If you are out on a recording set and handling audio equipment, it is extremely important to keep your headphones as dry as possible. If the headphones are wet, the built-in speakers are going to crackle and listening to the audio becomes impossible as you can't determine if the actual audio is crackling or if your headphones are damaged. Thankfully there are a few inexpensive ways to turn standard headphones into waterproof equipment.
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Place foam covers over the headpieces on your headphones. These foam devices can be purchased from any electrical store and are designed not only to pad your equipment but to prevent condensation from seeping in.
Wrap all the wiring of the headphones with electrical tape. Make sure the electrical tape is on tight as openings in the tape allows water to funnel into the equipment.
Place the headphones onto your head and slide a winter hat over them. This covers the headphones and your ears, preventing most water from ever touching the headphones. If you will be working in heavy rain it is a good idea to bring multiple hats.
Stand under an umbrella or any shade possible to reduce the amount of water that reaches you.