Final answer:
The soprano brass instrument with a brilliant timbre is the Trumpet, known for its bright and piercing sound.
Step-by-step explanation:
The soprano brass instrument that is sometimes described as possessing a brilliant timbre is the Trumpet. The trumpet is known for its bright, piercing sound that stands out in an ensemble, which is due to its relatively small size and the way it is played. As a brass instrument, the trumpet's sound is created when the player vibrates their lips against the mouthpiece, and pitch is changed by pressing valves to alter the length of the air column inside the instrument. This contrasts with instruments like the trombone, where a sliding tube changes the length, or the tuba, which has a much more expansive tubing for lower frequencies. Brass alloys are essential in making these instruments durable and giving them their unique sound qualities.