Page 184 - The Master Handbook Of Acoustics

159
REVERBERATION

the world. 8,9 At a given seat, the direct
sound arrives first because it follows the Arrival time
gap
shortest path. Shortly after the direct
sound, the reverberant sound arrives. Direct
The time between the two is called the
Reflected
arrival time gap or early time gap as
shown in Fig. 7-21. If this gap is less than
40 or 50 msec, the ear integrates the Rel. level - dB
direct and the reverberant sound suc-
cessfully. This gap is important in
recorded music because it is the cue that
gives the ear information on the size of
the hall. In addition to all of the delays Time
responsible for achieving echo density,
FIGURE 7-21
the initial time-delay gap is yet another
important delay that must be included in The introduction of an initial time-delay gap in the
artificial reverberation adds to its lifelike character.
digital reverberators.

The Sabine Equation

Sabine’s reverberation equation was developed at the turn of the cen-
tury in a strictly empirical fashion. He had several rooms at his dis-
posal and by adding or removing seat cushions of a uniform kind he
established the following relationship (adapted from the metric units
he used):
0.049 V
RT60 (7-1)
Sa
where
RT60 reverberation time, seconds
V volume of room, cu ft
S total surface area of room, sq ft
a average absorption coefficient of room surfaces
Sa total absorption, sabins

14
Young has pointed out that the absorption coefficients published
by materials manufacturers (such as the list in the appendix) are
Sabine coefficients and can be applied directly in the Sabine equation.
After a thorough study of the historical development of the offshoots
of Sabine’s work, he recommends that Equation 7-1 be used for all