What is a pure tone?

• A. A simple sine wave whose frequency stays constant over time.
• B. A complex waveform with many harmonics.
• C. A noise-like signal.
• D. A frequency-modulated tone.

Answer: (A)

A pure tone is a simple sine wave with a single, unchanging frequency. In acoustics, a sine wave contains only one frequency component and has no overtones, so the spectrum is a single line at that frequency. This makes the signal strictly periodic and constant in pitch over time, which is how a pure tone is defined. You can think of it as A(t) = A0 cos(2πf0 t + φ), where f0 stays the same forever.

The other descriptions describe more complex sounds: a complex waveform with many harmonics contains multiple frequency components beyond the fundamental, a noise-like signal has energy across a broad range of frequencies, and a frequency-modulated tone changes its frequency over time, so it’s not a single unchanging frequency.