Troels Dyhr Pedersen.indd - Solid Mechanics

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11.5.3 Filtering of sample data for frequency analysis
In the frequency analysis, the frequency content originating from the compression and
combustion in the cylinder are not of interest. Neither are the lower frequencies of the
acoustic noise. These low frequencies must be blocked so that only the higher frequencies
are left. The analysis of frequency content by discrete Fourier transformation (DFT) is
however not sensitive to phase delays, so IIR filters are suitable for this purpose.
The high pass frequency for cylinder pressure must be below the lowest resonant mode in
the chamber. This frequency is easily identified in a DFT of the unfiltered signal.
The high pass frequency for the acoustic noise can be selected as preferred, but with
necessary consideration to the sound field in the room. The acoustic property of a closed
test cell generally means that low frequencies cannot be attenuated properly. The
frequencies are furthermore not of interest since they are generated by intake and exhaust
pulsations as well as engine oscillation. It is therefore necessary to eliminate the
frequency content below a certain limit in order to evaluate the sound pressure generated
by the engine as a direct consequence of the combustion event. A high pass frequency of
1 kHz was considered appropriate for the current study.
11.6 Representation of acoustic measurements
11.6.1 Sound pressure
The sound pressure is the instant difference between the stationary (atmospheric) pressure
and that measured by the deflection of a diaphragm. The unit is Pa or uPa depending on
the amplitude of the signal.
11.6.2 Sound pressure level
The sound pressure level (SPL) is measured in decibel. It is a quantity defined by a
logarithmic scale.
p
SPL = 20 log
p
prms is the root mean square (RMS) of the sound pressure. The RMS value must be
obtained in a short term interval for transient noise. pref is the reference sound pressure at
the threshold of human hearing, 20 μPa. The reference value for acoustic measurements
is the threshold of hearing for a normal person.
The handheld device was calibrated with a piston calibrator that accurately produces 1
kHz with a sound pressure of 1 Pa, corresponding to 94 dB. The sensitivity was then
determined by measuring the rms value of the AC output signal. The unit for sensitivity is
V/Pa.
In many applications, weighting of the frequency content is applied, usually a -
weighting, which accounts for the sensitivity of the human hearing. This weighting was
rms
ref
[ dB]