Handbook for Sound Engineers

Loudspeakers 609

For example, a typical copper woofer voice coil has
a mass of 24 g and a gap heat transfer coefficient
(thermal resistance) of 1°C/W. Copper has a specific
heat of 0.092 cal/g°C or 0.0220 J/g°C. Therefore, using
Eq. 17-3, t= 0.528 s. This is a typical voice coil
response time. An aluminum coil will typically have a
shorter thermal time constant.

The time constant of the magnetic structure and
frame can be on the order of hours. For this reason, long
duration power tests are required to evaluate the
maximum power tolerance of transducers. Initially, the
voice coil might be at 280°F (137°C), but over the
course of 2 hours, the mechanical structure (typical 1 to
3°C/W) could rise another 200°F to 300°F (100°C to
150°C), bringing the voice coil well over the thermal
limit of its materials and adhesives. Heat transfer from
the frame and magnet to the air is another important
consideration. Although the rise time is large, the final
temperature may vary greatly due to the enclosure. A
vented enclosure with vents at the top and bottom with
no fiberglass insulation might provide adequate ventila-
tion for a hot loudspeaker. The same loudspeaker in a
closed box stuffed with fiberglass might be subject to a
dangerously high temperature rise. Attention to this
final thermal path is warranted in applications that will
demand maximum output from enclosed loudspeakers.
The efficiency of a loudspeaker has a direct bearing
on the thermal load it must withstand for a given
acoustic output level. The more efficient the loud-
speaker, the lower the self-heating for a given output
level; all else being equal, a loudspeaker with 3 dB
higher overall sensitivity for a given impedance will
experience one-half the thermal load for a desired
output level.

In concert touring use, loudspeakers are routinely
operated at and even beyond their design limits. Given
that a loudspeaker that operates at twice its voice coil
resistance due to heating will be 6 dB less sensitive,
sound quality can vary greatly over the course of a
performance. In failure situations, the nature of the
input signal will usually determine the type of failure
mode. Thermal failure can be precipitated by
compressed high-frequency content material (low
dynamic range). Mechanical failure is often due to
dynamic, percussive material, such as might occur in a
recording studio with drum channels set to solo, as well
as other signals that do not limit dynamic range.
Another cause of mechanical failure, most often in
high-frequency transducers, is the application of a
highly clipped signal that has been passed through a
high-pass filter. Such a signal will contain a
peak-to-peak voltage that is twice that of the input

signal. This phenomenon is illustrated in the section on

crossovers.

17.6.2 Heat Transfer Designs for High-Power

Woofers

Of all the components in a sound reinforcement system,

more heat is generated in low-frequency devices than in

any other. While high-frequency horn driver combina-

tions deliver 110–117 dB/1 W/1 m and midrange devices

deliver 100–110 dB, woofers rarely exceed 100 dB. A

typical woofer in a vented enclosure is in the

94–97 dB/1 W/1 m range. These devices are typically

2–8% efficient. The remaining 92–93% of the power

goes directly into producing heat. Adding to the problem

is the fact that much modern program material is

bass-heavy.

As understanding of heat transfer mechanisms in

loudspeakers grew, designs appeared that improved heat

transfer from the voice coil and gave improved thermal

power handling ratings, Fig. 17-26.

The heat transfer methods discussed here are simply

methods to transfer heat away from the voice coil. If

there were no heat transfer paths out of the magnetic cir-

cuit, the speaker’s temperature would continue to rise

without limit. In the cases of drivers on exposed horns,

natural convection transfers sufficient thermal energy to

prevent overheating. The thermal resistance of the direct

convection transfer path is on the order of 1–2qC/W. In

the case of a woofer in a fiberglass-lined enclosure, this

resistance may be five times greater. Heat buildup can

be substantial. This mechanism is often ignored. Several

proprietary loudspeaker systems have been developed in

an attempt to address this problem, but most sound rein-

forcement systems still provide no designed-in mecha-

nism for transferring heat out of the enclosures.

17.7 Radiator Types

In addition to converting electrical energy to mechan-

ical energy, a loudspeaker must include a means for

converting mechanical energy (e.g., the motion of a

diaphragm) into acoustic energy. For purposes of this

chapter, the various means for accomplishing this final

conversion are referred to as radiators. While there is

overlap in our definition of the terms transducer and

radiator, it is extremely useful in understanding loud-

speakers to consider the function of acoustic radiation

as a separate subject from electroacoustic conversion. In

general, there are two broad types of radiators: direct

radiators and horn radiators.