612 Chapter 17
Much more detailed information is available on the
subject of the acoustic characteristics resulting from
different rates of expansion from the sources cited in the
Bibliography. The models are useful in analyzing the
propagation of acoustic energy within a horn, but other
considerations become dominant in determining the
nature of the radiated sound beyond a horn’s mouth.
For this reason, practical horns have come to be
known more by salient details of their sidewall shapes
than by their flare rates. The more common types are
described below.
17.7.6.1 Radial Horns
Radial (or sectoral) horns were claimed to allow a
natural radial expansion of the sound wave from the
driver, while maintaining an exponential expansion rate.
Typically, a radial horn has straight horizontal sides and
top and bottom walls that are in the form of spherical
sectors. The design approach employed for a radial horn
involves positioning the sides at approximately the
desired angle for horizontal coverage. Given the area
expansion desired, the top and bottom surfaces are then
derived mathematically. The most popular materials
used in making radial horns are cast aluminum (now
relatively uncommon), molded plastic, laminated glass
fiber, and polyester resin. This type of horn was in
widespread use from the 1930s until approximately the
mid-1980s, by which time constant directivity types had
become more popular.
Fig. 17-28 is an Altec 311-60; it has a 60° horizontal
coverage and is intended for use above 300 Hz, using a
1.4 inch driver. Altec was well known for this design,
with its characteristic vertical vanes at the mouth of the
horn.
17.7.6.2 Multicell Horns
Multicell horns were the first horns to be employed
specifically for their directivity control attributes. The
design approach was straightforward — several small
horns were affixed together in an array, with each horn
to supply a portion of the total coverage angle. These
small horns were connected to a common manifold so
that a single driver could power them, Fig. 17-29.
Multicell horns first came into use in the late 1930s.
They were originally made of sheet metal soldered
together and either filled on the outside with sand or
covered with a mechanical damping material.
17.7.6.3 Controlled Directivity HornsThe first constant directivity type of horn appeared in- Developed by Electro-Voice, they employed a
hyperbolic-flare throat section coupled to a conical
radial bell section, as shown in Fig. 17-30. This horn
shape yielded good low-frequency loading and rela-
tively constant angular beamwidth in both vertical and
horizontal directions over a wide frequency range. At
the time, its design represented a major departure from
previous thinking. Don Keele, the designer of the horns,
presented an AES paper (“What’s So Sacred about
Exponential Horns”). In the paper, he disclosed several
empirically developed relationships between mouth
size, frequency, and maintenance of coverage angle.
The concept of a waveguide as applied to an acoustic
Figure 17-28. Altec Lansing 311-60 cast aluminum sectoral
horn with sound-deadening material. Courtesy Altec
Lansing Corp.Figure 17-29. Altec Lansing 1.4-inch throat, all soldered and
coated steel horn family showing throat plumbing fixtures.
Courtesy Altec Lansing Corp.