Subs -n- Ports Relationship

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[Doxquzme Premium Member]

Not trying to hijack your feeds, but me again...

If you are placing the subwoofer such that the port faces any one of a nearby interior wall, the distance to the metal or hard object will affect (lower) the box tuning, as the small area in between the wall and the port effectively extends the port length. If the speaker and port are firing "at the same area" this won't be an issue.

There are a few other issues related to ports including integration of the sound from the port with the direct sound from the driver, chuffing (air turbulence creating noise when the port is working hard), and midrange coloration (undesirable higher frequency sounds escaping the port - high-frequency compared with the port tuning ie 100hz when the tuning is 20hz). Chuffing can be exacerbated by placement close to a hard interior surface, while pointing the port away from the listener tends to reduce midrange coloration (still not an issue in your other build).

These are all "best case" recommendations. In your other build, it's not an issue because the driver and the port will be on the same face into the cabin, no reflective surface near to cause an issue. In your truck, it may just be a luxury and one that is not available. In cases where you can't have the port on the same side, then the next best is on the opposite side, If that is not option, then wherever it can be is used.

Bass port acoustic coupling connects the internal volume of a bass reflex enclosure to the outside environment. This coupling creates a Helmholtz resonance at the lowest frequencies of the system. The port is an opening in the enclosure that is tuned to create specific resonances and coupling with the air outside. The port creates more output at low frequencies, allowing the speaker to produce lower frequencies better.

The port is usually installed on the same side as the woofer. If this is not possible, the port can be installed on the side or back of the enclosure. The opening of the port should not be blocked, and multiple ports should be positioned on the same side of the enclosure.

 

[1aespinoza 10+ year member]

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 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.
I know JL designs their low power loaded boxes with a side port, but their heavy hitters port and sub face the same direction. So I think there may be something to it.

 

[1aespinoza 10+ year member]

Not trying to hijack your feeds, but me again...

 

[Buck 5,000+ posts]

Different sub and port placement can move pressure zones within the vehicle where the sub and port waves collide and create pressure peaks in relationship to how waves meet and collide while inside of the vehicle.

What matters most is the sub and port are in phase with each other, like sub and port exit into the exact same airspace. Sub and port location matter more for nuanced pressure directing, where a port tuned properly will phase properly with a sub, no matter where either one is. Certain types of boxes are more sensitive to woofer distance from port opening, but usually it's not a huge difference, especially in car audio where dimensions are so limited vs the 1/4 length of bass waves. Sub distance from port absolutely changes tuning, but it's complicated due to pressure changes outside vs inside of port, and would matter more in like a t-line or horn vs less so with ported, IMO.

It's all about properly phasing the pressure. Bass is omnidirectional, not angular like mids and highs become, so as long as everything is in phase, directions don't fundamentally matter (basically), but port and sub location does matter more when designing for different environments and cabins, where you move subs and ports to aim pressure zones, like to your ears or a SPL mic, so it does matter. It's hard to describe fundamentals vs reality, because everything you change matters somewhat.

It's all about phasing the pressure.

 

[Buck 5,000+ posts]

Also, resonance or energy buildup happens due to raw physics, like 1/4 wave design making pressure build in the chamber and port at tuning frequency. Impedances to specific frequencies in the port are what allow the pressure to build, so when it comes to air moving in and out of ports, it's AC, so it's frequency-based impedance changes to the movement of energy through the air, it's not a push like a balloon farting out all its air, that's resistance. Impedance is all frequency based, that's why your ported box will unload away from frequency until the sub destroys itself, because there's no air impedance restricting sub movement inside of the sub chamber away from tuning.

Impedance is just like how amperage burns smaller wires and not bigger ones, just it's based on frequency and resonant energy buildup. Even electrical wires have 1/4 wave resonances, that's what all antennas are.

 

[Water Bear]

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?

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.

Anyway I'm just pondering the answer as I sit here in the crapper, using some basic mathematical knowledge and with very little engineering expertise, so I could be way off. Interesting question you asked though.

 

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[Doxquzme Premium Member]

it's what i do while waiting for my HT Marantz Cinema 40 to arrive!

 

[Doxquzme Premium Member]

I suppose the same intrigued me thinking about passive radiators.

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...

 

[1aespinoza 10+ year member]

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!