Improved Port Area Calculator

[Triticum Agricolam 15]

I came up with this the other day and posted it on SMD, I figured I should post it here too.  Hopefully someone will find it useful.

I'm sure most people here have come across the "12-16 sq in of port area per cube" guideline for slot ports and the "9-12 sq in per cube" for aeros.  The problem with both of these is they only consider box volume.  Both input power and tuning frequency, along with box volume, have a great effect on how much port area you need.  This is why I've been recommending to people to NOT use those rules of thumb.  Online you can find several websites (such as carstereo.com) that have port area calculators using the formulas developed by Dickason or Small.  These have a similar problem of ignoring input power and these formulas aren't really relevant for our modern, high Xmax subs.  The best solution is to use box simulation software (WinISD, BassBoxPro, etc) to determine how much port area you need based on the specifics of your system.  However this is time consuming and not everyone has access to that software or has the desire to learn how to use it well.  I wanted to come up with something is quick and easy to use, and should hopefully give a lot better results than the rules of thumb or the obsolete formulas.  Here it is, to make this easy I'm using Google Docs:

https://docs.google.com/spreadsheets/d/1VREceNxz9YOcFlI8O_TG62-4O-l3Q2IjD5qsEkVVh28/edit?usp=sharing

This is what it looks like:

This will probably always be a work in progress, but so far I think it should work pretty well.  I encourage everyone to use it and give me your feedback!  Please note that due to the fact its a Google docs document multiple people can edit it at the same time.  So if the numbers are changing on you while you use it, its someone else, just have patience.

[Triticum Agricolam 15]

Here is how I developed the formulas that go into this. 

Most people tune their ported boxes to between 30-35 Hz, as long as you stay in that range you can ignore tuning frequency for the most part. So that leaves us with box volume and input power. I've noticed that if you view input power in terms of how many watts you have divided by box volume (which I'm calling "power density") there is a lot of similarities between different systems. For example, if you have a 4 cubic foot net volume box and you are going to be putting 2,000 watts of power into it, your power density is 500 watts per cube. Different size systems with the same power density are going to require very similar amounts of port area per cube to have similar port velocities. So a 4 cu ft, 2,000 watt system needs just about the same port area per cube as a 2 cu ft 1,000 watt system does.

When looking at systems with different power densities, a system with double the power density of a different system does not need twice the port are per cube to keep port velocities similar, it only needs about 50% more. Using that info if you calculate the power density of an enclosure (say 500 watts per cube) and then take the square root of that number (22.36) this gives us a value that can be used to calculate port area that will scale properly with different power densities. 

The absolute max port velocity I like to see through a slot port is 30 meters per second. To keep port velocities right around 30 m/sec you can multiply the square root of the power density by .605 and that will give you how many sq in per cubic foot of box volume you need. So in my example of a 500 watts per cube enclosure that means you need 13.5 sq in per cube.

Round or aero ports are tolerant of slightly higher port velocities and slot ports are, so with those ports your multiply the square root of the power density by .5445. This will give you a slightly lower required amount of port area. 12.2 sq inches in my example. 

Like I said, 30 m/sec is the max I like to see. At that point you will still be loosing some output to port compression and port noise is fairly likely. If possible using more port are than that is a good idea. I like to keep port velocities under 22 m/sec if I can. This applies to both slot and round/aero ports. To do this the number you multiply by is .82, so for my example we end up with 18.3 sq in per cube. 

Previously I said as long as you are tuning between 30-35 Hz, you can ignore what the tuning frequency is, but what if this is a bandpass box with its much higher tuning? Or what if you want to tune to the low 20s? Fortunately differences in tuning frequency affect how much port area you need similar to changes in input power. To compensate for different tuning frequencies I take the square root of the tuning and multiply it by .17677 and then multiply that by the results of my formulas above. This results in very similar peak port velocities regardless of tuning frequency.

[ohio45113 24]

Damn haven't seen you on here in a minute. Nice calc. 

[Triticum Agricolam 15]

22 minutes ago, ohio45113 said:

Damn haven't seen you on here in a minute. Nice calc. 

Yup, its been a while.

[jcarter1885 995]

Thanks for the gem man, need to design a new enclosure soon so this will definitely help.

[Notorious97200 339]

Thanks

I'll try it on my current enclosure.

 

[nem 2,019]

Very nice and use full.