is it Valid to Compare dB in Air and Water?
The case for taking dB levels, especially across species bounds,
with a sea of salt. . . . (BE SURE TO READ TO THE END OF THE PAGE!)
From an AEI report on seismic airguns, which contains footnotes and bibliography, detailing all references .
[DOWNLOAD REPORT (rtf)]
While, as discussed elsewhere, it is difficult to compare the experience of sound in the water with the experience of sound in the air, it can still be somewhat helpful to offer a few points of reference to bear in mind as we traverse the gauntlet of decibels. The following chart can give a rough sense of how common human sounds in air may sound to humans underwater:
||dBA re 20uPa at 1m in air (RMS)
||Equivalent Energy, dB re 1uPa2.s in water
||RMS dB re 1uPa in water
|Indoor crowd noise
|Pain threshold (human)
In-air sound levels (RMS) taken from Berger (2003), OSHA standards from comparison chart of noise regulations [DOWNLOAD CHART(pdf)]
Equivalent energy value: Difference calculated using 62dB standard correction, less 13dB correction of mean to equivalent energy, to match the figures shown on the transmission loss page.
Mean squared value: To be as modest as possible in these comparisons, this column adds the full 62dB correction to create a comparable RMS level in water; figures for peak levels would be higher by 15dB.
For more info, see the differing measurement systems page.
90dBA: OSHA permissible exposure limit; prolonged exposure causes permanent hearing loss; hearing protection required; NIOSH requires hearing protection at 85dbA
100dBA: short exposure can cause permanent hearing loss
115dBA: no employees may be exposed without hearing protection
140dBA: OSHA ceiling for impulse noises and NIOSH ceiling for all noise, with or without protection
What is dBA? Adding a straw that will hopefully not break our camels back, dB levels in air, when being rated in order to judge effects on human hearing, are weighted to better match our frequency sensitivity. Since our hearing is not very sensitive at lower frequencies, the absolute dB value of lower frequency sounds is REDUCED by this weighting system (to reflect the fact that, due to our poor sensitivity, they sound less loud to us). There are a couple of weighting sytems in use; dBA, or A-weighted is the one most commonly used when setting safety thresholds. A-weighted values are lower than absolute dB levels in frequencies below 1kHz, the range we are most interested in. The corrections are much higher at the lowest frequencies, and become smaller up to1kHz, where there is no correction applied (26dB are subtracted at 63Hz, 16 decibels are subtracted at 125Hz). By contrast, we do NOT create weighted dB scales for each ocean species; though each species does have its own frequency response curve (ie can more easily hear certain frequencies, with others at which sounds appear quieter than their absolute value); rather, we are content with absolute dB measurements. The bottom line of all this is that if we want to compare the sound of, say a jet take-off on land measured in dBA with the sound of seismic airguns underwater in unweighted dB(peak, mean, or energy), there is a case for adding roughly 25dB to our water values when considering frequencies under 100Hz, and 15dB if considering frequencies 100-200Hz (these are the frequency ranges in which airgun energy is its highest). Of course, if our point is to create a comparison that shows how we might hear a sound in air and water, then there is no need to consider this correction, since low frequency sounds in water will likewise need to be weighted to suggest how human ears would perceive them (ignoring the fact that our eardrums are generally very poor sound conductors in higher pressures of underwater
). For many reasons, comparisons such as this can only be valid when comparing how the same species, in this case, humans, would perceive a sound. Thus for this chart, we are only making the previously noted mathematical corrections for the different dB references, density of water, and differences between ocean sound measuring systems. Please do bear in mind that, given all the translations involved, all parties are on very thin ice in making any definitive claims about how sea creatures may respond to any given dB level. This is why scientists tend to rely on behavioral responses and observed physiological damage caused by ocean-based sound, and to avoid trying to make these sorts of comparisons with human hearing, and especially human safety thresholds, in the air.
For many reasons, comparisons such as this are only really valid when comparing how the same species, in this case, humans, would perceive a sound. Every species (and, to a lesser degree, every individual) has its own "frequency response curve"; that is, we each hear certain frequencies much more easily than others. Humans, for example, do not hear low frequencies very well; a 63Hz sound measured at 100dB sound will sound to us as though it is only 74dB, while a 2kHz, 100dB sound will indeed sound like 100dB. Every species has its own range of hearing where sounds are perceived optimally, and likewise ranges where sounds are not perceived as well. Of course, when dealing with exceptionally high intensity sounds, physiological awareness or damage (and perhaps behavioral responses such as avoidance) may be caused even when we are not hearing the given frequency very well.
Please do bear in mind that given all the translations involved (from these individual species differences, to the many measuring scales and conversions mentioned above), all parties are on very thin ice in making any definitive claims about how sea creatures may respond to any given dB level. At best, an underwater dB rating is a decent estimate of the sounds physical power, and a somewhat useful benchmark by which to make informed guesses about its possible effects. This is why scientists tend to rely on behavioral responses and observed physiological damage caused by ocean-based sound, and to avoid making direct comparisons with human hearing, and especially human safety thresholds, in the air.
That being said, it is especially important to base our decisions about operational safety, mitigation, and regulation on careful observation of the actual responses of many species at a range of distances from seismic survey vessels, rather than to over-rely on ideas about the danger of specific dB levels or on models of how we think that the dB level will change as the sound moves out into the ocean environment. As we will see, researchers do use dB measurements as a tool in determining thresholds of effects; when measured in the field, this can provide a more concrete handle on the animals reasons for responding the way they do, despite the cross-species uncertainties in how a given dB level is perceived. We will also see that many studies are based on observed responses at various distances, without regard to received dB levels (at times, such observations are paired with models of likely sound propagation, and more rarely, with measured sound readings); such studies offer less concrete data and more uncertainties as to the reasons for the observed responses. Both approaches offer valuable information; the most useful insight with which to inform our decisions about operational standards is likely to follow from a judicious use of all data, within a context of appreciating the limits of each approach.