Siren Design and Evaluation in Mud Flow Loops and Wind Tunnels - Measurement While Drilling Signal Analysis, Optimization and Design - page 271

Impulse

Mud

t

x

Figure 9.25. Conceptual attenuation test fixture.

A conceptual “attenuation test fixture” is sketched in Figure 9.25 that
avoids the problems associated with acoustic standing waves. Essentially, one
has a very long, thick-walled metal tube that is rigid at the right end while a
movable piston is installed at the left end. Pressurized mud free of bubbles fills
the test chamber within the pipe. A sudden impulse excitation, e.g., a bullet
fired from an oblique angle, is used to displace the piston momentarily. The
pipe is instrumented with piezoelectric transducers that are connected to a digital
oscilloscope. The speed of sound in the mud can be readily measured from the
first few wave traverses by dividing total travel distance by total time. Then,
attenuation can be inferred by counting the number of reflection cycles until the
acoustic signal completely disappears. The effects of fluid shear are not
accounted for in this procedure, so the results are optimistic. Still, the data so
obtained is less problematic that that found in standing wave environments.