[hahnmeister]

The dual standpipe method works better, FWIW.

You use a primary standpipe (no durso needed) which is shorter and has a valve all the way at the bottom outlet to 'back up' the water. This prevents the noise, bubbles, etc by adjusting the water level in this pipe. Then, you have the backup drain plumbed normally, with a standpipe that is taller than the primary. This is the backup/safety should the primary get clogged or need adjustment.

Its silent, its easy, etc. Much better than any stockman, durso, etc.

On my secondary, I utilize a 'S' trap to keep the water from falling more than 12" at any given time.... kind of like a weir or levy system to slow the water down and prevent bubbles.

Piping at an angle CAN help... it will slow down the water's rate of acceleration due to gravity, but if given enough height, it can still reach the velocity needed to drop in pressure and educt/suck in air bubbles as a consequence. This is why the 'S' trap system (although its not a sealed 'S'... there is an air inlet at the top of the 'S') or the dual standpipe methods are better. No stockman/durso/hoffer system can prevent the acceleration of water in the pipes, which is why the air gets sucked in, the noise starts, the salt spray spreads...

As water falls, it picks up velocity, and as water picks up velocity, its pressure drops as well, causing air to be sucked in. So putting a muffler on it doesnt solve anything... you will still end up with air being sucked in, noise either in your sump or in the tank because the velocity of air/liquid at the top will be lower than that at the bottom no matter what. To solve the problem rather than deal with it, you need to slow down the water's velocity.