Why choose a ported design?

I sometimes like to use External Ports

 

• They give you the freedom to go to a larger port which means no more port noise
• They are easily constructed from PVC pipe which is available in many sizes and bends
• They allow your box to be smaller
• They ensure your design will be unique

Naturally, there are some challenges ......

Box Size - The ported enclosure is double the size of a sealed enclosure. In order to achieve a reasonable SAF (Spouse Acceptance Factor), this bulk has to be softened or "explained away" somehow. A few strategies for dealing with SAF can be found in this subwoofer psychology thead at AVForums

• The 90litre “Beer Fridge” was initially presented in white melamine (basically chipboard covered in laminex) and had exposed Class3 Galvanised self-drilling-screws which gave it a sort of "can do" military look. Estimated SAF 2
  Later it was covered in imitation woodgrain vinyl that looked terrible. Revised SAF -2
  
  
• The 65litre “Sidewinder” was veneered and initially achieved a good SAF rating. The addition of the external ports however blew this out the window, and it has to rely on its unique appearance to earn its place in the living room. Estimated SAF +/- 5
  
  
• The 130litre “Blast Furnace” make no apologies. Moving some of the volume to a lower chamber helps slightly, but standing over a metre tall, this one basically says "get outta da way!" Estimated SAF -10

Group Delay - The sound coming from a Vented speaker will de delayed. This is less important in subwoofers than normal speakers because the long wavelength makes it difficult to "image" the sound. Home Theatre usage which relies on the sub to convey explosions etc. is more forgiving than musical usage

Cost - A ported box will cost more than a sealed box. PVC pipe and fittings become expensive, particularly in the larger sizes as utilised in the "Blast Furnace". There is also the cost in time to install and finish the ports

Transient Response - Bummer this one! Vented designs do not follow the signal as "tightly" as sealed ones. This means that gunshots don't sound quite as crisp as they could. (but they will be louder)

Cone Excursion - Below the resonance of the vent, cone excursion can become an issue. If this is going to be excessive, you should consider a hi-pass filter.

Port Noise - This is the biggie! If the port is too small, the air has to move through it too quickly and you WILL hear it! The only solution is to increase the size and / or the number of ports. Unfortunately the length of the port increases with the square of the diameter, so doubling the size of the port means it has to be four times longer. With the Sidewinder and the Blast Furnace, the solution was to run the ports externally, much to the detriment of the SAF.

For an unflared port, staying below 10 m/sec, which is a pedestrian 36 kilometers per hour, ensures no noise from turbulence.

Using a flared port allows a higher speed. For example adding a 35mm flare to a 90mm port lifts the acceptable speed to around 17 m/sec. See the next section for more detail

   There is an important upper limit to port length, which is pipe mode resonance

As the port gets longer, the resonant frequency of the port decreases. For example:

The Dolby spec says the subwoofer should not be fed with anything above 120hz, but your surround amp will rolloff above the frequency, so you can still get some excitation of resonances above this. The "Blast furnace" doesn't seem to have problems with its 936mm ports, but I wouldn't want to go any longer than this without an additional low-pass filter.

Despite these challenges, it is possible to build a ported subwoofer that doesn't have port noise and isn't as big as a refrigerator. Use a good design package such as WinISD to model potential drivers before you spend your money and you'll be pleased with the results

The next section has some ideas on port flares, including making your own.