discussion / Acoustics  / 2 April 2024

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! :)

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 not: 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.