First aero port build, question about port area

[Triticum Agricolam]

I think those specs are a good compromise given the limited space you have to work with.

Having the port 4" from the rear of the box will probably lower the tuning a bit, but that's most likely to your advantage.

[IKnowImLost224]

I think those specs are a good compromise given the limited space you have to work with.

Having the port 4" from the rear of the box will probably lower the tuning a bit, but that's most likely to your advantage.

Awesome thank you very much! Appreciate everyone's help and detailed responses!

[Nightshade]

Lots of good discussion in this thread. Here are my thoughts.

I disagree with Krakin, box volume does effect port area requirement, all else being equal. With a larger box volume the subwoofer is acting on a larger mass of air, more air being moved needs more port area to move though. This is closely related to why larger boxes produce a larger gain in output around tuning. If a larger box produced the same gain as a smaller box did then they would need the same port area, but this is not what happens. Where people go wrong with things like "port area per cube" rules is when they don't consider anything else other than box volume when trying to determine adequate port area, which is what I think Krakin is speaking against.

As far as impedance (or box rise as its commonly called) goes, I largely agree with what Nightshade is saying but I would like to expand on it a bit. Its easy to think that higher impedance is always bad, but this is not the case. Impedance is closely related to how hard the sub has to work. The harder it works the lower impedance is. Just because a sub is working harder doesn't always mean its putting out more output though. Also its important to remember that amps do not put out watts, they put out volts. When you are trying to get as loud as possible efficiency (which is dB/watt) is less important than sensitivity (which is dB/volt).

With a sealed box (or infinite baffle) there is a peak in output at the resonant frequency of the sub. Due to this, sealed subs are VERY efficient when playing content at their resonant frequency, you get a lot of output relative to the amount of watts the system is consuming. This is because the sub naturally wants to move easily at the frequency so very little power is wasted just overcoming the suspension of the sub, thus high impedance. The sensitivity isn't much better or worse at that frequency though, which is why there isn't (or shouldn't be) a peak in output at Fs. With a ported box there is a dip in impedance at the tuning frequency. This is because a ported box is a resonant system and the sub has to work hard to make that system resonate, thus low impedance. Due to the low impedance efficiency is also low at this point since the amp has to put out a lot of watts. Sensitivity is usually very high though, which is why people build ported boxes in the first place, they get you a lot of output per volt (which like I said earlier, is what amps put out). Ideally you want high sensitivity and high efficiency since you get lots of output and its easier on you amp and electrical system, but that's not always easy to do.

Now the trick is determining when high impedance is good and when it is bad. When impedance is raised due to compression its bad. Port compression is what you get when your port is too small and the port velocity is too high. This is why Nightshade saw his impedance go down with larger ports. Too small of a port restricts the amount of work the sub can do, this may seem counter-intuitive, but its just like when you plug the hose on a vacuum cleaner and the motor speeds up, you restricted the amount of work it can do. In addition to port compression there is power compression which is what happens when the voice coil gets hot and its resistance increases, also bad. When higher impedance is bad it decreases sensitivity.

Higher impedance is good when its corresponds to an increase in sensitive. This is what happens when you make a ported box bigger, it will have higher impedance, so your amp will do less work, but you will get greater output due to higher sensitivity. If the impedance is raised enough, you may be able to wire to a lower nominal impedance load and get even higher sensitivity out of your system. Higher impedance isn't always bad.

Good information. Thank you for adding that to the discussion.

[Krakin]

I as well agreed with almost all you said, I just disagree completely with any aspect of designing a port around the port area per cubic foot idea.

[DLHgn]

This is an awesome thread. Learned a lot.

[Nightshade]

I as well agreed with almost all you said, I just disagree completely with any aspect of designing a port around the port area per cubic foot idea.

I understand you have your own opinion on what is important when designing a proper enclosure. My particular method is somewhat of a shorthand approach for a good starting point that takes out a lot of guess work. By no means is it a full blown guarantee that using a specific amount of port area per cube will work the same in every situation either. However, for quick reference and especially SPL centric builds it works very well.

[Krakin]

I as well agreed with almost all you said, I just disagree completely with any aspect of designing a port around the port area per cubic foot idea.

I understand you have your own opinion on what is important when designing a proper enclosure. My particular method is somewhat of a shorthand approach for a good starting point that takes out a lot of guess work. By no means is it a full blown guarantee that using a specific amount of port area per cube will work the same in every situation either. However, for quick reference and especially SPL centric builds it works very well.

The basic theory I've come up with adding as much port area as possible I haven't seen a negative to except for the fact you're designing for a car and up to the point of where your port has now turned into a pipe organ resonance. Which I forgot the length per frequency it does that since I've been out forever.