signal ground to the shield of the new RCA plug, and the summed center conductor to the center pin of the RCA plug. 5.17 5.17 How do I determine a speaker's polarity? [IDB] ========================================================== If you have a speaker and the terminals are no longer marked, you can do a simple test to determine which terminal is positive (+) and which is negative (-). This test is useful for midrange/midbass/subwoofers, but not for tweeters. Use a 1.5V battery (AA, C, D) and connect the (+) terminal on the battery to one terminal of the speaker, and connect the (-) terminal to the other terminal of the speaker. If the cone moves OUT, then the battery is connected "properly," i. e., the (+) terminal of the battery is connected to the (+) terminal of the speaker, and the (-) terminal of the battery is connected to the (-) terminal of the speaker. If, however, the cone moves IN, the battery has been connected "backwards," i. e., the (+) terminal of the battery is connected to the (-) terminal of the speaker, and the (-) terminal of the battery is connected to the (+) terminal of the speaker. 5.18 How can I use an oscilloscope to set the gains in my system? [AO] ======================================================================== This section assumes you are already familiar with your oscilloscope and will not go into setting it up. If you haven't already, spend a few minutes with your scope's manual. You'll need a test disc with a variety of test tones. I use the official IASCA test disc, but there are some of the "Bass Discs" that have test tones as well. You do NOT want to use sweeps, only pure tones. There is an AutoSound 2000 disc (#101?) which has a signal which is unclipped for 20 seconds, clipped for 5, and then unclipped for the last 5 seconds. I have never used the AutoSound 2000 discs, but know them to have excellent recommendations (as well as all the test tones you could ever need). Viewing this track on your scope's display could be useful if you've never seen clipping on an oscilloscope display before. You start by finding the clipping level of your head unit. Many of the better head units will not clip the pre-amp outputs, even at full volume, but it's always better safe than sorry. Disconnect the RCA's from your head unit. Pop in your test disc and skip to the track with a 1 KHz tone. If your CD player has a repeat function, set it to repeat just this track. That way you won't have to skip back at the end of the tone. Set your bass, treble, fader, and balance all to center. Turn the volume all the way up. Probe your right and left front (and rear if you have them) one at a time. Your scope should show a wave, either a sine wave or a clipped sine wave. If you have a standard pure sine wave then all is good, and you're ready to proceed. If you have a clipped wave then you need to turn down the volume, one click at a time until you see a perfect sine wave on your scope's display. Remember this point, as this is the highest you can EVER turn up your head unit. After you set the level for one of your outputs the rest should be the same, but check them all just to be sure. The results will be the same if you leave the RCA's plugged into the head unit and disconnect them at the other end (from your amp/EQ/processor/whatever) but unless you have a remote control you'll be running back and forth to change the volume. If your head unit has subwoofer pre-amp outputs you'll need to test them using a different tone. I usually test subwoofer outputs at a level midway between the crossover points. For example, if your subwoofer outputs are crossed over at 80Hz then you would want to use a 40Hz test tone. Other than that the procedure for testing subwoofer pre-outputs is the same as testing front or rear outputs. Once you've found the clipping level of your head unit it's time to proceed down the signal chain. If you are using an EQ, preamp, DSP, or other processor (NOT including crossovers) test them next. Leave your EQ set the way you usually use it. Hook up the processor and probe all the outputs of your processors in the same method as you did your head unit. You should probe each of the outputs using tones that match the bands of your EQ. For example, if you have a 9 band EQ with bands at 50/100/200/400/800/1.5k/3k/6k/12kHz you would probe your EQ 9 times, once with a 50 Hz test tone, once with a 100 Hz test tone, and so on. If your EQ also includes a crossover you'll need to follow the crossover procedure. If any of these processors are clipping you will probably need to turn down your head unit's volume control or make any adjustments on that unit that you can. For example, if you are testing an EQ and you have any bands excessively boosted, try bringing down that band first. That may be causing your clipping. To test your crossover you need to probe each output using a test tone that is midway between the high and low pass. For example, a channel which is crossed over between 100 Hz and 20 KHz (like a front channel) would be tested at 9950 Hz. Since you'll be hard pressed to find a 9975 Hz test tone on your CD use the 10Khz tone. For a rear channel crossed over with a lowpass of 3500 Hz you would use a 1750 Hz tone. As you again would have problems finding a 1750 Hz tone on a CD use a 2 KHz tone. For a subwoofer channel lowpassed at 70 Hz you would use a 35 Hz tone. This one you may find on your CD, if not use 30 Hz or 40 Hz. Assuming your crossover has level settings you will want to turn the level for whatever channel you're testing all the way up and probe the output. Assuming the output is clipping, back the level down slowly until you see a perfect wave on your scope. Now it's time to check your amp's outputs. Hook up your amps and play the same test tones you were using on the crossover. Disconnect the speakers and then turn the gains all the way up. Probe the first channel's output. Adjust the gain the same way you did your crossover. Back the gain down slowly until your wave isn't clipping anymore. Voila! You are done. You have just effectively eliminated clipping from your system. If you turn your bass or treble up, or boost up a channel on your EQ you may introduce some clipping. After major EQ work you may want to redo this procedure, starting at the EQ. 5.19 Why are kickpanels such a popular location for mounting speakers? [ES] ======================================================================== There is a lot more to mounting speakers in the kick panels than just equalizing the path length difference (PLD). Two of which are: on-axis response, and angling for pattern control. On-axis response refers to the fact that most speakers sound best when listened to on-axis, or as close to on-axis as possible. Second, after mounting your speakers in the kick panels you can then angle the speakers to take advantage of their off-axis response to use output level to overcome any PLD that is still present. The pattern control I am mentioning is one of the ways a horn loaded compression driver works very well, they not only use amplitude to overcome any PLD that is still present they minimize early reflections that can destroy imaging staging and spectral balance. PLD can be improved more than marginally when you consider the stock locations in a lot of vehicles, or the locations most installers choose. Measure the PLD between tweeters when mounted high in the dash or at the front corner at the top of the door and you will notice its probably on the order of 24". This mounting setup requires a lot of amplitude adjustment to correct the problems induced by this difference. The nearer tweeter is out phase from the opposite side and is arriving much sooner and with much greater amplitude due to the fact is not as far away. When all these factors are added together, it is very difficult for even the most flexible DSP unit to correct. On top of that, not many people or installers have access to the necessary tools to properly set up time delays using a DSP - TEF, MLSSA or other very expensive time domain measuring equipment are required to do the job properly. There will always be trade offs involved and deciding which trade offs to take can be very hard. A small dropout due to phase cancellation will probably not be noticed by most people but most people will quickly notice when a vehicle is not imaging properly, and if you can move the problem to higher frequency where we determine localization more from amplitude rather than phase differences, it will be much easier to deal with. Also, if you minimize the time/phase difference it will be much easier to correct with amplitude. Some people complain that kickpanel mounting gives a low sound stage. However, keep in mind that when any stereo system is imaging properly the point sources can no longer be localized. When our brains can no longer localize the point sources it will then hear things at eye level. 5.20 How can I build custom kickpanels? [MB] =================================================== Building custom kickpanels for your car is a fairly advanced task, and requires knowledge (and experience) in working with fiberglass. Auto Sound & Security published an article in the August 1996 issue that covered the basics of working with fiberglass. This is kick panels in a nutshell. It takes about 2 days to do this right, although it is possible to do overnight (a LONG night) in one vehicle. Step 1: Cover base area with plastic & duct tape. Step 2: Lay fiberglass over the entire area. Don't worry about getting the pieces cut to the exact size and shape, you will trim them later. Step 3: Build the baffles for your speakers. Step 4: After the fiberglass has cured, set the baffles (with the speakers properly mounted into the panels and use a backstrap to secure the baffle to the panels. Now, spend some time listening to the car and aim the baffles until you get the best image and stage in the car. Note: Take your time aiming the speakers - once you fix the baffles, you won't be able to readjust the speakers. You may want to spend a few days listening to different positions to determine what sounds best. Step 5: Remove the speakers from the baffle and fill from behind with self-expanding insulation foam. This will allow you to mold the baffle into the rest of the car. Step 6: After the foam has hardened, sand it to the shape you want the panel to have. Step 7: Lay fiberglass over the foam to form the top panel of the kick panel. Step 8: After the fiberglass has hardened, sand the surface smooth. This may require the use of a little bondo to get things perfect. Step 9: Remove the foam by grinding it out from the inside of the kickpanel. Step 10: Cover the panels with carpet, vinyl, leather, fleckstone or other substance of your choice. Step 11: Install the kick panels in the car. Step 12: Install the speakers in the panels. Step 13: Make speaker baffles from Plexiglas, and grilles. Step 14: Enjoy. 5.21 What's worse for a speaker, too much or too little power? [IDB] ====================================================================== Problems occur (in everyday operation) when distortion is fed to a speaker. This occurs MUCH more often when you are dealing with an underpowered system - typically the owner will turn up the volume too much or set the amplifier gains too high to try and get more volume from the system. These introduce distortion to the signal - this will destroy *any* speaker. ((see Section 5.22).) When a speaker is overpowered, however, it is not nearly as common to have these kind of problems, so speakers aren't blown as much. Of course, it is certainly possible to destroy a speaker (thermally) by overpowering it, but you'll have a pretty hard time doing this on your own, especially with standard car audio amplifiers. 5.22 Why is distortion harmful to my speakers? [RK] ========================================================== Distortion is hard on speakers for two reasons. Reason 1: Distortion causes the power spectrum to shift upwards in frequency. A bass note, when distorted, will have lots of high frequency energy. This will cause mid-ranges and tweeters to fry, if the amplifier is operating full range. It doesn't harm woofers, necessarily. Reason 2: Distortion causes the average power to be much higher. Typically, a music signal that never clips has an average power level of 1/4 the peak power level for even the most compressed speed metal or pop. More dynamic music will be 1/8 the peak level or less on average. When you clip the amp hard, the average output moves up to the full-rated output of the amp or more. The peak to average ratio can be less than 2 to 1, with the peaks being at double the rated power of the amp, and the average being at the rated power of the amp or higher. Thermally, the speaker can handle the average power being 1/4 the rated power of the amp (little to no clipping), but it will have a much harder time with the average power being the amp's rated power or more (massive clipping). As you might expect, this is pretty hard on the amp, too. For transients, most speakers can handle a ton of power. But for long term signals, the power handling is much less. 5.23 What tools do I need to cut Plexiglas? [PS] ======================================================= A good quality Carbide Tipped Triple Chip saw blade is preferred when cutting Plexiglas. This reduces the amount of chipout on the opposite side of the material and produces a smooth edge. A Router can be used to cut Plexiglas as well. The Router should be at least 1.5 HP or better and with changeable collets for a 0.5" Shank quality 2 flute Carbide bit. This reduces the vibration and chattering when cutting and will produce a more smooth edge. Since Plexiglas is an acrylic plastic (polymer), heat generated by the saw blade or router bit when cutting will melt the material and create a glazed imperfection viewed from the face if you do not maintain a steady rate of cut. It is best that when cutting you do a rough cut first, then do the finished size cut to reduce stress on the cutting blade and equipment. 5.24 Are there any other special requirements for working with Plexiglas? [PS] ===================================================================== Check the protective paper covering for any rips, tears or folding at the time of purchase. In most cases if visual inspection is not done at the place of purchase, it's very difficult to prove product defects or damage done by the seller. Inspect all surfaces that will come into contact to reduce tearing and scratching of the protective paper covering and the Plexiglas finish. Make sure that the protective paper covering remains in place until all machining and milling tasks are finished. -- ian d bjorhovde -> ianbjor@mobileaudio.com internet mobile audio -> http://mobileaudio.com