Śaiśirīya Śikṣā 24

24

saṃvṛtaṃ cetyakārasya sarvavarṇo nigadyate
uccāraṇaviśeṣeṇātrākṣarāśrayamucyate

And close [articulatory effort] of the sound a is declared for every phoneme. / According to distinct pronunciation here, it is said to depend on syllables.