NTi Audio Acoustilyzer AL1 - Manual (Page 16)

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Input Filters
Depending on the selected measurement function the following sets of
filters are available:
SPL/RTA function:
Filters Applications
FLAT
Flat frequency response
(no filtering)
Overall sound pressure level; all
sound signal components are included
without weighting - required for special
applications.
A-WTD
A-weighting filter in
accordance with IEC 61672
Applicable to most common sound
pressure levels if level weighting
compensation for human loudness
perception is required.
C-WTD
C-weighting filter in
accordance with IEC 61672
Applicable to higher sound levels if level
weighting compensation for human
loudness perception is required.
X-CRV -1
Inverted X-Curve filter in
accordance with ISO 2969
Required for cinema installations
RLB (Revised Low-
frequency B-curve)
Broadcast Loudness
filter in accordance with
AES Paper “Loudness
Assessment of Music and
Speech” 2004
Applicable for loudness measurement
of broadcast material. The revised
B-weighted Leq measurement (RLB)
has the best correlation to the subjective
experience of humans.