
Air pressures are measured in Pascal (Pa), which are Newtons per square metre (N/m^{2}). The ambient air pressure is about 100,000 Pa, and the lungs modify this in phonation only by a small amount, namely 200 to 1000 Pa (= 2 to 10 cm H_{2}O). Outside your body, the air pressures caused by your speech are much smaller again, namely 0.01 to 1 Pa at 1 metre from your lips. These are comparable to the values that you see for a typical speech recording in Praat's sound window, although these numbers in Praat can be interpreted as true air pressures only if you perform a sound pressure calibration (including the multiplication).
A normative human ear can detect a rootmeansquared air pressure as small as 0.00002 Pa, for a sine wave with a frequency of 1000 Hz. The sound pressure level can be expressed in dB relative to this normative threshold:
SPL = 10 log_{10} { 1/(t_{2}t_{1}) ∫_{t1}^{t2} x^{2}(t) dt / (2·10^{5} Pa)^{2} } 
where x(t) is the sound pressure in Pa as a function of time, and t_{1} and t_{2} are the times between which the energy (squared air pressure) is averaged. For a calibrated sound, Praat's sound window will show you the SPL as a function of time, if you switch on Show intensity and set its averaging method to energy (with Intensity settings...).
© ppgb, November 24, 2004