The only way to measure it would be in a silence of the musicians, and subtracting the surface noise of the disc. Usually empty room time is edited out.

Izotope RX detects and can remove or measure it, even show you a visual representation..

Perhaps if you describe why you want  to knwo wecould be more helpful.

Lou Judson
Intuitive Audio

On Oct 24, 2016, at 11:37 AM, Steven Smolian <[log in to unmask]> wrote:

> Ambience of the room.
> Steve
> -----Original Message-----
> From: Association for Recorded Sound Discussion List
> [mailto:[log in to unmask]] On Behalf Of Lou Judson
> Sent: Monday, October 24, 2016 2:21 PM
> To: [log in to unmask]
> Subject: Re: [ARSCLIST] Ambience
> Are you meaning signal to noise? Or the ambience of the room it was recorded
> in?