User Guide

Sound System Design Reference Manual
The last point is illustrated graphically in Figure
6-12, adapted from the Peutz paper. Each of the
diagonal lines corresponds to a particular
reverberation time. Each shelves at a point
corresponding to a direct-to-reverberant sound ratio
of -10 dB. Note that the shelf may lie above or below
the 15% figure depending upon the reverberation
time of the room. This agrees with other published
information on intelligibility. For example, Rettinger
points out that in rooms having a reverberation time
of 1.25 seconds or less, direct sound and early
reflections always make up the greater portion of the
total sound field. Intelligibility in such rooms is good
regardless of the direct-to-reverberant sound ratio at
any given listening position. Conversely, anyone who
has worked in extremely large reverberant spaces
such as swimming pools or gymnasiums knows that
intelligibility deteriorates rapidly at any point much
beyond the critical distance. According to the chart, a
15% articulation loss of consonants in a room having
a reverberation time of 5 seconds corresponds to a
direct-to-reverberant sound ratio of only - 5.5 dB.
Problems associated with speech intelligibility
in enclosed spaces have received a great deal of
attention prior to the publication of the Peutz paper.
The virtue of Peutz’ method for estimating speech
intelligibility is its simplicity. It must be remembered,
however, that a number of contributing factors are
ignored in this one simple calculation. The chart
assumes that satisfactory loudness can be achieved
and that there is no problem with interference from
ambient noise. It also postulates a single source of
sound and a well behaved, diffuse reverberant sound
field.
The data from the Peutz paper have been
recharted in a form more convenient for the sound
contractor in Figure 6-13. Here we have arbitrarily
labeled the estimated intelligibility of a talker or a
sound system as “satisfactory”, “good”, or “excellent”,
depending upon the calculated articulation loss of
consonants.
There often is a dramatic difference in the
acoustical properties of a room depending upon the
size of the audience. Calculations should be made
on the basis of the “worst case” condition. In some
highly reverberant churches particularly, it may turn
out that there is no practical way to achieve good
intelligibility through the entire seating area when the
church is almost empty. The solution may involve
acoustical treatment to lessen the difference between
a full and an empty church, or it may involve a fairly
sophisticated sound system design in which
reinforced sound is delivered only to the forward
pews when the congregation is small (presuming that
a small congregation can be coaxed into the forward
pews).
Figure 6-12. Probable articulation loss of consonants vs.
reverberation time & direct-to-reverberant sound ratio
6-12