I saw that as well and have to admit it confused me. I have heard many setups with ports facing differently than the subs that sound really good. Buck (great box designer) once told me to design my next box with the sub forward and port to the side closest to a wall. He said the offloading effect would give me nicer lows.So it was told in another thread of mine that the subwoofer and port should be firing the same direction so they play nice together.
I'm in way out of my depth here, but that was an interesting question that made me think. Let's just imagine that all changes in air pressure travel at the local speed of sound. If you think about it the distance from the back of your woofer through the port to the front means that it'll take a while for changes on each side of the woofer to reach each other. When the woofer extends, the air is compressed in front of the woofer, that expands in a spherical wave, and the low pressure zone inside the box starts to draw in higher pressure air from the environment. But that low pressure region travels at the same speed as the compression wave. So it'll take a few...milliseconds or less or whatever for the low pressure effects to reach the front of the woofer, and by then the woofer has started to draw back. Time it all just right and you get resonance - the low pressure from the port reaches the woofer right when it is decompressing the cabine. All this happens in a spherically radiating pattern (reflecting off the cabin interior) so it's hard to describe exactly where the peaks and valleys of pressure happen, be that constructive or destructive interference. Compared to having two woofers right next to each other firing out of phase, the destructive effects would align along much more of the wave fronts much more of the time.In my mind the sub is retracting therefore compressing the volume of air inside the enclosure. This causes an expulsion of air through the port. Shouldn't that cancel out the frequency of the speakers? Why wouldn't it be like wiring one speaker backwards?
it's what i do while waiting for my HT Marantz Cinema 40 to arrive!
Passive radiators are just another "port" type. They are generally harder to tune and there isn't any box volume savings. Unless the manufacturer has already done the numbers AND has a recommend or supplies the passive radiators, I usually pass. Parts Express is pretty helpful and has a good selection, for me and my lack of experience tuning the, hard to justify them in a car. I have done a couple modeling them in Bassbox Pro and WinSid, but...I suppose the same intrigued me thinking about passive radiators.
The late 80's early 90's car's OEM speakers were of the same quality as today's offerings. The main difference was the rear speakers were enclosed in a baffle that was made out of jute padding. Come to find out they were what is called an aperiodic membrane "enclosure". They sounded pretty bassy for what they were and I loved them. Coupled with an AC Delco radio we were cruising!I suppose the same intrigued me thinking about passive radiators.