Getting to Grips with Aircraft Noise
Getting to Grips with Aircraft Noise
Getting to Grips with Aircraft Noise
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9 - A BIT OF THEORY<br />
9.3.2. NOTION OF SOUND PRESSURE LEVEL (SPL)<br />
The effective pressure (as defined in equation 9.2.1-2) is linked <strong>to</strong> the sound intensity<br />
by the following equation:<br />
2<br />
pe<br />
I =<br />
ρρρρ c<br />
62<br />
∞<br />
It is worth noticing that the effective pressure is the easiest quantity <strong>to</strong> measure, as<br />
most of the microphones are sensitive <strong>to</strong> air pressure variations. Consequently, it is very<br />
practical <strong>to</strong> define a quantity in the same philosophy as for the SIL, called the Sound<br />
Pressure Level (SPL):<br />
Where: 0<br />
2 ⎛ p ⎞<br />
e<br />
SPL = 10.<br />
log<br />
⎜⎜<br />
2 ⎟⎟<br />
⎝ p0<br />
⎠<br />
in decibel (dB) (9.3.2-1)<br />
p is the reference pressure of 2.10 -5 N/m 2 .<br />
The value of the pressure is squared in order <strong>to</strong> get the homogeneity <strong>with</strong> intensity,<br />
since this what the human ear is sensitive <strong>to</strong>.<br />
The relationship between SIL and SPL can then be expressed:<br />
⎛ I ⎞<br />
0<br />
SPL = SIL + 10 . log ρρρρ<br />
⎜⎜<br />
2 ∞c<br />
(9.3.2-2)<br />
⎟⎟<br />
⎝ p0<br />
⎠<br />
At sea level in ISA conditions, SPL=SIL+0.2.<br />
Flight Operations Support & Line Assistance<br />
<strong>Getting</strong> <strong>to</strong> grips <strong>with</strong> aircraft noise
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