discussion / Acoustics  / 26 November 2020

Calling all hydrophone users: Tell us what you want!

Who are we?

We are Open Acoustic Devices, makers of the AudioMoth. AudioMoth is a low-cost, open-source acoustic monitoring device widely used for terrestrial monitoring and surveying. It has helped improve the accessibility of environmental research in this area by making acoustic data collection more accessible.

Our motivations

To prevent the continued decline in biodiversity, it is important to develop new approaches and tools to expand monitoring coverage beyond the terrestrial focus of our current device. How can we expand the monitoring coverage of marine environments? With the success of the original AudioMoth, we aim to do so through accessibility. Reaching out to the marine acoustics community is the first part of Open Acoustic Devices’ product development process for a device which does just this.

What are we doing?

We are calling all marine and freshwater hydrophone users to take part in an online survey about hydrophone usage. We wish to understand the context in which you use your hydrophones, where you are using them, and how you are deploying them for conservation and environmental research. 

This initial survey is intended for existing hydrophone users, or those that have been wanting to use hydrophones for specific tasks, but haven’t yet had an opportunity to do so. If you are not a hydrophone user, but know someone that is, we would really appreciate it if you could share and pass this survey on to them to partake. The more people we get to participate with, the more we’ll understand how hydrophones are used. 

What are we going to do with the results?

Soon after this survey we will share anonymised findings on the data we collect on the WildLabs ‘Marine Conservation’ community forum pages. 

These results will help steer the initial design of our first prototype for a low-cost, open-source hydrophone. We aim to iteratively develop this new device alongside users in real-world deployments going forward. By seeking these insights early in the design process, we hope to prioritise what our future users find most important in their marine monitoring equipment.

The survey

Take the hydrophone survey here.

Please share this survey with anyone else you feel would be willing and able to contribute to the development of accessible marine acoustic monitoring tools.

Thank you for your time,
The Open Acoustic Devices team.

I’ll stick some thoughts here that other folk can maybe comment on/add to. 


So there are a few (obvious) differences between terrestrial and marine acoustics. 

  • Devices are immersed in a highly corrosive and conductive liquid (sea water) with strong underwater currents, fishermen, storms etc. 
  • Boat’s are usually required to deploy devices - that can mean it is much more expensive to make recordings. 
  • Ultra high frequency species and noise sources (e.g. harbour porpoise @130kHz and broadband delphinid clicks @ >200kHz and echo sounders @ 200kHz) are relatively common. 

For PAM devices that means a few things: 

  • Longevity is crucial because it can greatly decrease the cost per hour of recordings (less boat time is required). There are a few strategies for maximising this;
  1. Devices such as FPODs and SoundTraps run onboard detectors. The SoundTrap in particular can run a click detector at high frequency (up to 576khz sample rate) whilst also contiously recording at lower sample rates.  This means that it detects high frequency delphinid and porpoise clicks (and saves the waveform for further analysis) and at the same time records all lower frequency sounds where almost all of the complex tonal vocalisations of dolphins, baleen whales and anthropogenic sounds occur; these types of sounds also happen to be more difficult to accurately automatically detect (especially on low powered hardware). This strategy results in an order of magnitude reduction in data without incurring a large cost in data degradation. 
  2. Run on board compression algorithms - the SoundTrap uses an open source X3 compression algorithm that often results in four times lossless data compression. 
  3. Have the space for a lot of batteries and/or the option of an external battery pack. 
  • Sensitivity is important because you want to maximise your monitored area (again making your survey as efficient as possible). That means choosing a hydrophone, amplifier and DAQ with a low noise floor and appropriate clip level. 
  • Devices must be robust and waterproof. They should be able to survive biofouling, getting knocked on the seabed and ending up in a fishing net etc. The two common housing designs are to have a completely sealed unit with an underwater connector (often SubConn) for downloading data or having a unit which can be opened to remove memory cards and batteries. Both approaches have their advantages and failure points. For example, devices which are sealed tend to be more simple to set up but the connector is often a failure point. Devices which can be opened are serviceable at sea but they can flood and are air filled which makes them a bigger acoustic target for echolocators (and so can mess up results). 

There are also a few ‘nice to have’ options. 

  • Two sample synchronised hydrophones allowing a bearing to be calculated. This can be useful for a whole host of stuff - for example using bearing tracks to figure out how many animals might be present, improving density estimation methods and allowing an extra data dimension for classification algorithms. It also means the devices are useful for a host of other applications - e.g. towed hydrophone surveys. 
  • Depth, temperature, tilt and light sensors are inexpensive low power and produce only a few gigabytes (at the very most) of data over a survey but can provide a wealth of oceanographic data that is also helpful in acoustic monitoring. 
  • If running detectors, then recording noise is critical because it allows us to model changes in the probability of detecting animals which is often important in density estimation. 
  • Accurate clocks are useful. The  SoundTrap clock drifts around 2s per day. Although manageable, this is often a pain and consumes valuable analysis time re-syncing devices together. 
  • The ability to daisy chain devices so they can be sample synchronised and used in acoustic arrays is very handy. For example SoundTraps have been used to make large aperture hydrophone arrays to localise Kogia. DOI: 10.1016/j.dsr.2020.103233 (if achievable this may supersede point 1. )

Ok, that’s everything I can think of for now. I would be keen to hear other opinions on this, additions, disagreements etc.  Jamie