A Series Active Monitor Speakers
Date printed: 29.02.00 Service E4/37
bly smoothed by one technique or another to improve measurement repeat-
ability and reduce the visible effect of reflections to a value more consistent
with a human’s ability to hear them. Even simple analog measurement tech-
niques, where no “smoother” appears as such on a block diagram, do in fact
typically smooth due to the sweep rates and measurement response times
used. Measurement instruments have some finite time constant or response
time; they cannot instantaneously follow a rapidly-changing signal ampli-
tude. If the measurement instrument has frequency selectivity (spectrum
analyzer or tracking filter), the response time is even slower due to the en-
ergy storage or ringing effect of the tuned filter. When the stimulus signal is
swept rapidly, the measurement instrument simply cannot follow the more
rapid, extreme variations and the displayed result is smoothed – peaks are
reduced and dips are filled in.
4.6.3 MLS Technology for Frequency Response Measurements
Loudspeaker frequency response measurement under free-space conditions
without requiring an anechoic room is an important application of Maxi-
mum Length Sequence (MLS) test systems (such as Audio Precision™
System One + DSP™ or System One Dual Domain™, both using the
MLS.TST program, or MLSSA™, a PC-based audio test system).
Nevertheless, the setup dimensions given in sections 4.3.2.2, 4.4.2.2, and
4.5.2.2 must be strictly followed.
MLS testing permits time-selective measurements in which one signal, such
as the direct sound from a loudspeaker, may be separated from another
similar signal, such as a room reflection. The time window may be adjusted
to allow measurement of any arrival in a complex reverberation pattern.
These signals may be examined in the time domain (showing “energy” as a
function of time) or in the frequency domain (amplitude and phase vs fre-
quency). Impulse responses may be saved for further analysis.
A Maximum Length Sequence (MLS) is a special digital noise signal. For
typical loudspeaker measurement applications, this digital signal is con-
verted to the analog domain in the D/A converter and fed to a power ampli-
fier which drives the loudspeaker under test. A measurement microphone is
used to pick up the acoustical signal and return it to the analog input. The
MLS system then performs a cross-correlation between the received and
transmitted signals to obtain the impulse response which is stored in mem-
ory. The impulse response may be displayed on the computer, permitting the
user to select the portion of impulse response of interest. This portion may
be transformed into the frequency domain to study both magnitude and
phase response versus frequency.
These properties are of obvious use when measurement loudspeakers or
other electroacoustic devices. The time-selective capability permits separat-
ing the device-under-test response from that of the room in which the meas-
urements are made. Alternately, the room itself may be measured, studying
the reflection characteristics of each surface in the room or of the room
taken as a whole.
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