Audio Engineering

Acoustic Environment 101

The Greeks built their amphitheaters to take advantage of these acoustical facts:

1. They provided a back refl ector for the performer.


2. They increased the talker’s acoustic output by building megaphones into
   the special face masks they held in front of their faces to portray various
   emotions.


3. They sloped the audiences upward and around the talker at an included angle of
   approximately 120°, realizing, as many modern designers do not seem to, that
   humans do not talk out of the back of their heads.


4. They defocused the refl ective “ slapback ” by changing the radius at the edges of the
   seating area.

Because there were no aircraft, cars, motorcycles, air conditioners, and so on, the ambient
noise levels were relatively low, and large audiences were able to enjoy the performances.
They had discovered absorption and used jars partially fi lled with ashes (as tuned
Helmholtz resonators) to reduce the return echo of the curved stepped seats back to the
performers. It remained only for some unnamed innovative genius to provide walls and
a roof to have the fi rst auditorium, “ a place to hear ” ( Figure 3.12 ). No enhancement of
sound is provided in Figure 3.12 because there is no reverberation in a room whose walls
are highly sound absorbent.

Sometimes acoustic progress was backward. For example, the Romans, when adopting
Christianity, took over the ancient echo-ridden pagan temples and had to convert the
spoken service into a chanted or sung service pitched to the predominant room modes
of these large, hard structures. Today, churches still often have serious acoustical

16 8

Arbitrary loudness units

Sound absorbent

ceiling and walls

Figure 3.12 : Means of eliminating noise and weather while preserving outdoor conditions.