The Bass-Reflex Baffle

Author: N.H. Crowhurst

Two main facts bothered loudspeaker designers:

1. To get any radiation of power at low frequencies, the diaphragm has to move a lot of air.

2. The diaphragm moves air on both sides of it - so it would be an advantage to use both sides. The movements are, however, in opposition; the front pushes when the back pulls, and vice versa.

If only the movement from the back could be "turned around!" This is just what the bass-reflex does.

Achieving phase reversal.

In normal propagation of sound, the only way to get the waves from the back turned around would be to take them for a half-wavelength longer journey than the waves from the front. To reproduce 50 cycles per second, this requires a propagation path of eleven feet, which can hardly be contained in a livingroom piece of furniture.

When sound travels through air, the wave maintains natural relationships between the pressure fluctuations and particle movements, because the air is free to move and allows the wave to develop. When air is confined in a space or an opening, however, the natural relationship no longer exists. Air in the mouth of the box is free to move; so it will not compress, but moves freely. When air at the mouth moves in and out, the air inside the box compresses and rarefies, but it does not move much. Hence (relatively speaking the air in its mouth moves without appreciable changes in pressure; the air inside the box changes pressure without appreciable movement. Of course, there is no definite line where the air changes from one condition to the other, but most of the air associated with the box is in either one condition or the other.

Achieving in-phase action to make the practical bass-reflex baffle

When a box has two openings of the same size, the fact we just discussed means that air movement in both mouths must be approximately in-phase. The air inside the box does not move much, but only compresses and rarefies, whereas that in the mouths moves without compressing and rarefying appreciably. If the air in the mouths did not move in and out at the same time, there would have to be considerable air movement inside the box which does not occur. It is easier, particularly at the resonant frequency of the box, for air to move with both mouths working together, or in-phase.

In a bass-reflex cabinet, or enclosure as it is called, one of the mouths is occupied with the loudspeaker diaphragm, which drives the air in that mouth, whereas the other is just an open "port/* The dimensions of the second mouth, or port, are adjusted so that the air in the port is about equal in weight to the total of the diaphragm and the air it moves. This provides the correct condition for the two waves to emerge in phase at the resonant frequency of the box. The frequency is lower than could be obtained with the so-called infinite baffle of the same size, and helps radiate sound energy from both back and front of the diaphragm, without cancellation.

At higher frequencies, the box does not act in this way, but absorbs the movement from the diaphragm in the volume of air inside the box, without moving the air in the port mouth appreciably. Hence, at these frequencies, this arrangement works in the same way as the infinite baffle.