According to the change of intensity in time, noise can be described as which of the following?

• A. Permanent and impermanent
• B. Broadband and tonal
• C. Both permanent/impermanent and broadband/tonal
• D. None of these

Answer: (C)

Noise is described by two independent aspects: how it changes over time (continuous versus intermittent) and what frequencies it contains (broadband versus tonal). A noise source can be continuous (permanent) or intermittent (impermanent), and it can have a broad spectrum of frequencies (broadband) or be dominated by a single frequency or narrow band (tonal). Because both time behavior and spectral content are relevant, the most complete description is that noise can be both permanent/impermanent and broadband/tonal. For example, a continuously running machine that emits many frequencies is permanent and broadband; a brief tuned beep is impermanent and tonal.