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Why choose a ported design?

A ported subwoofer will have a greater output than a sealed sub at low frequencies

The following graph from WinISD, compares a ported sub (The Sidewinder) with a sealed box using the same driver and amp.
Notice how the port augments the output as the response from the driver is rolling off.
For Home Theatre operation, an extended low frequency response is essential.

Graph: Sealed vs Ported

The ported design makes it possible for DIY'ers to build an affordable subwoofer that is suitable for Home Theatre usage.
The advent of cheap high-excursion drivers (in the US at least), opens the door to sealed designs that use equalisation to boost the bottom end, but that's a whole other story!
For a good explanation of how a ported box works, see Colin Miller's essay complete with animations

Naturally, nothing in life is free ......

There are factors that have to be taken into consideration if one is to enjoy hassle-free ported subwoofering.....

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 solution is to increase the size and/or the number of ports, or use flares to control the noise.

Unfortunately the length of the port increases with it's area, so doubling the diameter of the port means it has to be four times longer.
There is an obvious advantage to keeping the area of the port as small as possible, whilst avoiding port noise
For an unflared port, you need to stay below 10 m/sec to ensures no noise from turbulence.
Using a flared port allows a higher speed for the same sized port. For example adding a 35mm flare to an 86mm port lifts the acceptable speed @30hz from 7 m/sec to around 22 m/sec.

There are quite a few factors that come into play when selecting a port and flare combination. These are explored in greater detail in later sections of this topic

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.
Note that this is not the tuning frequency of the sub (which is due to the mass of the air in the port working against the air in the box), rather it's the port acting as a tuned pipe, much the same as a flute or a bugle
Some examples:

Port LengthResonance
 600 mm286 hz
 800 mm215 hz
1000 mm172 hz
1200 mm143 hz
1400 mm122 hz

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.

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

Here's some of my experiences with SAF...

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.

Group Delay / Transient Response

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. Whilst gunshots don't sound quite as crisp as they could, they will be louder

Cost

A ported box will cost more than a sealed box. Commercial flared ports and / or PVC pipe and fittings become expensive, particularly in the larger sizes as utilised in modern subs. There is also the cost in time to install and finish the ports

I sometimes like to use External Ports

Sidewinder Vents
"I must confess that I have fallen in love with these devices"
The "Sidewinder" vents shown here never fail to elicit a comment from visitors
 

Despite the 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

 

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