Physics midterm question: changing sound wave air temperature

Physics 205A Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Questions 12.1, 11.3, Problem 12.1

For a sound wave traveling in air, if the temperature of the air were to decrease, its frequency would __________, while its wavelength would:
(A) remain constant; remain constant.
(B) remain constant; change.
(C) change; remain constant.
(D) change; change.

Correct answer: (B)

Wave speed v depends on the properties of the medium (in this case, the absolute temperature T, where v = (331 m/s)·sqrt(T/273 K). Frequency f depends on the properties of the source. These two parameters can be varied independently of each other.

The wavelength λ is the parameter dependent on both of the independent parameters:

λ = v/f,

such that decreasing the temperature T of the medium will decrease the speed v of sound waves in air, which will not affect the wave frequency f, but will cause the wavelength λ to decrease.

Sections 70854, 70855
Exam code: midterm02gL0u
(A) : 2 students
(B) : 22 students
(C) : 10 students
(D) : 20 students

Success level: 41%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.70