Main Difference
The main difference between Alto and Contralto is that the Alto is a voice range between soprano and tenor in choral music and Contralto is a type of classical female singing voice.
Alto
The musical term alto, meaning “high” in Italian (Latin: altus), historically refers to the contrapuntal part higher than the tenor and its associated vocal range. In 4-part voice leading alto is the second highest part, sung in choruses by either low women’s or high men’s voices. In vocal classification these are usually called contralto and male alto or countertenor.
Such confusion of “high” and “low” persists in instrumental terminology. Alto flute and alto trombone are respectively lower and higher than the standard instruments of the family, though both play in ranges within the alto clef. Alto recorder however is an octave higher and is defined by its relationship to tenor and soprano recorders, and alto clarinet is a fifth lower than B-flat clarinet, already an ‘alto’ instrument. There is even a contra-alto clarinet with a range B♭0 – D4.
Contralto
A contralto (Italian pronunciation: [konˈtralto]) is a type of classical female singing voice whose vocal range is the lowest female voice type.The contralto’s vocal range is fairly rare; similar to the mezzo-soprano, and almost identical to that of a countertenor, typically between the F below middle C (F3 in scientific pitch notation) to the second F above middle C (F5), although, at the extremes, some voices can reach the D below middle C (D3) or the second B♭ above middle C (B♭5). The contralto voice type is generally divided into the coloratura, lyric, and dramatic contralto.
Alto (noun)
A musical part or section higher than tenor and lower than soprano, formerly the part that performed a countermelody above the tenor or main melody.
Alto (noun)
A person or musical instrument that performs the alto part.
Alto (noun)
An alto saxophone
Contralto (noun)
The lowest female voice or voice part, falling between tenor and mezzo-soprano. The terms contralto and alto refer to a similar musical pitch, but among singers, the term contralto is reserved for female singers; the equivalent male form is counter-tenor. Originally the contratenor altus was a high countermelody sung against the tenor or main melody.
Contralto (noun)
the lowest female singing voice
“she sang in a high contralto”
Contralto (noun)
a singer with a contralto voice.
Contralto (noun)
a part written for a contralto voice.