But this attitude has changed in recent years after two Australian acousticians named Phiele and Small did a mathematical analysis of vented enclosures and came up with a new set of design equations. It is for this reason that vented loudspeaker enclosures were slow to win friends among audio purists. At its extreme, the effect is somewhat like talking into a barrel.
If the music contains bass notes near the natural resonance of such a vented enclosure, the box resonance will be added to the musical sound, falsifying its character and making it boomy. Any vented enclosure has a resonance of its own. Still, there's a hitch - or at least there used to be. They certainly provide an efficient way of converting amplifier wattage into acoustic energy. They not only render the troublesome back radiation harmless but also make good use of it. Vented loudspeaker enclosures would thus seem a fine way to have your cake and eat it, too. The net result is a gain in efficiency and a strengthening of the bass. On the contrary, the two wave trains re- enforce each other. The effect of this phase reversal is that the rear wave no longer cancels the front wave when the two meet. These liberated back waves do not cancel the waves emanating from the front of the speaker because - before being projected into the listening room - they have been passed through an internal duct of carefully calculated length that inverts the phase of those sound waves - i.e., it creates a wave crest where a wave trough had been before, and vice versa. The most common loudspeaker enclosures of this kind have openings - called a vent or a port - that let out the sound waves generated by the rear of the speaker cone. In consequence, they are less demanding in terms of amplifier power. Such speaker enclosures do not suppress the back radiation from the speaker but allow it to make an audible contribution. Such excess wattage, however, tends to be expensive, and fortunately there are alternative loudspeaker designs providing greater efficiency.
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