Low Interval Limits
The reason why some chord voicings sound muddy and unfocussed often lies in the fact that they violate the so-called low interval limits. These limits are guidelines for every interval structure and the lowest possible position they can be played together without sounding muddy. They are NOT definitive rules but a good guide to avoid muddyness.
Depending on the level of consonance, certain intervals like the perfect fifth can be put lower than for example the minor second. These are the low interval limits for each interval up to the major tenth (click to enlarge):
So to check whether THIS voicing will sound muddy or not as a C7 voicing (C1 in the external link being middle C), you simply check all the intervals within the structure and see whether they fit into the chart or not. In this case, the voicing is not muddy.
It’s a different matter with THIS voicing that contains a “muddy” major third between the F and the A which is actually a perfect fourth lower than the low interval limit for the major third allows, however note that the minor third between C and Eb would be within the limit. THIS case of a voicing for Am7 is a special case. By the looks of it, it seems like it fulfills the low interval limit, however in a case where the lowest note of the voicing is not the root note (for example this voicing could be a trombone voicing while the tuba plays the root), when checking for low interval limits, you have to assume that the root is present, so basically virtually adding it to the voicing, it would look like THIS which shows the violation of the low interval limits.
As usual, a word of how much importance to put into these limits. They are as many such charts and rules in music just a guideline. The amount of mud a voicing generates is highly depending on its dynamic and how many higher harmonics the sound contains. So soft low voicings on for instance strings are more forgiving than loud ones on brass. Also, there might be instances where you want a specific low rumbling sound where you deliberately want to ignore these rules for effect. So as a bottom line this chart helps you prevent mud in practically any case but there might be situations where you would want to ignore it.