Attenuation Of Sound In Fluids; Frequency-Dependent Attenuation - 3B SCIENTIFIC PHYSICS U10010 Manuel De L'utilisateur

Unfortunately, in most bodies and fluids there is a large
scattering component to the attenuation coefficient.
Since scattering is dependent on the ratio of the wave-
length to the size of the scattering object, this can lead
to wide variations in the frequency dependence of the
attenuation arising from (4).
When comparing with standard published values, it
should be noted that the values are usually given in
dB/cm so that the value for α results from (3) thus:
α [1 / cm] or [neper / cm] =
α α
[ ] [
dB cm / dB cm
=
( )
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4.5. Attenuation of sound in fluids

By measuring inside a fluid container with a movable
reflector it is possible to plot a curve of the reflected
amplitude for various values.
An external program can then derive the attenuation
coefficient α by finding a fit for the exponential func-
tion in (3) or more simply identifying a linear fit to a
line that matches (3) when it is rearranged in the form
(6):
=
y a x
A
= α 2 −
0
(
Ln x
i
A
i
where A is the amplitude of the peak closest to the
0
transducer. All subsequent measurements (i) are related
to this value so that the measuring error at greater dis-
tances becomes much smaller. If the speed of sound
in the fluid has already been determined (e.g. by using
the transmission method where measuring both
lengthways and across the width eliminates the effect
of the container's walls) and entered into the program,
the distances of the reflector from the transducer can
be read off directly from the software (Depth setting).
Attenuation of sound in water is too weak to measure
any alteration in amplitude over a distance of around
20 cm. The following diagram shows a graph as meas-
ured for sunflower oil.
]
/
(5)
.
)
x
(6)
0
By setting the amplifier appropriately, the entire avail-
able path may be used for measurements. Using a fre-
quency of 1 MHz gives a value of about 0.5 dB/cm for
the attenuation coefficient, which is close to the pub-
lished value of 1 dB/cm for frequencies of 1 – 5 MHz.

4.6. Frequency-dependent attenuation

Frequency-dependent attenuation can be studied very
well using a thin acrylic plate (thickness 1 – 2 cm
approx., see photograph).
Since parallel surfaces give a series of multiple echoes
when the transducer is placed straight against them,
the frequency components of individual echo pulses
can be investigated using the FFT function built into
the program. The following illustration shows the cor-
responding FFT analyses. It can clearly be seen that
higher frequencies are attenuated more acutely as the
distance traveled through the plate increases. Thus the
median frequency (the frequency component of the
highest amplitude) becomes shifted.
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