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CDT Audio Crossovers Technical Info:
Filter Comparison - Butterworth vs Linkwitz-Riley vs Elliptic
Crossover networks use filters in high pass low pass pairs. These may be symmetric or assymetric and may (usually significantly) involve driver characteristics which affect slope, shape, and rolloff at both the high and low end. All of these and many variations for which no particular name has been applied also exist. The elliptic differs from the others mentioned in having "zeros" in the stop band. This means the roll off shifts between two rates and can even result, if the parameters are so adjusted, to show increases in drive in the stop band. Since most drivers would sound excellent with no crossover at all but would burn out and overload almost immediately, crossovers are necessary to prevent low frequencies which are beyond a midrange or Tweeters capability from entering them. It is also then necessary to use many drivers to cover the frequency range to enable frequency and load sharing and power handling. Some cone type drivers can achieve nearly full range and play adequately loud in car and home environments. Generally some bass and highs must be sacrificed. This type is found in many OEM cars due to low cost and compact installation.
The Butterworth and Linkwitz-Riley
The drivers usually have to be inverted in phase relative to each other in the Butterworth and L-R. The overlap in the Butterworth can often also be reduced to provide good results especially in a reverberant environment such as found in the automotive sphere.
Because the ear-brain can separate the sound of reverberation from the sound of ringing it makes an audible difference to provide crossover designs that give better transient response.
Transient responsdegrades with greater order in practice. This audibly gives a system that has a character which remains embedded in the music over all musical sources. This character can overlay real information in the source thereby obscuring it. Usually the losses are barely noticeable up to an order of three. Beyond this the system can develop a character that is kind to various sources in that harshness is minimized but then information is lost when the very best recordings are used.
The louder the system will be run the more appropriate higher order designs become. The elliptic filter by allowing typically .1% to 1% of the input power to "bleed through" out of band gives faster roll off rates in the crossover region without degrading transient response. The gain is typically one to two orders of roll off added at no penalty. This gives a very clean sound and good power handling without overlaying the sound with excess character. The roll off of woofer power response is typically 24 db per octave and sometimes can reach 36 db or more if the driver is taken to the limit. This typically happens in most cases. The sound this makes is mitigated by the fact that part of the drop (12db/octave) is due to dispersion reduction. The roll-off rate is actually staggered in the majority of cases. Metal cone drivers display somewhat different character, must be expertly employed and are an exception to this rule of thumb.
Use of the elliptic in a fourth order configuration gives a transient response slightly better than a third order Butterworth and a roll-off rate slightly better than a fourth order of the same type.
The same advantages apply to the L-R but in somewhat different proportions depending on the order.
The net crossover in the CDT system is designed to interface the mid woofer to the soft dome Tweeter to give optimum overall performance and consistent results.
Only a loss in power handling and a custom in-car install and unusually delicate main drivers (not rugged miniature woofers) can provide a smoother more detailed easy listening experience. This can be accomplished with cone drivers that are both full range, crossed over to and interfacing with; a Tweeter that uses its own mechanical properties to augment filter response. Each car requires different positions and settings and the settings are hyper critical. Although the networks are simple there are more of them. Operating such a system with a high power subwoofer, especially the band pass type, is also a bad idea. The mid-high sound cannot keep up. Even marginally powerful systems will stress drivers like this. This is however an optimum approach for easy listening OEM cars. Pure, custom quality only, aftermarket product is also feasible but the market for such systems hardly exists. Most people stick with the systems found in their new cars. The value of such a system has to be demonstrated to see if the prospective buyer can hear the difference, and understand the high custom labor expense including the potential need to alter the car's interior. - usually not an option.
Maximum sound quality can be so configured by providing special large expensive exotic drivers not easily installed - to achieve a demonstration of very detailed and moderately loud (by autosound standards) sound. Some show cars like this have been attempted with varying success. The CDT system gives a wide margin of power and precision with the easiest installation possible. Overlay character is held to a minimum giving unprecedented results short of a full all out custom show car - with a genious for a designer - on an unlimited budget.