Hydrophone End-Cap Selection
March 3, 2015
The sensitivity and frequency range of microMARS is set by the hydrophone end-cap. The end-caps, which cost around $500, are easily replaceable. By keeping an inventory of end-caps, your recorders can be adapted for a variety of jobs.
End-caps should be selected depending on the characteristics of the sound source (marine mammal, industrial noise, etc.) to be studied, and the expected ambient or background noise level. Here are the basic considerations.
Determine the highest frequency needed
The microMARS end-caps are built around piezo-electric resonators that provide a flat frequency response (uniform sensitivity) up to a specified frequency. The part number specified that frequency, such as MH33-xx for end-caps with a flat frequency response up to 33 kHz, or MH125-xx for end-caps with a flat response up to 125 kHz. Because end-caps designed for higher frequencies are also less sensitive, select the lowest frequency end-cap type that still covers your frequency range of interest. For example, to study killer whales or great whales and industrial noise that all occur below 33 kHz, select an MH33-xxx type. However, to study dolphins that vocalize at higher frequencies, select an MH125-xx for operation up to 125 kHz.
Select Flat of Sloped Frequency Response
Flat Frequency Response: Underwater recorders are traditionally optimized to provide a flat frequency response, i.e. uniform sensitivity across the operating frequency range. The principal advantage of a flat frequency response is that the resulting .WAV files can be used directly to generate dB re. 1 µPa (or equivalent) referenced spectra, perform various sound measurements or play the files with a 'natural' sound quality. Hydrophones with a flat frequency response are designated by a number directly after the dash, such as MH33-1 or MH33-2. The -1 or -2 defines the gain programming of the hydrophone, which determines both the maximum signal the hydrophone can receive without clipping, and the noise floor.
Sloped Frequency Response:Sloped frequency response hydrophone end-caps increase in sensitivity with frequency. At low frequencies, they are less sensitive and can thus accommodate louder signals without clipping. At high frequencies, sensitivity is greater and the noise floor is lower, meaning that fainter signals can be detected. This sloped frequency response matches the typical noise profile in the ocean, with lower frequencies experiencing louder noise. Thus, by selecting a sloped frequency response end-cap, you may be better able to measure loud industrial noises without clipping while at the same time detecting faint biological signals such as from distant marine mammals vocalizing at higher frequencies. In other words, a sloped frequency response end-cap provides a greater dynamic range (great ratio of loudest to weakest discernable signal) than is available with a flat frequency response end-cap.
The downside of the sloped frequency response is that for most uses of the data, the .WAV files generated by microMARS must first be equalized, a process in which frequencies at which the hydrophone is less sensitive (low frequencies) are boosted relative to frequencies at which the hydrophone is more sensitive (high frequencies). This process however is performed quickly and easily by Audacity, a free open-source software available on-line. An Audacity compatible equalization curve is provided for each hydrophone configuration by Desert Star Systems. This curve also automatically 'normalizes' your sound files to a certain value, such as 180dB re. 1 µPa full scale for MH33-1 and MH33-2, 215 dB full scale for MH33-S1 and 175 dB full scale for MH33-S2.
Select to Accommodate the Strongest Expected Signal
Figure X1 shows the frequency-specific full-scale value for the four MH33-xx standard configurations along with observed loudness for a selection of events. In order to avoid signal clipping (i.e. signal exceeding the full-scale value), select a hydrophone configuration where the clipping curve is above the expected signal.
For example, MH33-1 can accommodate an environment with strong snapping shrimp, while the shrimp clicks would be clipped when using MH33-2 or MH33-S2. To monitor pile driving (a loud industrial noise) at 330m distance, MH33-S1 would be compatible but to monitor at 7m distance a special configuration not listed here would be needed.
The sound observations here can be corrected for other distances by subtracting 6dB for each doubling of the distance or adding 6dB when halving the distance. At 10% of the observation distance, the expected signal is up 20dB, and at 10X the observation distance, it would be down 20dB. For example, the ferry at 1000m had an observed noise level of about 150dB with a signal peak around 0.4 kHz. This signal is actually compatible with all MH33-xx configurations. However, at 100m distance, the signal would be around 170dB, requiring either MH33-1, or MH33-S1.
Figure X1: Full-scale sound curves for MH33-xx hydrophone end-caps with some sound observations.
Select for Best Weak Signal Performance
If the only selection criteria were strong signal performance (above), one would always select the MH33-S1 configuration among the MH33 types. This hydrophone can accommodate all but the strongest observed signals without clipping. However, this hydrophone also exhibits the highest noise floor or self-noise, as shown in figure X2. Conversely, configuration MH33-S2 with the lowest full-scale or clipping value offers the best (lowest) noise floor. Therefore, there is a trade-off: Select configuration MH33-S1 or MH33-1 to accept the loudest signals, but MH33-2 or MH33-S2 to get the best low-noise performance and thus detect a sound source at the greatest distance.
Figured X2: Noise floor of MH33 hydrophone end-caps
In order to judge this trade-off, consider the ambient or background noise level at the observation site. If the hydrophone's noise floor is less than the ambient noise level, the ambient noise is the limiting factor and there is little point in selecting a more sensitive hydrophone. Typical noise in the ocean is described by the Wenz curves (Figure X3). While actual conditions may vary strongly, these curves provide a good starting point. As an example, consider recorder use to monitor Killer Whales in calm waters such as the Salish Sea. From figure X1, we observed their clicks center around 25 kHz, where the Wenz curves predict noise down in the 20-30 dB range for low sea states. Consequently, use of the MH33-S2 configuration, which also reaches down into the 20dB range at these frequencies, can potentially increase the detection range. However, the high sensitivity of MH33-S2 is then expected to cause signal clipping when the animals are within 500m or so. Yet, if the location exhibits high ambient noise such as due to snapping shrimp or small boat traffic, (small boat engines cause high frequency noise due to cavitation around the fast spinning propeller), MH33-S2 may be too sensitive and it may be better to select MH33-2 or even MH33-1 to get better close range performance.
Figure X3: Wenz Curves for Ambient Noise in the Ocean
(Source: National Research Council; National Academy of Sciences)
Hydrophone Frequency Curves
The MH33 series of hydrophone end-caps are specified for use at frequencies up to 33 kHz. As compared to higher frequency end-caps, they offer better sensitivity. The end-caps include an integrated pre-amplifier, which is factory programmed for flat or sloped frequency response. High sensitivity types (MH33-2 and MH33-S2) are available for extended range monitoring in quiet waters. Medium sensitivity MH33-1 is for general use. Due to its sloped frequency response, MH33-S1 provides very high full-scale capability at low frequencies to monitor loud industrial noise while at the same time offering good sensitivity at high frequencies to monitor biological such as marine mammal vocalizations.
Audacity compatible equalization curves are available for all hydrophones where?, and yield a flat frequency response and a defined full-scale value post-equalization. Please note that equalized files should be saved in 24-bit .WAV format (rather than 16-bit) to avoid loss of very weak signals.
Figure X7: MH33-S2 sloped frequency-response hydrophone end-cap for extended range monitoring of weak high frequency signals and higher full-scale signal capability at low frequencies to reduce clipping in the presence of ship and other noises.
Hydrophone End-Cap Specifications
The microMARS hydrophone end-caps are user replaceable. The end-caps define the acoustic characteristics (frequency and sensitivity range) of the recorder. The table provides specifications for currently available and anticipated hydrophone end-caps.
|PART NUMBER||PRE AMP GAIN||FLAT FREQUENCY RESPONSE RANGE (3dB cutoff points)||ROLL-OFF BEYOND CUTTOFF POINTS (dB / Octave)||SIGNAL RANGE (dB re 1µPa)||NOISE FLOOR (dB re 1µPa / √Hz)||STATUS|
|MH33-1||18 dB||0.6-33 kHz||6 dB||70-166 dB||70 dB||Available|
|MH33-2||38 dB||6-33 kHz||6 dB||50-146 dB||53 dB||Available|
|MH125-1||40 dB||TBD - 125kHz||12 dB||TBD||TBD||In Design. High frequency optimized.|
|MH30-1||TBD||0.1-30kHz||12 dB||44-140 dB||
← 40 dB target
|In Design. High sensitivity and low frequency optimized.|