From the archives...
The following question was originally posted on the (now discontinued) LOE Forum.
As in architecture (IPA ɑr kɪˌtɛk tʃər), the sound "T" is not /t/ as in your phonogram but it is tʃ sound. Can you explain that?
Yes! What's happening here is called assimilation.
In architecture, as well as many words where a long U follows a T or S (like sure, sugar, picture), and sometimes when it follows D (educate), the shape of the consonant in the mouth is distorted by the long /ū/ sound that comes after it.
In these words, the consonant sound assimilates to the sound of the vowel that follows it, taking on a new shape that is closer to the shape of the vowel in the mouth. This way, the pronunciation shifts to one that is easier to say. If you try saying the 'true' sounds - /tyoo/ or /syoo/ - and say the word more and more quickly, you can feel why this has happened and how the assimilated consonant sound is easier to pronounce. Similarly, the vowel sound /ū/ in these words becomes a schwa, an unstressed vowel sound, for ease of pronunciation.
Assimilation is actually quite common. In some words, the consonant is always pronounced in an assimilated way (nature, architecture, picture, factual, treasure). In other words, you will hear a more assimilated consonant in some speakers' pronunciations than others (endure, mature).
If you are using LOE curriculum, you'll find information about assimilation in in Foundations D and in lessons 28 and 29 of Essentials. It is also discussed in Chapter 16 of Uncovering the Logic of English.