Should I use "air" or "the air" when talking about waves in the context of physics?

Question

This post by @choster gives a nice explanation about using 'in the air' rather than 'in air'.

Unfortunately, when I google: how does sound wave travel through air, I found that lots of guys are using ‘in air’ when talking about physics, in particular, when talking about waves.

example_1:

Sound travels in air as a series of compressions and rarefactions.

example_2:

Sound waves traveling through air are indeed longitudinal waves with compressions and rarefactions.

Q: Should I use “air” or “the air” when talking about waves in the context of physics?

Answer 1

You generally use the definite article the when you are referring to a specific thing and both you and the listener/reader understand which particular thing you are talking about. This is the case in the answer that you referred to: the answerer is talking about "all of the air surrounding the planet".

When talking about a thing in general, we don't use the definite article. if it's a mass noun (air, water, sugar) we don't use an indefinite article either.

Compare these two sentences:

I don't like the cheese.
I don't like cheese.

The first sentence refers to a particular piece of cheese and the speaker/writer and the listener/reader both know what particular piece of cheese this refers to. The second sentence refers to cheese in general: the speaker/writer does not like cheese of any sort.

When talking about air in a scientific context, we are usually talking about air as a medium- air in general. We therefore do not use the definite article, although it would be grammatically correct to do so since, as far as this planet is concerned, air in general is the same as the air that surrounds our planet.

Looking at typical actual usages of air in a scientific context, for example as search for "[speed] of sound in air" vs. "[speed] of sound in the air", google NGrams finds very few instances where the air is used, and generally they are references to a specific volume or type of air, for example "the speed of sound in the air in the tube".

Answer 2

_{Many thanks to the OP's question which got me thinking}

When an object moves — whether it’s a vibrating guitar string or an exploding firecracker — it pushes on the air molecules closest to it.

Here the definite article is needed because it specifies *which * air molecules are affected, e.g., the ones closest to the object making the sound.

Those displaced molecules bump into their neighbors, and then those displaced molecules bump into their neighbors. The motion travels through the air as a wave. When the wave reaches your ear, you perceive it as sound.

If the author was referring to the theory, they might have written

Motion travels through air as a wave.

We see the same construction in the OP's example

Sound travels in air as a series of compressions and rarefactions

But in the original text, the author of the article is referring back to the sound produced by the hypothetical guitar string or firecracker, the writer is reminding his or her reader where they are in the “story”.

As a sound wave passes through the air, the air pressure in any given spot will oscillate up and down; […]

Text: There Actually Is Sound in Outer Space