BS ISO 18406:2015 pdf download – Underwater acoustics — Measurement of underwater radiated sound from percussive pile driving NOTE1 The one-third octave bands may be calculated using either base-10 or base-2, and the choice is statedwhen presenting the results. The two calculation methods will give slightly different results, and base-10 is thepreferred method (IEC 61260-1:2014). (Note that the base-10 representation of a one-third octave band isreferred to as a “decidecade” in lSO 18405:— ). 4.2.3 Directivity The hydrophone used shall have an omnidirectional response such that its sensitivity is invariant withthe direction of the incoming sound wave to within a tolerance of 2 dB over the frequency range ofinterest. This requirement is not difficult to satisfy at frequencies up to 20 kHz. However, one issue that can cause enhanced directionality is where the hydrophone is deployed closeto another structure that is capable of reflecting the sound waves. The combination of the direct andreflected waves causes interference, the nature of which will change depending on the arrival angle forthe sound wave.This effect can be evident at kilohertz frequencies if the hydrophone is deployed closeto a support structure such as a heavy mooring or support, or a recorder case that houses electronicsand batteries but is mostly air-filled. Similarly, if the hydrophone has a guard deployed around it (aprotective cage to prevent damage of the element by impacts), this can influence the directivity atkilohertz frequencies. If necessary, the above effects can be quantified by directional responsemeasurements of the hydrophone together with the mounting, in a free-field environment. 4.2.4System self-noise To achieve acceptable signal-to-noise ratio when measuring acoustic signals, the system self-noise(expressed as the equivalent bandwidth noise pressure level) should be at least 10 dB below the lowestsignal level to be measured in the frequency range of interest.For measurements of percussive piledriving where high amplitude signals are commonplace, this criterion is only likely to be challenging atsignificant range from the source (tens of kilometres). NOTE1 In the context considered here, the system self-noise is considered to be the noise originating from thehydrophone and recording system (for considerations of deployment and platform noise, see 5.3). The systemself-noise is the noise generated by the system in the absence of any signai due to an external acoustic stimulus.This noise is electrical in nature, and is generated by the hydrophone itself and any electronic components such asamplifiers and ADCs. This is normally expressed as an equivalent bandwidth noise...
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