A trombone has a variable length. Blowing into the mouthpiece causes air in the tube of the horn to vibrate, and the waves set up by the vibrations subsequently reflect back and forth in the horn to create standing waves.

As the length of horn is made longer, what happens to the frequency?

The frequency either increases or decreases, depending upon the diameter of the horn.

The frequency either increases or decreases, depending upon how hard the horn player blows.

The frequency increases.

The frequency decreases.

The frequency remains the same.