Biomedical Engineering Reference
In-Depth Information
FIGURE 7-11
The
original binaural
sonic aid.
(a) Photograph of
device mounted in a
glasses frame.
(b) Device in use.
(Courtesy of
Zabonne Ltd
http://www.zabonne.
co.nz/.)
relevant information needed by the user, whereas Kay strives for more information-rich
sonar-based displays.
Heyes argues, for instance, that if some object moves away from users they do not
need to know this in regard to safe and efficient travel. In addition, this minimalist approach
results in less confusion by minimizing interference with hearing normal environmental
sounds (e.g., traffic). The Sonic Pathfinder uses brief tones on a musical scale to denote
distance, with pitch descending when approaching an object. Perception of objects on the
left and on the right is further supported by tones for the corresponding ear. Two off-center
sonar beams are used to detect objects on the left or right to ensure that users are aware of
obstacles in their periphery.
7.5.1.4 Binaural Sonic Aid (Sonicguide)
As can be seen in Figure 7-11, this device comes in the form of a pair of spectacle frames,
with one ultrasonic wide-beam transmitter mounted between the spectacle lenses and one
receiver on each side of the transmitter (Kay, 1974). FMCW signals from the receivers are
demodulated and presented separately to the left and right ear. The resulting
interaural
amplitude difference
allows the user to determine the direction of an incident echo and
thus of an obstacle.
In addition to providing wide field-of-view returns, the newer Trisensor is fitted with
a narrow-beam transmitter to provide information about the size and location of objects
directly ahead. In both the Sonicguide and the Trisensor, distance to an object is encoded in
the frequency of the demodulated low-frequency tone, whereas surface texture is returned
by the timbre of the audio signal.
Psychophysical studies conducted at short range (within arm's reach) showed that
users were able to judge the direction to a small cylinder to an accuracy of about 5
◦
compared with 1
◦
for a source of natural sound. In contrast, range estimation was improved,
with subjects able to judge the distance to an object to an accuracy of 50 mm, which was
significantly better than their ability to judge the distance to a natural sound. At longer
ranges (up to 5 m), users were able to judge direction to an accuracy of 6
◦
and distance to
within 300 mm (Warren and Strelow, 1984).
7.5.1.5 NavBelt
This device was developed as part of a Ph.D. thesis in 1989 and provided imaging and
guidance operational modes. In the imaging mode, the device output a crude 120
◦
wide
