Theatre Design & Technology - Summer 1983 - 22

Fundamentals of Loudspeakers
'ollIi/lued from page 10

Jower balance between woofer and
weeter very exactly. Also by having
ieparate power amplifiers, any harmonc distortion products generated in the
ow-frequency amplifier are unable to
find their way into the tweeter. Thus,
Nith high-power operation, the highfrequency sounds tend to remain very
TIuch cleaner than with a high-level
:rossover.
Modern built-in systems for high
Jower operation almost always use lowlevel crossovers and bi-amplification.
Even three- and four-way systems are
beginning to be more common. Portable loudspeakers in the 100 to 200 watt
range usually employ high-level crossJvers for economy and for ease of hookup (only two wires rather than the four
required by bi-amping).
Amplifier Power
There is much confusion about
matching the loudspeaker and the amplifier. A full treatment of the subject
would require another article. But there
are two basic factors that must be
:hecked for compatibility: load impedance and power ratings.
Almost all modern solid-state amplifiers will operate with any load impedance from 4 ohms up. A few will operate into 2 ohms. Most loudspeakers
are rated at 8 ohms so anyone loudspeaker should be compatible with any
amplifier. Two 8-ohm loudspeakers
connected together will have a load of 4
ohms. Again, this should work with
most amplifiers. Three 8-ohm loudspeakers would have a load equal to 8
di\·ided by 3 or 2.7 ohms, \\·hich would
probably overload most amplifiers at
full power. A safe rule is to connect no
more than two loudspeakers to anyone
amplifier. (These rules apply only to
modern, solid-state, direct-output amplifiers. They do not apply to a 70-volt
distribution system, which is an oldfashioned world unto itself.)
How much power will a loudspeaker
take? What does the specification mean
when it says 100 watts mu ic power? or
50 watts rms? or sinewave? or shaped
noise? Different manufacturers rate
their loudspeakers in different ways because there are not yet any industry standards and there are real differences of
opinion on how (he rating should be
measured. If you have a question about

a particular loudspeaker, call the manufacturer's home office and ask to speak
with an applications engineer. He or she
can tell you in detail how the loudspeaker is rated and how you should use it.
A good guide is your ears. If it
sounds distorted, turn the power down.
Distortion is a sure warning that you are
pressing failure.
There is a saying that goes: Use an
amplifier equal to or less than the loudspeaker's rated power and all will be
safe. Don't believe it. By Q\·erdriving an
amplifier-called clipping-the tweeter
can be more damaged by the clipped
signal than by a clean signal of twice or
three times the power rating. Clipping
in the amplifier is often death to highfrequency drivers. The fact that clipping
is so dangerous is the reason many, if
not most, professional amplifiers have
clip indicators to warn of dangerous
operation.
So two easy rules for long loudspeaker life are don't clip the amplifier
and don't drive the loudspeakers to distortion. A well-designed system should
have an electronic limiter to protect
against accidents and inattentive operators. The limiter can keep the amplifiers
out of clipping and prevent overload of
the loudspeakers. When properly adjusted, the limiter will not be acting in
the "normal" program range. A good
limiter can be had for as little as Si400about the same cost as repairs on a
couple of compression drivers.
A word about fuses to protect your
loudspeakers-don't bother. They
don't work reliably. They sometimes
blow on loud but harmless signals and
other times fail to blow on fatal blasts.
Get a limiter, not fuses.
Attributes of Interest
The following reviews the major attributes of interest in a loudspeaker and
looks at what is usually listed on a specificat ion sheet.
Natural Sound
An evaluation of natural ound can
only be made by your ears. Does it
have a smooth mu ical sound? Does the
transient response allow a gunshot to
sound real? 0 set of numbers can gi\·e
a true appreciation of the sound of a
loudspeaker.
seful Frequency Range
On the high end, contrary to what
the hi-fi buffs think, 10,000 or 12,000
Hz are all that is needed, and that is all

there is going to be at high levels in a
large space. On the low end, it depends
upon purpose. For voice-only, 150 Hz is
fine. For a stage monitor, there is no
need to go belo\\' about 75 Hz. For recorded music, 40 to 50 Hz is all you will
be getting off commercial recordings.
For li\·e reinforcement, the lowest note
on a string bass is about 42 Hz; but if
electronic music is to be reproduced,
then 30 Hz is not too low. Remember
that bottom octave (from 30 to 60 Hz) is
costly in dollars, box size, and amplifier
power.
Another important point is to note
how the response range is stated. Does
the specification sheet give a range of
variation such as ± 3 dB or ± 5 dB?; or
does it just say a "useful range." "Useful range" usually means ± 10 dB or
more; a better description would be a
"barely useful" range.
Coverage Pattern
The coverage pattern is measured between the - 6 dB points on the polar response curves at various frequencies.
The most important band is between
1000 and 4000 Hz. The loudspeaker
should be positioned and aimed at the
audience based on the average beam
width and height in this band.
Power Handling Capacity
This rating is likely to be fairly
meaningless unless there is some explanation of how it is derived. The amplifier may be as large as the long-term
average power rating of the loudspeaker
and may be larger if used with care. It
is important to note that power alone
does not give a good indication of maxim um loudness and cannot be used in
comparing loudspeakers without simultaneous consideration of sensitivity.
Sensitivity
Sensitivity is a measure of a loudspeaker's ability to convert electrical
energy into acoustical energy. Sensitivity is usually expressed as a sound pressure level at a stated distance (generally 4 feet or I meter) when the loudspeaker is receiving a power of I watt.
(See insert block for procedure in applying sensitivity and power ratings to
calculate maximum loudness from a
loudspeaker.)
c=;
Rollins Brook IS a member of the
acoustical firm of Bo(t, Beranek and
e\\"man and an associate editor of
TD&T

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