Question: In what century was the the style of voices popular that the Liebeslieder Walzer, Op. 52 was in?

Context paragraphs:
Moravian Duets: Moravian Duets (in Czech: "Moravské dvojzpěvy" ) by Antonín Dvořák is a cycle of 23 Moravian folk poetry settings for two voices with piano accompaniment, composed between 1875 and 1881.  The Duets, published in three volumes, Op. 20 (B. 50), Op. 32 (B. 60 and 62), and Op. 38 (B. 69), occupy an important position among Dvořák's other works.  The fifteen duets of Op. 32 are the most famous part of the cycle.  Popular interest in the "Moravian Duets" was the starting point for subsequent works which propelled Dvořák to international fame.

The Love Songs of Hafiz: The Love Songs of Hafiz (German: "Des Hafis Liebeslieder)" is the name of two song cycles by Karol Szymanowski, Op. 24 with piano accompaniment, and Op. 26 with orchestral accompaniment.  There are six songs in Op. 24, three of them orchestrated in Op. 26, and five additional songs, unique to Op. 26.  They were composed in Vienna, Austria in 1911 and 1914, respectively.  The works represent a transition between the first and second periods of the composer's style.

List of Liebeslieder Walzer stagings: This is a chronological list of stagings of New York City Ballet co-founder and balletmaster George Balanchine's Liebeslieder Walzer, made to the eponymous music by Brahms.

52 chorale preludes, Op. 67: 52 chorale preludes, Op. 67, is a collection of 52 settings of popular Protestant hymns for organ by Max Reger, composed between 1900 and 1902.  Originally published in three volumes between 1900 and 1903 with the cover title "52 Choralvorspiele für Orgel" (52 chorale preludes for organ), the full title of the collection was "Zweiundfünfzig leicht ausführbare Vorspiele zu den gebräuchlichsten evangelischen Chorälen".

Liebeslieder Walzer (ballet): Liebeslieder Walzer is a ballet by New York City Ballet co-founder and ballet master George Balanchine, based on the Brahms' "Liebeslieder Walzer", Op. 52, and their sequel "Neue Liebeslieder", Op. 65.

Liebeslieder, Op. 114 (Strauss): "Liebeslieder " op. 114 is a waltz by Johann Strauss II written in 1852 (not to be confused with Brahms's similarly titled "Liebeslieder Walzer" and "Neue Liebeslieder").  At the time it was conceived, the waltz was titled 'Liebesgedichte' or "Love Poems" and during its first performance, it was even announced as 'Liebesständchen' or "Love Serenade".  The first performance was at the famed Vienna Volksgarten on 18 June 1852 under the composer's direction.

Violin Concerto No. 2 (Paganini): The Violin Concerto No. 2 in B minor, Op. 7, was composed by Niccolò Paganini in Italy in 1826.  In his Second Concerto, Paganini holds back on the demonstration of virtuosity in favour of greater individuality in the melodic style.  The third movement of Paganini's Second Concerto owes its nickname "La Campanella" or "La Clochette" to the little bell which Paganini prescribes to presage each recurrence of the rondo theme.  The character of the bell is also imitated in the orchestra and in some of the soloist's passages featuring harmonics.  The outcome is a very transparent texture, which gains extra charm of gypsy coloration of the rondo theme.  This movement has served as the basis of compositions by other composers, such as the Étude S.140 No.3 "La campanella" by Liszt, and Strauss I's Walzer à la Paganini Op.11.

Neue Liebeslieder: Neue Liebeslieder (New love songs), Op.  65, written by Johannes Brahms, is a collection of Romantic pieces written for four solo voices and four hands on the piano.  They are also known as "Neue Liebesliederwalzer".  "Neue Liebeslieder" were written during the Romantic period between 1869 and 1874.  The text of the songs is adapted from folk songs of various areas of Europe including Turkey, Poland, Latvia and Sicily.  The text for songs 1 through 14 were translated and compiled by Georg Friedrich Daumer in his poem series, "Polydora"; the text for the fifteenth and final song, entitled "Zum Schluß" (In Conclusion), was written by Johann Wolfgang von Goethe.

Ländler: The Ländler (] ) is a folk dance in  time which was popular in Austria, south Germany, German Switzerland, and Slovenia at the end of the 18th century.

Liebeslieder Walzer, Op. 52 (Brahms): Johannes Brahms' Liebeslieder Waltzes ("Liebeslieder-Walzer") are distributed across two opus numbers: Op. 52 and Op.  65.  The waltzes are a collection of love songs in Ländler style for voices and piano four hands.  The lyrics for the "Liebeslieder" come from Georg Friedrich Daumer's "Polydora", a collection of folk songs and love poems.  While there is no concrete record indicating the exact inspiration for the Waltzes, there is speculation that Brahms' motivation for the songs was his frustrated love for pianist Clara Schumann, composer Robert Schumann's wife.

Supporting paragraphs: Liebeslieder Walzer, Op. 52 (Brahms) Ländler
Answer: 18th century