Port flares make a large difference in how much air a port can pass before turbulence becomes audibleBeing able to increase airflow means you can use a smaller diameter port. Since the port length is proportional to the square of the port diameter, this can make a large difference to the length of your ports. A shorter port is more likely to fit into your enclosure without any bends, further saving on noise and build difficulty. It also has less chance of "pipe mode" resonance intruding on the operating range of your subwoofer. The reduction in volume occupied by the port also means a smaller box.
Some options for making your own flares
Routed flares - up to 13mm or 36mm in radius
Donuts and Smoothing Rings - up to 13mm or 36mm in radius
Heat moulded PVC Flares - up to 25, 30 or 40mm
Whilst making the moulds requires some work, this can be offset if you make ports for a few friends. For countries where commercial 30mm radius flares are not available, or are too expensive to import, this method can give a professional finish if you take your time.
Heat moulded PVC Flared ends
Custom built - larger flares
Flares for slot ports
Effective LengthA flared port shares part of the air in the flare with the surrounding air, meaning that the port appears shorter. The generally accepted adjustment is to subtract half the flare radius from the physical length to obtain the effective length. If both ends of the port are flared, the adjustment is done for each flare. See the Test Ports page for examples
How large a flare is needed?I have conducted a series of experiments to determine what velocity of air can be passed through flared ports before port noise becomes audible. Fifteen ports in a range of diameters and flare sizes were tested, leading to the following observations:
- Maximum usable velocity varies with flare radius
- There is a limiting velocity for any given diameter port, regardless of flare radius
- Larger diameter ports have a higher usable velocity than smaller ports
- Port performance varies with frequency
The following pair of tables show the default recommendations for some common port / flare combinations. Selecting different options, such as allowing some port compression can change these values.
They show the maximum allowable port velocity in metres per second, so that turbulence will not be noticable at the typical Home Theatre seating position. The core limit is where the air in the "core" of the port becomes turbulent, regardless of flare size. Combinations that are core limited are shown in red
The first table is for a sub whose peak velocity occurs at 30z. The second table shows what happens when the velocity peak is at 20hz - note the drop in performance
Useable velocity at 30hz
|Port diameter -->||50 mm||86 mm||100 mm||150 mm|
|30 mm flare||15||19||21||31|
|40 mm flare||15||24||27||38|
Useable velocity at 20hz
|20hz Port diameter -->||50 mm||86 mm||100 mm||150 mm|
|30 mm flare||10||12||13||20|
|40 mm flare||10||16||17||24|
Click here for all the details on how the experiments were done, and to download the Flare-it calculator
As a general rule, the minimum port diameters should be as shown below.
Always check the velocity in your design. For high excursion drivers you may need more portage.
|Driver size||Minimum Port Diameter|
|10 inch||4 inch|
|12 inch||5 inch|
|15 inch||6 inch|
|18 inch||8 inch|
Commercial port flaresIf DIY flares are not for you, I've compiled a list of some some commercial options. If you live in the USA, you have access to Precision Ports and to the PNR Aeroports, both of which have a flare radius of around 30mm
Clearance around port intakeWhere possible there should be no wall closer to the port intake than one port diameter. When using a flared port, the ideal clearance should be the diameter of the flare exit.
Where this is not possible, aim for one port diameter clearance from the outside edge of the flare. If your port is a bit closer to the back of the box, maintaining plenty of clearance around the sides of the port exit will minimise possible turbulence and tuning changes.
BendsWith regard to bends, I have had no noise problems with 90degree and 180degree bends at 10m/sec. For the higher velocities that a flared port allows, it is probably best to aim for a straight port.
If you are purchasing PVC elbows, bear in mind that one of the papers on the AES site recommends that the minimum bend radius should be 20% of the port diameter. If your supplier only sells bends where the inside bend is a sharp 90 degree step, try and find an alternate source. See my Good Bend, Bad Bend page for some guidelines on selecting your bends
Port CompressionUsing flares allows boundary layer turbulence, generated as air leaves the port, to be controlled. Above a certain velocity, excessive turbulence within the core of the port occurs. This type of turbulence, which cannot be controlled by flares, represents the limiting velocity of the port, and the onset of compression
Other experimentersSince airing these results on several discussion forums, I have been told about the wealth of information available from the the Audio Engineering Society website. After accessing some of their documents, I can recommend a look.....
Further workAll the work referenced on this page uses flares with a circular cross-section. As the air travels out of a flared port, it slows down in the process. Theoretically this would allow the flare radius to be decreased as the air gets closer to the end of the port. This would result in an exponential profile, or a tractrix profile depending on the exact shape used.
A benefit of this approach is that the flare requires less overall diameter for a given starting radius. When applied to the heat moulded flares, it would allow the initial radius to be increased, meaning a higher usable velocity than that which could be obtained with a circular flare profile.
Expect to see more on this subject.....