It's commonly practiced to avoid music during at least part of Sefirah and the entirety of the Three Weeks. Many of those who hold of this stringency also hold that certain types of a capella are permissible.
What is considered an instrument for the purposes of this minhag, according to those who permit vocals?
- Anything which isn't a voice (which would prohibit, for example, clapping, snapping, or whistling)?
- Anything which isn't produced by a human mouth (which would prohibit clapping and snapping, but not whistling)?
- Anything which requires the aid of a device external to the human body (which would not prohibit clapping or snapping, either, but would prohibit, for instance, banging rhythmically on the table)?