Final answer:
The 'A' value in the wavelength and velocity equation represents the amplitude of a wave, which increases with louder sounds and is tied to the wave's energy but is independent of its velocity.
Step-by-step explanation:
The real-world measurable property of waves that the "A" value in the equation relates to is the amplitude of the wave. When speaking about the relationship between wavelength and velocity of a wave, the amplitude does not directly factor into the calculation; velocity (v) is the product of the wave's frequency (ν) and its wavelength (λ). However, in terms of the physical characteristics of a wave, when you speak louder, the amplitude of the sound waves you produce increases, causing the sound to be louder. This is because amplitude represents the maximum displacement of points on a wave, which directly correlates with the wave's energy. Hence, a wave with greater amplitude will carry more energy, which in the case of sound waves, translates to higher volume. This concept is separate from the wave velocity, which is independent of amplitude but depends on the medium's properties and the wave's frequency and wavelength.