866 Chapter 29
where A is the total sound absorption in m^2 , which is computed by multiplying each room
surface by its sound absorption coeffi cient and summing these to give the total absorption
present, or
RT
lV
Sa016.
,
where a is the average absorption coeffi cient.
In acoustically dead rooms, the following Eyring formula should be used:
RT
lV
SaMVe016
1
.
log,
()
where V is the volume of the room in m^3 and M is an air absorption constant.
Achieving a uniform room reverberation time (absorption) characteristic does not
necessarily ensure good acoustics; the effect and control of specifi c refl ections must also
be fully taken into account. For example, strong refl ections can strongly interfere with
the recorded or perceived live sound, causing both tonal colorations and large frequency
response irregularities. Such refl ections can be caused by poorly treated room surfaces,
by large areas of glazing, by off doors, or by large pieces of studio equipment including
the mixing console itself.
Figure 29.14 illustrates the effect well, being a frequency response plot of a monitor
loudspeaker (with a normally very fl at response characteristic) mounted near to a
refl ective side wall.
Apart from causing gross frequency response irregularities and colorations, side wall
refl ections also severely interfere with stereo imaging precision and clarity. Modern
studio designs go to considerable lengths to avoid such problems by either building the
monitor loudspeakers into, but decoupled from, the structure and treating adjacent areas
with highly absorbing material or by locating the monitors well away from the room
walls and again treating any local surfaces. Near fi eld monitoring overcomes many of
these problems, but refl ections from the mixing console itself can produce comb fi ltering
irregularities. However, hoods or careful design of console-speaker geometry/layout can
be used to help overcome this.