TermAbbreviationDescriptionFurther information
absorptionSound is absorbed during its passage through seawater at a rate that is higher for higher frequencies of sound. The characteristics of broadband clicks consequently changes during transmission with the higher frequencies being reduced more than the lower frequencies. This difference increases with the distance from the source.
Acoustic Doppler Current ProfilerADCPThese current speed measuring instruments typically operate at1Mhz or higher but often produce lower frequencies that may affect the distribution of cetaceans and be detected by C-PODs.
ambient noise/soundThe 'background' sound at a site. As higher frequencies, expecially above 20kHz, are absorbed by seawater the source of this are mainly local, while very low frequencies can include sound from very distant sources
amplitude profileThe sequence of heights of the waves in a click. All clicks have three amplitudes logged – the loudest and the one before and after. Clicks with full waveform data have more.
The amp profile of NBHF clicks and boat sonars is fairly flat. That of dolphin clicks and many non-cetacean sources is much further from flat.
amplitudeThe height of the pressure wave of a sound. Also referred to as SPL = sound pressure level. This is used here as a simple instantaneous pressure scale value, but in some systems, it refers to a value in decibels of the RMS value of the pressures of the whole wave.
AmpReversalCountThis is the number of times the cycle amplitude trend (whether a wave peak is higher or lower than the one before) within a click switches from rising to falling. It is assumed to be rising at the start.
It is lower in cetacean clicks than noise and lowest in NBHF clicks.
AvPkIPIthe average value of the time between peaks (= wave period) of the loudest cycle in each click in a train. Frequency is the reciprocal of the wave period, so this is useful to get a frequency value based solely on the loudest cycles in each click in the train.
bandwidthA measurement of the frequency width of the FFT spectrum. The width at half the maximum is often used = the 3dB bandwidth. Shorter cetacean clicks give higher bandwidths. FPOD data does not support application of the FFT and a value for bandwidth is dervied from the variation, within the click, of the wave periods and amplitudes. It does not have a linear relationship with the conventional FFT. It is used in the discrimination of click sources.
beam structureClicks are emitted from the cetaceans’ melon as a narrow beam of sound that is loudest near the center. In dolphins the mix of frequencies changes strongly across the beam, with low frequencies dominating the outer edges of the beam, and vice-versa. This can often be seen in POD data, with surface echoes revealing the frequency content of a different part of the beam from that forming the direct path to the POD. For NBHF clicks there is little change in frequency content across the beam.
Broad Band Transients BBTShort clicks of high bandwidth, as often made by cetaceans other than porpoises.
burst pulseA short sequence of rapidly repeated clicks usually with an abrupt start and end and known to be often a social communication
calibrationRadial meaurements of the sensitivty of PODs are made at intervals of 5 degrees, and at two frequencies.
clickClick is normally taken to imply a brief spike in amplitude. Whistles, by contrast, are much longer tonal sweeps
click rateThe number of clicks per second. Also called PRF – pulse repetition frequency. It is the reciprocal of inter-click-interval.
click trainA more or less regular series of clicks, each similar to the one before.
clippingThe pressure of a loud clicks may reach the maximum output level of an amplifier or ADC (analogue to digital converter) before it has reached the peak of that cycle. It is then ‘clipped’. The time taken between crossing zero and clipping can be used to extrapolate the peak value.
cropMake a new shorter file that is a copy of part of a file with time removed from one or both ends.
cycleOne wave of sound pressure going from zero up, then down below zero then back up to zero.
date formatMinuteN refers to the minute number in FPOD time – it is the number of minutes since the start of the year 1900. Divide this by 1440 to get the time used in Excel and Access, which is a floating point number representing the number of days and fraction of the last day since the same start date.
DecibelsdBDecibels are intended to represent intensity (= power) of a sound and is 10 times the log to base 10 of the ratio of rms values. It is a long out-dated unit that is rightly deprecated by the SI system of units. It is not a physically precise unit of comparison because it depends on the acoustic impedance of the media being identical, it is troublesome to learn and it contributes to errors! The alternative is to report pressures directly as Pascals or rms pressures.
Detection Positive MinutesDPMWidely used as a measure of density of animals or habitat use. if DPM/h or DPM/day exceed about 30% the statistic is starting to be come pregressively less sensitive to changes in density of cetaceans
Detections and EnvironmentA type of export from CP3 or FP3 files. It adds information to each logging period about the angle, deadband, time lost (to ‘maxing out’ = reaching the limit for the minute) and some measures of the soundscape.
Detection Positive Ten Minute periodsDP10MThese are 10minute periods set by the clock, so their boundaries always correspond with the hour ends. Compared to DPM or smaller units this unit reduces the effect of variations between loggers but will saturate (reach the maximum possible) before DPM if the density of animals is high.
Detection Positive MinutesDPMDPM/hour or DPM/day is a good general purpose measure of the level of acoustic activity of cetaceans so long as it is not going above about 30%, as it becomes less and less sensitive as the level rises.
Click durationdurDuration of clicks in FPOD data is often represented as the number of cycles in a click. The duration in 250 nanosecond unit is also stored and used to calculate the frequency of the click.
Effective Detection RadiusEDRThis is the radius of a circle that would contain as many animals as were detected in total i.e. the number missed inside this radius is the same as the number detected outside it. There should be a time period specified i.e. ‘the 1 minute EDR’ as it will vary with this time period.
It is not the same as the mean distance at detection, which is typically much larger.
envelopeAn imaginary line stretched over the amplitude peaks of the waves in a click.
event‘Clicks’ are detected by PODs as short periods in which successive wavelengths are similar. These are tonal ‘events’, and there is no requirement that they must be louder than the sound stream before or after, although they usually are.
Export SPL as PaTicking this box will show the amplitude of each click in Pascals by correcting for the frequency-dependent sensitively of the system and the non-linearity of the amplitude scale. However, it makes it less clear when a frequency has reached and may have exceeded the maximum response for that frequency.
F-PODChelonia suggest using this reference that gives a technical description of an FPOD in a paper or report.
False negativeClicks made by cetaceans that were missed by the detector. Because it includes clicks from cetaceans at all distances from the detector the idea is better represented by the ‘detection function’ that describes the proportion of false negatives in relation to the distance from the detector.
False positiveSound wrongly identified as a coming from the species or group identified. To understand the process false positive rates should be expressed /h /day etc or as a fraction of the sounds made by their real source, if known e.g. 1% of dolphin clicks are classified as porpoises.
Fast Fourier TransformFFTThis takes in a set of amplitudes, measured at uniform intervals, and generates half as many complex numbers. One of the pair of values in each complex number represents the intensity of one frequency within the set of amplitudes. The set typically has 256, or 512, or 1024 (or other power of 2) values.
Field Programmable Gate ArrayFPGAA type of microprocessor suited to parallel processing which enables complex logic at low clock speeds and low power cost
frequency domainThe information within the frequency spectrum such as the peak frequency of click or the bandwidth. This includes no information on the order in which frequencies appear in a sound.
frequency profileThe F-POD stores the wavelengths of the successive waves in a click. These give information about the frequencies in the click and their time sequence, and can be graphically displayed as the frequency that would have that wavelength in a pure sine wave. Frequency profiles cohere in trains.
As with the amplitude profile it is flatter in clicks from NBHF sources and boat sonars than cetaceans and noise sources.
generalization performanceThe success of a classifier when applied to data that was not part of its training set. It is always worse and sometimes very much worse
guard bandA simple technique of detecting a narrow band signal is to compare the frequency of interest with a ‘guard band’ of higher or lower frequencies. The very first POD used 3 guard bands with used defined threshold ratios for each.
Inter-click-intervalICIThe time between the starts of successive clicks in a train. For click trains identified by KERNO-F this ranges between 1 second and 0.5ms but cetaceans sometimes click more slowly.
Inter-peak-intervalIPIThe time interval between the peaks of the waves in a click. In the F-POD and BatBug this is measured in 250ns units. The maximum logged is 255. That BatBug uses a compressed count making the maxium 511.
KERNOChelonia’s automated classifier for data from C-PODs.
KERNO-FChelonia’s automated classifier for data from the F-POD. The KERNO-F classifier seeks coherent trains of clicks within each minute of stored data. Coherence is assessed on the similarity of successive clicks in a sequence in terms of their features including time-spacing, amplitude, frequency and multipath cluster size. Multiple hypothesis testing is used to find possible trains which are allocated to quality classes and species guilds using a set of train features.
specificityhow well a detector picks the real target and rejects false positives
Narrow Band High Frequency clicksNBHFLong, tonal clicks, around 125 kHz Characteristic of all porpoises and a few dolphin species and Kogia sp.
Passive acoustic monitoringPAMThe detection of cetaceans by listening for the noises they make. Active acoustic monitoring means sending out very loud sounds and listening for the weak echoes from animals, and is used in fish finders and depth sounders. PODs, however, are PAMs and are silent when logging.
Programable Interface ControllerPICa type of micro-controller
sensitivityhigh sensitivity = able to detect very weak sounds. Sensitivity can often be increased at the cost of reduced specificity
standardisationsetting the sensitivity to a common standard
Static Acoustic MonitoringSAMpassive acoustic monitoring by a static logger of any type
toothed whalesOdontocetes - the toothed whales - all the cetaceans except the Baleen Whales that do not have teeth but filter small prey
train detectionToothed cetaceans make long series of similar clicks - called trains - made with similar time gaps between successive clicks. The gaps usually show continuous variation - getting shorter or longer, and dolphins also vary the sound characteristic of the clicks. Trains are also produced by boat sonars and can arise by chance from random sources of clicks. The software filters that extract cetacean trains are essential to the POD's ability to accurately identify the presence of the animals and work by identifying coherent trains - those in which the variation in timing and character between successive clicks is lower than might occurs by chance among the many background noise clicks that are logged.
uniform thresholdall types of logger tend to be more senstive in quiet conditions, and this can bias detection rates. Fixing a static and relatively high detection threshold is a way of avoiding this variability.
wavenumber of peakeach cycle or wave in a click has an amplitude. If the highest in the click is the third this value is 3
Weak Unknown Train SourcesWUTSSound interference. Weak unknown train sources exist in some water bodies. Their identity is not known but small crustaceans that may colonise the transducer housing surface are on the list of suspects. IN some places large numbers occur and dominate teh acoustic record but their identity is not yet known.
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