a cappella vs vocal

a cappella

noun
  • A vocal performance with no instrumental accompaniment. 

adj
  • Singing solely or mainly without instrumental accompaniment. 

  • Related to a form of purely vocal music mostly associated with American college performance groups. 

adv
  • In a manner of a choir with no instrumental accompaniment; literally, "in the style of the (Sistine) Chapel (in Rome)", such as a musical Mass done a cappella. 

vocal

noun
  • A musical performance involving singing. 

  • A part of a piece of music that is sung. 

  • A man in the Roman Catholic Church who has a right to vote in certain elections. 

  • A vocal sound; specifically, a purely vocal element of speech, unmodified except by resonance; a vowel or a diphthong; a tonic element; a tonic. 

adj
  • Relating to, composed or arranged for, or sung by the human voice. 

  • Of, pertaining to, or resembling the human voice or speech. 

  • Consisting of, or characterized by, voice, or tone produced in the larynx, which may be modified, either by resonance, as in the case of the vowels, or by obstructive action, as in certain consonants, such as v, l, etc., or by both, as in the nasals m, n, ng. 

  • Used in the production of speech sounds. 

  • Synonym of expressive. 

  • Uttered or modulated by the voice; expressed in words. 

  • Full of voices. 

  • Expressing opinions or feelings freely, loudly, or insistently. 

  • Having or exercising the power of producing voice, speech, or sound. 

  • Synonym of vocalic. 

How often have the words a cappella and vocal occurred in a corpus of books? (source: Google Ngram Viewer )