Which property of a sound wave is most closely related to loudness?

• A. Amplitude
• B. Frequency
• C. Wavelength
• D. Phase

Answer: (A)

Loudness comes from how strong the air pressure variations are in the wave, which is determined by the amplitude. When a sound wave has a larger amplitude, the pressure fluctuations are bigger, sending more energy toward the ear and producing a louder sensation. In physics terms, the intensity is tied to the square of the amplitude, so increasing amplitude makes the sound noticeably louder. The other properties describe different aspects: frequency sets pitch, wavelength relates to spatial spacing of the wave, and phase concerns the alignment of wave cycles. None of those, by itself, determines loudness the way amplitude does.