BASIC GUIDELINES
FOR THE USE OF HORNS AND DRIVERS WITHIN A SOUND SYSTEM.
DESIGNING
FOR INTELLIGIBILITY AND ADEQUATE SPL
The Basic
Many sound systems would have better performance
if
the following basic
principles are kept in mind Speakers
the appropriatecoverage patterns
should bechosen, aimed and powered to
uniform direct field in the
highly absorbtive audience, with no sound aimed at the reflective wall and ceil-
ing surfaces Where multiple speakers are required in order to achieve a
uniformdirect field, their coverage patterns should beonlyslightly overlapped,
so that each section of the audience is covered by a single speaker To the
extent this ideal is achieved reverberation is minimized and intelligibility is
maximized
The following
explains these concepts in more detail and illustrates
two design approaches
What is Reverberation?
Reverberation is the persistence of sound within an enclosure, such asa room,
after the original sound has ceased Reverberation may also be considered
as aseries of multiple echoes so closely spaced in time that they merge into
a single continuous sound These echoes decrease in level with successive
reflections, and eventually are completely absorbed by the room
Non-Reverberant Environments
An open, outdoor space is considered
to
be a non-reverberant environment,
as virtually all sound escapes the area without reflection.
Variations in Level Due to Distance for Non-Reverberant Environments
In non-reverberant environments, such as outdoors, sound pressure level will
be reduced by half
(6
every
the distance form thespeaker is doubled
(this is called the inverse-square law). Figure
A
shows the
losses
to
be
expected as distance from the speaker is increased from the one-meter
(3 28-foot) measuring distance typically used in SPL specifications.
Reverberant Environments
Wheresound is reflectedfrom
other surfaces, thereisapoint beyond
which the "reverberant field" dominates and the sound pressure level is
higher and more constant than predicted by using the inverse-square law
alone
Variations in Level Due
to
Distance for Reverberant Environments
The reverberant field will begin todominatetypicallyatdistancesof
feet This distance is greatest for the least reverberant rooms and speakers
with narrow beamwidth angles The frequency and beamwidth
provided by the datasheet are still required
to
obtain satisfactory distribution
of the direct sound (or direct field) from the
which still follows
the inverse square law
It
is the direct signal that contributes
to
speech
intelligibility This is why the sound system designer should seek a uniform
directfield, with
reverberantfieldas possible For example, consider
a single speaker with awide beamwidth angle used
to
cover a long, narrow
reverberant room The direct field will be
so far below the reverberant field at
the
the room that speech will probably be unintelligible
Calculating Variations in Level Due to Changes in Electrical Power
Each time the power delivered
to
the speaker is reduced by one-half, a level
Thenomographof
to be expected as the power varies from the one-watt input typically used in
SPL specifications
Power Handling
The power rating of a speaker must be known to determine whethera design
is capable of meeting the sound pressure level requirements of the system
The power rating combined with the sensitivity will enable a system designer
to calculate the maximum sound pressure level attainable at a given distance
DISTANCE FROM
CHANGE IN
FIGURE
A
FIGURE
Level Variation
Level Variation
With Distance
With Power
Powering to Achieve Both Average and Peak SPL
The average power that must be delivered to the
to achieve the
desired average SPL can be determined from the previously presented
material on speaker sensitivity, level variation with distance and level varia-
tion with power. Enough additional power must be available to reproduce
without distortion the short-term peaks that exist in voice and music program.
Thisdifference between the
averagecapability of a sound system,
when expressed in
is often called "peak-to-average ratio," "crest factor"
or "headroom." The peaks can be large, as noted earlier: at least
times
the average
(1
0
The better sound
designed for peaks that are 10
above the
average, although 6
of headroom is sufficient for most general-purpose
voice paging systems The
1
peaks require amplifier power ten
that
requiredfor theaveragesound levels The 6-dB peaks require fourtimesthe
power.
Utilizing Speaker Beamwidth Information for Maximum Intelligibility
Knowing the beamwidth angle
of a loudspeaker can aid in providing a
direct field in the listening area After selecting a desired speaker location, the
beamwidth angle needed
to
adequately cover the listeners without spilling
over
to the walls or ceilings must be determined Once these angles are
known the correct speaker can be found by using catalog specifications
Using Easy-VAMP
TM
and Floor-Plan Isobars
In some circumstances is desirable
to
use an approach that is more detailed
than using the basic horizontal and vertical beamwidth angles Environments
which have excessive reverberation or high ambient noise levels make
it
especially difficult to achieve the desired SPL and intelligibility
In recent years a number of computer based techniques have been
developed
to help sound system designers Some of the more complex
systems use personal computers with relatively sophisticated graphics
Simpler systems, such as Electro Voice VAMP
TM
(Very Accurate Mapping
Program) utilize clear overlays and require programmable scientific
calculators However, the
and training investment required
to
utilizeeven thesimpler systems are not attractivetosomesound systems
designers Because of this University Sound has developed a special
adaptation of VAMP, called Easy VAMP
TM
which providesa similar design
aid without the complexity and cost of the VAMP programs
More information
on
both the Easy VAMP
TM
and floor plan isobars can be
found in the University Sound Guide
Part Number 531291-923
in
US 42-02-0261 17
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