Handbook for Sound Engineers

Acoustics for Auditoriums and Concert Halls 179

restrictive role for the possible mass of the reflectors.
For spoken performances an area-related mass of 5 to
7 kg/m^2 (0.2 to 0.3 lbs/ft^2 ) may still produce acceptable
results, to which effect plastic mats of high surface
density are suitable. The additional room-acoustical
measure usually employed for enhancing the sound

reflection of bass instruments with music performances

consists in appropriate wall surfaces, so that the installa-

tion of heavy panels can be abandoned. In this case an

area-related mass of 20 kg/m^2 (0.8 lbs/ft^2 ) is sufficient.

If a multiple reflection occurs close to edges of

surfaces, there results, if the edge is at right angle to the

surface, a sound reflection with a path parallel to the

direction of the sound incidence, Fig. 7-40. In corners,

this effect acquires a 3D nature, so that the sound

always gets reflected to its source, independently of the

angle of incidence. With long travel paths it is possible

that very disturbing sound reflections are caused at

built-in doors, lighting stations, setoffs in wall paneling,

which for the primary structure of a room are known as

“theater echo” (see Section 7.3.3.2.2).

7.3.4.2 Sound Reflection at Smooth Curved Surfaces

If the linear dimensions of smooth curved surfaces are

much bigger than the wavelength of the effective sound

components, the sound is reflected from these surfaces

according to the laws of concentrating reflectors.

Concavely curved 2D or 3D surface elements may,

under certain geometrical conditions, lead to sound

concentrations while convex curvatures always have a

sound scattering effect.

For axis-near reflection areas (incident angle less

than 45°) of a surface curved around the center of

curvature M, it is possible to derive the following

important reflection variants, Fig. 7-41.

Circular Effect. The sound source is located in the

center of curvature M of the reflecting surface, Fig.

7-41A. All irradiated sound rays become concentrated

in M after having covered the radius twice, so that a

speaker may, for instance, be heavily disturbed by his

own speech.

Figure 7-38. Geometrical sound reflections.

Figure 7-39. Minimum size of surfaces for geometrical

sound reflections.

a (^1) a 2
AB
b
Sound source
Listener
Figure 7-40. Multiple reflection in room edges.