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Aperiodic Loudspeaker Enclosure Design


Aperiodic Loudspeaker Enclosure Design

Aperiodic Loudspeaker Enclosure An Aperiodic Speaker Enclosure is essentially a poorly sealed box (i.e. leaky). The aperiodic enclosure vents the inside of the box to the outside air through a damped ported (a port stuffed with damping material), which is sometimes referred to as an "acoustic resistor" or "resistive port/vent". These Aperiodic ports or vents are sold primarily by Dynaudio, (called "Variovents") and by Scan-Speak (called a "flow resistance vent"). Photographs of a Dynaudio Variovent and a ScanSpeak Flow Resistance Vent are shown below.

Dynaudio Variovent

Photograph 1: Dynaudio Variovent

ScanSpeak Flow Resistance Vent

Photograph 2: Scanspeak Flow Resistance Vent

The purpose of an aperiodic design is to allow a smaller enclosure size than would normally be possible in a sealed box. If a driver is placed into a sealed box that is too small, the result will be a high Q, which will cause a peak in the lower frequency response and a high impedance peak. By allow air to leak from the enclosure, both the frequency response and impedance peaks are tamed. This results in clearer, better defined bass, with more amplifier power and control into the lower frequencies. The impedance response of a sealed versus an aperiodic enclosure is shown in Figure 01 below.

Impedance Response - Sealed vs Aperiodic Enclosure

Figure 1: Impedance Response - Sealed vs Aperiodic Enclosure

Aperiodic box designs roll off at about 18 dB/octave. Enthusiasts of aperiodic enclosures will often compare the performance with transmission line enclosures, in that the design avoids "ringing" effects of ported enclosures, while alleviating the "pressure effect" of a sealed box.

A typical enclosure is shown in Figure 2. Generally, about 60% of the cabinet volume should be filled with damping material, while being careful to leave an open path from driver to the vent. For enclosure volumes up to about 50L, one vent is generally sufficient. For cabinet volumes of about 50 to 80L, two vents are needed, and for volumes of more than 80L, three or more vents may be required.

Aperiodic Speaker Enclosure

Figure 2: Aperiodic Speaker Enclosure

Additional Aperiodic Loudspeaker Information