Sonomicrometry1

Sonomicrometry is the measurement of distances using sound. Transducers made from piezo-electric ceramic material (commonly called "crystals") transmit and receive sound energy. Typically, these transducers operate at ultra-sound frequencies (1 Mhz and higher). To perform a single distance measurement, one crystal will transmit a burst of ultrasound, and a second crystal will receive this ultrasound signal. The elapsed time from transmission to reception is a direct and linear representation of the physical separation of the crystals.

Any sonomicrometer is therefore a timing device, repeating this measurement of transit-time hundreds or thousands of times per second. The Digital Sonomicrometer measures this transit time similar to the operation of a digital stop watch; that is, in discrete time steps (about 15 nano-seconds). The resulting transit time is easily converted to a distance if the speed of sound in the material being measured is known. Typically, in biological materials, this speed is 1540 meters per second. Given the speed of the ultrasound propagation and the size of the discrete sampling steps, the resulting resolution of the sonomicrometer would be 1540 m s-1 x .000000015 s = 0.000023 m (0.023 mm). The resolution of any sonomicrometer depends on its ability to accurately detect the received ultrasound signal and on its ability to measure transit time.

Sonomicrometry is used in the Flight Lab to measure the contractile properties (strain, or change in fiber length/total resting fiber length; strain rate) of avian skeletal muscle. Thus far, the technique has been used to measure the regional contractile properties of pectoralis muscle, and to measure the fiber length change of the pectoralis (while simultaneously measuring muscle force with strain gauges) to estimate muscle power output.

Sonomicrometry crystals used in bird pectoral muscles.

The following graph contains raw sonomicrometric and strain gauge data from a magpie flying in the windtunnel at 6 m s-1. Note that the peak muscle force (top series) is being produced during mid-downstroke as the aerodynamic force produced by the wing reaches its peak. This event is also marked by a reduction in the rate of contraction (that is, the curve flattens slightly; bottom series) of the pectoralis muscle.

1(Adapted from the Sonometrics "general principles of sonomicrometry."

For more information, visit the Sonometrics Corporation home page.