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  The Silverado, a small d'Appolito loudspeaker 
  Sep 28, 2005, 07:30am EDT 


By: Sander Sassen

Loudspeakers come in all shapes in sizes, some too large to conveniently place in a normal living room others so small they抣l get lost if dropped into the couch. For my next loudspeaker project I was looking for a small, yet full range loudspeaker that can be used in stereo applications but also for surround sound, i.e. 5.1-channel and up. The ideal configuration for surround sound, especially if you keep in mind the requirements for DTS and Dolby Digital EX playback, is five, or more, similar loudspeakers with identical response curves and tonal balance. Because both DTS and Dolby Digital EX playback calls for full range loudspeakers on all channels, the sound stage and coherency of the reproduced recording will be much better if all loudspeakers are identical, supplemented by a subwoofer of course.

I wanted to use drivers that have low, or virtually no, internal damping in the cone material so as to end up with as much transparency and detail as possible. Aluminum is a material that certainly has these characteristics. However cone breakup, i.e. the cone no longer operating as a whole at a set frequency, means that many aluminum drivers need steep filter curves to keep the cone breakup resonance peak out of the actual reproduction. Aluminum dome tweeters usually have their resonance peak well above the highest audible frequency, i.e. >20kHz hence it isn抰 much of a problem. With aluminum low and midrange drivers this however can be problematic if you want to use as few drivers as possible. For example a 17-cm, 6.5-inch, aluminum driver will have a peak at around 4 to 5kHz that抣l be hard to get rid of.

However for this loudspeaker I want to use as few drivers as possible, simply because I want to keep the number of drivers per loudspeaker to a minimum as to prevent phase shifting with five or more loudspeakers in a surround configuration, at various distances from the listener. This will result in some frequencies being cancelled out, and others amplified, more so than with a two-way loudspeaker. So a two-way loudspeaker it is. That leaves me with determining what drivers I抎 like to use and the size of the cabinet. As mentioned I抎 like something small, but not too small, as that will impact faithful reproduction. The subwoofer should only be used for frequencies below 60Hz, otherwise the main loudspeakers will sound too thin, missing the low-end extension.

Because I抎 like to use aluminum drivers for the low/midrange I抣l need a driver that has a resonance peak at a frequency that抯 at least twice the crossover frequency of the filter. With a two-way system the crossover frequency is usually around 2.5 to 3kHz, which means that the resonance peak of the low/midrange driver will need to be at more than 6kHz. Furthermore I mentioned I抎 like a small loudspeaker which can also be used as a center speaker. Unfortunately the usual tweeter-woofer, or inverted tweeter-woofer (woofer-tweeter) doesn抰 lend itself well for a center speaker as the sound axis will be skewed due to the loudspeaker being used on it side rather than in a normal upright position.

Fortunately there抯 a solution to that problem which is used on a large number of center loudspeakers and that抯 using a d Appolito configuration. A d扐ppolito configuration consists of two woofers and a tweeter in between, or rather a woofer-tweeter-woofer configuration. The d扐ppolito configuration has a much wider off-axis reproduction hence it is better suited for loudspeakers that抣l be used in either a vertical or horizontal orientation. The upside of all this is we抣l also get a boost in efficiency as two identical drivers will be used for the low/midrange and they抣l be able to handle a higher load. So we抣l use a d扐ppolito configuration for all loudspeakers which will also make the soundstage much more coherent due to the much better off-axis reproduction.

1. Introduction
2. Selecting drivers
3. Bass-reflex enclosure
4. Cross-over
5. Measurements
6. Conclusion
7. References and credits

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