What is Speech Intelligibility

speech intelligibility

Intelligibility of speech is the percentage of speech that a listener can understand. If you can only understand half of what a child is saying then their speech intelligibility rating would be 50%.

​Speech Intelligibility changes with a child’s age. Speech development begins with babbling and then speech matures until older children can say all the sounds in their primary language/s and everyone can understand them.

​Speech Intelligibility is Expected to Improve with a Child’s Age

Younger children are expected to be harder to understand then older children. Speech Sound Developmental Checklists and Speech sound charts can help parents, teachers and carers to see if the level of speech intelligibility is at expected levels for a child’s age.

​The graph below shows that an 18 month old child will have lower speech intelligibility levels than a 3 year old child. This does not mean that a 3 year old will not still make some speech sound errors. It means that they are only using a few speech sound errors and that most people will be able to understand what they are saying. Lynch, Brookshire & Fox (1980), p. 102, cited in Bowen (1998)

​Why does Speech Intelligibility Change with Speech Developmental Stages?

speech intelligibility
There are 3 main areas to consider when looking at speech intelligibility
1 Articulation
Toddlers do not have the fine motor control of their tongues and lips to be able to say all the sounds correctly and so their speech is characterised by speech sound developmental errors.

​These are called articulation errors. For example, sounds like the “r” sound can be difficult for toddlers. The “r” sound requires a fine curling of the tongue and young children often will substitute the “r” sound for an easier sound (e.g., w). You will often hear a 2 year old say “wed” for “red”.

2 Motor speech co-ordination 

​Young children also cannot move their tongue and lips fast enough to keep their speech clear as they try to say longer words and sentences. This is called motor speech co-ordination. It is a bit like doing buttons up. Young children may be able to do easy buttons but might take a long time. As their fine motor co-ordination increase their ability to dress becomes easier and faster.

3 Phonological Processes
​Toddlers use more phonological processes that reduces speech intelligibly than older children. As toddlers do not have the oral motor skills to say words 100% correctly, they use speech sound patterns that simplify words.

​One example of a phonological process is called “cluster reductions”. Speech sound clusters like “sp, sk, dr, bl” are very difficult for young children to say. Most young children reduce the cluster to one sound (e.g., “dep” for “step”, “back” for “black”). Young children use lots of phonological processes reducing speech intelligibility. As children get older they use less and less and so speech intelligibility improves.

​As speech develops children use less articulation errors, their rate & co-ordination of speech improves and they use less phonological errors.

​See the Speech Sound Developmental Checklist to see the ages speech sounds and phonological processes are typically present.

​Speech Intelligibility as a measure of Severity of Speech Delays and Speech Disorders.

speech intelligibility

Children with speech delays and speech disorders will often have lower speech intelligibility percentages than same age children.

​If a 3 year old child has speech delays they may be still making speech sound errors like a 2 year old would be making. This can reduce their speech intelligibility for their age.

​Speech disorders such as dyspraxia of speech (CAS) is characterised by significant reductions in speech intelligibility. Speech intelligibility may be one of the criteria used to determine how functional a child’s speech is in their community. If most people they interact with them cannot understand them, then it would be considered a significant speech impairment.

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Sandra McMahon

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