Loudspeaker Enclosures

Here you will find information/guidelines about the design of a loudspeaker enclosure. I have seperated the topic into three categories:
First the geometrical design is discussed, followed by considerations regarding time alignment (vertical alignment of the loudspeakers). The last topic revolves around the use of absorption materials.

Enclosure Geometry

A loudspeaker enclosure should contribute with as little coloration of the sound as possible. The main artefacts of annoyance in the enclosure is standing waves inside the enclosure and undesirable, uncontrolled diffraction due to the finite dimensions of the front baffle.

The standing waves introduce a delayed sound besides the immediately radiated sound. Standing waves are effectively avoided if no parallel surfaces are present in the enclosure, and therefore a pyramid shape will be a good choice. The pyramid should be odd-sided (3-sided, 5-sided 7-sided …) in order to neglect parallel surfaces. However, even sided pyramids (4-sided, 6-sided 8-sided …) are much easier to build, and therefore, a 4-sided construction was used combined with diagonals inserted inside the enclosure for minimizing the amount of standing waves and enforcing the stiffness of the enclosure.

Olson has made investigations revolving around the influence of the front baffle dimensions on the radiated sound and on the basis of his research it can be concluded that non symmetric baffle shapes introduce the least significant transitions in radiated sound pressure. In order to minimize the effect of coloration due to diffraction, the loudspeaker units were mounted asymmetrically on the baffle.

Alignment in Time

In order to time align the loudspeaker units, providing the same arrival time at the listening position for all loudspeaker units, each unit must be aligned differently vertically. The midrange and tweeter loudspeaker has the narrowest mainlobe in the frequency range in which they are used and therefore these should preferably point directly towards the ear of the listener.

This is accomplished by mounting the midrange and tweeter in the height of the ear of the listener. The bass loudspeaker is nearly omni-directional in the frequency range where it should be used and therefore it can be mounted anywhere in the enclosure. It is in this design located in the proximity of the floor and thereby, the reflections from this surface are inaudible and a doubling (approximately) of the sound pressure for low frequencies is obtained for free.

Absorption Materials

The method for attenuating the backward radiated sound (radiation into the enclosure) is to apply damping materials inside the enclosure, which effectively attenuates the reflected sound. Sound waves are adiabatic processes, meaning that the instantaneous temperature depends on whether a compression or rarefaction of the sound particles occur, and therefore the characteristic of the loudspeaker (mounted in the enclosure) will have varying characteristics, depending on the operating level/time.  By applying porous damping inside the enclosure, the thermodynamic processes will approximate isobaric processes, thus revealing a more constant operating temperature.

The application of porous damping materials effectively decreases the mechanical resistance of the loudspeaker unit + enclosure –system, thereby decreasing the overall Q of the system.

A loudspeaker enclosure will experience high strains at the boundaries. This strain can be decreased by applying a thin layer at the boundaries either glued or stapled on. Alternatively, several layer may be used, utilizing the reflection coefficients of specific materials in conjunction.

It is also wise to apply damping materials in the middle of the enclosure, where the velocity of the sound particles is largest.