Loudness in sound waves depends on amplitude

Steps:

• Identify that loudness is the human perception of sound intensity.
• Recall that sound intensity is proportional to the square of the wave's amplitude.
• Note that frequency determines pitch, which stays constant for the same note.
• Confirm that speed and wavelength are fixed for a given note in air.

Why A is correct:

• Loudness increases with amplitude because intensity $I \propto A^2$, where $A$ is amplitude, per the wave energy formula.

Why the others are wrong:

• B: Frequency affects pitch (note), not loudness; higher frequency raises pitch without changing volume.
• C: Speed of sound is constant in a medium like air, independent of loudness.
• D: Wavelength $\lambda = v/f$ depends on frequency and speed, not loudness.

Final answer: A