NTi Audio Acoustilyzer AL1 - Manual (Page 17)

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RMS/THD 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. For example,
measuring residual noise of a device in
sound broadcasting applications.
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 - for
special applications.
HP400
Highpass 400 Hz in
accordance with DIN 45045,
-120dB/dec.
Removes any mains frequency (50/60Hz)
components from the test signal.
HP19k
Highpass 19 kHz
Removes any low frequency components
from the test signal, e.g. to measure
critical public announcements systems
with no audible change to the signal
(using a 20kHz pilot tone level)