The DAEDAL Acquisition System

By Karen Topp.

Acoustic measurements of all types, especially imaging data acquisition, require a precise knowledge of the relative position of the sample in the sound field. Measurements in the Bioacoustics Research Lab are performed using a Daedal micro-positioning system which allows movement along the three translational axes, and around two angular axes, with positional accuracy of about 2 micrometers and 0.02 degree respectively. Most experiments are carried out in distilled, degassed water (see clear Plexiglas water tank in figure) where either the transducer or the sample to be measured is held in a fixed position while the other is moved by the Daedal system.

The positioning system is computer controlled, and the direction and velocity of its motion are easily selectable. Discrete steps in a 2-dimensional plane perpendicular to a transducer beam axis are typically used for imaging and positional averaging of ultrasound information, but continuous scanning along one axis is possible, and useful for rapid imaging measurements. Motion parallel to the transducer axis is used in mapping beam profiles and for imaging different depths of a sample.

The ultrasound transducers used in the lab are typically spherically focussed, with a center frequency in the 1-30 MHz range. They are excited by a mono-cycle pulse from a pulser/receiver (Panametrics 5800) which receives and amplifies the signal in pulse-echo experiments, and can be used to receive a hydrophone signal in through measurements (where a sound pulse is sent one-way through the sample and detected by the hydrophone on the opposite side). Signals are then displayed on digitizing oscilloscope (LeCroy 9354TM, up to 2 GS/s) and recorded by a PC for processing off-line.