Sealed versus Ported Enclosures

Car Audio enclosures come in a variety of designs each with their advantages and disadvantages. Depending on your goals, the type of enclosure should be based on the desired applications.

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Sealed Enclosures

The loudspeaker driver’s moving mass and compliance (slackness or reciprocal stiffness of the suspension) determines the driver’s resonant frequency (Fs). In combination with the damping properties of the system (both mechanical and electrical) all these factors affect the low-frequency response of sealed-box systems. Output falls below the system’s resonant frequency (Fc), defined as the frequency of peak impedance.

In a closed-box, the air inside the box acts as a spring, returning the cone to the ‘zero’ position in the absence of a signal. A significant increase in the effective volume of a sealed-box loudspeaker can be achieved by a filling of fibrous material, typically fiberglass, bonded acetate fiber (BAF) or long-fiber wool. The effective volume increase can be as much as 40% and is due primarily to a reduction in the speed of sound propagation through the filler material as compared to air. The enclosure or driver must have a small leak so internal and external pressures can equalise over time, to compensate for barometric pressure or altitude; the porous nature of paper cones, or an imperfectly sealed enclosure, is normally sufficient to provide this slow pressure equalisation.

Ported/Vented Enclosures

Also known as vented (or ported) systems, these enclosures have a vent or hole cut into the cabinet and a port tube affixed to the hole, to improve low-frequency output, increase efficiency, or reduce the size of an enclosure. Bass reflex designs are used in home stereo speakers (including both low- to mid-priced speaker cabinets and expensive hi-fi cabinets), bass amplifier speaker cabinets, keyboard amplifier cabinets, subwoofer cabinets and PA system speaker cabinets. Vented or ported cabinets use cabinet openings or transform and transmit low-frequency energy from the rear of the speaker to the listener. They deliberately and successfully exploit Helmholtz resonance. As with sealed enclosures, they may be empty, lined, filled or (rarely) stuffed with damping materials. Port tuning frequency is a function of cross-section and length. This enclosure type is very common, and provides more sound pressure level near the tuning frequency than a sealed enclosure of the same volume, though it actually has less low frequency extension since the “rolloff” is steeper (24db/oct vs. 12db/oct for a sealed enclosure). Malcolm Hill pioneered the use of these designs in a live event context in the early 1970s.

Vented system design using computer modeling has been practiced since about 1985, when researchers Thiele and Small first systematically applied electrical filter theory to the acoustic behavior of loudspeakers in enclosures. While ported loudspeakers had been produced for many years before computer modeling, achieving optimum performance was challenging, as it is a complex sum of the properties of the specific driver, the enclosure and port, because of imperfect understanding of the assorted interactions. These enclosures are sensitive to small variations in driver characteristics and require special quality control concern for uniform performance across a production run. Bass ports are widely used in subwoofers for PA systems and sound reinforcement systems, in bass amp speaker cabinets and in keyboard amp speaker cabinets.

This article includes material from “Loudspeaker enclosure.” Wikipedia.  Licensed under Attribution-ShareAlike 3.0 United States (CC BY-SA 3.0 US) Authors: