Audio Engineering

110 Chapter 3

the sound. This kind of diagram is called a refl ection pattern. The initial-signal-delay gap
can be measured from it.

Figure 3.14 illustrates the decay of the reverberant fi eld. Here the direct sound enters at
the left of the diagram. The initial-signal-delay gap is followed by a succession of sound
refl ections. The reverberation time of the room is defi ned as the length of time required
for the reverberant sound to decay 60 dB.

We will encounter the effects of delay versus attenuation again when we approach the
calculation of articulation losses of consonants in speech.

Figure 3.15 shows measurements from an analyzer made in both large and small rooms.
Figure 3.16 shows that the sound arriving at the listener has at least three distinct divisions:

1. The direct sound level LD.


2. The early refl ections level LRE.


3. The reverberant sound level LR.

The direct sound, by defi nition, undergoes no refl ections and follows inverse-square-
law level change. The reverberant sound tends to remain at a constant level if the sound

Figure 3.15 : Vivid proof that there is a fundamental difference between a small reverberant

space and a large reverberant hall.

(Continued)

(a) Envelope Time Curve (ETC) of a small room

showing lack of a dense field of reflections

6 dB

Horizontal : 0 msec or 0 ft

T 1 T 2

(b) Small room without reverberant sound field but

with room modes

6 dB

Horizontal : 20.35–9868.43 Hz

T 1

T 2