Loudspeakers come in all shapes in sizes, some too large to conveniently place in a normal living room others so small they’ll 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’t 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’ll 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’d like to use and the size of the cabinet. As mentioned I’d 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’d like to use aluminum drivers for the low/midrange I’ll need a driver that has a resonance peak at a frequency that’s 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’d 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’t 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’s a solution to that problem which is used on a large number of center loudspeakers and that’s using a d’ Appolito configuration. A d’Appolito configuration consists of two woofers and a tweeter in between, or rather a woofer-tweeter-woofer configuration. The d’Appolito configuration has a much wider off-axis reproduction hence it is better suited for loudspeakers that’ll be used in either a vertical or horizontal orientation. The upside of all this is we’ll also get a boost in efficiency as two identical drivers will be used for the low/midrange and they’ll be able to handle a higher load. So we’ll use a d’Appolito configuration for all loudspeakers which will also make the soundstage much more coherent due to the much better off-axis reproduction.
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