102 Chapter 3
shortcomings and require a very carefully designed sound system in order to allow the
normally spoken word to be understood.
It is also of real interest to note that in large halls and arenas the correct place for the
loudspeaker system is most often where the roof should have gone if the building
had been designed specifi cally for hearing. A loudspeaker is therefore usually an
electroacoustic replacement for a natural refl ecting surface that has not been provided.
3.13 The Acoustic Environment Indoors
The moment we enclose the sound source, we greatly complicate the transmission of its
output. We have considered one extreme when we put the sound source in a well-elevated
position and observed the sound being totally absorbed by the “ space ” around it. Now, let
us go to the opposite extreme and imagine an enclosed space that is completely refl ective.
The sound source would put out sound energy, and none of it would be absorbed. If we
continued to put energy into the enclosure long enough, we could theoretically arrive at a
pressure that would be explosive. Human speech power is quite small. It has been stated
by Harvey Fletcher in his book Speech and Hearing in Communication that it would
take “ ... 500 people talking continuously for one year to produce enough energy to heat a
cup of tea. ” Measured at 39.37 in (3.28 ft), a typical male talker generates 67.2 dB-SPL,
or 34 μ W of power, and a typical female talker generates 64.2 dB-SPL, or 18 μ W. From
a shout at this distance (3.28 ft) to a whisper, the dB LP ranges from 86 to 26 dB, or a
dynamic range of about 60 dB. Not only does the produced sound energy tend to remain
in the enclosure (dying out slowly), but it tends to travel about in the process.
Let us now examine the essential parameters of a typical room to see what does happen.
First, an enclosed space has an internal volume ( V ), usually measured in cubic feet. Second,
it has a total boundary surface area ( S ), measured in square feet (fl oor, ceiling, two side
walls, and two end walls). Next, each of the many individual surface areas has an absorption
coeffi cient. The average absorption coeffi cient ( a ) for all the surfaces together is found by
asa s a s a
s^11 2 2 nn (3.15)where s1,2,...n are the individual boundary surface areas in square feet, a–1, 2,...n are the
individual absorption coeffi cients of the individual boundary surface areas, and S is the
total boundary surface area in square feet.