The singer Mohammed Reza Shajarian is one of Iran 's most celebrated classical singers , and he is joined on their third United States tour together by Hossein Alizadeh on tar -LRB- lute -RRB- and Kayhan Kalhor on kemancheh -LRB- spike fiddle -RRB- in new pieces rooted in the improvisations of Persian classical music and mystical Sufi poetry .