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Because all harmonics are present, and because the fundamental frequency is given by (18.11) Despite the similarity between Equations 18.7 and 18.11, you must remember that v in If a pipe is closed at one end and open at the other, the closed end is a displace- ment node (see Fig. 18.18b). In this case, the standing wave for the fundamental mode 1 " v/4L. As Figure 18.18b shows, the higher-frequency waves that satisfy our condi- tions are those that have a node at the closed end and an antinode at the open end; 1 , 5f 1 , . . . . f n " n
v 2L
n " 1,
2,
3, . . . S E C T I O N 18 . 5 • Standing Waves in Air Columns 561 In a pipe closed at one end, the natural frequencies of oscillation form a harmonic We express this result mathematically as (18.12) 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 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- f n " n
v 4L
n " 1,
3,
5, . . . Quick Quiz 18.7 A pipe open at both ends resonates at a fundamental frequency f open . When one end is covered and the pipe is again made to resonate, the fundamental frequency is f closed . Which of the following expressions describes how these two resonant frequencies compare? (a) f closed " f open (b) f closed " f open (c) f closed " 2 f open (d) f closed " f open Quick Quiz 18.8 Balboa Park in San Diego has an outdoor organ. When the air temperature increases, the fundamental frequency of one of the organ pipes 3 2 1 2 Natural frequencies of a pipe open at both ends Natural frequencies of a pipe closed at one end and open at the other |