Drop-deployed HydroMoth

Hi all, I'm looking to deploy a HydroMoth, on a drop-deployed frame, from a stationary USV, alongside a suite of marine chemical sensors, to add biodiversity collection to our payload.
Max deployment will be 100 m, but unlikely we will reach this depth.

Has anyone already deployed one to this depth, and what kind of protection did you give it, for enough protection, but still allowing the hydrophone to function?

Thanks in advance! :)
Sol

Matthew Duggan

@MattyD797 | He/Him

PhD Student | Cornell University | Smithsonian Institution; My focus is in computational ecology within fishery acoustics

5 April 2024 2:04pm

Hi Sol! This seems like an awesome project! I have a few questions in response: Where were you thinking of deploying this payload and for how long?

Regarding hydromoth recorders, there have been several concerns that have popped up in my work with deploying the them at this depth because it's a contact type hydrophone which means it utilizes the case to transmit the sound vibrations of the marine soundscape to the microphone unlike the piezo element based hydrophones.

At 30-60m you will likely have the case leak after an extended period of time if not immediately. The O-ring will deform at this depth, especially around the hinge of the housing. The square prism shape is not ideal for deep deployments you describe.
After that depth and really starting at about 50m, a major concern is synthetic implosion from the small air pocket of the hydromoth not having a pressure release valve and lithium ion batteries getting exposed to salt water. This type of reaction would cause your other instruments to probably break or fail as well.
You are unlikely to get a signal with a reinforced enclosure. The signal is generated via the material and geometry of the housing. The plastic will probably deform and mess with your frequency response and sound to noise ratio. If you place it against metal, it will dampen the sound quite a lot. We tried to do this, but the sensitivity is quite low with a large amount of self noise.
A side note: for biodiversity assessments, the hydromoth is not characterized and is highly directional, so you wouldn't be able to compare sites through your standard aocustic indices like ACI and SPL.
That said if you are deploying for a short time, a hydrophone like an Aquarian H1a attached through a penetrator of a blue robotics housing that contains a field recorder like a zoom recorder may be optimal for half a day and be relatively cheaper than some of the other options. You could also add another battery pack in parrallel for a longer duration.

Sol Milne

5 April 2024 2:04pm

Hi Matthew,

Thanks for your advice, this is really helpful!

I'm planning to use it in a seagrass meadow survey for a series of ~20 drops/sites to around 30 m, recording for around 10 minutes each time, in Cornwall, UK.

At this stage I reckon we won't exceed 30 m, but based on your advice, I think this sounds like not the best setup for the surveys we want to try.

We will try the Aquarian H1a, attached to the Zoom H1e unit, through a PVC case. This is what Aquarian recommended to me when I contacted them too.

Thanks for the advice, to be honest the software component is what I was most interested in when it came to the AudioMoth- is there any other open source software you would recommend for this?

Best wishes,

Sol

Matthew Duggan

5 April 2024 2:04pm

Hey Sol,

No problem at all. Depending on your configuration, the Audiomoth software would have to work on a PCB with an ESP32 chip which is the unit on the audiomoth/hydromoth, so you would have to make a PCB centered around this chip. You could mimic the functionality of the audiomoth software on another chip, like on a raspberry pi with python's pyaudio library for example. The problem you would have is that the H1A requires phantom power, so it's not plug and play. I'm not too aware with the H1e, but maybe you can control the microphone through the recorder that is programmable through activations by the RPi (not that this is the most efficient MCU for this application, but it is user friendly). A simpler solution might be to just record continuously and play a sound or take notes of when your 10 min deployment starts. I think it should last you >6 hours with a set of lithium energizer batteries. You may want to think about putting a penetrator on the PVC housing for a push button or switch to start when you deploy. They make a few waterproof options.

Just somethign else that occured to me, but if you're dropping these systems, you'll want to ensure that the system isn't wobbling in the seagrass as that will probably be all you will hear on the recordings, especially if you plan to deploy shallower. For my studies in Curacao, we aim to be 5lbs negative, but this all depends on your current and surface action. You might also want to think about the time of day you're recording biodiversity in general. I may suggest recording the site for a bit (a couple days or a week) prior to your study to see what you should account for (e.g. tide flow/current/anthropogenic disturbance) and determine diel patterning of vocalizations you are aiming to collect if subsampling at 10 minutes.

Cheers,

Matt

Alex Rogers

5 April 2024 2:04pm

Hi Sol,

If the maximum depth is 30m, it would be worth experimenting with HydroMoth in this application especially if the deployment time is short. As Matt says, the air-filed case means it is not possible to accurately calibrate the signal strength due to the directionality of the response. For some applications, this doesn't matter. For others, it may.

Another option for longer/deeper deployments would be an Aquarian H2D hydrophone which will plug directly into AudioMoth Dev or AudioMoth 1.2 (with the 3.5mm jack added). You can then use any appropriately sized battery pack.

If you also connect a magnetic switch, as per the GPS board, you can stop and start recording from outside the housing with the standard firmware.

Alex

Drop-deployed HydroMoth

Sol Milne

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Matthew Duggan