Loudspeaker EnclosuresHere 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 GeometryA
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. |