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:— ).
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.
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 pressure level. With a typicalrecording system, it should be possible to approach the Knudsen sea-state zero levels (which include distantshipping noise) at 63 Hz and 125 Hz, the values for which are approximately 64 dB re 1 uPa”/Hz and 59 dB re 1uPa2/Hz respectively,125]
NOTE 2 lf measurements of background noise are necessary then the system self-noise shall be sufficiently lowto enable good quality measurements of background noise. It might not be appropriate to use the samehydrophone for the background noise measurements as that used for the measurement of the noise radiated fromthe pile-driving.For measurement of background noise a hydrophone with low-noise performance and highsensitivity is generally required.For a system designed to measure very low sound levels, a maximum system self-noise of 47 dB re 1 uPa2-/Hz at 63 Hz and 43 dB re 1 uPa2/Hz at 125 Hz is recommended.l25]
4.2.5 Dynamic range
The system dynamic range should be chosen to be sufficient to enable the highest expected soundpressure, at the measurement position, to be recorded faithfully without distortion or saturation causedby the hydrophone, amplifier, and ADC.
The dynamic range of the measuring system is the amplitude range over which the system can faithfullymeasure the sound pressure. This ranges from the noise floor of the system (which defines the lowestmeasurable signal) to the highest amplitude of signal that can be measured without significantdistortion.
NOTE1 High amplitude sounds which are beyond the maximum capability of the measuring system will causedistortions in the measured data.For example, clipping can occur where the peaks of the signal are missing fromthe data (the peaks being truncated at the full-scale value of the system ADC).The measuring system is required tobe linear over the full dynamic range, requiring that the system sensitivity is constant over the full range ofmeasurable sound pressure. Systems with dynamic ranges of in excess of 60 dB are recommended formeasurement of pile driving noise.For some systems, when approaching the high amplitude limit, the responsemight no longer be linear due to limits in the performance of components such as amplifiers. Therefore, it isadvisable that a measurement system is not used close to the limit of its dynamic range unless the linearity hasbeen checked.
NOTE2 A method to mitigate problems with dynamic range is to have some flexibility in the sensitivity, oftenachieved by use of adjustable gains for amplifier stages and scale settings on ADCs.However, where a system hasbeen deployed remotely (for example, an autonomous recording system), control over the system settings afterdeployment might not be possible. In this case, some knowledge of the likely range of sound pressure levels isrequired to optimise the available dynamic range (this knowledge can be obtained from reported levels in thescientific literature or from approximate theoretical calculations).BS ISO 18406 pdf download.