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"?

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    'Light speed', 'the speed of light', 'sound speed' and 'the speed of sound' are all equally grammatical. But 'sound speed' is the unidiomatic one. – Edwin Ashworth Aug 26 '16 at 1:14
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    I've certainly read "sound speed", in "soft" scientific articles, but probably only where the context had already been established. I see nothing wrong with the term, especially in scientific articles -- certainly "light speed" is readily accepted by the general public. – Hot Licks Aug 26 '16 at 1:17
  • And there's nothing "inherently incorrect" with the use of those words in that order. It's a normal English construction. – Hot Licks Aug 26 '16 at 1:18
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    I suspect that sound speed was used because the author wanted some variety, having already used speed of sound many times in the same article. – David Handelman Aug 26 '16 at 14:51

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:

  1. 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:

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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

  • Thank you for an educational answer! The specific article I was talking about did assume a constant speed of sound, so it doesn't fall in the categories you offered. But very interesting, thanks! – Whyka Aug 26 '16 at 2:55
  • I agree with Edwin and I want to add that normally "the speed of sound" is specific, ie 1,100 ft/sec whereas "sound speed" might merely imply “around the magnitude of the speed of sound, as opposed to light speed or a snail’s pace.” – Robbie Goodwin Sep 5 '16 at 17:06

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:


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.

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