The Infinite Baffle

Author: N.H. Crowhurst

Next we consider ways to "tidy up" the baffle. A big board suspended in space, with a loudspeaker mounted in a hole at its middle, is not an ideal piece of furniture. And unless it is very big, it still has limitations in low-frequency response. Of course, if the baffle could be made infinitely large, by extending it up to the sky, it would be perfect, but this is obviously not practical.

The purpose of an infinite baffle is to prevent any access from back to front around the edges. Putting the loudspeaker in a box that is completely closed except for a hole for the diaphragm does the same thing. This works perfectly, as far as enabling the diaphragm to radiate frequencies right down to the lowest is concerned. However, this arrangement gives the diaphragm two jobs to do; as well as radiating sound from the front, it must alternately compress and rarefy the air inside the box.

The air in front of the diaphragm moves much more readily than that behind it (which is restricted by being contained in the sealed box). For this reason, the air in the box places a greater "loading" on the diaphragm movement, and only a fraction of the driving force produced by the voice coil is used to radiate sound from the front; most of the energy is wasted compressing and rarefying the air inside the box. The smaller the box, the bigger the waste, and the less efficient the complete assembly is as a loudspeaker.

In the box-type baffle the cone has two jobs to do: to push sound waves out front and to compress and expand air inside the box.