The Traditional Approach
Chances are, you’re already familiar with the traditional approach for teaching sounds. The traditional method teaches children the specific sounds they have difficulty pronouncing, one by one.

Remember Elmer Fudd from Looney Tunes (“Shhh. Be vewy vewy quiet, I’m hunting wabbits”)? A traditional approach would teach Elmer how to pronounce “r.” This would be great for Elmer, since the “r” sound is the only sound he has a hard time pronouncing.

But what if there are a lot of sounds my child can’t pronounce?
Oftentimes, parents seek out speech therapy when their child is “hard to understand.” This is absolutely a valid concern! Intelligibility, or how “understandable” the child’s speech is, is important for communicating wants and needs.

If a child can’t be understood, it’s not surprising that they can become upset or frustrated (and parents can get frustrated too!). To help a child with multiple sound errors to make quick progress we can use the phonological complexity approach. The complexity approach teaches more challenging (or complex) sounds first, which in turn leads to the acquisition of more simple sounds without necessarily having to practice them.

What is the Complexity Approach?
The short explanation is this:
If you teach the child a challenging or “complex” sound:
They will learn that sound, AND
They will also learn a bunch of “easier” sounds too… automatically!

It might sound like magic, but it’s based on over 30 years of research demonstrating its effectiveness. The longer explanation has to do with the different categories that sounds fall into, and how the different types of sounds relate to each other in a hierarchical manner (scientifically referred to as implicational laws, language universals, or language laws).

Based on these sound relationships, along with knowing what sounds a child can and can’t yet pronounce, a speech-language pathologist using the complexity approach will know which specific complex cluster sounds to teach.

Some examples of complex clusters are:

“spl” like in “splat”

“spr” like in “sprinkles”

“str” like in “string”

Sources Cited (APA): 

Gierut J. A. (2007). Phonological complexity and language learnability. American journal of speech-language pathology, 16(1), 6–17. https://doi.org/10.1044/1058-0360(2007/003)