"D'Appolito Configuration" (was: Speaker question).

[Henry A. Pasternack]

The midrange-tweeter-midrange arrangement, often called the D'Appolito
configuration, is becoming very popular these days. Although D'Appolito
didn't invent this type of driver arrangement, he has done a lot of work
with the design, and has written papers on the subject. His "Swan IV"
speaker design, published in "Speaker Builder" during 1988, is very popular
with amateur builders, although I am convinced that better sound can be
had by re-engineering the system with improved cabinet construction and
drivers. A variety of D'Appolito-designed speakers and D'Appolito knock-offs
can be bought on the hobby kit market. The highly-regarded Hales Signature
loudspeakers use a pair of Dynaudio drivers and an MB titanium dome tweeter
in the M-T-M arrangement. These are often cited as one of the best dynamic
speaker designs in existence today.

The advantage of the M-T-M arrangement is that it guarantees a symmetrical
vertical polar response even when non-in-phase crossovers are used. When
a 3rd-order Butterworth target crossover response is obtained, the combination
of the physical arrangement and the amplitude/phase response of the drivers
produce smooth vertical dispersion within +- 15 degrees of the listening axis
and broad off-axis nulls that prevent energy from beaming to the floor and
ceiling. D'Appolito has begin using 4th-order L-R crossover target functions
in his later designs, which are convenient for certain reasons and which also
seem to work well.

Subjectively, D'Appolito speakers produce very good imaging that is
stable over a wide vertical range of listening positions. There is no
"Venetian Blind" effect caused by vertical lobing. The largest drawback
to this design is the cost of two additional drivers and the need to build
a larger cabinet. These are serious constraints for commercial designs,
but less so for the home builder.

The symmetrical driver arrangement has certain advantages that become
obvious as one begins to design a crossover for such a speaker. I am
a strong believer in the merits of this design. My current speakers were
designed by D'Appolito (Signature 717's). I am very pleased with the
performance. In addition, I have just completed construction of a pair of
M-T-M satellites using a pair of Scan Speak 5" midranges and a Scan Speak
soft dome tweeter per channel.

-Henry