Pronunciation: Assimilation of /d/ and /b/

source: BBC Learning English     2016年12月23日
Tim's  finding out what happens when one word ends in /d/ and the next begins in /b/ - and talking speed boats with some Londoners...
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Voxpops: It’s a speed boat. Speed boat.

Tim: In everyday speech, when we’re speaking fluently, if one word ends in a vowel sound followed by a /d/ and the next begins in a /b/, then the /d/ changes to a /b/ and then merges with the /b/ from the following word. So ‘speed boat’, becomes ‘speebboat’. Watch again, this time in slow motion. Look at the mouth shape. Can you see /d/ or is it just /b/?
This is another case of assimilation. Here are some more examples.

Examples
My son is a really good boy.
I’m sorry if I was rude before.
My car had to be towed back to the garage.
You should buy him a present.