What does the audio term 'air' mean?

Cey, I think you are right about smooth upper frequency extension, but I think there is something also to the way the musical notes sound on both uppers and mids. I'm not sure about bass. People love tube gear because it brings "bloom" to the sound, which to me means that the notes are voluptuous, as opposed to lean. (Sorry about the metaphors, but I can't find other words.)

In a way, the notes have inner air, and bass notes count too, I guess. I think of a jazz bass where the notes seem to float on air as opposed to cutting the air. So hard to explain. Perhaps a system with air embraces the listener rather than impresses the listener with its strength. 

I'm trying to compare very good systems in my mind. A Wilson speaker or Dytnaudio speaker which I think of as more lean and analytical versus Sonus Faber or other speakers that people think of as "musical." They both could have "smooth upper frequency extension," but one would seem to float on air and the other to penetrate the air. And that's the best I can do at describing what I think I am talking about.

Low noise floor is an electronic design concept...

We can improve it in a various way by filtering DC for example..

By grounding our pieces of gear ....

by protecting it from EMI interference...

"Air" between notes and instrument is the fluid quality, a respiration, uniting but  separating the instruments and the music  as an organized perceived whole by the listener...

A better grounding of the gear pieces for example can increase the perception of "air" between instruments already in the recording because of a lower noise floor...

But acoustics control of the speakers/ room  pressure zones distribution can contribute to the spatial perceived attributes of sound as it must be translated optimally for my ears in my room ...

When listening to an opera in my room if i perceive the singing actors walking and if i see them when singing turning their head it is because the original recording acoustics is well translated in my own room acoustics parameters there is "air" between the singers and the instruments, a space of their own but linked to the others, because they were recorded in the same room now transported in my room  and perceived  as if i was there....

I agree with what most have said. Generally, an open high end that doesn't sound muffled or rolled off at all. 

My ears only hear up to about 12.5KHz now, but even I can hear "air".  I notice it very much when comparing concert recordings done with decent microphones and preamps versus great microphones and preamps. 

The ones done with better equipment sound more transparent and open - more "real", especially upon close listening with good headphones. You feel like you are in the room and not listening through a microphone. It is difficult to describe, but it is like a thin sheet is taken away from your speakers and you can better hear everything. Whether that is "air" I'm not sure, but it is to me. 

When can talk about frequencies associated with what many people might call “air” but for me it comes down to a type of perception and not necessarily a particular frequency range.  Because I’ve heard some speakers that don’t go stratospherically high in frequencies, but still have a sense of “ air.”

What that generally means to me is a sense of openness to the sound, generally aided by a type of high frequency character.

So for instance, if I’m listening to recordings on quite a number of systems there’s a distinct difference between the frequencies and the recording, and the frequencies of how things sound in the real world, represented by the room I’m in.

You play a track where somebody is tapping some drum rims or the symbols or clapping.  And then in the same room, I snapped my own fingers or lightly clap my hands.

This reveals a difference, where the  sound coming from the speakers sounds “ canned” or sort of artificially limited in the high frequencies, which distinguishes the sound from a “ live happening now” presentation.  

In other words, snapping my fingers are clapping in the room or hearing about talking in the room  there is no sense at all anything but the sound occurring in open air.

So when a speaker has a proper type of airiness I’m thinking of, the high frequencies essentially dissolve into and meld with the sense of the air in the room. 

No sense of artificial rolloff  or canned sound.  The atmosphere of the recording merges with the sense of air in the room I’m listening in, And so there is a more realistic “ that’s a real instrument happening in front of me in real acoustic space” sensation that also means that the timbre of the sound appears to have all the harmonics in the upper frequencies, to describe a realistic depiction of that instrument.

The thing is that this doesn’t necessarily have to be the type of stratospheric frequencies one might associate with a super tweeter or something.   It can come with the dispersion characteristics of the speaker or what is doing in the lower treble… what counts is the perception that the sound I’m hearing is unfettered and that I am sharing the same acoustic space.

(just to be clear: I’m not talking about getting everything to sound like it’s in the room with me.  When I have dialed in my system properly, which includes paying attention to the acoustics of my room, what I get is more sense of being transported to the real space of a recording)

prof, great explanation. On my system, I am sometimes suprised by a sound like drumsticks banging against each other. It comes from somewhere behind and off to the right of the right speaker. It will surprise me and I will look up from what I'm doing, like, huh? Where did that come from? Was it on my stereo or was it real? This happened sometimes when I had far less expensive speakers, but happens a lot with my new speakers.