In the context of sound waves, what does the term 'loudness' refer to?

Loudness is a perceptual attribute of sound that relates specifically to the amplitude of the sound wave. Amplitude refers to the height or strength of the wave, and greater amplitude results in a louder sound. In practical terms, a sound wave with a higher amplitude will produce a greater pressure variation in the air, which the human ear interprets as increased loudness.

While the other concepts such as the speed of sound, quality of the sound, and frequency are important characteristics of sound waves, they do not determine loudness in the same way. Speed relates to how fast the wave travels, quality involves the timbre or color of the sound, and frequency affects the pitch of the sound, not its loudness. Thus, loudness is directly connected to the amplitude of the wave, making that the correct understanding in relation to sound waves.