Science Tutorial: Sound Pressure Levels and Sound Exposure Levels
Regulators use sound pressure levels (SPLs) and sound exposure levels (SELs) to characterize potential effects of sound on marine animals.
Sound pressure level (SPL) is a means of characterizing the amplitude of a sound. There are several ways sound pressure can be measured. The most common of these are the root-mean-square (rms) pressure, the peak pressure, and the peak-to-peak pressure (see: Introduction to Signal Levels)
Sound pressure levels are reported in units called decibels (db). It is important to tell whether the pressure being used to compute decibels is root-mean-square (rms), peak, or peak-to-peak pressure. There can be a significant difference in the sound pressure level value depending upon which measure is used. Which measure is appropriate depends on the type of signal (e.g. peak pressure or peak-to-peak pressure for impulsive signals and rms pressure for continuous signals). On the DOSITS website, underwater dB is used to indicate decibels computed using root-mean-square pressure unless otherwise indicated. For situations in which the peak pressure is used, the unit of dB peak will be indicated. If the peak-to-peak pressure is used, the unit of dB peak-peak will be indicated.
Sound exposure level (SEL) is a measure of energy that takes into account both received level and duration of exposure. SEL is a common metric since it allows sound exposures of different durations to be related to one another in terms of total acoustic energy. However, the duration of a sound event should be specified because there is no accepted standard duration over which the summation of energy is measured. SEL can be computed for a single pulse or signal, which in studies of pile driving is often referred to as single-strike SEL (SELss). It can also be computed for multiple pulses or signals to generate a value equivalent to a single exposure for the cumulative sound energy (SELcum). If the individual pulses or signals are approximately the same, then the SELcum can be estimated as SELss + 10 log10(N), where N is the number of pulses. The number of pulses should be specified, as should the total duration over which sound energy is being accumulated.
Some studies on the equal energy hypothesis have shown that SELcum can be used to predict the risk for hearing loss in mammals. This implies that sounds received at lower levels for a longer duration may have similar effects as sounds received at higher levels for a shorter duration. It is important to consider the time between individual pulses or signals since hearing can be recovered if the interval between pulses is long enough. In addition sound exposure conditions vary. Therefore a number of factors need to be considered in determining potential sound impacts.
Science of Sound Tutorial Sections:
- Chapman, D.M.F. and Ellis, D.D. 1998, “The elusive decibel: Thoughts on sonars and marine mammals.” Canadian Acoustics 26(2): 29-31.
- Finneran, J. J. and C. E. Schlundt. 2010, “Frequency-dependent and longitudinal changes in noise-induced hearing loss in a bottlenose dolphin (Tursiops truncatus).” The Journal of the Acoustical Society of America 128:567-570.
- Finneran, J. J. and C. E. Schlundt. 2013, “Effects of fatiguing tone frequency on temporary threshold shift in bottlenose dolphins (Tursiops truncatus).” Journal of the Acoustical Society of America 133:1819-1826.
- Finneran, J. J., D. A. Carder, C. E. Schlundt, and R. L. Dear. 2010, “Growth and recovery of temporary threshold shift at 3 kHz in bottlenose dolphins: experimental data and mathematical models.” The Journal of the Acoustical Society of America 127:3256-3266.
- Finneran, J. J., D. A. Carder, C. E. Schlundt, and R. L. Dear. 2010, “Temporary threshold shift in a bottlenose dolphin (Tursiops truncatus) exposed to intermittent tones.” The Journal of the Acoustical Society of America 127:3267-3272.
- Kastak, D., C. J. Reichmuth, M. M. Holt, J. Mulsow, B. L. Southall, and R. J. Schusterman. 2007, “Onset, growth, and recovery of in-air temporary threshold shift in a California sea lion (Zalophus californianus).” Journal of the Acoustical Society of America 122:2916-2924.
- Kastelein, R. A., L. Hoek, R. Gransier, M. Rambags, and N. Claeys. 2014, “Effect of level, duration, and inter-pulse interval of 1-2 kHz sonar signal exposures on harbor porpoise hearing.” The Journal of the Acoustical Society of America 136:412-422.
- Madsen, P.T. 2005, “Marine mammals and noise: Problems with root mean square sound pressure levels for transients.” Journal of the Acoustical Society of America 117(6), 3952-3957.