An echea, or sounding vase (literally echoer), is a pot, chamber or vessel that is similar in function to a modern-day bass trap. They were used in ancient Greek theaters to enhance the voices of performers through resonance. They were typically made of bronze, but were also (more economically) of earthenware.
Echea were used with a, "due regard to the laws and harmony of physics," according to Roman writer Vitruvius. The vases operated by resonance, enhancing key frequencies of the performers' voices and absorbing those of the audience, which altered the sound in the theater to make the performers' voices clearer and more lush. The size and shape of a theater determined the number of echea used, and their positioning within it.
Both their use in Roman times and usefulness have been debated. Thomas Noble Howe wrote in his commentary on Vitruvius' Ten Books on Architecture, "These vessels, bronze or clay, may be another example of Vitruvius singling out a highly technical feature of Greek architecture that was uncommon, but between eight and sixteen potential sites with evidence of echea have been identified. It is debatable whether such vessels amplified or deadened sound."
Similar devices were used in early churches. Some were discovered in the vaulted ceiling of the choir of Strasbourg Cathedral, as well as in mosques dating to the 11th century.