1916 Chambers's Jrnl. Jan. 61/1 Expression has to be ignored to a very considerable extent during the task of producing or recording, because the sound-line, into which the effects of a full orchestra have to be compressed, is of microscopic proportions.
extracted from soundn.3
sound-line
4.sound-line, ‘the tow-line carried down by a whale when sounding’ ( Cent. Dict.).