Now whether you want to create sound libraries for others to use or simply want to record your own sounds, when choosing a hydrophone for field recording there are several factors to consider, including:

1. Sensitivity: This refers to the ability of the hydrophone to pick up sounds. A more sensitive hydrophone will be better at capturing quieter sounds, but may also pick up unwanted noise.
2. Frequency response: This is the range of frequencies that the hydrophone can pick up. A wider frequency response is generally better, as it allows you to capture a greater range of sounds.
3. Self-noise: This is the amount of noise that the hydrophone produces itself. A lower self-noise is better, as it means that the hydrophone will pick up more of the desired sound and less of its own noise.
4. Directionality: Hydrophones can be either omnidirectional (picking up sound from all directions) or directional (picking up sound from a specific direction). The choice depends on the type of sounds you want to capture.
5. Price: Hydrophones can range in price from less than $100 to several thousand dollars. The price you pay will depend on your needs and budget.
6. Phantom power is an important factor to consider when choosing a hydrophone for field recording. Cheaper hydrophones may not require phantom power, making them easier to use with recording devices without XLR inputs. However, these products often have a higher self-noise and may not reproduce the same frequency range as hydrophones with phantom power.