Sound System Design 1255beration times. Eq. 34-26 will accurately predict attenu-
ation for distances Dc, which are greater than the
calculated critical distance, Dc, whenever an on-site
measurement shows the actual LP to be 1 to 5 dB below
the predicted LP at a distance equal to twice the calcu-
lated Dc from a source.
34.3 Loudspeaker Systems for Sound
ReinforcementThe answer to Question 2: “Can everybody hear?”
comes from evaluating the success of the loudspeaker
system, in particular, how well the loudspeakers have
been aimed to cover the audience and how well the pat-
terns of individual loudspeakers combine to cover areas
with complex shapes. To answer Question 2, then, the
following section discusses loudspeaker system compo-
nents, types of loudspeaker systems, and loudspeaker
system design.
34.3.1 Loudspeaker Components
A transducer is any device that converts one form of
energy to another. Loudspeaker components are trans-
ducers because they convert electrical energy into
acoustic energy. Packaged loudspeaker systems and line
arrays are designed from loudspeaker components
including cone-type loudspeakers and their enclosures,
compression drivers and their horns, and other compo-
nents such as ribbon drivers and ring radiators. In the
past, sound reinforcement systems often used clusters of
individual high-frequency horns and low-frequency
woofers (cone loudspeakers in enclosures). Today, most
systems use packaged loudspeaker systems or line
arrays to cover the audience. See Chapter 19 for more
detailed information on loudspeakers and Chapter 20
for additional information on cluster design.
34.3.1.1 Cone Loudspeakers
Large cone loudspeakers (15 and 18 inch diameters) are
normally used as the low-frequency components of
two-way, three-way, or multiway systems. Also, 12 and
10 inch cone loudspeakers may be used as the low-fre-
quency component in a low-power, two-way system or
as the lower midrange component in a three-way or
multiway system.
Smaller cone loudspeakers (8 and 4 inch) may be used
as low-frequency or midrange components in a packaged
loudspeaker system. Other 8 and 4 inch cone loud-
speakers are designed for relatively full-range perfor-mance and are used in ceiling-type distributed systems
and as the components in column loudspeaker systems.34.3.1.2 Cone Loudspeaker EnclosuresThere are three basic types of loudspeaker enclosures in
use in professional systems: sealed (often improperly
called infinite baffle ), vented (also called ported or bass
reflex), and horn-loaded. Some manufacturers also offer
combination vented and horn-loaded enclosures.
A sealed enclosure is relatively simple to design and
construct; it has a smooth frequency response curve,
good transient response, and helps protect the loud-
speaker from overexcursion at low frequencies. Sealed
enclosures are most common in home entertainment
systems.
A vented enclosure works as a Helmholtz resonator
to boost the low-frequency response of a loudspeaker
above the response of a similarly sealed enclosure
design. Transient response and frequency response
smoothness may suffer somewhat, although these prob-
lems are small in a good design. An electrical high-pass
filter should be used to help protect the loudspeaker
against overexcursion at frequencies below the enclo-
sure resonance frequency fb. Because of their greater
output at low frequencies, vented enclosures are
common in professional systems.
Horn-loaded enclosures place a horn in front of the
loudspeaker and a sealed compression chamber behind
the loudspeaker. The loudspeaker thus becomes a
compression driver. Properly designed, a horn-loaded
enclosure boosts the overall efficiency of the loud-
speaker-enclosure combination above a sealed or vented
enclosure and provides some measure of control over
the dispersion pattern. In addition, the sealed chamber
behind the loudspeaker helps prevent overexcursion at
low frequencies. Horn-loaded enclosures are most
common for midrange applications, Fig. 34-9.
For low-frequency applications, one type of
horn-loaded enclosure, often called a vented horn, adds
a vented chamber behind the loudspeaker (instead of the
sealed chamber) to boost the low-frequency response
below the horn’s cutoff frequency.
Another type of low-frequency, horn-loaded enclo-
sure, known as a folded horn, is a relatively long horn
that has been folded back on itself to reduce the external
package size, Fig. 34-10.
Because of their efficiency, horn-loaded enclosures
were popular in the early days of sound when power
amplifiers were small and expensive. Unfortunately, a
horn designed to work well at low frequencies is quite