1524 Chapter 40
imported into the GuidePORT system. The base station
PC and/or central control unit is only needed when
configuring or reconfiguring the system, and could be
substituted with a temporary PC or notebook.
40.5 Narrow Beam Loudspeaker System
The directivity (narrowness) of any wave producing
source depends on the size of the source compared to
the wavelengths it generates.* Audible sound has wave-
lengths ranging from a few inches to several feet, and
because these wavelengths are comparable to the size of
most loudspeakers, low- to medium-frequency sound
(20 Hz to 10 kHz) generally propagates omnidirection-
ally. Only by creating a sound source much larger than
the wavelengths it’s producing can a narrow beam be
created. To accomplish this with standard loudspeakers
would require loudspeakers 50 ft in diameter. A narrow
beam of sound from a small acoustic source is accom-
plished by generating a beam of ultrasound, which
becomes audible as it travels.
Ultrasound, whose wavelengths are only a few milli-
meters long, are much smaller than the source, and
consequently travel in an extremely narrow beam.
Ultrasound contains frequencies far outside of our
range of hearing, and is completely inaudible, but as the
ultrasonic beam travels through the air, the inherent
properties of the air cause the ultrasound to distort
(change shape) in a predictable way. This distortion
gives rise to frequency components in the audible band-
width, which can be accurately predicted, and therefore
precisely controlled. By generating the correct ultra-
sonic signal, we can create, within the air itself, essen-
tially any sound desired.
Note that the source of sound is not the physical
device you see, but the invisible beam of ultrasound,
which can be many meters long. This new sound
source, while invisible, is very large compared to the
audio wavelengths it’s generating, so the resulting audio
is extremely directional, just like a beam of light.
Often incorrectly attributed to so-called Tartini
tones, the technique of using high-frequency waves to
generate low-frequency signals was in fact pioneered by
physicists and mathematicians developing techniques
for underwater sonar over 40 years ago.
Dr. F. Joseph Pompei, then a researcher at MIT,
solved the problems of using ultrasound as an audible
source that plagued earlier researchers. His design of
the Audio Spotlight®* sound system has become the
very first, and still the only, directional loudspeaker
system which generates low-distortion, high-quality
sound in a reliable, professional package, Fig. 40-9. Fig.
40-10 shows the sound field distribution with
equal-loudness contours for a standard 1 kHz tone. The
center area is loudest at 100% amplitude, while the
sound level just outside the illustrated beam area is less
than 10%.
Audio Spotlight systems are much less sensitive to
listener distance than traditional loudspeakers, but
maximum performance is attained at roughly 1–2 m
(3–6 ft) from the loudspeaker.
Typical levels are 80 dB SPL at 1 kHz for the AS-16,
and 85 dB SPL for the AS-24 models. The larger AS-24
can output about twice the power and has twice the low-
frequency range of the AS-16.
The most common use of the Audio Spotlight system
is to deliver sound to a specific, isolated area. Just as
with lighting, the Audio Spotlight system is bestFigure 40-8. GuidePORT receiver. Courtesy Sennheiser
Electronics.
* Much of this section was copied with permis-
sion from copywrited text by Holosonic
Research Labs, Inc.* Audio Spotlight is a registered trademark of
Holosonic Research Labs, Inc.