How does a rear reflex port opening work? What are the benefits of such design?

The key benefits of a reflex port opening in the rear of the cabinet. The main benefit of a reflex port in an enclosure is that it enables the loudspeaker to produce low frequencies at a very low distortion level, close to the frequencies where the bass reflex has been tuned. This enables very linear low frequency reproduction systems to be designed. The bass reflex port allows air to move in and out. The reflex is effective if the energy losses of this air movement remain small. Because of this reason the reflex port is designed so that the speed of the air moving in the reflex port remains fairly small while the port is in resonance. The direction where the bass reflex port is facing does not greatly influence the work of the bass reflex. While it is in resonance the reflex port generates low frequency sound. This sound sums with the low frequency sound output from the woofer at other frequencies. The distance and location of the bass reflex port is designed so that this summation works correctly.

If the reflex port is not sufficiently large, the air motion in the port can become turbulent (non-linear). This will dramatically increase air flow losses in the reflex port. A high flow loss can cause the bass reflex to stops working. This will change the low frequency characteristics of the loudspeaker. In Genelec designs we take care to dimension the reflex port to have sufficient capacity to support the system up to the maximum sound level output.

Our reflex port design also minimizes the audibility of air flow in the port. That's because the port opening is faced away from the listener. When the air flow in the port generates noise, this noise is typically 10…20 times higher in frequency than the port tuning frequency. The flow noise is therefore rather directional, and by having the port opening in the back, this noise is well attenuated compared to having a port in the front and directing such noises towards the listener.

Summarizing the key benefits:

- Allows to have a maximized waveguide (DCW) area on the enclosure front

- Minimizes acoustic diffraction as the front of the enclosure has no holes, vents or slots. This is especially significant near the tweeter. The port opening, or flare, can be significantly larger, ensuring laminar flow up to high sound level outputs, high linearity and low system distortion at woofer frequencies.

- Minimizes the audibility of air flow in the port.

Tips for a proper studio monitor placement:

In several Genelec products the bass reflex port opening is on the back side of the enclosure. When the loudspeaker is placed close to a wall, a small gap (50mm or 2") between the speaker and the wall is needed to enable the bass reflex to work at full capacity. Our experiences show that a distance of twice (2x) the port diameter from the wall to the enclosure back is a very safe distance.