The Concise Oxford Dictionary of World Religions (2033 page)

(i) 
spho
a
, arising from the eternal, unmoving principle with illuminating power (
akti
);
(ii) 
n
da
, perceptible only to a poet or
i
;
(iii) an
hata, potential (e.g. a thought) but not expressed;
(iv) 
hata, sound of all kinds, whether humans can hear it or not.
abda has power in its own right, not just in speech, especially in
mantras
or in bells and drums:
iva's
drum (
amaru
) manifests creation.
abda-brahman is the ultimacy of sound devoid of attributes, the realization of
Brahman
. Initially, this was equated with the
Vedas
, but in the
Upani
ads
it is Brahman.

ABAD