Ask Language Log: Easy but unused initial clusters?

From Bob Moore:

I have recently become interested in an important Alaska native weaver named Jennie Thlunaut. The linguistic question is about the initial consonant cluster of her last name, "thl". My initial reaction on seeing the name was that this consonant cluster was not phonotactically possible in English, and that it would be hard for me to pronounce. I was surprised to find that it was very easy for me to pronounce, without the perception of a highly reduced vowel separating the initial consonants that I usually experience when trying to pronounce a foreign word containing a consonant cluster not found in English.

I confirmed that "thl" does not seem to be a possible word-initial consonant cluster in English by grepping for all case-insensitive instances of " thl" in the English Gigaword corpus. I found something between 100 and 200 of these, and I examined all of them, finding then all to be either (1) foreign words or names, (2) attempts to represent the pronunciation of foreign words or names, (3) representations of "lisping" in English, or (4) typos.

I am puzzled that there would be an easy-to-pronounce phonological sequence that is completely unused in a language. It seems like coding efficiency would favor using any sequence that is easy to pronounce. Is there a more general phonological principle in English that would block the use of "thl"? Are there other easy-to-pronounce consonant clusters that are not used in English?

There are two different questions here, I think:

1. Why are some of the consonant clusters (or other sound sequences) that are not used in a given language easier for native speakers to pronounce, while others are harder?
2. Why are some of the easy-to-produce sound sequences nevertheless not used in a given language?

The answer to the first question probably has a universal part and a language-specific part. (I say "probably" because I don't know of any research results, I'm just reasoning from general principles.)

The universal part is that syllable centers are generally sonority peaks, with sonority rising from the start of the syllable to the center, and then falling to the end. Whether this is exactly the right way to think about it or not, it's true that fricative+liquid onset sequences are more "natural" in some general sense than liquid+fricative onset sequences would be — and the opposite is for syllable-final sequences.

And the language-particular part is that English has lots of /fl/ and /sl/ onsets, with /ʃl/, /vl/, /zl/, etc. being more marginal but certainly possible. So adding /θl/ is not nearly as big a step as if English had no initial clusters at all, or no fricative+liquid clusters.

What about the second question, "Why are some of the easy-to-produce sound sequences nevertheless not used in a given language?"

Presumably this is because phonotactic redundancy — which is a fancy way of saying that some of the natural sound sequences are not used — creates a sort of error-correcting code at the phonetic level.