Mach Wave Elimination
A new
method for quieting high-speed jets
(U.S.
Patents 5590520 and 5916127)
NOISE DEFINITIONSThis is a brief
overview of the various measures of noise and annoyance used in our study.
The microphone output is a time trace of voltage, V(t). The
voltage is prortional to the pressure fluctuation p'(t) via a constant
provided by the manufacturer. The pressure flutuation p'(t) is
normalized by a commonly-used reference pressure, p_{a}=20 mPa. The spectrum of the normalized pressure fluctuation,
S(f), is computed using a fast Fourier transform (FFT) algorithm.
The spectrum is corrected for the response of the microphone and for
atmospheric absorption. In our experiments, the resolved
frequency f ranges from 500 Hz to 150 kHz.The following noise metrics
are then computed: Sound Pressure Level (SPL) Spectrum Overall Sound Pressure Level (OASPL) E = ò S(f) df OASPL
= 10 log_{10 }(E) [units: dB] A-weighted (dBA) spectrum
PNL PNL
(Perceived Noise Level) and its variants are metrics used for aircraft
noise certification. For an overview
of the PNL calculation the reader is referred to this web page. To compute the PNL, the spectra are
extrapolated to higher frequencies using a drop of 30 dB per frequency decade.
The extrapolation allows resolution of scaled-up frequencies up to 10,000 Hz
necessary for computation of PNL. For the scale-up factors used here (50-
100) the calculated PNL is very insensitive on this extrapolation. |