L-Acoustics V-DOSC, Operator'S Manual

Page: 126 / 158

L-ACOUSTICS V-DOSC Manual Version 4 6/29/2005 Page 126 of 158
More sophisticated measurement instruments can also provide additional information related to
room acoustics that can be useful for the tuning process – for example, RT
60
at various frequencies
and waterfall plots that help identify low frequency resonances for a given room.
Measurement Tips
Performing accurate measurements and interpreting them correctly requires being aware of a few
potential problems. Here are some typical ones:
Measure one source at a time. Measuring two sources radiating the same signal
simultaneously will display interference cancellations which occur due to path length
differences (unless the microphone is exactly equi-distant from both sources).
Place the microphone on the floor. When the measurement microphone is mounted on a
stand or near reflecting surfaces, the frequency response will show a cancellation in the low-
mid frequencies due to the path difference between the direct signal and the reflected signal
(which arrives a few milliseconds later). The resulting frequency response cancellations are
not due to the system and should not be equalized. More sophisticated measurement
systems such as MLSSA, WinMLS and TDS allow the user to apply a time window to remove
such reflections from the measurement, however, this can be at the expense of low
frequency resolution (depending on the length of the time window).
When placing the microphone on the floor, if the floor is absorbent (e.g., thick carpet or
grass in open-air situations) the measured response may display a high-frequency loss – in
this case, a sheet of plywood can be useful for reducing this effect on your measurements.
Time aligning the “sine wave characteristics" of the sub and low section impulse responses
provides the best sub/low summation results (see Section 1.15). Measurement systems such
as MLSSA or WinMLS are recommended for this purpose since they measure the impulse
response directly in the time domain and it is possible to obtain a clean impulse response
even in highly reverberant indoor environments.
Equalizing subwoofers is difficult since measurements taken at a single location can be
misleading. Indoors, there are room modes to consider and you may be located in a
pressure null or maximum depending on the location and the frequency. Be sure to verify
the effect of your adjustments throughout the audience by walking the room and performing
spatial averaging.
Watch out for wind effects and be sure that measurements are stable and repeatable at high
frequencies.