WordNet (r) 3.0 (2006):

sousaphone
    n 1: the lowest brass wind instrument [syn: bass horn,
         sousaphone, tuba]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "sousaphone":
   alpenhorn, alphorn, althorn, alto horn, ballad horn, baritone,
   bass horn, brass choir, brass wind, brass-wind instrument, brasses,
   bugle, bugle horn, clarion, cornet, cornet-a-pistons,
   corno di caccia, cornopean, double-bell euphonium, euphonium,
   helicon, horn, hunting horn, key trumpet, lituus, lur, mellophone,
   ophicleide, orchestral horn, pocket trumpet, post horn, sackbut,
   saxhorn, saxtuba, serpent, slide trombone, sliphorn, tenor tuba,
   tromba, trombone, trumpet, tuba, valve trombone, valve trumpet