Which statement about amplitude is true?

• A. It can have units of any acoustic variable.
• B. It is independent of the medium.
• C. It is always unitless.
• D. It is always measured in decibels.

Answer: (A)

Amplitude is the magnitude of a wave’s strength, defined for different physical quantities. Because you can describe a sound wave by how much its pressure varies, how far particles move, or how much energy passes through, the amplitude can have units tied to whichever quantity you’re using. That means amplitude can be expressed in pascals (pressure amplitude), meters (displacement amplitude), meters per second (particle-velocity amplitude), or watts per square meter (intensity amplitude), among others. So the statement that amplitude can have units of any acoustic variable is correct.

It’s not independent of the medium—the medium’s properties, like acoustic impedance and attenuation, influence the observed amplitude for a given source. It isn’t always unitless, since it reflects a real quantity with specific units. And it isn’t inherently measured in decibels; decibels are a logarithmic way to express a ratio or level relative to a reference, not the fundamental amplitude itself.