If a trumpet or a flute is flat, the pipe should be shortened to increase the pitch of the sound.
Explanation :
Musical instruments such as trumpet or flute work on the concept of sound waves. A flute or trumpet acts like a hollow tube with some holes. When the air is blown inside it, longitudinal sound waves are produced due to different wave frequencies.
If we reduce the length of the pipe; it produces higher frequency inside the tube. Whereas, longer tubes create sound waves of lower frequencies, generating low sound.
Hence, the length of a flute or trumpet should be shortened if it’s flat.