It is interesting to investigate what happens to the frequencies of instruments based
on air columns and strings during a concert as the temperature rises. The sound
emitted by a flute, for example, becomes sharp (increases in frequency) as it warms up
because the speed of sound increases in the increasingly warmer air inside the flute
(consider Eq. 18.11). The sound produced by a violin becomes flat (decreases in fre-
quency) as the strings thermally expand because the expansion causes their tension to
decrease (see Eq. 18.8).
Musical instruments based on air columns are generally excited by resonance. The
air column is presented with a sound wave that is rich in many frequencies. The air col-
umn then responds with a large-amplitude oscillation to the frequencies that match
the quantized frequencies in its set of harmonics. In many woodwind instruments, the
initial rich sound is provided by a vibrating reed. In the brasses, this excitation is pro-
vided by the sound coming from the vibration of the player’s lips. In a flute, the initial
excitation comes from blowing over an edge at the mouthpiece of the instrument. This
is similar to blowing across the opening of a bottle with a narrow neck. The sound of
the air rushing across the edge has many frequencies, including one that sets the air
cavity in the bottle into resonance.