leaving mics in a cave during fieldwork

Question

I'm recording bats in a cave with many small (Knowles SPU-0410), and big (Sanken CO-100) mics.

The cave is mostly isolated from the outside, with a temperature between 10-15C inside, and it could get moist every now and then based on outside conditions.

Id really like to leave the mics in the cave over multiple nights without moving them ( and only taking the ADC + batteries outside to dry). Does anyone know if it okay to do so, or have personal experience? How worried should I be about moisture seeping into the mic internals? Also, what about potential changes in sensitivity?

Answer 1

To be safe, I would use microphones which the specifications explicitly say that they can handle such humidity.

RF condenser technology is described as "weather-proof" by the manufacturer (e.g. here) and Widkipedia says:

The RF biasing process results in a lower electrical impedance capsule, a useful by-product of which is that RF condenser microphones can be operated in damp weather conditions that could create problems in DC-biased microphones with contaminated insulating surfaces. The Sennheiser "MKH" series of microphones use the RF biasing technique.

Otherwise, other microphones could do the job, depending on your constraints, e.g. DPA 4017 (directional) or DPA 4060 (miniature, omnidirectional) which the manufacturer says they are resistant up to 90% relative humidity.

Answer 2

While I cannot speak to the effect on your microphone, I do have an alternative to consider:

My bioacoustics professor, Dr. Bill Evans, told me stories of the early days of underwater bioacoustics where they used condoms to protect their microphones. Determined to have backup before I embarrassed myself... I found this question on the Sound Design Stack Exchange and found that in addition to unlubricated condoms, people also recommended balloons (especially those used for balloon animals). I highly encourage everyone to enjoy this very exceptionally entertaining and informative thread!!!

I feel like I have now gone full circle-- and I tip my hat to Dr. Bill, whose stories live on! (and who would be proud to have been cited for this type of conversation!).

Answer 3

I have not tried this myself, but I would be concerned with moisture damage. As far as I know, all condenser mics have vents to equalize the (static) pressure behind the membrane to the ambient pressure in front of the membrane, so you don't end up capturing variations in atmospheric pressure. This means that moisture can always enter the device through the vent (and perhaps via other paths depending on the mic construction) and potentially cause damage. I have read about certain outdoors test setup that include a small heating element to keep the mic at a slightly higher temperature than ambient temperature to avoid condensation, but I have no idea how easy and/or cheap it is to purchase/build one.

Also note that even a thin membrane over the mics will likely change their frequency response, especially at higher frequencies (I'm assuming you are looking into high frequency / ultrasound signals since you're working with bats?). The change in frequency response can be estimated by measuring test signals with and without the membrane, with some luck it might be very small and not matter for your analysis.

[Quick thought that just occurred to me: if you encapsulate the mic with a balloon/condom, the air inside the membrane will still have some moisture in it, and it might create some small condensation inside the balloon. Again, I have no experience with this, so I don't know if this is a significant problem or just a theoretical one!]