I am looking to purchase a hydrophone and speaker system that I can use to identify singing humpback whales while at sea, in order to deploy different tags to better understand song behavior on feeding grounds. I want to be able to lower my hydrophone over the side of the boat and listen for song - most of the options I have found are quite expensive (>\$1000) and I was hoping to find something cheaper - ideally less than $500 total. What hardware have other folks used for in the field listening? What's the cheapest option? It doesn't have to be high quality recordings - we just want to be able to have some confidence that we're tagging a singer.
I have used Cetacean Research's SQ26-H1 Portable Underwater Sound Recording System for listening for fish sounds from a boat, similar to what you're proposing with whales, and I believe the system should be at least close to within your budget. I was able to get a student discount for mine, so I can't recall the usual full cost of it.
The paper discussing the relatively new HydroMoth prototype (Lamont et al. 2022) also presents the results of a systematic review of recent acoustic studies and the costs of the recorders used by their authors, to quantify the lack of low-cost underwater recording equipment, especially when compared to terrestrial recording equipment. So you're not alone in struggling to find affordable options!
Hydrophones are notoriously the most expensive part of underwater acoustic sensing systems. If you look to some affordable commercial systems that allow listening to underwater sound, maybe you could start with the offerings of aquarian audio. Have not used them myself, I cannot comment on use, but certainly others are willing to comment.
I have made hydrophones from scratch using elements like: https://www.piezoelements.com/piezo-ceramic/piezo-cylinder/pzt5-piezo-ceramic-tube.html If you solder some wires to them (I think we used "silver-solder") and encase them in silicone, you've got yourself a hydrophone.
I used mine for embedding in suction cups attached to the melon of harbour porpoises for monitoring the click-timing (ABR/AEP research). Worked a charm!
Depending on your cable length requirements, you might have to install some sort of pre-amplifier in your system close to the element.
You'll likely need a small amplifier before you recorder (e.g: https://www.aquarianaudio.com/pa1.html ask them for compatibility, they're very helpful!).
Next step is analog-to-digital conversion: I think the latest "audiomoth" (https://www.openacousticdevices.info/audiomoth) has a jack input and can sample up to 384 kHz (but for humpback whale song anything will really do, it's in the human hearing range). I don't thing the audiomoth supports real-time monitoring.
Using this approach you could be up and running for 50-100 USD.
Over 5 locations and 20 years, the Orcasound hydrophone network in Washington State (USA) has tested many makes and manufacturers, but for our live-streaming 24/7 web application the hydrophone and cable still require 60-90% of the expense budget for each network node! So, like you and the International Quiet Ocean Experiment, we are interested in reducing the cost of the hydrophones. To this end, and thanks to some of you who helped crowdfund it, we have an on-going project to engineer a low-cost (<$100) hydrophone via Experiment.com which all are welcome to join or study.
For your purpose -- which doesn't require recording, I gather -- you may get away with a kayaker-style hydrophone with a short (<5m) cable and a speaker. On a calm day (low ambient noise) in the ocean with a strong source like singing humpbacks nearby, your hydrophone can be just a few meters below the sea surface. On windy/wavy days, getting 5-10m down will yield a better SNR.
Aquarian offers their H1a with 3 meters for 150 dollars. Their recommended amplifier/speaker won't put you over $500 U.S. even with Washington's 10% sales tax.
For many hydrophones with preamps, your speaker will also need to power the hydrophone, typically via a line-in or hot-mic jack. One combination we've enjoyed is the CRT SQ26-08, powered by the Zoom H1n recorder. The H1n has a tiny speaker built-in, but we prefer to use the headphone jack for more careful monitoring. You could add a headphone splitter and an external speaker if you need multiple people to be able to hear the underwater sounds on a noisy boat or a windy day.
If you want more options, we'd appreciate your comments and new data (especially recent quotes) in Orcasound's shared Google spreadsheet of hydrophone manufacturers and models. (There are other tabs for cable if you like to splice or build your own, plus other hardware, ADCs, power supplies, and historic makes/models).
Jez Riley French makes good, affordable hydrophones that can be used with relatively cheap PCM recorders (note you'll need a XLR impedance matching adapter though if you use via XLR port) - with a bespoke cable length too: https://jezrileyfrench.co.uk/hydrophones.php
I've used these and Aquarian hydrophones for freshwater recordings (obviously a bit shallower!) and got good clear recordings back.
If you go for Acoustic Devices instead note that you need the AudioMoth 1.2 if you want to use a 3.5mm jack input, and you'll need to buy the jack separately and solder it on yourself (easy to do though). They now also have a HydroMoth in a waterproof case which is up for sale periodically on GroupGets: https://store.groupgets.com/products/hydromoth-with-underwater-case
At risk of sacrificing my scientific credibility, but because I love options on the silly-side, I give you the following suggestions. These aren't cheap hydrophones, but could be the cheapest ways to check whether the humpback is a singer:
In the intro of Urick's Principles of Underwater sound he recounts a quote from none other than Leonardo da Vinci from 1490: "If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear you will hear ships at a great distance from you." I believe said "long-tube" can probably be readily scrounged for free, or purchased for less than the cost of a hydrophone.
And assuming that your boat is suitable and that your health, safety, and ethics guidelines are allowing, one could always use their own ears as a pair of dipping hydrophones.
And finally, there's always the option of building your own hydrophone(s). I have used and can recommend Jay Barlow, Shannon Rankin and Steve Dawson's guide to building hydrophone arrays (NOAA Technical Memo NMFS-SWFSC-417) though unless you've already got or can scavenge all of the "capital" equipment, piezo elements, and urethane required, I doubt this is likely to be the cheapest option. Maybe it would start to become cost effective if you wanted to make dozens of very small hydrophones. But I can vouch for the guide and, the hydrophoens do work reliably -- for long enough to complete a PhD project anyway. And all but the piezo could potentially be swapped out for cheaper/scrounged parts to keep costs down (for example, I used ethernet cable instead of "proper" hydrophone cable).
Anyway, hope this helps, or at the very least generates a few laughs or smiles on faces.
You could try the LSTN2 from Seiche: https://m.youtube.com/watch?v=1CnZSrEZ6o8
It’s a very simple small unit with headphone out, and line out option to a recorder (I use a Zoom F3 with it) or an iPhone for a real-time spectrogram.
Seiche (hands up - I work for Seiche) do a high spec portable recorder (PAMGo), but that’s probably too pricey for what you are looking for in this case.
I and my colleagues have used https://www.aquarianaudio.com/ hydrophones (mentioned in WMXZ's answer) for years. They are relatively cheap, robust, sound good (I am an artist rather than a scientist ;) and they have a wide frequency response. We have already had a generation of students misusing them at the Institute of Sonology here in The Hague and they have lasted pretty well.
For amplification / recording I suggest using a recorder such as a SoundDevices MixPre - you can connect this easily to headphones or a monitor loudspeaker.
If using a loudspeaker system, you might want to filter out some low frequencies as the hydrophones are quite sensitive to LF and having a speaker in your boat may cause feedback. Aquarian audio can make long cables for you if their standard lengths are not suitable.
I thought the YouTube clip might illustrate audio out from a LSTN2, on a small boat around cetaceans.
Here is a link to a datasheet - i'll post a permanent link next week: https://we.tl/t-Bb9yymmi8X