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Port Flares

Port flares make a large difference in how much air a port can pass before turbulence becomes audible

Being 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

Routed port rollovers
Make your own by routing a rollover on the supporting panel. As above, the flare radius is limited by the router bits available - a small router will get you 13mm, full-sized routers can do up to 36mm (1-1/2")
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Donuts and Smoothing Rings - up to 13mm or 36mm in radius

Donuts and smoothing rings can provide flares where space is limited. These are also popular with sonosub builders. The flare radius is limited by the router bits available - a small router will get you 13mm, full-sized routers can do up to 36mm (1-1/2")
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Heat moulded PVC Flares - up to 25, 30 or 40mm

DIY moulded port
PVC pipe can be heated with a hot air gun and pushed onto a mould with good results. This method will produce flares up to 25mm for port diameters up to 100mm, flares up to 30mm for 150mm pipe, and flares up to 40mm for 225mm pipe. Flared ends and joiners can be produced, but the nicest feature of this method is that you can make a one-piece port. (you can't buy those anywhere....)
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.
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Heat moulded PVC Flared ends

Flare made from flat sheet
I recently had another look at making flared ends from flat PVC sheet heated and pressed between a pair of moulds. Unfortunately this method doesn't allow flares any larger than the heated pipe method. It may be useful if you require a flare with a mounting flange
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Custom built - larger flares

Custom built flare
Flares larger than those described above need to be individually built. I've built ports using an "inside out" version of the "heat moulded" technique.
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Flares for slot ports

Slot flare
Slot ports sometimes require a flare which is larger than you can achieve with a router. Splitting some PVC pipe lengthwise can do the trick
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Effective Length

A 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

Effective length of flared port

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: The results have been incorporated into the "flare-it" downloadable calculator. It is suitable for designing flares for ports up to 150mm (6 inches) in diameter, and which operate with a velocity peak below about 35hz.
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
Unflared 4 7 8 16
10mm flare 7 10 12 21
13mm flare 9 11 13 22
25mm flare 15 17 19 29
30 mm flare 15 19 21 31
35mm flare 15 22 24 35
40 mm flare 15 24 27 38
Core Limit 15 24 30 38

Useable velocity at 20hz

20hz Port diameter --> 50 mm 86 mm 100 mm 150 mm
Unflared 2 4 5 10
10mm flare 5 6 8 13
13mm flare 5 7 8 14
25mm flare 9 11 12 18
30 mm flare 10 12 13 20
35mm flare 10 14 15 22
40 mm flare 10 16 17 24
Core Limit 10 16 20 25

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
08 inch -
10 inch 4 inch
12 inch 5 inch
15 inch 6 inch
18 inch 8 inch

Commercial port flares

If 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
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Clearance around port intake

Where 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.


With 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 Compression

Using 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
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Other experimenters

Since 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.....
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Further work

All 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.....



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Page modified: Wednesday, 17-Oct-2018 03:13:09 EDT
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