The National Physical Laboratory in the U.K. has developed a new laser-driven technique allows remote, non-invasive and rapid mapping of sound fields, which will provide loudspeaker manufacturers with reliable data on which to design their technology. |
High-performance
loudspeaker manufacturers have been able to improve sound quality
dramatically over the years, but still face the issue of dead spots.
While
HIFI loudspeakers can be designed to deliver the full frequency range
of audible sound, it is difficult to achieve a smooth frequency output
in all directions. Dead spots are caused by deconstructive interference
as a result of radiating sound waves overlapping and cancelling each
other out. The biggest issue being where the sound is radiating from two
or more sources, which commonly occurs in the mid-frequency ranges
where both the “woofer” and “tweeter” loudspeaker cones are both active.
This creates areas where the frequency response of the loudspeaker is
less smooth, and sound quality is diminished.
Determining
the nature of these dead spots has proven difficult until now. High
accuracy acoustic measurements can be made using a microphone, but to
build up a picture of the spatial distribution of the sound many point
measurements are required within the 3D space. Manufacturers can conduct
computer-aided simulations, but these can prove inaccurate to the
actual loudspeaker performance through the variability of the
manufacturing process.
Now
The National Physical Laboratory (NPL), the U.K.’s Measurement
Institute, has developed a solution. The new laser-driven technique
allows remote, non-invasive and rapid mapping of sound fields, which
will provide loudspeaker manufacturers with reliable data on which to
design their technology.
The
technique builds on a piece of technology developed for the study of
mechanical vibration; the laser vibrometer, and on research for its
application to the 3D characterisation of underwater sonar arrays. This
NPL work has shown that in air, the acousto-optic effect, the resulting
optical phase change of light as it passes through an acoustic field, is
significant enough to be detected. To measure the acoustic output from
the loudspeaker, the laser is positioned to the side of the loudspeaker
and is rapidly scanned through a series of points in front of the
loudspeaker, being reflected back to the laser vibrometer by virtue of a
retro-reflective mirror on the other side. By measuring the laser as it
returns to its source, the technology can rapidly provide spatially
distributed phase shift data, enabling an image, or video, of sound
propagation around the source to be constructed.
“This
is a significant breakthrough for loudspeaker manufacturers. By having
actual data to rely on, they will be able to better understand how
different designs impact the loudspeaker’s directionality, and design
out the dead spots which could limit the quality of the loudspeaker,”
says Ian Butterworth, project lead at NPL.
“The
main applications are likely to be for high-end in-home loudspeaker
manufacturers who want their products to deliver the perfect surround
sound experience, and outdoor loudspeaker manufacturers who want to
eliminate the noticeable spatial changes in levels experienced at music
festivals and other live events,” says Butterworth. “We’re now looking
to conduct further studies, scanning larger areas with higher
definition, to get a better picture of how sound is propagating away
from these loudspeakers.”
The
measurement technique should ideally be performed in conditions that
minimise sound reflection, such as NPL’s hemi-anechoic chamber. However
measurements can also be carried out outdoors given the natural
hemi-anechoic nature of fields.
