This 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, pa=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 log10 (E) [units: dB]
A-weighted (dBA) spectrum
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.