EVENTS, ADVANCES, AND NEWS
■ BY JEFF ECKERT
Audio engineers are pretty good at designing loudspeakers that reproduce the full audible frequency range, but generating a smooth
output in all directions is a challenge. The problem is the result of a
phenomenon called deconstructive interference in which overlapping
sound waves cancel each other out, creating dead spots. In the past,
engineers have been able to create a map of the sound output's spatial
distribution by using microphones to make high-accuracy measurements,
but these measurements must be made at many points within the 3D
space. An alternative is computer simulation of the loudspeakers'
performance, but this has proven to be insufficiently accurate, primarily
because of manufacturing process variability. Now, however, the Brits' National Measurement Institute ( www.npl.co.uk)
has come up with a way to apply laser technology to perform remote non-invasive mapping of sound fields. The
technique should provide manufacturers with much better data to use for design purposes.
The NPL technique is based on an instrument originally developed to study mechanical vibration — the laser
vibrometer — and makes use of the fact that the speed of light in air changes slightly when it passes through an
acoustic field. This — known as the acousto-optic effect — causes a phase shift that the vibrometer can detect. In
practice, all that is necessary is to position the laser beside the speaker and scan it through a series of points in front of
it. The light is reflected back to the instrument by a retroreflective mirror and measured as it returns to the source.
Finally, this information is used to generate an image of the sound propagation around the source. The bottom line is
that engineers will find it easier to design dead spots out of tomorrow's loudspeaker systems. ▲
■ Laser-generated sound propagation
map inside the NPL hemi-anechoic
There's nothing unusual about materials that radiate visible light after being exposed to sunlight. Glow-in-the-dark toys and stickers have been around for many years.
Until recently, however, scientists have been stumped in
efforts to come up with a useful material that can emit
light in the near-infrared range. In a recent paper,
University of Georgia ( www.uga.edu)
Prof. Zhengwei Pan described a new
compound that does exactly that. At
its base is a well-known emitter
called the trivalent chromium ion.
When exposed to light, its electrons
move quickly to a higher energy
state. When the electrons return to
the ground state, near-IR light is
released. The problem is that the
flash lasts only a few milliseconds.
Prof. Pan discovered that putting
the material into a matrix of zinc
and gallogermanate generates "traps"
that can capture the excitation
energy and store it for longer periods. Using a thermal
process, the energy is slowly released back to the chromium
ions over a period of up to two weeks, allowing it to
provide continuous illumination. Just a one minute exposure
to sunlight can create a 360 hour release of near-IR.
■ Night vision image of researchers
Zhengwei Pan and Feng Liu standing in
a room lit only with their near-infrared