Elephants, whales, and many other animals use low frequencies to communicate over very long distances. Low-frequency sounds travel further than high-frequency ones.

It's possible to make a drum in the forest for emergency communication.

1. Setup your tarp between two trees, and make sure the ropes are very tight. (the trucker's hitch is good for this)
2. Find a suitable branch, maybe 2' long and 1" diameter. If it's sharp at the end, round it over with your knife/axe so it doesn't damage the tarp.
3. Bang on the "drum head" with your "drumstick".

Since the tarp is large, it should produce a fairly low note. Tarps that are thicker should produce a better note.

Another possible method:

1. Setup your tarp, parallel with the ground, and make sure the ropes are very tight.
2. Setup a rope that's vertical. The bottom of the rope should attach at the center of the tarp, and the top of the rope should attach to a tree branch directly above.
3. Attach the rope to the center of the tarp using the "improvised tarp attachment point" trick (TODO: find a link for this).
4. Make sure the rope is very tight.
5. Pluck the rope like a guitar string.

Here, the tarp acts as a speaker diaphragm, and the center rope acts as a voice coil.

For this version, you should be able to vary the frequency of the note by varying the length of the rope.

However, note that even primitive drums included a drum shell of some sort.