A signal sent through the
air will diminish as a function of
distance. So, just how fast will
the ultrasonic signal diminish?
We’ll ignore acoustic absorption
of air since this is a relatively
minor effect at distances of a
few feet, and instead just concentrate on signal
attenuation as the result of wavefront expansion.
Figure 5 depicts what is happening. We know that
the wavefront expands in both dimensions normal to the
transmission axis. So, the signal is proportional to (1/r2),
where r is the distance from the transducer.
Once the wavefront hits the target, it’s dispersed by
the target and similarly expands, also resulting in a
decrease in signal as a function of r2. So, the signal
experiences a 1/r4 overall reduction.
Thus, if the return from the
target at a two foot distance is
S, at twice that distance (four
feet) the signal will be down to
It’s easy to see that a return
signal that is a relatively large
amplitude one or two feet away
may require a huge gain
increase or other non-trivial
processing method to be reliably
recognized at, say, 10 feet.
With all of these problems, it’s not surprising that
hobbyists sometimes find using ultrasonic devices
frustrating. Fortunately, these issues can be solved with
some basic understanding about how they function,
making them a popular choice for obstacle detection and
avoidance in robotic applications. SV
SERVO 02.2018 57
Figure 5. Ultrasonic transmission and