The Sages enacted a prohibition against playing musical instruments on Shabbat or Yom Tov, out of concern that one may come to build or fix such an instrument, which would constitute a Torah violation. They further forbade engaging in other activities related to music; thus, for example, according to Sephardic practice, one may not clap or dance on Shabbat or Yom Tov. (See the Daily Halacha entitled "Door Knocking, Whistling, Clapping, & Making Sounds on Shabbat", dated Dec. 26, 2004)
The question thus arose as to whether a Chazan may use a tuning fork on Shabbat. A tuning fork is a metal apparatus that a singer strikes to create a sound in the desired pitch. He puts the fork to his ear to hear the sound, thus helping him find the proper pitch for the given melody. Would striking a tuning fork be forbidden on Shabbat due to the prohibition of "Hashma'at Kol" – playing music or producing sounds on Shabbat?
Chacham Ovadia Yosef addresses this issue at length in his work Yabia Omer (vol. 3, Orach Chayim 22), where he cites two different views on this issue. Many authorities permitted the use of tuning forks on Shabbat because it creates a very faint sound; the Chazan must hold the fork near his ear to hear the sound it produces. Secondly, the tuning fork plays only a single note, and not any sort of tune. This was the position taken in the work "Ve'zot Le'yehuda," and it is recorded that the cantors in the synagogue of Rabbi Yom Tov Yisrael in Egypt used a tuning fork on Shabbat with the Rabbi's consent. Likewise, the Aruch Ha'shulchan (work of Halacha by Rabbi Yechiel Michel Epstein, Byelorussia, 1829-1908) justified the practice to use tuning forks on Shabbat.
Many other authorities, however, disagreed, and forbade the use of this instrument on Shabbat. This is the position taken by the Mishna Berura (commentary to the Shulchan Aruch by Rabbi Yisrael Meir Kagan, the "Chafetz Chayim," Lithuania, 1839-1933), in Siman 338 (Se'if Katan 4).
In his work Halichot Olam (vol. 4, p. 73; listen to audio for precise citation), Chacham Ovadia Yosef rules that one should preferably refrain from using a tuning fork on Shabbat and Yom Tov, in deference to those authorities who rule stringently on this issue. However, one should not object forcefully to those who are lenient in this regard, as they do have authorities on whom to rely.
Summary: Chazanim should not use tuning forks on Shabbat, though one should not forcefully object to those who do use these instruments, as many authorities indeed rule leniently in this regard.
