To prevent your water fall from sounding like you are filling the bathtub you need to have the water strike the pond surface at variable times. Having part of the falls hit a rock before going into the pond always makes a pleasant sound, of course there are many other ways to break up the water as well.
You do not have to have a tall waterfall to get a beautiful melody. A drop of only 6 inches makes noise while a drop of 6 feet can be deafening or a whisper. It is not the height of the falls that matters, it is how it strikes the pond surface. An 18 inch waterfall can be loud enough to be heard around the yard while still being soft enough to talk by.
Make the waterfall proportional in size to the pond. Nothing looks more out of place than a waterfall that is much bigger than the pond it feeds.
If the water water coming out of the pipe looks like it could be a fire hose (because it shoots out so fa)r all you have to do is increase the diameter of the pipe the last couple of feet and it will push just as much water without so much force.
Waterfalls are the most popular additions to a pond and are the most confusing to make. They add oxygen to pond water and add an enticing melody to the yard.
Waterfalls come in many different shapes and sizes. Whether there are many smaller falls in a waterfall or one big sheet falling into the pond, no one type is better or worse than another.
Waterfalls must be watertight, just like ponds and streams. Use EPDM liner to prevent the water from seeping into the ground. This liner must overlap on top of the liner for the pond. If it goes behind the pond liner the water will leak into the ground. The waterfall liner must also overlap the pond liner by at least 6 inches as well.
If you are looking for that sheet of water falling into the pond you will need a flat rock at least 1 foot long for the water to run over the edge of. The flat rock does not have to protrude a foot out and would not hold the weight of the water if you did. No more than half of the length of flat rock should protrude out over the cliff it sits on. The edge farthest from the falls needs to have rocks laid on it to hold the flat rock from falling into the pond when the water runs over the edge. How much of this rock should be exposed? As much as you want, but at least half should be covered with rocks to hold it down or the sides held down by larger rocks.
How much water needs to be pumped over the fall to look nice? Although you will see varying amounts the general rule of thumb is 100 gallons for every inch of width. So a 12 inch wide waterfall would need at least 1,200 gallons/hour. More water/hour does not just mean deeper water it also means the water will shoot out farther before drooping into the pond. Keep in mind that a 1,200 gallon/hour pump will not pump 1,200 gallons at a height of 6 feet. The higher the falls the more the pressure drops.