Sound speed or sound velocity profiling: which is technically correct?

Hydrographer and researcher Elias Adediran unpacks a common misconception in hydrography: is it really ‘sound velocity profiling’ or should we be saying ‘sound speed profiling’? Drawing on industry experience and academic insight, he explores the science behind sound propagation in water, why accurate terminology matters, and how correct profiling improves the quality and reliability of hydrographic data.

As a hydrographer with experience in both industry and academia, I frequently encounter the term ‘sound velocity profiling (SVP)’ to describe the process of measuring sound speed in the water column – even in industry-standard software. But is this term technically correct? The prevalence of its usage suggests acceptance, yet from a technical standpoint, ‘sound speed profiling (SSP)’ is the more accurate term. Here, I aim to clarify the concept of sound speed, its measurement methods and its significance in hydrography while keeping it relatively simple.

What is sound speed?

Sound speed refers to the rate at which an acoustic wave propagates through a medium. It is a scalar quantity, meaning it has magnitude but no direction. The speed of sound in a medium depends on its mechanical properties, specifically compressibility, rigidity and density, and is expressed in metres per second (m/s). The general equation for sound speed in a solid is:

Thus, sound speed in water depends on bulk modulus and density, both of which vary with temperature, salinity and pressure (or depth). This dependency leads to empirical equations that provide estimates of sound speed. Some widely used models include those by Wilson (1960), Mackenzie (1981), Coppens (1981), Chen and Millero (1977) and Del Grosso (1974). These models have various degrees of accuracy for given ranges of temperature, salinity and pressure/depth.

How is sound speed measured?

There are two primary methods for measuring sound speed in the ocean:

1. Direct measurement (time-of-flight method): This method determines sound speed by measuring the time taken for an acoustic pulse to travel a known distance. Sound speed profilers and some moving vessel profilers (MVPs) employ this method.
2. Indirect calculation: This method derives sound speed from measured temperature, salinity and pressure using the empirical formulas mentioned above. CTDs (conductivity, temperature and depth sensors), bathythermographs (XBTs), expendable CTDs (XCTDs) and some MVPs employ this method.

In addition to these two primary methods for measuring sound speed in the ocean, sound speed can also be estimated using oceanographic climatology databases such as the World Ocean Atlas or the Matthew/Carter tables, which are based on historical data. This approach is particularly useful in the absence of direct measurements and is sometimes used to supplement data in deep waters where conditions remain relatively stable.

Furthermore, sound speed measurement can be categorized into two techniques: static profiling, where the vessel remains stationary while using instruments such as self-contained CTDs and CTD rosettes, and underway profiling, which allows data collection without stopping the vessel, using tools such as MVPs, XBTs and XCTDs.

Why is sound speed measurement important?

Accurate sound speed measurements are critical for:

• Depth conversion: Converting acoustic travel times to depth using the harmonic mean, not average or in situ sound speed.
• Ray tracing: Determining the true path of sound waves in non-vertically incident beams.
• Beam steering: Ensuring accurate steering angles at the transducer face in multibeam echosounding.

Thus, accurate sound speed measurement is essential not only for generating high-quality hydrographic products but also for enhancing acoustic positioning techniques.

Why ‘sound velocity profiling’ is a misnomer

The term ‘velocity’ implies a vector quantity (magnitude and direction), whereas sound speed is purely a scalar quantity. The measurement process does not involve tracking the directional propagation of sound waves but rather determining their speed under specific oceanographic conditions. Perhaps the widespread use of SVP stemmed from the concept of sound ray bending due to variations in water properties, which influences acoustic propagation paths. However, this effect pertains to acoustic travel paths rather than the direct measurement of speed.

Another possible source of confusion is the profiling direction of the instrument – downcast or upcast. While this describes the direction of instrument movement through the water column, it does not imply that the measured sound speed itself has a directional component; sound speed remains independent of profiling direction. In contrast, true velocity profilers, such as acoustic Doppler current profilers (ADCPs), measure both speed and direction of currents, making them more accurately aligned with the definition of velocity.

Conclusion

Given that sound speed is a scalar quantity, the term ‘sound speed profiling’ is the more technically correct term. While ‘sound velocity profiling’ has gained traction in industry and software, it is a misnomer that does not accurately describe the measurement process.

Acknowledgements

The author would like to express his gratitude to his professors at the Center for Coastal and Ocean Mapping, University of New Hampshire for their invaluable guidance and knowledge: “I am deeply appreciative of the opportunity to learn from you all – it is truly a privilege.”