---
title: "A sub-audible sound source emitting a continuous wave of frequency \\(f_0 = 20 \\text{ Hz}\\) moves to the right at a constant speed of \\(v_s = 170 \\text{ m/s}\\) through still air, where the speed of sound is \\(v = 340 \\text{ m/s}\\). Observer 1 is stationary and positioned directly ahead of the source along its line of motion. Observer 2 is stationary and positioned at an angle of \\(60^\\circ\\) relative to the line of motion, as shown in the diagram. Which of the following correctly gives the frequency \\(f_1\\) measured by Observer 1 and compares the wave speed \\(v_1\\) measured by Observer 1 to the wave speed \\(v_2\\) measured by Observer 2?"
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url: "https://nerd-notes.com/ubq/117453/"
date_modified: "2026-08-04T06:52:12+00:00"
---

# A sub-audible sound source emitting a continuous wave of frequency \(f_0 = 20 \text{ Hz}\) moves to the right at a constant speed of \(v_s = 170 \text{ m/s}\) through still air, where the speed of sound is \(v = 340 \text{ m/s}\). Observer 1 is stationary and positioned directly ahead of the source along its line of motion. Observer 2 is stationary and positioned at an angle of \(60^\circ\) relative to the line of motion, as shown in the diagram. Which of the following correctly gives the frequency \(f_1\) measured by Observer 1 and compares the wave speed \(v_1\) measured by Observer 1 to the wave speed \(v_2\) measured by Observer 2?

A sub-audible sound source emitting a continuous wave of frequency \(f_0 = 20 \text{ Hz}\) moves to the right at a constant speed of \(v_s = 170 \text{ m/s}\) through still air, where the speed of sound is \(v = 340 \text{ m/s}\). Observer 1 is stationary and positioned directly ahead of the source along its line of motion. Observer 2 is stationary and positioned at an angle of \(60^\circ\) relative to the line of motion, as shown in the diagram. Which of the following correctly gives the frequency \(f_1\) measured by Observer 1 and compares the wave speed \(v_1\) measured by Observer 1 to the wave speed \(v_2\) measured by Observer 2?

![A horizontal dashed line represents the straight-line trajectory of a sound source moving to the right. The source is represented as a small filled circle labeled S with a rightward vector arrow labeled v_s. Concentric circular wavefronts surround the source, spaced closer together on the right and farther apart on the left. Observer 1 is shown as a small square located on the horizontal trajectory line directly to the right of the source, labeled Observer 1. Observer 2 is shown as a small square located above and to the right of the source, labeled Observer 2. A line segment connects the source to Observer 2, forming an angle labeled \theta = 60^\circ relative to the horizontal path. No other labels, lines, text, or axes appear.](https://nerd-notes.com/wp-content/uploads/ubq-frq-generated/stem-fig-1-1785826332-3izH1n.jpg)

- **A.** \(f_1 = 30 \text{ Hz}\), and \(v_1 > v_2\)
- **B.** \(f_1 = 30 \text{ Hz}\), and \(v_1 = v_2\)
- **C.** \(f_1 = 40 \text{ Hz}\), and \(v_1 > v_2\)
- **D.** \(f_1 = 40 \text{ Hz}\), and \(v_1 = v_2\)

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/117453/*