User guide

NOCTILUCA

NOCTILUCA turns an Android phone into a hydrophone recorder. Plug in your underwater microphone, see the sound live, and save WAV files you can study on a computer. This guide takes you from install to your first recording.

← Back to Hydrophone Download NOCTILUCA (.apk)

What you need

Your hydrophone with a 3.5 mm jack (built in Steps 1–2).
An Android phone or tablet, Android 7.0 or newer, with a headphone jack. No jack? A USB-C audio adapter works too.
The NOCTILUCA app (download button above).

NOCTILUCA is Android only for now. There is no iPhone version yet.

Install the app

NOCTILUCA is a personal project, so it is not on the Play Store. You install it from the file you download. Android will warn you that it does not recognise the app — this is completely normal and safe. It is the standard message for any app installed outside the Play Store. You just have to confirm the installation.

On the phone, tap Download NOCTILUCA above. The file noctiluca.apk goes to your Downloads.
Open it (Files app → Downloads → noctiluca.apk).
Android says installs from this source are blocked. Tap Settings and turn on Allow from this source, then go back.
Android shows a warning that the app is unknown / not recognised. This is expected for a homemade app. Tap Install anyway (it may be hidden behind More details).
Tap Install, then Open. You should see the NOCTILUCA icon on the phone.

In short: every warning you see is just because the app comes from outside the Play Store, not because anything is wrong. Confirm each step and the install will go through.

The three buttons

Button	What it does
Listen	Starts capturing the microphone and shows the sound live, without saving. Use it to check that the hydrophone signal is coming through.
Record	Starts saving the sound to a file. A red dot and a timer show that it is recording.
Stop	Stops listening or recording. When you stop a recording, the WAV file is saved automatically.

Make your first recording

Plug the hydrophone into the phone's jack before opening the app.
Open NOCTILUCA. The first time, it asks to use the microphone: tap While using the app.
Tap Listen. The spectrogram should start scrolling and react to sound (tap the sensor gently, the colours move).
Tap Record to capture. Watch the red dot and timer.
Tap Stop. A message shows the saved file name, like noctiluca_2026-07-15_14-32-08.wav.

Find and share your recordings

On the phone

Tap My recordings in the app to see the list. Tap a recording to Play it or Share it (email, cloud, messaging).

On a computer

Connect the phone by USB and choose File transfer. The files are in Internal storage → Music → Noctiluca. Copy the WAV files to your computer.

Reading the spectrogram

Up and down is the pitch (frequency): low sounds at the bottom, high sounds at the top.
Left to right is time: the picture scrolls, the newest sound is on the right.
Colour is loudness: dark is quiet, bright (green, yellow, red) is loud.

Different animals leave different marks: a fish, a dolphin whistle or a passing boat do not look the same.

Study the sounds on a computer

The WAV files are uncompressed, so they keep all the detail for analysis. Open them in Audacity (free) to listen and zoom in, or use the Python spectrogram code on the hydrophone page to turn a recording into an image.

Safety and respect for wildlife

Always stay with an adult near the water. Never play loud sounds underwater, and keep your distance from animals: the goal is to listen without disturbing them. Keep the phone away from the water; only the hydrophone goes in.

If something does not work

Problem	What to try
The spectrogram reacts to your voice near the phone, not to the hydrophone	The phone may be using its built-in mic. Plug the hydrophone in fully before opening the app, and push the jack all the way in.
The spectrogram looks dark or empty	The signal is weak. Turn up the gain on the preamp, and check the preamp battery.
“Microphone unavailable” message	Allow the microphone: phone Settings → Apps → NOCTILUCA → Permissions → Microphone.
No files in Music/Noctiluca on the computer	Make sure you tapped Stop (files save on stop). Use a data USB cable and choose File transfer mode.