In a recent Dynaudio post, the headline read, “Why are all loudspeakers square boxes?” and this caught my eye, since I could not think of ANY square box loudspeakers. In fact, loudspeaker designers usually avoid this shape like the plague. In the following post, we will briefly explore the shape of loudspeaker cabinets.
Let's get the "square box" statement out the way: The shape of the typically loudspeaker is a so-called rectangular prism, also referred to as a rectangular cuboid. If the width and depth are equal, you can rightfully term the loudspeaker a "square box" or square cuboid, and the special case of width=depth=height is termed a cube. Square box designs are rarely seen.
So why are loudspeakers shaped this way; parallel walls with one longer height dimension, and two smaller, but typically unequal (no squares in sight), dimensions; width and depth. Well, first it comes down to tradition. This is the how it has always been done, and it is a proven concept. It is easier to produce the rectangular cuboid shape, than some intricate design with twists and turns, and so there is a financial incentive to make them this way. In addition, the tweeter is most often placed at ear height, and if the loudspeaker is not on a stand, it naturally has to have a certain height to accommodate this. We will get back to the other two dimensions shortly.
Those frustrated with this stagnation in loudspeaker design have sometimes used the term “monkey coffin” derogatorily to describe this typical look. You should note, however, that several designs that are more courageous do exist, such as for example those shown below.
When it comes to width and depth, designers very often avoid the square box design, in that they do not want width and depth being equal. But WHY do they avoid this design? Well, it probably has to do with eigenmodes. The structure of the loudspeaker has a number of inherent vibroacoustic modes, or vibration patterns, associated with it. Each eigenmodes has a particular eigenfrequency associated with it. These modes, once exited, can in principle vibrate at their eigenfrequencies forever, but losses in the vibroacoustics setup make them die out after a short time. There is no way around having such modes, but the dimensions of the loudspeaker can be adjusted to change the location and distribution of the modes. This is probably why the square box is avoided, since such a design would result in so-called degenerate eigenmodes; modes that coincide in frequency. At least for perfectly square boxed... The thought amongst designers may be that at this particular frequency the vibration is now “stronger”, since it has contributions from more than one mode. To answer this, you really need to include the fully coupled vibroacoustics with driver, the box and the driver placement in said box, and have some knowledge about how the degeneration affects the distribution of modes. We will address these technicalities in an upcoming post.
One issue with having rectangular cuboid loudspeakers is that they have parallel walls. This is often thought of as being a negative attribute, since the loudspeaker may be thought to have more eigenmodes for a given frequency range, compared to having non-parallel walls. This statement will also be addressed in an upcoming post. A couple of possible downsides to having non-parallel walls are difficulties when it comes to production, and packaging issues for complex designs.
One final thing to consider is the exterior surfaces of a rectangular cuboid, in that there are sharp edges and corners. This can lead to issues with unwanted sound diffraction, and the loudspeaker designers often make sure to round the edges for this reason. Again, diffraction will be addressed in an upcoming post.
The main takeaway from this post is that most loudspeakers are NOT square boxes, and that perhaps square boxes may in fact not be so bad after all, but a more thorough analysis will illuminate this, see upcoming posts.