Ported & Sealed Subs


Novice Member
Hello all,

Calling all the experts out there......................

What are the main differences between ported subs (as all the SVS subwoofers seem to be) and sealed-box designs (such as those made by B&W).

Reading around the topic, it appears that a sealed-box design is technically more difficult to execute than a ported design.

Is there a particular kind of sound that each of these designs imparts to a sub-woofers? And what are the trade-offs that each type of design requires?

I am intrigued because some companies seem to 'prefer' one type over the other.


Ed Selley

AVF Reviewer
Originally posted by TheSeer
Reading around the topic, it appears that a sealed-box design is technically more difficult to execute than a ported design.

In our development experience it is the other way around- a convincing amount of slam can be had by producing an infinite baffle downward firing sub but it is wasteful of power and efficiency. The THX 502 sub is infinite and once we cured a resonance issue it almost designed itself whilst the ported 309 and 909 took a great deal more work. As a result we locked an engineer in a cupboard and wouldn't let him out until we had plotted data on port characteristics. I have no idea what experiences other manufacturers have had but for us ports were tricky.


What is rather more important than choosing reflex or closed box designs is the size of the enclosure and the size of the driver(s).

Trying to get high SPLs in the deep bass from a small cone in a small box, whatever the design, requires enormous power. And will still fail in comparison with large enclosures due to higher distortion levels.

The smaller the cone, the less air is moved, despite extreme cone movements. The larger the cone the smaller the movement required to produce the same sound level.

This size problem becomes worse the lower the frequency required to be reproduced at full power. Fortunately room effects help to lift low bass output. It is a good job we don't usually listen to subwoofers out in the garden. Otherwise many subs wouldn't even qualify for the name "subwoofer".

The danger with reflex porting is poor reflex port flare design (or lack of any flare at all) causing wind noise due to turbulence at high air speeds. The larger the port diameter the lower the air velocity at any frequency.
Small boxes can't use a big port. Big enlosures can.

What a shame most AV nuts "better halves" don't want a a huge sub in their lounge. Perhaps such people should invest in a wall mounted baffle with hidden enclosure behind an array of large drive units. Usually referred to as an "infinite baffle". These take up no floor space and a built-in cupboard will supply the enclosure requirements. Size matters. But it can be hidden away.


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