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.
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.....
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
|600 mm||286 hz|
|800 mm||215 hz|
|1000 mm||172 hz|
|1200 mm||143 hz|
|1400 mm||122 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.
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...
- 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
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
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
- 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
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