Carroll E. Tschiegg

Tables of the Speed of Sound in Water

An equation and tables for the speed of sound in water are reprinted from a recent paper published in The Journal of Research of the National Bureau of Standards.

Ultrasonic‐transducer power output by modulated radiation pressure

We have set up and are using an apparatus for the measurement of total sound power output of a piezoelectric transducer radiating into water. This apparatus combines the better features of previously used methods which depend on radiation pressure. The input is modulated at a low frequency and the output power is intercepted by a target which experiences a force at the modulation frequency. The target is mounted on the armature of an electromagnetic receiver provided with an independent coil through which a current at the modulation frequency is adjusted in amplitude and phase, either manually or automatically by feedback, to arrest the motion of the armature. When the armature is stationary the force depends only on the current, and the apparatus can be calibrated using direct current and dead weights. It is thus absolute. In practice, the carrier frequency is swept over any part of the range 0.1–15 MHz while a recording of power output versus frequency is made. The results of comparisons made with those of other mehtods are encouraging. Examples of curves from normal and defective transducers are shown.

Effect of Dissolved Air on the Speed of Sound in Water

A sing‐around ultrasonic velocimeter was used to show that dissolved air has a negligible effect (less than 10 ppm) on the speed of sound in water at 31.8°C and 0°C.

Temperature Coefficient of the Speed of Sound in Water near the Turning Point

A sing‐around ultrasonic velocimeter was used to locate the temperature for which the velocity of sound in water is a maximum at 73.95°C. It is suggested that water held near this temperature is a suitable standard for intercomparison and calibration of instruments.

Some Laboratory Experiments with a Sing‐Around Velocimeter

The velocimeter described in the preceding paper has been used to locate the temperature at which the velocity of sound in water has a maximum. The temperature coefficient has been measured in the neighborhood of this turning temperature (73.81°C). It turns out that the velocity‐temperature curve is very flat near the maximum so that water at 73.81°C provides an excellent standard liquid for calibration and intercomparison purposes, especially in the field. The velocimeter has also been used to measure the effect of dissolved air on the sound velocity in water 0°C and at 31.8°C. Measurements were made in water about 10% saturated and 100% saturated with air. The difference was less than 1 part in 105 at either temperature.

Subharmonic Crystal Oscillators

Relaxation oscillators are described of which the fundamental frequency is a high‐order (up to 10 000) subharmonic of the frequency of a quartz crystal which serves as the controlling element. Except for the addition of the crystal, the circuits are no more complicated than conventional ones.

A Progressive‐Wave Velocimeter and the Speed of Sound in Water

The speed of sound in water at 5 temperatures near room has been determined from measurements of variation of phase with distance on the axis of a progressive wave emitted by a small piston‐like radiator. The minimum distance from source to receiver was large enough to render negligible the Fresnel‐type interference errors; errors arising from the finite diameter of the receiver were also negligible. Reverberation effects were reduced partly by use of absorbing baffles and partly by pulsing the sound at a rate such that the reverberation died out between pulses. Readings of the screw were taken each half‐wavelength, as indicated by Lissajous figures, for 30 wavelengths (total distance of 18 mm for the frequency of 2.6 Mc). The variation of distance with phase was accurately linear; the standard deviations of the points were about 1 micron, which is also the least count of the screw. The standard deviations of the speed of sound were in the range 0.04 to 0.12 m/sec. The results agree with our earlier ones ...

An Ultrasonic Velocimeter for Liquids

A device using the “Sing‐Around Principle“ will be described. The instrument consists of a pair of barium‐calcium‐lead‐titanate transducers and a hard rubber reflector to form a 10‐cm sound path. The sending crystal is actuated by an 0.2‐microsecond blocking‐oscillator pulse. This pulse after passing through the liquid is received and reshaped by a high gain amplifier. It is then allowed to retrigger the blocking oscillator so that the system continually regenerates a sound pulse whose repetition rate depends upon the combined acoustical and electrical delays. Although not an absolute instrument, its high sensitivity and stability make it well suited for detecting small changes (1 part in 105) in the sound velocity. The effective path length and circuit delay are found by calibration against liquids of known sound velocity and having attenuation characteristics similar to those of the liquids to be measured.

Precise Measurement of the Speed of Sound in Water

An X‐cut quartz crystal is optically wrung on each end of a stainless‐steel tube of which the length as a function of temperature is known. Electrical pulses are applied to one crystal at such a rate (PRF) that the pulses, received on the other crystal and viewed on an oscilloscope, coincide. The coincidence is among the first received pulse corresponding to a particular electrical pulse, the first echo corresponding to the electrical pulse next preceding, and so on. The speed of sound is twice the path length times the PRF, which is set by a stable sine‐wave oscillator having fine control and then measured with a counter. The result is meaningful for nondispersive liquids only. The results, which are easily reproduced to within 1 part in 35 000, will be presented for water over the temperature range 0 to 100°C.