That's dumb. The majority of the conversational vocal scale is below 500 Hz, no wonder it sounds like mumbling. Most people aside from trained stage actors and some radio/television broadcasters do not enunciate very well at all. Lazy, sloppy, monotone voices are difficult to understand, but that's what people give us by and large. Some of the worst are the ones from the orthodontics generations, starting in the 1980s, where braces and fixtures cramped kids teeth into their mouths so much that there is no room for their tongues to move and they speak like they have a mouth full of gravy. Another side effect of those generations is being television-educated instead of reading-educated which results in an absolute lack of vocal punctuation.