I read a scientific paper repeatedly using the term "sound speed" instead of "speed of sound". It sounds strange to my ears, but in fact why not say "sound speed"? Is it inherently incorrect, or does it sound strange just because we are used to "speed of light"?
There's nothing inherently incorrect about the expression "sound speed", which is an analogous construction to "light speed" and is representative of the preference in English of dispensing with unnecessary words - e.g. "a car horn" instead of "the horn of a car".
However, I'd recommend using "speed of sound" instead of "sound speed" in most situations. As a general rule, it's undesirable for the reader to pause mid-sentence because of an unfamiliar or ambiguous expression, even if it's a valid construction. "Sound speed" is certainly the less uncommon usage, as this Ngram shows; and without an adequate context, it's also potentially ambiguous because of the other possible meaning of sound as an adjective:
- Based on valid reason or good judgement
In other words, "sound speed" might mean "wise speed": "She was travelling at a sound speed in the school zone". Note that native speakers would tend to avoid this construction for exactly the same reason - that "sound" might mean "noise" rather than "wise".
TL;DR: the expression "sound speed" is correct but uncommon, no doubt because the double meaning of "sound" can create an ambiguity.
Here's the Ngram chart of "sound speed" vs "speed of sound" since 1880:
As an addendum to the discussion, it's interesting to note that the use of "speed of sound" picks up significantly starting in the 1930s, coinciding with the development of gas turbine ("jet") engines. The primary stimulus in developing jet engines for powered flight was the limit to propellor efficiency, which declines dramatically as the blade tips approach the speed of sound.
The use of "sound speed" takes off (pun intended) around WW2, although I haven't researched whether this related more to "wise speed" than "speed of sound", given the number of crashes as plane speed increased. Chuck Yeager became the first person to break the sound barrier in October 1947.
Sound speed is fine. It is used when you are fiddling with the conditions affecting the speed of sound while making measurements. It also works in many non-isotropic situations involving mixed phase materials or foams or solvent gradients such as haloclines. Here's a doc that uses sound speed in what I consider to be the normal manner. Speed of sound is a material property of isotropic materials that don't scatter sound. When you have more complicated things going on than just a spherical wave front being propagated, it's better to find another term. Another use of sound speed is to form compounds such as sound speed gradient
The speed of sound is extremely fast.
Sound speed is extremely fast.
They both mean the same thing.
In the first sentence, the subject and noun "speed" is modified by the adjective prepositional phrase "of sound." The phrase, like an adjective that it represents, answers what an adjective would ask, "What kind?" "What kind of speed?" Lightning speed? Jet speed? The speed that sound travels, which is over 750 miles an hour.
The object of the preposition in the phrase is "sound" which is a noun. You can take a noun and use it as an adjective, as in "sound speed."
A few examples of how an adjective prepositional phrase is really the adjective but in a phrase instead of a single word modifier:
NOUNS USED AS ADJECTIVES
The kitchen light is on.
The Chicago and New York airports are crowded.
This is a house key.
Now, lets put them in ADJECTIVE PHRASES
The light in the kitchen is on.
The airports in Chicago and New York are crowded.
This is a key for the house.
John E. Warriner. Warriner’s English Grammar and Composition. Third Course. Liberty Edition. Orlando, Florida: Harcourt, Brace, and Jovanovich. 1986. 90-91.