1867 A. M. Bell 40 For ‘Back’ Vowels, the back of the tongue forms a narrow aperture with the soft palate, or (for the ‘low-back’) with the pharynx.
1901 1 19 In these and similar words the r must invariably be preceded by a low-back vowel.
1965 41 346 A low-back /ə/.
1994 S. Chalker & E. Weiner 231 The sound../ɑː/ as in hard and heart is a low back vowel.