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Chapter 2: How Loudspeakers Work > 2.2 Reflex (Ported) Loudspeakers

2.2  Reflex (Ported) Loudspeakers

The alternative loudspeaker types all, in different ways, return low-frequency energy from the rear of the drive unit in the correct phase to reinforce that radiated from the front.

Reflex or ported loudspeakers, (sometimes called vented boxes) as shown in Figure 2.1b, have a pipe-like port which allows the passage of air in and out of the box. The mass of air in the port resonates with the compliance of the air in the box, causing the port output to be in phase with the forward radiation of the drive unit at low frequencies, and allows the bass response to be extended without the lower efficiency that results when that is done with a sealed box. At frequencies below the port resonance the port output is in anti-phase, causing the output to fall at 24 dB/octave rather than 12 dB/octave as for the sealed box. The response looks like Figure 2.2, but with steeper rolloffs. This does not result in a notch in the response, as occurs in the ABR system, but the increased slope makes things more difficult if you are planning low-frequency response extension by equalisation in the crossover, as the matching of equaliser and driver/box responses is more critical.


  

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