Metrical Phonology - Metrical Trees

Metrical Trees

Linguistic prominence in metrical phonology is partially determined by the relations between nodes in a branching tree, in which one node is Strong (S) and the other node or nodes are Weak (W). The labels Strong and Weak have no inherent phonetic realization, and only have meaning relative to the rest of the labels in the tree. A Strong node is stronger than its Weak sister node. The most prominent syllable in a phrase is the one that does not have any Weak nodes above it. This syllable is called the Designated Terminal Element. In the example tree (1), the syllable '-ci-' is the Designated Terminal Element.

(1)

Metrical trees allow us to change the stress pattern for a phrase by switching S and W sister nodes. The tree in (1) represents the metrical structure for the sentence "Doctors use Penicillin." when the sentence is providing all new information. This is called broad focus, and might be used in response to a question like "What did you learn at the hospital today?" The same metrical structure would be used when the sentence has narrow focus on the word 'penicillin'; for example, if it was used in response to a question like "What do doctors use to treat that disease?". However, we need a new metrical structure to put narrow focus on the word 'doctors', for example, if the phrase is used in response to the question "Who uses penicillin?" In this case, the S and W nodes at the intermediate phrase (ip) level have to switch, resulting in (2).

(2)

In Metrical Phonology there has been debate about whether nodes on metrical trees must have two children, making them binary branching, or whether they can have any number of children, making them n-ary branching . Proponents of binary branching trees have claimed that such trees can constrain the restructuring of very long and very short constituents because new nodes created in this restructing have to correspond to nodes in the original tree. Proponents of n-ary branching trees point out that only multiple branches allow a limited number of tree levels, which can correspond to predetermined levels of prosodic constituents, whereas binary branching trees require intermediate levels that do not correspond to any prosodic constituent. A number of levels of prosodic constituents have been proposed, including: moras, syllables, feet, phonological words, clitic groups, phonological phrases, intermediate phrases, intonational phrases, and phonological utterances. The relations between prosodic constituents at different levels is commonly thought to be governed by the Strict Layer Hypothesis (SLH). This hypothesis states that in metrical trees, all prosodic constituents at a particular level consist exclusively of constituents from the level below. The SLH forbids a number of types of tree structures, including trees in which: a node has two parents in the level above, a node has two or more different types of children, a node has children from a level that is not the level immediately below it, a node does not correspond to any of the specified levels, or a node has children of the same type as itself.

The various levels of the prosodic hierarchy are independently justified by the phonological phenomena that make reference to them. For instance, in English the sounds /p/, /t/, and /k/ are aspirated (followed by a puff of air) only if they are the first segment in a foot. Similarly, in the Gorgia Toscana variety of Italian, the intonation phrase is the domain of a rule that changes voiceless plosives (/p/, /t/, /k/) between vowels into fricative consonants, like /θ/ (th) and /h/.

In addition to describing prominence relations between words, metrical trees can also describe prominence relations within words. Indeed, a set of rules developed by Liberman and Prince can be used to quite accurately predict stress in English words. Their Lexical Category Prominence Rule states that the second node in a pair of sister nodes is labeled W unless one of a number of conditions are met, such as the node branching or dominating a particular suffix, in which case it is labeled S. Allowable tree structures and node labels for a particular word in Liberman and Prince's system are constrained by the two-value feature, which can be assigned to segments or syllables by separate rules that refer to the number and type of segments in the syllable and the syllable's position in the word. Syllables that are can only be immediately dominated by a W node. However, syllables that are can be immediately dominated by S or W nodes.