Figure 1: The
big picture.
Overmeasuring
a room’s decay
time produces
better signal-to-
noise ratio and
increased frequency
resolution. (Figure
1-5 measurements
with Smaart.)
Figure 2: Zooming
in on the time
record reveals
that the room
is live (specular
reflections) but
not reverberant
(absence of
dense tail).
Figure 3: In contrast
to Figure 2, this
room has a dense
tail that decays over
several seconds.
The designer
must consider
the build-up of
reverberation in the
space.
Figure 4: A
frequency-domain
view of Figure 2
shows good modal
density above 100
Hz. This is a large
room for most of the
audible spectrum.
SMALL ROOM
CHARACTERISTICS
When the impulse response is viewed in
the frequency domain, the modal density
of the space is easily observed. On a log
scale, modes tend to be widely spaced at
low frequencies, with increasing density
as frequency increases. The objective is
to observe the region in which the modes
become sufficiently dense to overlap and
produce a smoother response.
Figure 4 shows the impulse response
of a 1,200-seat auditorium after having been transformed to the frequency
domain via the Fast Fourier Transform
(FF T) – a feature found on any analyzer
that can measure the impulse response.
Note that the modes get pretty
dense by 100 Hz. This means that they
are unlikely to produce significant coloration in the vocal range, but could
significantly affect the sound of a subwoofer or an organ pedal note.
In a smaller space, this transition
region shifts up in frequency and more
of the spectrum can be adversely affected
by room modes. Good room geometry is
the first line of defense against poor low
frequency performance.
Treatment is necessary in severe cases
– a good time to bring in an acoustical
consultant. (You do have an acoustical
consultant, right?)
carefully planned treatment specified
by your acoustical consultant.
An acoustically large space is one
in which the wavelengths of the sound
waves are small relative to the spacing
between surfaces. This is true of most
rooms at high frequencies.
AMBIENT NOISE
Noisy rooms produce a number of problems for sound reinforcement, depending on the use of the space. Noise forces
the system to be operated at excessive
levels, which reduces its stability and
forces close-mic’ing of sound sources.
The main offenders regarding noise are
