51 The precise meaning of the term translated “polished bronze” (χαλκολιβάνῳ, chalkolibanō), which appears nowhere else in Greek literature outside of the book of Revelation (see 2:18), is uncertain. Without question it is some sort of metal. χαλκολίβανον suggests “fine brass/bronze.” L&N 2.57 takes the word to refer to particularly valuable or fine bronze, but notes that the emphasis here and in Rev 2:18 is more on the lustrous quality of the metal.
52 Or “that has been heated in a furnace until it glows.”
53 Grk “sound,” but the idea is closer to the roar of a waterfall or rapids.

In other versions

his feet were like burnished bronze, refined in a furnace, and his voice was like the roar of many waters.

His feet were like bronze glowing in a furnace, and his voice was like the sound of rushing waters.

His feet were like fine bronze as it is fired in a furnace, and his voice like the sound of cascading waters.

His feet were like polished bronze refined in a furnace, and his voice thundered like mighty ocean waves.

and his feet were like fine bronze when it has been fired in a furnace, and his voice was like the sound of many waters,

And his feet like unto fine brass, as if they burned in a furnace; and his voice as the sound of many waters.

Public Domain