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midband frequency, the output signal level measured on
any level range minus the level of the conesponding
electrical input signal.
Level non-linearity. In decibels, the level difference measured
on any level range, at a stated one-third octave nominal
midband frequency, minus the corresponding reference
level difference, all input and output signals being relative
to the same reference quantity.
Background The combined present in a measure- Level range. In decibels, an operating range determined by the
ment system from sources other than the test aircraft, which setting of the controls that are provided in a measurement
can influence or obscure the aircraft noise levels being system for the recording and one-third octave band analysis
measured. Typical elements of background noise include of a sound pressure signal. The upper boundary associated
(but are not limited to): ambient noise from sources around with any particular level range shall be rounded to the
the microphone site; thermal electrical noise generated by nearest decibel.
components in the measurement system; magnetic flux
noise ("tape hiss") from analogue tape recorders; and
digitization noise caused by quantization error in digital
converters. Some elements of background noise, such as
digitization noise, can obscure the aircraft noise signal,
while others, such as ambient noise, can also contribute
energy to the measured aircraft noise signal.
Broadband noise. Noise for which the frequency spectrum is
continuous (i.e. energy is present at all frequencies in a
given range) and which lacks any discrete frequency
components (i.e. tones).
Calibration checkfiequency. In hertz, the nominal frequency
of the sinusoidal sound pressure signal produced by the
sound calibrator.
Calibration soundpressure level. In decibels, the sound pressure
level produced, under reference environmental conditions,
in the cavity of the coupler of the sound calibrator
that is used to determine the overall acoustical sensitivity of
a measurement system.
Free-field sensitivity level of a microphone system. In
decibels, twenty times the logarithm to the base ten of the
ratio of the free-field sensitivity of a microphone system
and the reference sensitivity of one volt per pascal.
Note.- The free-jeld sensitivity level of a microphone
system may be determined by subtracting the sound pressure
level (in decibels re 20 pPa) of the sound incident on the
microphone from the voltage level (in decibels re 1 at the
output of the microphone system, and adding 93.98 dB to
the result.
Linear operating range. In decibels, for a stated level range
and frequency, the range of levels of steady sinusoidal
electrical signals applied to the input of the entire measurement
system, exclusive of the microphone but including the
microphone preamplifier and any other signal-conditioning
elements that are considered to be part of the microphone
system, extending from a lower to an upper boundary, over
which the level non-linearity is within specified tolerance
limits.
Note.- It is not necessary to include microphone
extension cables as configured in the field.
Measurement system. The combination of instruments used
for the measurement of sound pressure levels, including a
sound calibrator, windscreen, microphone system, signal
recording and conditioning devices, and a one-third octavc
band analysis system.
Note.- Practical installations may include a number of
microphone systems, the outputs from which are recorded
simultaneously by a multi-channel recording/analysis device
via signal conditioners as appropriate. For the purpose ofthis
section, each complete measurement channel is considered to
be a measurement system to which the requirements apply
accordingly.
Microphone system. The components of the measurement
system which produce an electrical output signal in response
to a sound pressure input signal, and which generally
include a microphone, a preamplifier, extension cables, and
other devices as necessary.
APP 2-3 24/11/05
Annex 16 - Environmental Protection Volume I
Reference direction In degrees, the direction of sound
incidence specified by the manufacturer of the microphone,
relative to a sound incidence angle of 0°, for which the
fiee-field sensitivity level of the microphone system is
within specified tolerance limits.
Reference level diffence. In decibels, for a stated frequency,
the level Qfference measured on a level range for an electrical
input signal corresponding to the calibration sound
pressure level, adjusted as appropriate, for the level range.
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