bumper

bumper
 n.— «The Konzertstuck for four horns, which will preface the BBC SSO’s performance of Manfred, also looks like being allowed to speak for itself. When, some years ago, the work was presented by another Scottish orchestra, the audience must have been baffled by the sight, at the front of the platform, of five hornists. The fifth was what has come to be called a bumper, an extra player employed to relieve the principal hornist and “bump up” the volume of tone.» —“Too much Mozart? Try the music of angels” by Conrad Wilson Ther Herald (Glasgow, Scotland) July 5, 2006. (source: Double-Tongued Dictionary)