1. Port compression - this is a reduction in output from the vent due to poor aerodynamics where high velocity compresses the output and reduces dynamics. As a result, the sub doesn't live up to its potential.
2. Port chuffing - an audible and distracting sound as vent velocity becomes extreme. It can be louder than the bass itself. These two problems are in fact one problem to a different extent.
Typically, many will follow a basic rule of thumb, such as using a 4" vent for a 12" driver, 5" for a 15" and 6" for an 18" driver. Others go by the velocity, aiming for arbitrary numbers like 34 m/s. This is a very hit and miss approach and I don't recommend it for anyone serious enough to design and build their own subwoofer. Just a little extra effort and you can design a high performance vented subwoofer. If this seems like too much effort, then I suggest a sealed design.
A few basics
The performance of the vent is determined by the velocity and the aerodynamics of the vent itself.
Port velocity is increased by:
- larger drivers
- high excursion
- reducing the diameter of the vent
- increasing the vent diameter
- using a rumble filter below tuning
- lowering tuning
Note: commercial subs often use high excursion subs in a sealed box or lower excursion drivers with a vented design which tends to avoid making the vent design critical. It's not common to see too many designs with very high excursion drivers vented. Typically there are too many compromises involved. Also keep in mind that many subs claiming to use high excursion drivers have an xmax of around 12mm. Frankly, subwoofers have progressed a long way since 12mm xmax was impressive.
Increasing vent diameter. Sounds like a great solution, but this makes our vent longer. As soon as you make the vent big enough for your 25mm xmax driver, you have something that is huge. Oops! Now this is where we want to start getting away with higher velocity. Now we work on aerodynamics.
If we build a lazy vent, with no flares on the ends, it will chuff even with low velocity ie 7m/s. Let's say we have a 100mm vent. Add a flare with an 18mm radius and we can now live with around 10m/s. If we increase that radius to around 50mm then we can now get up to 20m/s. At that point, we can't improve further because the vent reaches its "core limit" at which the vent walls will start to cause problems. To go to the next level, we need a bigger vent.
Stepping it up to 150mm, the core limit now goes up to 25m/s. If we start getting really fanatical, we might go for 200mm, just so we can say "my vent is bigger than yours!" At this point, we can deal with 34m/s.
So where am I plucking all these numbers from?
I have to give some credit where it's due. Bill Collison (Collo) did some experiments on vents to determine where chuffing becomes audible. He then used the results to create an application called Flareit. The numbers here all come from that application. If designing a vented subwoofer, I suggest you open up Flareit along with WinISD. It's an essential tool.
Download Flareit here >
Driver: Exodus Audio Tempest X 15"
Box: 400L vented tuned to 16 Hz
With the right settings, we have a textbook result with -3 db points at 20 and 80 Hz. With around 800w we use up all of the xmax capability.
A rumble filter keeps excursion under control below tuning. The cone will be moving up to 54mm from peak to peak.
If we use a 100mm vent, we have a problem:
Vent velocity goes off the chart! 54 m/s. At that velocity your subwoofer will work quite well as a fan to keep you cool in summer, and it will be noisy and chuff like mad. Things change as we use a bigger vent.
Now we have something workable. What happens if we turn off the rumble filter?
Again we have a problem. Very low bass that is present in movies will cause chuffing.
Given that our box is very large, we can live with a bigger vent. This vent is only 400mm long, so we can easily go up to the next size and have very low compression.
Now we have very low velocity, and that means we don't have to pay as much attention to aerodynamics and more difficult to construct vents with large flares.