Biomedical Engineering Reference
In-Depth Information
FIGURE 7-9
Sound attenuation
as a function of
frequency and
relative humidity in
air at 20
◦
C.
A far more comprehensive model (Burnside, 2004) has been used to generate the results
shown in Figure 7-9. These results give an attenuation of 0.88 dB for a frequency of 30 kHz
at 20
◦
C and a relative humidity of 30%. This result is in reasonably good agreement with
the results from (7.1).
The maximum range that can be achieved by ultrasound sonar systems is determined
by a combination of the beam divergence, the atmospheric attenuation, and the target size.
If the target is a fixed size, like a street sign, then the received power decreases with
R
4
,
made up from an
R
2
term on the way to the target and a second
R
2
term on the way
back. For a flat target like a wall, the received power decreases with the square of the
range,
R
2
, because the reflecting area increases with
R
2
, which cancels one of the terms
of the previous case. These losses in conjunction with an atmospheric attenuation of about
1 dB/m limit the maximum range to about 5 m. However, that is more than sufficient as
an extension to the white cane.
The beamwidths of most ultrasound transducers are diffraction limited, with the result
that the larger the transducer diameter the narrower the beam. As a rule of thumb, this
relationship can be calculated as
25000
df
θ
3
dB
=
(7.2)
where
θ
3
dB
(deg) is the half-power beamwidth of the transducer,
d
(mm) is the diameter
of the transducer, and
f
(kHz) is the frequency.
The sizes of the transducers used, and hence their beamwidths, are determined by the
application. For example, a unit mounted on the frame of a pair of eyeglasses would have
to be much smaller diameter than one that is handheld or attached to a belt, and as a result
its beamwidth would be much larger.
