The Fifth Element #35 2.5 Cheers For Sealed Boxes!

Sidebar 1: 2.5 Cheers For Sealed Boxes!

Most loudspeaker designs today use ported boxes to achieve greater bass extension than a sealed box of the same size. What many loudspeaker purchasers may not understand is that the use of a ported enclosure involves several technical tradeoffs that can adversely affect sound quality, especially when the designer pushes the envelope of bass extension.

Maxing out the port's contribution to a speaker's bass extension characteristically results in skewed frequency and time responses that often merit the tag "one-note bass," that note being the one closest to the port's resonant frequency. (The other bass notes are still there, just not as loud and long.) This is not to say that ported designs are inherently invalid; it's just to point out that, as Pliny the Elder might have said, nunque liberum prandium (never a free lunch).

Using a port to get more bass from a small box brings complications that sealed boxes do not. Just like adding a counter-rotating balance shaft to a V6 auto engine, these complications can be engineered around. But if you want pure performance, isn't it better to start with an uncompromised design in the first place?

The prime problem is that what makes a ported design work is the very thing that works against it. A ported enclosure is a resonant system. It therefore takes some time to excite it and some more time to stop it. The bass notes, especially those whose frequencies are near the port's resonance, will take longer to begin to speak than the midrange and treble will.

By the same token, a ported design can let bass notes hang on longer than they should. This is because the air within a ported enclosure takes on a life of its own—or, if you will, becomes a loose cannon. Once that air has begun to resonate, unlike the bass driver itself, the air is no longer under the amplifier's direct control.

Controlling a driver's potential for wayward behavior is what an amplifier's "damping factor" is supposed to do, which is sometimes a more important factor in an amplifier's bass performance than is wattage or current. The amplifier can directly damp the driver's motions, but it is less effective at damping the resonating air within the ported enclosure.

In addition to having a bass characteristic that is more controlled, a sealed box presents a smoother load to the driving amplifier, and a sealed box's bass response rolls off at half the rate of a ported enclosure's.

The advantages of a sealed box are so compelling that, when he was visiting me to hear and measure the ESP Concert Grand SIs, John Atkinson was moved to query, "Why doesn't everybody use sealed-box designs?" He then answered his own question: "Because nobody has ever gone broke selling a loudspeaker that booms."—John Marks

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