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SOUND PRESSURE LEVEL (SPL) MEASUREMENT
One of the most important is-
sues to be tackled when dealing
with sound as a physical pheno-
menon is the calculation of its
intensity. Sound pressure me-
asured in free-field conditions
is always expressed in dB SPL,
(that is to say decibels calculated
at a specific Sound Pressure Le-
vel. In order to ensure good intel-
ligibility of speech, the average
sound pressure level of the mes-
sage must exceed the existing
ambient noise level by at least 6
to 10 dB. In order to design the
acoustics properly, therefore, it is
essential to know the noise level
in the area in which sound is to
be broadcast. The table shown
here provides the approximate
average levels found in normal
areas, both indoors and out of
doors.
The sensitivity (or efficiency) of a loudspeaker is the sound pressure
that the speaker unit produces at a distance of one metre when it ab-
sorbs a power equal to 1 Watt. The sound level produced by a speaker
unit lowers as the distance from the listening point increases. Theo-
retically, if the absorption effect due to the environment is ignored, it
can be stated that the effect of doubling the distance will contribute
to attenuating the sound pressure level by -6 dB (Fig. 7). In any case,
the sound pressure of a loudspeaker
SPL
m
at a given distance
D
m
is
given by the following formula:
SPL
m
= SPL
max
– 20 log (D
m
)
where
SPL
max
is the sound pressure at a distance of one metre.
Another essential parameter of sound pressure is the power absorbed
by the speaker unit. Again in this case, there is a proportional rela-
tionship that can be summarised as follows: each time the electrical
power of the emitter doubles, the sound pressure increases by 3 dB
(likewise, each time the sound pressure is halved, the sound pressu-
re decreases by 3 dB). The exact formula for calculating the sound
pressure
SPL
max
of a speaker unit with a power
P
applied to it is as
follows:
SPL
max
= S + 10 log (P)
where
S
is the sensitivity (efficiency) of the speaker unit.
To conclude, it can be said that the efficiency of a speaker unit is
the most important parameter for sizing an audio system. The greater
the efficiency, the less power has to be applied in order to achieve
the same sound pressure, this being the aspect to be considered for
achieving the required intelligibility.
ENVIRONMENTS
Noise (dB)
Sports stadium
85÷95
Railway station
70÷80
Workshop
60÷70
Gym
60÷70
Beach
50÷70
Swimming pool
50÷60
Conference hall
50÷60
Supermarket
50÷60
Car park
50÷60
Restaurant
45÷65
Offices
45÷55
Gardens
40÷50
Hotel lobby
40÷50
Schoolroom
30÷50
Theatre
30÷50
Church
30÷50
Shop
30÷50
Hotel room
30÷45
Hospital side ward
30÷45
Fig.7
Connection of the speaker units
Constant-impedance system
This type of connection is normally used for systems based on a limi-
ted number of speaker units or for hi-fi systems, with distribution lines
not longer than a few dozen metres. With this type of connection, in
order for the amplifiers to drive the speaker units at their rated output
power, the total power must be equal to the output power of the actual
amplifier. Amplifiers normally have three standard output impedances,
i.e. 4, 8 and 16 Ω, and these values are marked on the output termi-
nals. It is therefore necessary that the technician be able to determine
– sometimes by means of complex calculations – the total impedance
of a number of units, regardless of how they are connected (parallel,
serial or mixed serial and parallel connections).
Constant-voltage system
The advantages introduced by systems featuring constant-voltage
connections are so many that it is the ideal system for sound-broadca-
sting systems of any size. This connecting system requires each spea-
ker unit to have its own line transformer, which adapts the impedance
of the loudspeaker (which is usually low: 4, 8 or 16 Ω) to the far higher
impedance of the actual line. Unlike constant-impedance connection
systems, in which it is the loudspeaker that is the load for the ampli-
fier, in a constant-voltage system it is the transformer (connected to
the loudspeaker), with its high impedance, that constitutes an almost
constant load for the booster. Each amplifier has its own transformer
featuring constant-voltage outputs, which have now become standar-
dised at 50, 70 and 100 V (high impedance). All the loudspeakers are
connected in parallel to the output of the booster. Thus, should expan-
sion of the system become necessary (and provided an amplifier with
a higher than strictly necessary output power was chosen at the time
of the original installation), this will be extremely simple to accomplish,
branching out from any of the speaker units installed beforehand. It
is assumed that both the amplifier (that is to say its output power)
and the type of speaker unit , with its power absorption, have been
defined. If this is so, the maximum number of speaker units that can
be connected to the line units is determined according to the following
formula:
number of loudspeakers = amplifier power / loudspeaker power
In more general cases, in which the speaker units are of several dif-
ferent types and/or are connected with different power outputs, it is
always important to check that the overall power required by the spe-
aker units (obtained simply by working out the sum of the power of
the single units) is lower than the rated power output of the amplifier.
Fig. 7
A few rules
Public Address Systems
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