An article by Oceana back in September 2013 made me consider how they can justify publishing such misinformation and distortion of the facts and science and, why they do it. Unfortunately, their misinformed claims are subsequently repeated by the media and other environmental groups and rapidly become equivalent to myths as opposed to facts. Have a look at their CEO’s blog and note their claim that seismic sounds are “100,000 times more intense than the roar of a jet engine”.
Even if the sound of a jet taking off were 140dB at 1m, as Oceana incorrectly claim, a sound in air that would be 100,000 times louder would have to be louder than an atomic bomb (248dB – about 2,000 times louder) and close to the Krakatoa eruption (310dB – about 131,000 times louder. Note that 300dB is 65,500 times louder). The sound and pressure wave from the Krakatoa eruption cracked walls and broke windows up to 160km away, created a 37m tidal wave and was heard up to 4700km away! Common sense tells us that seismic pulses are nowhere near the loudness of atomic explosions and pale into insignificance compared to the Krakatoa eruption.
Thus, where (or why?) has Oceana gone wrong in their calculations?
The sound of a seismic pulse is actually slightly lower than the sound of a jet taking off – basic physics and sound measurements tell us that. As part of their efforts to distort the truth, in their calculation of this absurd claim, Oceana quoted the sound of a jet engine as 140dB and a seismic pulse as 250dB. However, they are incorrect on at least 5 points:
1. As can be seen in any table of typical sound levels in air, the sound of a jet taking off would be 140dB at 50m. Yes, 50m! Not 1m, which is the distance that should be used to quote sound source levels. The sound at 1m would actually be over 180dB.
2. They then compared the sound of a jet taking off at 50m with the sound of a seismic pulse at 1m. NOT a valid comparison!
3. Furthermore, the value of 250dB for the pulse from a seismic array is the THEORETICAL value. This is calculated on the basis of all the elements (the compressed air containers, or “airguns”) of the seismic array being at the same point. This is not physically possible! The array is spread over an area of about 10m x 15m, so the actual sound level at the centre of the array is generally about 10-20dB less than the theoretical level quoted by Oceana. More misinformation!
4. Oceana have “conveniently” ignored the 62dB difference between pulses of the same loudness (amplitude or pressure) in air versus water. This is the result of two factors: a) the different reference levels used by acousticians for air and water (ie. “re 20 µPa” in air versus “re 1 µPa” in water, where “re 1 µPa” means “relative to 1 micropascal”) which lead to a 26dB difference; and b) the difference in density which contributes to a further 36dB. Thus, 180dB re 20 µPa in air is the same loudness (ie amplitude or pressure) as 242dB re 1 µPa in water.
5. Finally, Oceana has further misled the reader by confusing the difference between loudness (amplitude or pressure) and intensity. As shown in the table of typical sound levels (in dB) in air relative to loudness, 70dB is twice as loud as 60dB. It is well known that when the level of a sound in air is increased by 10dB, the loudness (amplitude or pressure) of that sound doubles, although the sound power (or intensity) increases by a factor of 10. Oceana has incorrectly chosen to use intensity and hence a factor of 10, rather than the more relevant parameter (loudness, amplitude, pressure) and a factor of 2.
Bear in mind that the values quoted by Oceana are relative to pressure (ie. re “x” µPa or relative to “x” micropascals) and therefore equate to loudness, not intensity. It is the amplitude or pressure of a sound signal that has a behavioural or physical effect on the receiver.
Thus, to say “100,000 times greater”, when the loudness (amplitude or pressure) of a seismic pulse is slightly lower than a jet taking off, leads one to question Oceana’s motives.