Biomedical Engineering Reference
In-Depth Information
FIGURE 7-8
Photograph of the
SensComp 600
series transducer.
(Courtesy of
SensComp
http://www.
senscomp.com/.)
In this case the same transducer operates as a receiver with the following specifications:
−
=
μ
•
Sensitivity:
42 dB at 50 kHz relative to 0 dB
1V/
bar (for a 150 V direct
current [DC] bias)
Unfortunately, the wider bandwidth results in a much poorer receive sensitivity than that
of the narrow band resonant transducer variety.
As an acoustic signal propagates through the atmosphere, it attenuates. The main
reason for this is the beam divergence, which reduces the signal power density with
the square of the distance traveled. The second mechanism is atmospheric absorption,
which converts some of the acoustic energy into heat. A reasonably simple model for
the atmospheric attenuation,
(dB/m), which includes the effects of both temperature
and humidity, has been derived from measured data and is accurate for relative humidity
greater than 30% (Knudsen and Harris, 1950).
α
3
/
2
f
1000
0
.
283
20
+
φ
t
α
c
=
(7.1)
where
f
=
frequency (Hz)
φ
t
=
φ
20
(
1
+
0
.
067
t
)
φ
20
=
relative humidity % at 20
◦
C
t
=
the temperature difference from 20
◦
C
Consider the attenuation,
α
c
(dB/m), determined using this model at a frequency of 30 kHz
for a temperature of 20
◦
C and a relative humidity of 30%.
30
3
/
2
10
3
1000
×
0
.
283
α
c
=
30
=
0
.
93 dB
/
m
20
+
