Tried a couple more ports designed to dampen the pipe resonance. While I've made some great examples that really do the job well, all of them suffer from much increased harmonic distortion and chuffing…
Hi Augerpro,
Very nice work on the perforated port here! The addition of 1/4-wave stubs at strategically located spots can be easily simulated and fine tuned using Akabak before making hardware. I don’t think micro perforations are necessary, nor desired. What you want is something the actually *disrupts* the formation of a standing wave resonance. A simple duct on the side wall circa 1/10 the CSA of the main vent will work wonders.
The way I go about it is like this:
1. Calculate the fundamental resonance frequency of the duct. It’s a 1/2 wave resonator since open at both ends. Let’s assume you have 8in long duct. That’s 0.203m and 1/2 wave resonance is 842Hz.
2. A 1/4 wave stub will be exactly half this length or 4in long. The most effective place to put the stub is at the duct midpoint where pressure variations are the largest. Fill the stub with loosely packed fiberglass.
3. So the model in Akabak would have a 4in T connected to the midpoint of the 8in duct.
4. Make the stub about 5/8in dia for a 2in dia vent. This gives about 10:1 ratio as a starting point.
When you run the sim of the reflex box with vent, adjust the dia of the stub and the stuffing density until you get the best suppression of the resonance.
So in real life get a 2in PVC duct and some 1/2in or 3/4in PVC duct and drill a hole at the midpoint of the 2in duct and glue on a 4in long stub. Use a removable end cap or threaded pipe joint connection to allow adjustment of stuffing.
It should work well. The problem with little micro perforations is that the holes have a lot of pressure loss so it takes a lot of them. You have to make (relatively expensive 3D prints or drill them carefully). But I am not sure it’s any better than a simple T joint with a larger pipe.