Click here to purchase the entire book in PDF format. dBspl The dBspl is a measurement of sound pressure (spl stand for Sound Pressure Level). What you do is take a measurement of, say, the sound pressure of a jet at takeoff (measured in Pa). This provides Pressure1. Our reference Pressure2 is given as the sound pressure of the softest sound you can hear, which we have already said is $20 * 10^{-6}$ Pa. Let's say we go to the end of an airport runway with a sound pressure meter and measure a jet as it flies overhead. Let's also say that, hypothetically, the sound pressure turns out to be 200 Pa. Let's also say we want to calculate this into dBspl. So, the sound of a jet at takeoff is : $$\textrm{Pressure (in dBspl)} = 20 \log \left ( \frac{\textrm{Pressure 1}}{\textrm{Reference}} \right )$$ (3.28) $$= 20 \log \left ( \frac{\textrm{Pressure1}} {20 * 10^{-6} \textrm{Pa}} \right )$$ (3.29) $$= 20 \log \left ( \frac{200 \textrm{ Pa} }{ 20 * 10^{-6} \textrm{ Pa}} \right )$$ (3.30) $$= 20 \log 10 000 000$$ (3.31) $$= 20 \log 10^{7}$$ (3.32) $$= 20 * 7$$ (3.33) $$= 140 \textrm{ dBspl}$$ (3.34) So what we're saying is that a jet taking off is 140 dBspl which means ``the sound pressure of a jet taking off is 140 dB louder than the softest sound I can hear.''