A sonority hierarchy or sonority scale is a hierarchical ranking of speech sounds. Sonority is loosely defined as the loudness of speech sounds relative to other sounds of the same pitch, length and stress, therefore sonority is often related to rankings for phones to their amplitude. For example, pronouncing the vowel [a] will produce a louder sound than the stop [t], so [a] would rank higher in the hierarchy. However, grounding sonority in amplitude is not universally accepted. Instead, many researchers refer to sonority as the resonance of speech sounds. This relates to the degree to which production of phones results in vibrations of air particles. Thus, sounds that are described as more sonorous are less subject to masking by ambient noises.