The True Audio Experience
We all know high fidelity means faithful to the original.

Your ultimate goal in pursuit of a car audio system is a believable rendition of the live performance. Our subjective judgment is all that¹s required to know we have arrived and is a rare experience in the real world. The special perspective of an automotive environment and the fact that the vast majority of recordings were electronically mixed from a variety of tracks makes us realize the goal is really to produce a credible illusion from an exciting perspective. The smaller enclosed volume in a car will always differentiate autosound from home audio and likewise home audio from theater sound. The larger the listening venue the longer the reverberation times are. The close-up environment of autosound naturally results in a more intimate sound approaching that provided by headphones. This is a different perspective from which and in which we experience the performance. Within these parameters our goals include clarity, focus and ambiance. Stunning results are possible. Some amateurs and installers end up trying to win the battle toward accuracy by substituting quantity for quality. Unable to achieve the goal they pile on equipment to hide the drawbacks. Transparent high quality has not been achieved.

Car audio almost always involves listening off axis due to the fixed seating and interior locations available for mounting speakers. Considering this the car stereo perspective resembles home theater surround sound. This small ³enveloped² sonic perspective is in many ways more dramatic than home audio. All that remains is to provide the building blocks and to install them correctly.

  1. Basic System Considerations Source material
  2. Head unit
  3. Speaker mounting positions
  4. Satellite Speakers
  5. Passive crossover networks
  6. Amplifiers and Electronics
  7. Subwoofer
  8. Car
8) In reverse order, the car determines what can be achieved. Interior details can be enhanced through modification if the car is not a new luxury model. In most cases one has to settle for a compromise.

Quietness on the road and freedom from vibrating panels and other interior items sets the limit on the quantity and quality of sound that can be achieved. A book could be written on how to disassemble and reassemble the interior of various automobiles. In general the easiest way is to own a clean quiet luxury car. Barring this one can remove interior items such as door panels and rework them and the interior areas behind them. The headliner can be a problem if it vibrates. Tapping items in the car you are selecting will alert you to the relative merit of these items. Thin cheap panels of any kind are suspect.

A number of methods can be used to improve each panel. Experts use sealants, coatings, sheets of foam, undercoats, expanded foams, padding etc to achieve the best sounding cars. Unless you have plenty of time you may want to rely on an installer if your car needs a lot of attention or if you want optimal results.

7) The sub should optimally be in a separate sealed enclosure. Ported enclosures or exotic multi-chambered boxes are often used for maximum loudness. These types are inherently inaccurate for quality use in a car. The reason is that the car¹s interior models as a simple volume for long wavelengths (bass). Because of this it can be shown that the pressure response is perfectly flat if a sealed box that would normally roll off at 12 decibels per octave becomes flat to theoretically zero Hz in this ³chamber² formed in the car. Exotic resistively damped enclosures not generally covered can provide an over-damped second order response and are also effective. They do the same job as a lower Qt woofer in the same size box when fully sealed. Once the box size has been mated to the driver or drivers, the only problem is setting the cutoff frequency. In a typical car this is about 66 Hz. In a big van the frequency will be lower, perhaps 50Hz.

Another nasty problem afflicting virtually all installations is a narrow peak about one octave above this frequency. That would be typically 132 Hz. This resonance happens because one wavelength of sound just fits in the cars¹ largest dimension and builds on itself. Some installers ³gap² the crossover to produce a dip at this frequency. This works but the transient response remains so poor there actually seems to be a ³hole² at this frequency on any music or voice that changes rapidly. Slow moving electronic or organ music is only slightly affected. A better way to produce an ideal correction is to drive another set of speakers in the car with this same problem frequency and then cancel the sum by phase interference with the subwoofer.

This restores ³speed² to the lower midrange/upper bass where this peak occurs. It allows the bass attack (represented by good transient response at this problem frequency) to remain. The speaker inter-mounting distance phase shifts and delays the cancellation. This both ultimately controls the exaggerated musical ³sustains² at that critical problem frequency with good transient response restored. In short the crossover is tuned to provide a critical amount of destructive interference between spaced speakers. To achieve this the ³satellite² speakers should be capable of ³keeping up² with the sub at this frequency. A very powerful sub played very loud might require this other speaker (probably in a door panel) to be larger than usual or to have an excellent Xmax if it is smaller. When the smaller speaker gives out the bass will become uncontrolled and ³muddy² especially since it is required to destructively interfere. Rear seat passengers who are equidistant between interfering speakers will experience a dryer sound. Ideally no riders should be equidistant.

6) Amplifier and ancillary electronics such as interfaces, equalizers and crossovers. Amplifiers for autosound now contain crossovers that can be used for low pass of the subwoofer if desired and high pass. Many are equipped to provide high and low pass filtering over a wide range. These built-ins are necessarily fixed in slope for the majority. A few provide a dual slope on the sub. In order to get most subs to roll of at 6db per octave a 12 db per octave slope must be used to counteract a gently rising response from the woofer. 18 db slopes would provide good roll off and with phase cancellation the net transient response could be good. Localizing the subwoofer sound (a drawback) would then be avoided. To interface the satellite speakers some high passing is desirable. Very low frequencies driving these moderately sized speakers can exceed speaker Xmax and limit the maximum clean sound that can be achieved. Virtually all of these panel mounted speakers have resonate near the typical in car pressure response rollover frequency (about 66 Hz).

This means these speakers could possibly provide full range sound. Most new cars use this idea and mount cheap wide range speakers in the doors and call it a day. Luxury cars do better. However in practice only the softest background music can be reproduced cleanly and in many cars this level is too low to provide adequate listening levels with the car in motion. The mounted panels do not have adequate rigidity for low bass and this potential is not realized in practice as a result.

All these factors inveigh against trying to run the doors full range in the bass. The door speakers would have to be built like miniature sealed box subwoofers. Space rarely allows this except in a Van. All these considerations point to using an amplifier with a built in variable high pass filter. Slopes of 6 or 12 db should be adequate and a setting around 120 HZ or less depending on position and interaction with the sub. Power levels are a personal matter. In a small car satellites will do OK with 50 watts per channel. More than 200 watts per channel in the doors is excessive. Subwoofer power is altogether another matter. Levels up to 1000 watts can be used in ultra compact high power multi driver subs. A deck mounted (free air) woofer could get by with 50 watts since the large chamber implies high electro-acoustic efficiency. Door panel amps should be chosen for sound quality with high bias mosfets generally being smoother sounding. Here there is no rule of thumb. Each amp is evaluated on it¹s own. You don¹t necessarily need either high bias or mosfets if the design is righteous.

An equalizer is usually a last resort for systems that must be designed in a hurry. However a separate electronic crossover can provide a better way to set up your crossovers and relative levels without shopping for amps that have the right built in qualities. Now the amps can be run full-range and this master set-up box can be located near the amps. Some head units require special interface boxes to isolate noise and provide level shifting. This occurs when OEM systems are upgraded without replacing everything.

5) Passive crossover networks, not the variable electronic type discussed above are almost universally used for the higher crossover frequencies. Mid-woofer to Tweeter crossovers and some midrange to woofer crossovers are in this design group.

We now come to the area where considerations such as mounting position are going to interact to provide the best design.

Hearing considerations are also involved. Human perception of music is insensitive to phase above 5khz. The automotive environment is highly reverberant and listening cannot be done (on-axis) which means equidistant between two stereo speakers. The best solution that has evolved is to crossfire the left and right systems and in some cases even to reverse the back channels. The key is not to fire the speaker right next to you right at you (the driver of the car). Direct it to the passenger. The reason is all your attention will be pulled to this speaker collapsing the surround image. A delay in the reflected energy adds spatiality to the sound and the longer the delay the more pleasant the sound. Naturally this is frequency dependant. Low frequencies won¹t be delayed much at all because the wavelengths are large. Tweeters should be relatively close to keep the time delay reasonable. Tweeters that are mounted low and fired up don¹t need to be cross-fired as much ­ they are already unobtrusive and diffused. Very high frequencies can be spatially injected to add ³air² to the sound since the ³ear² and the environment combine to make this effective. The ³ear² refers to the laws of human aural perception.

Inter-mounting distances are ideally small but most cars will sound better if the drivers are separated. This is because the ideal does not apply outside of an acoustically treated studio. These studios can sound very dry and ³dead² as opposed to a concert or performance hall that purposely includes some reverberation.

With regard to crossover rates the same rules do not apply to woofers and Tweeters. The car environment since it tends toward reverberant lessens the need to handle crossovers strictly according to a fixed set of rules but in general these3 rules ar4 good practice.

Energy response and frequency response are both the sound of a speaker but the frequency response is one fraction of the energy response taken at one specific angle to the speaker. This discussion is limited to forward radiating drivers like cones, domes and ribbons all of which basically reciprocate to move the air. The sound in a car is basically the energy response whereas in a studio it is basically the frequency response. The blend of the two is measured with an RTA. This usually measures a random noise signal representing all frequencies. It sounds like a waterfall. Some of the sound is reverberant and some of it is direct. This is the normal listening situation. This does not tell you how much of a given frequency band is relatively reverberant or what specific directions are most responsible for the direct or reverberant sounds. The reverberation time ­ another factor is also frequency dependant. The complexity of all these and other parameters not mentioned places the design of autosound in the art category. No mathematically definite answers are fully predictive. A skilled practitioner applies known techniques to produce excellent results. Sometimes changes are required. The process is partially experimental unless a formula for a specific car model is being followed.

Crossovers from the woofer to the Tweeter require a slow decline in the mid-woofer top end to avoid audible coloration. This is because the power response of these drivers can already be falling at 12 db per octave due to dispersion declines. The ³ear² is also sensitive to phase shifts in this frequency range and can identify ringing in the low pass characteristic since it is temporally anchored to the mid-woofer cone which covers a wide middle range. The Tweeter above this mid-woofer has a very strong energy response and thus wide dispersion at the crossover (typically) to the mid-woofer. It can dominate the sound if it overlaps the mid too much. It has relatively low power handling and is not temporally connected to the broad midrange sound. Because of this a faster crossover high pass characteristic is a better match. Since the power response of the mid-woofer is probably falling at 18 db per octave a Tweeter high pass characteristic of 18 db per octave would be a match. CDT crossovers add a response ³zero² below the crossover frequency to suppress Tweeter chamber resonance. Even when suppressed by ferrofluid soft dome Tweeters reach peak distortion in this frequency area. A sharp drop virtually removes all drive to the Tweeter in the problem area.

4) Speaker mounting positions are virtually limited to specific places by the design of the car and esthetics. In the discussion of other components various comments have already been made about speaker location. In summary a minimal set of front and rear stereo speaker systems should be installed with a subwoofer. The front speaker system should dominate the sound and raise the image to listening height. The rear speaker system should provide good sound to rear seated passengers without obtruding in the front. In general new sedan designs place the rear speakers in the side panels or rear doors and not in the back deck. The subwoofer system goes in the deck. Below this deck a sealed box gives the cleanest bass although a very tight luxury car trunk area can accommodate an effective panel mounted (free air) woofer. This is not really consistent with a bigger luxury car budget. This budget will easily allow for a more powerful amplifier which will be needed to drive a smaller sealed box system. Car systems designed for maximum volume should use a ported or chambered box. The sound is actually evaluated at a distance from the vehicle and the design should follow home system parameters. The port frequency should be at or below 40Hz to attain some semblance of quality

Copyright © 2008 by CDTAUDIO

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