Path: bloom-beacon.mit.edu!hookup!europa.eng.gtefsd.com!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 2/3) Supersedes: Followup-To: rec.audio.car Date: 6 Apr 1994 05:47:16 GMT Organization: University of Illinois at Urbana Lines: 785 Approved: news-answers-request@mit.edu Distribution: world Expires: 6 May 1994 05:00:00 UT Message-ID: 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:14904 rec.answers:4773 news.answers:17721 Archive-name: car-audio/part2 Rec-audio-car-archive-name: FAQ/part2 Version: 2.1 Last-modified: 5 Apr 94 3 Components This section describes various components that you can have in a car audio system, along with common specifications, desirable features, some of the best and worst brands, and so on. Be aware that there is no standardized testing mechanism in place for rating car audio products. As such, manufacturers are open to exaggerating, "fudging", or just plain lying when it comes to rating their own products. 3.1 What do all of those specifications on speakers mean? [JSC,CD] "Input sensitivity" is the SPL the driver will produce given one watt of power as measured from one meter away given some input frequency (usually 1kHz unless otherwise noted on the speaker). Typical sensitivities for car audio speakers are around 90dB/Wm. Some subwoofers and piezo horns claim over 100dB/Wm. However, some manufacturers do not use true 1W tests, especially on low impedance subwoofers. Rather, they use a constant voltage test which produces more impressive sensitivity ratings. "Frequency response" in a speaker refers to the range of frequencies which the speaker can reproduce within a certain power range, usually +/-3dB. "Impedance" is the impedance of the driver (see 1.1), typically 4 ohms, although some subwoofers are 8 ohms, some stock Delco speakers are 10 ohms, and some stock Japanese imports are 6 ohms. "Nominal power handling" is the continuous power handling of the driver. This figure tells you how much power you can put into the driver for very long periods of time without having to worry about breaking the suspension, overheating the voice coil, or other nasty things. "Peak power handling" is the maximum power handling of the driver. This figure tells you how much power you can put into the driver for very brief periods of time without having to worry about destroying it. 3.2 Are component/separates any better than fullrange or coaxials? [JSC] Usually, yes. Using separates allows you to position the drivers independently and more carefully, which will give you greater control over your imaging. For rear fill applications, however, coaxial speakers will perform fine, as imaging is not a primary concern. 3.3 What are some good (and bad) brands of speakers? [JSC] People will emotionally defend their particular brand of speakers, so asking what the "best" is is not a good idea. Besides, the best speaker is the one which suits the application the best. In general, however, various people have claimed excellent experiences with such brands as Boston Acoustics, MB Quart, a/d/s/, and Polk. Also, most people agree that you should avoid brands like Sparkomatic and Kraco at all costs. 3.4 What do all of those specifications on amplifiers mean? [JSC,BG] "Frequency response" refers to the range of frequencies which the amplifier can reproduce within a certain power range, usually +/-3dB. "Continuous power output" is the power output of the amplifier into one channel into a certain load (usually four ohms) below a certain distortion level (usually at most 1%THD) at a certain frequency (usually 1kHz). A complete power specification should include all of this information, e.g. "20W/ch into 4 ohms at < 0.03%THD at 1kHz" although this can also be stated as (and be assumed equivalent to) "20W/ch at < 0.03%THD". The amplifier should also be able to sustain this power level for long periods of time without difficulties such as overheating. "Peak power output" is the power output of the amplifier into one channel into a certain load (usually four ohms) below a certain distortion level (usually much higher than the continuous rating level) at a certain frequency (usually 1kHz). A complete power specification should include all of this information, e.g. "35W/ch into 4 ohms at < 10.0%THD at 1kHz" although this can also be stated as (and be assumed equivalent to) "35Wch at < 10.0%THD". Consumer warning: some manufacturers will state the "peak power output" rating by including the amount of power which can be drawn from "headroom", which means power supply capacitors. They usually will not tell you this in the specification, however; indeed, they tend to prominently display the figure in big, bold letters on the front of the box, such as "MAXIMUM 200W PER CHANNEL!!!" when the continuous rating is 15W/ch and the unit has a 5A fuse. "Damping factor" represents the ratio of the load being driven (that is, the speaker - usually four ohms) to the output impedance of the amplifier (that is, the output impedance of the transistors which drive the speakers). The lower the output impedance, the higher the damping factor. Higher damping factors indicate a greater ability to help control the motion of the cone of the speaker which is being driven. When this motion is tightly controlled, a greater transient response is evident in the system, which most people refer to as a "tight" or "crisp" sound. Damping factors above 100 are generally regarded as good. "Signal to Noise" or "S/N" is the ratio, usually expressed in decibels, of the amount of true amplified output of the amplifier to the amount of extraneous noise injected into the signal. S/N ratios above 90 to 95dB are generally regarded as good. 3.5 What is "bridging"? Can my amp do it? [JSC] Bridging refers to taking two channels of an amplifier and combining them to turn the amplifier into a one channel amplifier. In normal operation, one wire which goes to a speaker from the amplifier is "neutral", that is, the potential never changes (with respect to another fixed point, like ground). The other wire is "hot", that is, it carries the fluctuating AC speaker signal. The speaker "sees" a potential between these two leads, and so there is a voltage applied to the speaker. When an amplifier is bridged, both leads are "hot". However, one signal must be inverted, or else the speaker will never see a potential, as both wires are carrying roughly the same signal. With one signal inverted, the speaker will see a signal that is twice as great as one signal alone. Thus, if your amplifier does not have a switch or button of some sort which inverts one channel, you cannot bridge your amplifier (unless you build an external inverter). With respect to power, the commonly accepted definition is that when you bridge an amplifier, you add all of the characteristics of the bridged channels together. Thus, if you bridge an amplifier that is 50W/ch into 4 ohms at < 0.05%THD, your bridged channel is 100W/ch into 8 ohms at < 0.10%THD. Therefore, an amplifier which is 2 ohm stable in stereo mode is only 4 ohm stable in bridged mono mode, and an amp which is 4 ohm stable in stereo is only 8 ohm stable in bridged mono. 3.6 What is "mixed-mono"? Can my amp do it? [JSC] Some amplifiers which are both bridgeable and able to drive low impedance loads also allow you to use "mixed-mono" mode. This involves driving a pair of speakers in stereo mode as well as simultaneously driving a single speaker in bridged mono mode. What happens is that you put your amp in bridged mode, which inverts one output signal. You then connect the mono speaker as you normally would in bridged mode. To the channel which is not inverted, you connect your stereo speaker as you normally would. To the channel which is inverted, you connect the other stereo speaker with its leads reversed (+ to - and - to +) since the signal is inverted. 3.7 What does "two ohm stable" mean? What is a "high-current" amplifier? [JSC] An x ohm stable amplifier is an amp which is able to continuously power loads of x ohms per channel without encountering difficulties such as overheating. Almost all car amplifiers are at least four ohm stable. Some are two ohm stable, which means that you could run a pair of four ohm speakers in parallel on each channel of the amplifier, and each channel of the amp would "see" two ohms. Some amps are referred to as "high-current", which is a buzzword which indicates that the amp is able to deliver very large (relatively) amounts of current, which usually means that it is stable at very low load impedances, such as 1/4 or 1/2 of an ohm. Note that the minimum load rating (such as "two ohm stable") is a stereo (per channel) rating. In bridged mode, the total stability is the sum of the individual channels' stability (see 3.5). 3.8 Should I buy a two or four (or more) channel amplifier? [JSC] If you only have one line-level set of outputs available, and wish to power two sets of speakers from a single amplifier, you may be able to save money by purchasing a two channel amplifier which is stable to two ohms rather than spending the extra money for a four channel amp. If you do this, however, you will be unable to fade between the two sets of speakers (without additional hardware), and the damping factor of the amplifier will effectively be cut in half. Also, the amp may run hot and require fans to prevent overheating. If you have the money, a four channel amp would be a better choice. You would need to add a dual-amp balancer in order to maintain fader capability, however, but it is more efficient than building a fader for a two channel amp. If you wish to power a subwoofer or additional speakers as well, you may want to purchase a five or six channel amp. 3.9 What are some good (and bad) brands of amplifiers? [JSC] As with speakers, people emotionally defend their amplifier, so choosing the best is difficult. However, some brands stand out as being consistently good while others are consistently bad. Among the good are HiFonics, Phoenix Gold, a/d/s/, and Precision Power. 3.10 What is a crossover? Why would I need one? [JSC] A crossover is a device which filters signals based on frequency. A "high pass" crossover is a filter which allows frequencies above a certain point to pass unfiltered; those below that same point still get through, but are attenuated according to the crossover slope. A "low pass" crossover is just the opposite: the lows pass through, but the highs are attenuated. A "band pass" crossover is a filter that allows a certain range of frequencies to pass through while attenuating those above and below that range. There are passive crossovers, which are collections of purely passive (unpowered) devices - mainly capacitors and inductors and sometimes resistors. There are also active crossovers which are powered electrical devices. Passive crossovers are typically placed between the amplifier and the speakers, while active crossovers are typically placed between the head unit and the amplifier. There are a few passive crossovers on the market which are intended for pre-amp use (between the head unit and the amplifier), but the cutoff frequencies (also known as the "crossover point", defined below) of these devices are not typically well-defined since they depend on the input impedance of the amplifier, which varies from amplifier to amplifier. There are many reasons for using crossovers. One is to filter out deep bass from relatively small drivers. Another is to split the signal in a multi-driver speaker so that the woofer gets the bass, the midrange gets the mids, and the tweeter gets the highs. Crossovers are categorized by their "order" and their "crossover point". The order of the crossover indicates how steep the attenuation slope is. A first order crossover "rolls off" the signal at -6dB/octave (that is, quarter power per doubling or halving in frequency). A second order crossover has a slope of -12dB/octave; third order is -18dB/octave; etc. The crossover point is generally the frequency at which the -3dB point of the attenuation slope occurs. Thus, a first order high pass crossover at 200Hz is -3dB down at 200Hz, -9dB down at 100Hz, -15dB down at 50Hz, etc. It should be noted that the slope (rolloff) of a crossover, as defined above, is only an approximation. This issue will be clarified in future revisions of this document. The expected impedance of a crossover is important as well. A crossover which is designed as -6dB/octave at 200Hz high pass with a 4 ohm driver will not have the same crossover frequency with a driver which is not 4 ohms. With crossovers of order higher than one, using the wrong impedance driver will wreak havoc with the frequency response. Don't do it. 3.11 Should I get an active or a passive crossover? [JSC] Active crossovers are more efficient than passive crossovers. A typical "insertion loss" (power loss due to use) of a passive crossover is around 0.5dB. Active crossovers have much lower insertion losses, if they have any loss at all, since the losses can effectively be negated by adjusting the amplifier gain. Also, with some active crossovers, you can continuously vary not only the crossover point, but also the slope. Thus, if you wanted to, with some active crossovers you could create a high pass filter at 112.3Hz at -18dB/octave, or other such things. However, active crossovers have their disadvantages as well. An active crossover may very well cost more than an equivalent number of passive crossovers. Also, since the active crossover has separate outputs for each frequency band that you desire, you will need to have separate amplifiers for each frequency range. Furthermore, since an active crossover is by definition a powered device, the use of one will raise a system's noise floor, while passive crossovers do not insert any additional noise into a system. Thus, if you have extra money to spend on an active crossover and separate amplifiers, and are willing to deal with the slightly more complex installation and possible noise problems, an active crossover is probably the way to go. However, if you are on a budget and can find a passive crossover with the characteristics you desire, go with a passive. 3.12 How do I build my own passive crossovers? [JSC] 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 crossover 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 x pi x f x Z). The needed inductance in henries is Z/(2 x pi x 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 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. 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. 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. 3.13 Should I buy an equalizer? [JSC] Equalizers are normally used to fine-tune a system, and should be treated as such. Equalizers should not be purchased to boost one band 12dB and to cut another band 12dB and so on - excessive equalization is indicative of more serious system problems that should not simply be masked with an EQ. However, if you need to do some minor tweaking, an EQ can be a valuable tool. Additionally, some EQs have spectrum analyzers built in, which makes for some extra flash in a system. There are two main kinds of EQs available today: dash and trunk. Dash EQs are designed to be installed in the passenger compartment of a car, near the head unit. They typically have the adjustments for anywhere from five to eleven (sometimes more) bands on the front panel. Trunk EQs are designed to be adjusted once and then stashed away. These types of EQs usually have many bands (sometimes as many as thirty). Both types sometimes also have crossovers built in. 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? [JSC] You can use any portable CD player in a car provided that you have either an amplifier with line level inputs (preferred) or a tape deck. If you have the former, you can simply buy a 1/8" headphone jack to RCA jack adapter and plug your CD player directly into your amplifier. If you have the latter, you can purchase a 1/8" headphone jack to cassette adapter and play CDs through your tape deck. The cassette adapters tend to be far more convenient; however, there is a significant tradeoff: by using cassette adapters, you limit your sound to the frequency response of the tape head, which is sometimes as much as an entire order of magnitude worse than the raw digital material encoded onto the CD itself. Portable CD players which were not designed for automotive use will tend to skip frequently when used in a car (relatively). CD players that are specially designed for automotive use, such as the Sony Car Discman, tend to include extra dampening to allow the laser to "float" across the bumps and jolts of a road. Some people have indicated success with using regular portable CD players in a car when they place the CD player on a cushion, such as a thick shirt or even on their thighs. 3.22 What's that weird motor noise I get with my portable CD player? [JSC] Many people report problems while playing CDs from a portable CD player into their car audio systems. The problem, stated very simply, has to do with the stepping of the motor requiring a varying amount of current and non-isolated power and audio signal grounds. Using a liberal application of capacitors and inductors, this voltage variance can be restricted to a window of 8.990 to 9.005V for a 9V CD player, yet even the swing between these two levels is enough to cause annoyingly loud noise on the outputs. It has been reported that this entire problem can be solved by using a true DC-DC inverter at the power input to the 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? [HK] MiniDisc (MD) seems to have a better future than Digital Audio Tape (DAT) or Digital Compact Cassette (DCC) which don't seem to have appeal to the public. Ease of use seems to be an important factor and the CD formats allows direct access to musical tracks at an instant. Although MD doesn't match the sound quality of the standard CDs it will probably be popular since the players have a buffer to eliminate skipping. DAT will remain as a media for ProAudio for recording purposes before pressing CDs. 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? [HK, JSC] If a proper center image isn't achievable via a two channel configuration, installation of a center channel can help. Since the majority of recordings are done in two channel, a two channel system designed correctly should be able to reproduce a center image which was captured during recording. A center channel is not simply a summation of the left and right channels, like bridging an amplifier; rather, it is an extraction of common signals from the left and right channels. This usually means the lead vocals, and perhaps one or two instruments. These signals will then be localized to the center of the stage, instead of perhaps drifting between the left center and right center of the stage. A signal processor is usually required in order to properly create a center channel image. The image should then be sent to a driver in the physical center of the front of the car, at an amplification level somewhat lower than the rest of the speakers. The correct frequency range and power levels will depend on the particular installation, though a good starting point is perhaps a pass band of 250-3000Hz at an amplification level of half the power of the main speakers (3dB down). 3.28 Should I buy a sound field processor? 3.29 What are some good (and bad) brands of signal processors? 4 Subwoofers This section describes some elements necessary for understanding subwoofers - how they operate, how to build proper enclosures, how to pick the right driver for you, and how to have a computer do some of the work for you. 4.1 What are "Thiele/Small parameters"? [CD,RDP] These are a group of parameters outlined by A.N. Thiele, and later R.H. Small, which can completely describe the electrical and mechanical characteristics of a mid and low frequency driver operating in its pistonic region. These parameters are crucial for designing a quality subwoofer enclosure, be it for reference quality reproduction or for booming. Fs Driver free air resonance, in Hz. This is the point at which driver impedance is maximum. Fc System resonance (usually for sealed box systems), in Hz Fb Enclosure resonance (usually for reflex systems), in Hz F3 -3 dB cutoff frequency, in Hz Vas "Equivalent volume of compliance", this is a volume of air whose compliance is the same as a driver's acoustical compliance Cms (q.v.), in cubic meters D Effective diameter of driver, in meters Sd Effective piston radiating area of driver in square meters Xmax Maximum peak linear excursion of driver, in meters Vd Maximum linear volume of displacement of the driver (product of Sd times Xmax), in cubic meters. Re Driver DC resistance (voice coil, mainly), in ohms Rg Amplifier source resistance (includes leads, crossover, etc.), in ohms Qms The driver's Q at resonance (Fs), due to mechanical losses; dimensionless Qes The driver's Q at resonance (Fs), due to electrical losses; dimensionless Qts The driver's Q at resonance (Fs), due to all losses; dimensionless Qmc The system's Q at resonance (Fc), due to mechanical losses; dimensionless Qec The system's Q at resonance (Fc), due to electrical losses; dimensionless Qtc The system's Q at resonance (Fc), due to all losses; dimensionless Ql The system's Q at Fb, due to leakage losses; dimensionless Qa The system's Q at Fb, due to absorption losses; dimensionless Qp The system's Q at Fb, due to port losses (turbulence, viscousity, etc.); dimensionless n0 The reference efficiency of the system (eta sub 0) dimensionless, usually expressed as % Cms The driver's mechanical compliance (reciprocal of stiffness), in m/N Mms The driver's effective mechanical mass (including air load), in kg Rms The driver's mechanical losses, in kg/s Cas Acoustical equivalent of Cms Mas Acoustical equivalent of Mms Ras Acoustical equivalent of Rms Cmes The electrical capacitive equivalent of Mms, in farads Lces The electrical inductive equivalent of Cms, in henries Res The electrical resistive equivalent of Rms, in ohms B Magnetic flux density in gap, in Tesla l length of wire immersed in magnetic field, in meters Bl Electro-magnetic force factor, can be expressed in Tesla-meters or, preferably, in meters/Newton Pa Acoustical power Pe Electrical power c propagation velocity of sound at STP, approx. 342 m/s p (rho) density of air at STP 1.18 kg/m^3 4.2 What are the enclosure types available, and which one is right for me? [JLD] Only the order of the enclosure First Order itself is shown here. The addition Infinite-Baffle or Free-Air of a crossover network increases the order of the system by the | order of the crossover. | Example: If a First-Order, 6dB/Oct. / crossover (single inductor in series / with the speaker) is used with a || Fourth Order enclosure, the total || system is a fifth order. \ Note: Air volumes and ratios shown \ here may not be to scale. This is | designed to provide order information | only. Second Order Second Order Acoustic- or Air-Suspension Isobaric* Acoustic-Suspension or Sealed (Compound Loaded) _______________________ _______________________ | | | _____| | / | / / | / | / / | || | || || | || | || || | \ | \ \ | \ | \____\ |_______________________| |_______________________| Fourth Order Fourth Order Fourth Order Bass-Reflex or Passive Radiator Isobaric* Vented or Ported Bass-Reflex Bass-Reflex _______________ _______________ _______________ | | | | | ____ | | / | / | / / | / | / | / / | || | || | || || | || | || | || || | \ | \ | \ \ | \ | \ | \____\ | | | | | | | | | / | | | | | / | | | ____| | | | ____| | | | | | ____ | \ | ____ | | | \ | | |_______________| |_______________| |_______________| Fourth Order Fourth Order Single-Reflex Bandpass Isobaric* Single-Reflex Bandpass _________________ ____ _______________________ ____ | | | | | | | | | | | / | | | | / \ | | | | / | | / \ | | || | | || || | | || | | || || | | \ | | \ / | | \ | | \ / | |_________|_______________| |_______________|_______________| Fourth Order Fourth Order Three Chamber Three Chamber Isobaric* Single-Reflex Bandpass Single-Reflex Bandpass ____________ ____________ ______________ ______________ | | | | | | | | | | | | | / | | \ | | / \ | | / \ | | / \ | | / \ / \ | | || || | | || || || || | | || || | | || || || || | | \ / | | \ / \ / | | \ / | | \ / \ / | |______|_____________|______| |_______|_______________|_______| Fifth Order = Fourth Order Enclosure + First Order Crossover = Third Order Enclosure + Second Order Crossover, etc. Sixth Order Sixth Order Dual-Reflex Bandpass Isobaric* Dual-Reflex Bandpass ____ _____________ ____ ____ ____________ ____ | | | | | | | | | | | | | | | | | / | | | | | | / \ | | | | | | / | | | | / \ | | || | | || || | | || | | || || | | \ | | \ / | | \ | | \ / | |_______________|_____________| |______________|_____________| Sixth Order Three Chamber Quasi-Sixth Order Dual-Reflex Bandpass Series-Tuned Bandpass _ _________ _________ _ _________________ ____ | | | | | | | | | | | | | | | | | | / | | \ | | | | / | | | | / \ | | / | | || || | | || | | || || | | || | | \ / | | \ | | \ / | | \ | |________|_____________|________| | ____| | | | | ____ | | | | |___________|_____________| Seventh Order = Sixth Order Enclosure + First Order Crossover, etc. * Isobaric or Coupled Pair (Iso-group) Variations: A variety of configurations may be used in the isobaric loading of any order enclosure. Physical and acoustic restrictions may make one loading configuration preferable to another in a particular enclosure. Composite or Push-Pull Compound or Piggy-Back or Face-to-Face Loading or Tunnel Loading _________________ ___________________________ | | | ____| | / \ | / / | / \ | / / | >>> || || >>> | >>> || || >>> | >>> || || >>> | >>> || || >>> | \ / | \ \ | \ / | \___\ |_________________| |___________________________| Back-to-Back Loading Planar Loading _________________________ ___________________________ | _________| | | | | \ / | / | | \ / | / | | >>> || || >>> | || >>> | | >>> || || >>> | || >>> | | / \ | \ | | /_______\ | \ | |_________________________| |________________________| | | | / | / | || <<< | || <<< | \ | >>> indicates direction of \ | >>> simultaneous cone movement. |__| 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? [MH] Various enclosure design software is available via ftp from csd4.csd.uwm.edu in the directory "/pub/high-audio/Software". The most popular program there is Perfect Box, which is in the file "perf.uu" (or "perf.zip"). 4.6 What is an "aperiodic membrane"? [CD] An aperiodic membrane is one part of a type of subwoofer enclosure. It is an air-permeable sheet which has frequency-dependent acoustical resistance properties. The original design goes back to Naim, for use in home systems, but has been applied by several individuals and companies in car audio. The completed system will be aperiodic, which means it will prove to be over-damped with a Q below 0.7. In contrast, most car audio systems range from sort of to grossly underdamped, with Q's > 0.8 and higher. These high-Q systems have poor transient response, nasty peaks in frequency response, and high rates of roll-off. Aperiodic systems will feature excellent transient response, smooth frequency response, and extended very-low frequency reproduction. Another benefit of the system is that you can pretty much choose whichever driver you'd like to use, as long as they are big. The Thiele/Small parameters (which would normally determine what kind of box would be used) are taken into consideration by the membrane designers so that the response is extended and overdamped, regardless of the characteristics of the driver. Physically, the aperiodic membrane isn't for every car. It requires sealing the trunk from the passenger compartment in an air-tight manner, as well as sealing the trunk from the outside for best results. The drivers are then mounted into the baffle between the passenger compartment and the trunk, as would be standard in an infinite-baffle/free-air set-up. The aperiodic membrane is then placed either in front of the driver or behind the driver, depending on the type. When mounting behind the driver, the membrane is used as the rear-wall of a very small box which the driver sits in (as in Richard Clark's infamous Buick Grand National). So, in short, it's not suitable for trucks, jeeps, R/V's, or hatchbacks. You should probably only get an aperiodic membrane if you've got money to burn, lots of amplifier power, some big subs, a sedan, a desire for trunk space, and no wish to boom. If your tastes lean towards bass-heavy booming, as opposed to well-recorded acoustic instruments, you're not going to be pleased with the result. -- 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